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1/98 mIRCIDanOmpGS-pjd

DEVELOPMENT OF NEW SYNTHETIC ROUTES TO CHIRAL
INTERMEDIATES AND SYNTHESIS, CHARACTERIZATION OF NOVEL
MEMBRANE BASED ADVANCED MATERIALS

By

Guijun Wang

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Chemistry

1999

ABSTRACT
DEVELOPMENT OF NEW SYNTHETIC ROUTES TO CHIRAL
INTERMEDIATES AND SYNTHESIS, CHARACTERIZATION OF NOVEL
MEMBRANE BASED ADVANCED MATERIALS
By

Guijun Wang

The main objective of this dissertation is to develop chiral technology for the synthesis of
chiral building blocks especially for use in the pharmaceutical and advanced material
industries. The two major thrusts are carbohydrate and amino acid based asymmetric
synthesis and the design and synthesis of novel, stabilized, membrane lipid based systems
which have potential applications in the fabrication of molecular photonic-electronic

devices, biosensors, and drug delivery systems.

There are two parts in this dissertation. The first part (chapters 2, 3) describe about the
development of new asymmetric synthetic routes to chiral building blocks and chiral drugs
for the pharmaceutical industry. This includes the development of new synthetic routes to
chiral 3-carbon synthons which are key building blocks for many important compounds such
as antiviral drugs, cardivasicular agents, chiral membrane lipids and other glycerol ‘
derivatives. It is also describes the development of new asymmetric synthesis of important
drugs such as L-carnitine, R-y-amino-fi-hydroxybutyrate (GABOB) and S-beta blockers.
New synthetic routes to chiral cis-l-amino-Z-indanols, key building block for HIV protease
inhibitors and important catalysts in asymmetric synthesis are also demonstrated. These new

routes have Significant advantages over the existing routes, giving high yields and high

optical purity and by Simple processes that are highly efficient and applicable to industry.
The chirality of the products are conserved from the chiral starting carbohydrate and amino

acids.

The second part of the dissertation (chapters 5, 6 &7) is about the design, synthesis and
properties of new advanced materials based on membrane mimics. These include stabilized
phospholipid analogs and liposomes, chiral multifunctional self assembled 2-dimensional
polydiacetylene containing systems, 2-dimensional conducting polyamide conducting thin
films and other advanced materials. A tail-to-tail dimer of phosphatidyl ethanolamine was
prepared and shown to readily form very unifome flat self assembled larnellar
supramolecular arrays and liposome that are stable at temperatures up to 80°C. This
extremely stable and readily functionalizable dimeric phospholipid has potential uses in the
fabrication of biomaterials, stable membrane models and liposome drug delivery systems.
In chapter 6, two new, very accessible, chiral, self-assembling phospholipid analogs
containing diacetylenic units in the middle of their acyl chains were prepared by very Simple
and highly efficient routes. They formed very uniform, extremely flat, thin films which are
readily polymerized to give extensively conjugated systems which absorbed well out into the
near infrared region unlike typical polydiacetylenes. The results indicate that this is an
excellent approach for preparing ordered polydiacetylene systems for use in designing
advanced materials. The last chapter introduces a new approach for obtaining long range
order and perfect alignment of long chain polydiacetylene by anchoring them along a

polymer backbone.

ACKNOWLEDGMENTS

I would like to thank my advisor Dr. Rawle I. Hollingsworth sincerely for his valuable
advice, guidance and support. I am grateful that he has led me into a new world of scientific
frontiers, including organic chemistry, life science and material science. I would like to
extend my sincere gratitude to my committee members Dr. C. K. Chang, Dr. G. J.
Blanchard, Dr. K. A. Berglund for their support and suggestions. My special thanks go to
Dr. M. W. Rathke, Dr. G. L. Baker for their suggestions. I also would like to thank the.
NMR facility of Chemistry Department and the computer graphics facility of Biochemistry
Department. And I also would like to thank the past and current group members in

Hollingsworth's lab.

Finally, Iwould like to thank my family and my friends for all their support and

understanding.

iv

LIST OF SCHEMES .................................................. viii
LIST OF FIGURES ..................................................... x
Chapter 1. Introduction for Synthesis of Chiral Building Blocks .......... 1
1.1. About chirality ............................................... 3
1.2. Methods for obtaining chirality .................................. 6
1.3. Chiral 3 and 4-carbon building blocks and their applications .......... 16
1.4. Introduction of cis-l-Amino-2-indanol ........................... 29
1.5. References ................................................. 36
Chapter 2. The Direct Conversion of (S)-3-Hydroxy-y-Butyrolactone to
Chiral 3-Carbon Building Blocks and Facilitation of Synthetic
Routes to L-Carnitine, (R)-GABOB and (S)-beta-Blockers . . . .43
2.1. Introduction ................................................ 45
2.2. Results and disussions ....................................... 52
2.2.1. Preparation of chiral 3-carbon synthons .................... 52 .
2.2.2. Synthesis of L-carnitine and R-GABOB intermediates ......... 54
2.2.3. Exploration of new practical routes to chiral S-B-blockers ...... 60
2.3. Conclusions ................................................ 63
2.4. Experimental Section ......................................... 64
2.5. References ................................................. 71
Chapter 3. The Exploration of New Synthetic Routes to Important Chiral
Intermediates from Amino Acids ........................... 74
3.1. Introduction ................................................ 76
3.2. Results and discussion ........................................ 87
3.3. Conclusions . . . . . . . ......................................... 89
3.4. Experimental Section ........................................ 90
3.4.1. Preparation of C-4 carbon intermediates .................... 90
3.4.2. Preparation of aminoindanol precursors .................... 91
3.4.3. Synthetic routes of arninotetrolin precursors ................. 94
3.5. References ................................................. 96

TABLE OF CONTENTS

(1

(11

[ha

(ha

Chapter 4. Introduction for Advanced Materials Based on Self-Assembling

4.1.
4.2.

4.3.
4.4.

Chapter 5.

5.1.
5.2.
5.3.
5.4.

5.5.
Chapter 6.

6.1.
6.2.
6.3.
6.4.
6.5.

Chapter 7.

7.1.
7.2.
7.3.
7.4.
7.5.

Lamellar Supramolecular Arrays and Two Dimensional Polymers

................................................. 100

Membrane lipid structure and properties ........................ 102
Liposmes, stabilized phospholipid analogs and their biological
properties ................................................. 106
Mutifunctional self assembling thin films ....................... 115
References ................................................ 130
Synthesis and Properties of a Bipolar, Bis-Phosphatidyl
Ethanolamine that Forms Stable 2-Dimensional Self-Assembled
Bilayer Systems and Liposomes ........................... 140
Introduction ............................................... 142
Results and Discussions ...................................... 147
Conclusions ............................................... 1 56
Experimental sections ....................................... 157 .

5.4.1. Characterization of physical properties .................... 157

5.4.2. Synthesis .......................................... 159
References ................................................ 164
Synthesis and Properties of Chiral Self-Assembling Lamellar
Polydiacetylene Systems with Very Long Range Order ...... 167
Introduction ............................................... 169
Results and Discussions ....................... ' ......... 174
Conclusions ............................................... l 85
Experimental .............................................. 186
References ................................................ 19]
Preparation of 2-Dimensional Polydiacetylenes by Monomer

Chain Alignment Along a Polar Polyamide Backbone: A
General Strategy for Preparing Highly Ordered, Stabilized

2-Dimensional Systems ................................... 194
Introduction ............................................... 196
Results and Discussions ...................................... 202
Conclusions ............................................... 21 1 '
Experimental sections ....................................... 2 12
References ................................................ 2 16

vi

Scheme 1.1. The synthesis of C-3 chiral synthons in the literatures .............. 13
Scheme 1.2. The direct conversion of carbohydrate to S-3-hydroxy-y-butyrolactone
................................................. 15
Scheme 1.3. The synthesis of L-camitine and R-GABOB by asymmetric
dihydroxylation. ............................................ 1 9
Scheme 1.4. The synthesis of L-Carnitine and S-beta-Blockers by asymmetric
hydroxylation of or-amino ketone derivative ....................... 20’
Scheme 1.5. The stereoselective bio-assimilation of C-3 building units. ........... 21
Scheme 1.6. The commercial synthetic routes to L-carnitine, top is by Sigma Tau,
bottom one is by Lonza. ...................................... 23
Scheme 1.7. The synthesis of S-Propranolol Sharpless epoxidation method ........ 26
Scheme 1.8. The synthetic approaches to ciS-l-amino-2-indanol. ................ 32
Scheme 1.9. The synthesis of cis-l-amino-Z-indanol by Merck’s early method ...... 32
Scheme 1.10. The synthesis of cis-l-amino-Z-indanol by chemoenzymatic method . . . 33
Scheme 1.11. The current most efficient synthetic routes to cis-l-amino-Z-indanol
by Jacobson’s catalyst and Ritter type reaction .................... 34
Scheme 2.1. The conversion of carbohydrate to C3-chiral building blocks in literature
...................................................... 48
Scheme 2.2. The synthesis of S-camitine from S-3-hydroxy-y-butyrolactone ....... 51
Scheme 2.3. The synthesis of 3-carbon chiral molecules from S-3-hydroxy-y-
butyrolactone ............................................... 53
Scheme 2.4. The direct conversion of S-3-hydroxy-y-butyrolactone to R-carnitine . . 55
Scheme 2.5. The exploration of an alternative route to S-camitine via Hoffman

LIST OF SCHEMES

reaction. .................................................. 58

Vii

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)‘A‘Ua

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~v,,L

Scheme 2.6.

Scheme 2.7.

Scheme 2.8.

Scheme 3.1.

Scheme 3.2.

Scheme 3.3.

Scheme 3.4.

Scheme 3.5.

Scheme 5.1

Scheme 5.2

Scheme 6.1.

Scheme 6.2.

Scheme 7.1.

Scheme 7.2.

The synthesis of R-carnitine and GABOB via Curtius reaction ........ 59

Synthetic explorations to S-B-blockers fiom S-3-hydroxy-y-

butyrolactone ............................................... 6 1
Synthetic routes to S-beta-blockers from C-3 synthons. ............. 62
The conversion to various 4-carbon synthons from t-Boc glutamine . . . 79

The synthesis of chiral indanone from phenylalanine in the literature. . . 80

The current most efficient routes to the synthesis of cis- 1 -amino-2-

indanol by Merck and Sepracor. ............................... 82
A new synthetic route from phenylalanine to chiral aminoindanols

and other chiral indanol skeletons ............................... 84
The synthesis of arninotetralins from phenylalanine ................ 86
The synthesis of triacotanedioic acid 8. ......................... 145
The synthesis of dimeric phosphatidyl ethanolamine 2. ............ 146
The synthesis of bilpolar molecule 1 ............................ 173
The synthesis of lipid analog 2. ............................... 173

The formation of the polymer by reaction of the long chain amine
with 2(5H)-furanone. ....................................... 198

The synthesis of the primary long chain amine containing
diacetylene groups. ........................................ 199

viii

Figure 1.1.
Figure 1.2.
Figure 1.3.

Figure 1.4.

Figure 1.5.

Figure 2.1.
Figure 4.1.
Figure 4.2.

Figure 4.3.

Figure 4.4.
Figure 4.5.

Figure 4.6.

Figure 4.7.

Figure 4.8.

LIST OF FIGURES

The structures of some 3-carbon chiral synthons. .................... 12
The important compounds that can be derived from chiral C-3 synthons . . 17
The structures of some important beta-blockers. ..................... 24
The structures of some antiviral agents and their synthesis from C-3

synthons. ................................................... 28
The structures of cis- l -amino-2-indanols, important HIV-protease inhibitors, 1
chiral catalysts derived from the substructure of cis-aminoindanol ...... 30
The structures of some chiral building blocks and their derivatives ...... 46
The general structures of some phopholipids. ...................... 103
Lipid shapes and their packing characteristics ...................... 105
The structure and dimensions of unilamellar (SUV and LUV) and
multilamellar (MLV) liposome. ................................. 107
The membrane lipids from Sarcina Ventriculi ...................... l 13
The structures of some archaebacteria membrane lipid models. ........ 1 l4

Schematic structure of the PDA-vesicles modified with a carbohydrate
capture molecule, sialic acid. The sugar binds to the influenza virus lectin,
hernagglutinin. A variety of capture molecules may be co-assembled with

or grafted onto the PDA-vesicle. ................................ l 19

The tow modes of polymerization of diacetylene and the atomic positions of
the PDA backbone structures based on the ab initio calculation. The average
value from the molecular mechanics calculation is shown in parentheses. 122

Schematic diagram of the structure and polymerization of diacetylene
monolayers on gold or other surfaces with a high ration of step to terrace
sites. The polymerization requires strict Spatial arrangement of the diacetylene
groups. a= 4.7-5.2A, b= 3.4-4.0 A. Polymerization across step sites is
greatly hindered because the spatial tolerances are exceeded. (c and d are too
large). .................................................. 125

ix

 

«"4

 

Figure 4.9.

Figure 5.1.

Figure 5.2.

Figure 5.3.

Figure 5.4.

Figure 6.1.

Figure 6.2.

Figure 6.3.

Figure 6.4.

Figure 6.5.

The structures of three type of diacetylene containing lipids in the
literature. .................................................. 128

Laser scanning confocal microscopy images of hydrated compound 2 and 3.
The first and second images are phase-contrast pictures of 2. The circular
structures are liposomes. Note the uniformity of their Size. The third image

is a dark field image of compound 2 Showing the Spherical outline of the
liposomes. The last (bottom) image is a phase contrast picture of 3. Note

the variation in Size of the liposomes as well as the presence of several

other lamellar structures corresponding to sheets and ribbons. ......... 148

The atomic force micrographs of the thin film formed by compound 2.

(a) is the section analysis over an 800 nm range, (b) is a top View of the

same area from which the section analalysis was taken and (c) is the surface
plot of the same area as above two images. ........................ 149

The relationship of TI relaxation time versus the reciprocal of temperature
(VT) for bulk methylene (1.22ppm), methyl (0.83ppm), methylenes next to
amino group (2.84ppm), methylene a to carbonyl groups (2.27ppm and
2.16ppm) and methylene B to carbonyl groups (1.55ppm). ........... 152

The 300MHz ' H NMR spectra of the water signals in a liposome suspension
of compound 2 in PBS-deuterium oxide at different temperatures ...... 155

The structure of a typical phospholipid (phosphatidyl choline)» containing
diacetylenic fatty acyl groups. ................................. 170

The structure of a membrane lipid from a thermotropic bacterium ...... 172

Space filling models showing the conformations of compounds 1(lefi)

and 2 (right) as indicated by NMR spectroscopy and molecular mechanics
calculation. Red is oxygen, blue is nitrogen, gray is carbon and light blue is
hydrogen. .................................................. 176

AFM images of a film formed by compound 1 on a freshly-cleaned mica
surface. A. Section analysis over a 4 mm length. B. The top view of the same
area as in A. C. Surface plot of the same area. .................... 177

AF M images of a film formed by compound 2 on a freshly-cleaned mica
surface. A. Section analysis over a 2 pm length. B. The top view of the same
area as in A. C. Surface plot of the same area ....................... 178

Figure 6.6.

Figure 6.7.

Figure 6.8.

Figure 7.1.

Figure 7.2.

Figure 7.3.

Figure 7.4.

Figure 7.5

Figure 7.6.

Figure 7. 7.

Laser scanning confocal micrograph of hydrated films of compound 1 (top, a,
b) and compound 2 (bottom, c,d). The images on the left (a,c) were acquired
using cross polarizers and the images to the right(b, d) were obtained using
dark field optics. The films are the areas to the right of the fields. ...... 180

Proposed packing models from compound 1 (top). and 2 (bottom) based on ‘
X-ray powder diffraction information and molecular modeling. ....... 181

Near infrared spectra of films formed from 1 (top 4) and 2 (bottom 4). In a,
b, e, f, the film was polymerized by UV irradiation and in c, d, g, h, the films
were iodine doped. Spectra on the left were aquired in the range of 600-
1050nm, and those to the right were acquired between 880 and 1700 nm. 184

1H NMR spectrum of the oligomers formed in the reaction shown in
Scheme 1. .................................................. 200

1H NMR spectrum of the large molecular weight polymer formed by heating
of the oligomer to further the polymerization along the amide backbone of
the product. ................................................ 201

The photograph of the oligomer solid after UV(254nm) irradiation of 10
minutes. ................................................... 203

The film formed by the oligomer amide by dissolving in chloroform, left
(colorless strip) is the film prepared by casting the material on to the glass
slide by solvent evaporation; the middle (blue) is the film prepared as the
left one being exposed to UV light for 7 minutes, the right (red) is the blue
film prepared as the middle one then was heated at 80 °C for 1 minute. 204

The photograph of the polymer upon treating with UV for 10 minutes

and then doped with iodine. .................................... 206
The near IR spectra of the oligomer cross linked by UV 1ight(a, b) and

the near IR spectra of the polymer cross linked by UV 1ight(c, d). ...... 206
The proposed model of packing structure of the polymer. ............ 207

Figure 7.8. The laser scanning confocal micrograph of the oligomer. A, by polarized

light; B, by phase contrast method, C, by fluorescence ............... 209

Figure 7.9. The atomic force microscopy of the oligomer. A, section analysis; B. The top

view of the cross section area the same with A, C. the surface contour plot of
the same area as above. ....................................... 210

xi

Chapter 1

Introduction for Synthesis of Chiral Building Blocks

Abstract
Chirality or handedness is an important property of many molecules. It has significant
impact in biological systems, pharmaceutical industry and advanced materials science.
It is of great interest to develop Simple and efficient methods to obtain these chiral
compounds. Many methods to chiral molecules have been developed including
asymmetric synthesis and racemate resolution. In recent years asymmetric synthesis to
chiral compounds have been developed rapidly. Several approaches include the use of '
chiral auxiliaries to induce chirality, biotransforrnation using fermentation, enzymes, and
chiral pool approaches which use optical active raw materials as the chirality source for the
desired products. In this chapter, these methods of obtaining chirality especially the chiral
pool approaches are discussed. Chiral 3-carbon, 4-carbon molecules are pivotal building
blocks of many biological important compounds. Chiral amino indanols are important
chiral molecules for drug design and catalysts for asymmetric synthesis. The importance
of these chiral compounds have long been recognized, however their syntheses are not
very straightforward, the application and synthesis of some of these compounds are

reviewed.

1.1. About chirality
Handedness or chirality is an interesting feature of the structure of some molecules

including many drug substances. Such molecules are structurally identical but are actually

mirror images of each other. They have the relationship that the left hand has to the right '

hand. They look identical but cannot be interchanged in the same way that a glove that is
made for one hand cannot fit the other. There are many molecules that we are familiar with
share this left hand / right hand relationship. They have the same molecular weights,
densities, boiling points, crystal structure etc and are therefore very difficult to separate.
However, they smell differently and taste differently. This is because they impact very
differently on living systems. Most biomolecules are chiral or handed, existing in two
mirror image forms called enantiomers. Non living systems normally contain equal

numbers of the left (L) and right (D) handed forms of a given molecule. Biological

systems are sensitive to handedness or chirality, a characteristic hallmark of life is its high

degree of homochirality. Hence amino acids are predominately in the L—form and sugars
are predominately of the D-form. The designation D or L is based on a set of rules
proposed by Fischer at 1919 [1]. There is a set of general rules stipulated by the Cahn-
Ingold-Prelog convention which is the currently used nomenclature to describe the
stereogenic center in a molecule. It is also known as the sequence rule or the R and S rule
[2,3 ]. This rule can be used to designate configurations rapidly and unambiguously. Chiral
molecules are very common in our everyday lives. They are the active constituents not
only of many pharmaceuticals but of vitamins, flavours and fragrances, and herbicides and

pesticides. One very familiar pair of chiral substances is the right and left handed versions

r“ ..

L.)

(23‘

(mirror images) of carvone. The right hand version smell like Spearmint but the left hand

version smells like caraway (a Spice used in cooking).

The impact of the different chiral forms of chiral molecules. often goes far beyond the
senses of taste and smell. The consequence of having the wrong chiral form present can be
tragic. One well documented case of the awful consequences of this is the story of the use
and eventual ban of the drug thalidomide in the early 19608 [4]. The use of a mixture of
both left and right handed versions of this drug (it was too costly to separate them) led to
severe birth defects in over 10,000 children. It was later found that the left handed
molecule was the cause. Another well known case is Ritalin, a drug prescribed for millions
of children, and some adults, with attention-deficit hyperactivity disorder (ADHD). ADHD -
occurs in 3 to 5 percent of school-aged children, it is the most common psychiatric disorder
in childhood. The present form of Ritalin is racernic and with Side effects such as stomach
aches, nervousness, loss of appetite, insomnia and a temporary slowing of growth. It has
been found that 50% of the Ritalin in each pill has no effect on ADHD and may cause the
above Side effects [5]. The importance of optically pure drugs is being realized more and
more as it is demonstrated that only one enantiomer accounts for the drug activity and
the other form is either inactive or may cause side effects which sometimes are serious.
There is increased evidence that the chiral or single -isomer form of the drug often has a
better therapeutic profile. Because of tragedies such as the thalidomide one, finding a way

of making only one chiral form of a drug or drug intermediate is now one of the most

important, challenging and competitive areas for research in chemistry. Nowadays, drugs

4

 

in preclinical and clinical development are dominated by chiral compounds. It has been
estimated that 80% of all drugs in development are single isomer versions of chiral drugs
[6]. Pharmaceutical companies are being forced to take notice of the importance of chiral

technology.

Besides its importance in chiral drugs production, chirality is becoming more and more
an issue in material science too. Researchers at Molecular OptoElectronics Corporation
have reported on developments based on chiral molecules at ACS meeting [7]. It has been .
found that enantiomerically pure chiral molecules can produce polymers with optical
properties four times as stable as those made using conventional precursors. The
development of polymers with nonlinear optical (N LO) properties has been hampered by
the fact that such properties tend to fade over time, this has made it impractical to make
certain fiber optic devices, light source or optical signal processing device out of plastic,
it has been proposed that chirality may offer a solution. Some carbon nanotubes have
chirality in their atomic arrangements. The chirality may influence the tubule conductivity
[8]. In liquid crystal science, chirality is also an important aspect which being utilized
more and more by material scientists [9]. In the area of materials related to biomembrane
mimics, it has been found that chiral fatty acid vesicles are more stable and able to self
reproduce, but the racemic vesicles destabilized during hydrolysis, causing phase
separation [10]. Many long chain phospholipids self assemble into spherical bilayer
structures, known as liposomes or vesicles, however in one study, researchers found that

one class of synthetic phospholipids with polyrnerizable diacetylenic moieties in the acyl

.
um

chains can self assemble into hollow, cylindrical structures, known as tubules [11]. Such
tubules have potential for long term release applications such as marine antifouling.
Several theories based on molecular chirality have been developed to explain the
formation of tubules. They are all based on the principle that chiral interactions cause the
molecules to pack at a nonzero angle with respect to their nearest neighbors. This chiral .
packing induces a twist in the bilayer, which results in the formation of a cylindrical
structure [1 1]. Chirality is an important factor in the assemblies of surfactant too. A long
chain L-histidine derivative can lead to a stereo dependent expression of molecular

chirality at the supramolecular level [12].

1.2. Methods for obtaining chirality

Given the importance of chirality, the big issue now is how to obtain it. The challenges
here are cost and technology. Optical pure molecules are extremely difficult to make thus
making the development of some drugs virtually impossible. In one recent case the cost
of manufacture of a particular AIDS drug was so high and the manufacturing process so .
complicated that the price was outside of the reach of most people and the quantities were
limited. The company eventually could not keep up with demand and many people had to
go without the drug. This is tragic and is a direct result of the cost and difficulty in making

chiral drugs. There is a tremendous need for methods and expertise that make this possible.

There are a number of different methods for producing chiral chemicals. The first one is

by enantiomer separation. This includes classical resolution, which uses a resolving agent

such as tartaric acid to precipitate salts containing only one isomer. Other methods are
crystallization, kinetic resolution, chiral chromatography, etc. Another general way to
chirality is by asymmetric chemical synthesis. This includes enantioselective reactions
using optically active agents, or catalysts, or by diastereoselective reactions involving
chiral auxiliaries. The third strategy is to employ biological agents. This includes
biocatalysis, enzymatic and microbial assimilation. The fourth approach is the use of
compounds from the natural pool of readily available optically active compounds (such
as amino acids, carbohydrates, lactic acids etc) as starting points for synthesis. This last
method in which Simple chemical reactions are performed on molecules with existing
chiral centers to generate optically active building blocks is becoming more and more

important and is the most straightforward one.

Racemate resolution still constitutes the main method for the industry synthesis of pure
enantiomers. There are three methods for the racemate resolution, direct preferential
crystallization, crystallization of diastereoisomeric salts and kinetic resolution. Direct
preferential crystallization is a crystallization-induced asymmetric transformation of a
racemate or deracernization under suitable conditions which include the type of solvent,
temperatures and concentration, one enantiomer will preferentially crystallize while the
other remain in solution. If a racemate is a homogeneous solid phase of the two
enantiomers that coexist in the same unit cell, then it cannot be separated by the above
crystallization method but diastereomer crystallization can be used instead. In this case,

the pair of enantiomers are allowed to interact with a pure enantiomer which act as a

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resolving agent that allows the formation of a mixture of diastereomers that can be
separated by crystallization. The most commonly used resolving agents are based on
natural products available from the chirality pool. These include such as L-tartaric acid, D-
carnphorsulfonic acid and various alkaloid bases. The theoretical yields is 50% unless the
diastereomeric salt remaining in solution can epimerize in what is called a crystallization
induced asymmetric transformation. In kinetic resolution, the success of which depends
on the fact that the two enantiomers react at different rates with a chiral entity. The chiral
entity is usually used only in catalytic amounts and can be either a biocatalyst or system
such as an enzyme or a microorganism or a chernocatalyst such as a chiral acid or base or
a chiral metal complex. The separation of enantiomers by chiral chromatography-chiral
HPLC columns is a well established technique. The principle is similar to the above
mentioned crystallization methods, which all use the different physical properties of the

two isomers to separate them.

Asymmetric synthesis. The asymmetric synthesis strategy for obtaining optical pure
materials involve chemical synthesis utilizing enantioselective reactions employing
optically active agents, or catalysts, and diastereoselective reactions involving chiral
auxiliaries. Catalytic asymmetric synthesis has virtually become synonymous with
asymmetric synthesis catalyzed by chiral transition metal complexes [13]. This is a very -
active field in organic chemistry. Many stereospecific reactions have been developed and
many catalysts have been found or designed. Many types of reactions have been explored

in this field. These include asymmetric hydrogenation of olefins, carbonyl compounds, and

s...-

as!
l

prochiral ketirnines by rhodium complexes or chiral phosphines [14,15] and asymmetric
oxidation. One well known example is the Sharpless epoxidation. In this reaction, an
allylic alcohol is oxidized to a chiral epoxide with reagents made from ruthenium ions,
enantiopure tartaric acid and tert—butoxyl peroxide. There are Some other reagents for the
Sharpless asymmetric dihydroxylation (AD) based on rhodium complexes with pseudo-

enantiomeric ligand [16, 17]. Jacobsen’s catalayst, is another well documented one. This

catalyst is a cobalt complex of (salen) ligands and can be used for the hydrolytic kinetic '

resolution of terminal epoxides and for the enantioselective catalytic ring opening of meso
epoxides [18,19]. Gregory Fu at MIT has developed a new type of chiral transition metal
complex that contains planar dimethylarnino pyridine or related structures. The DMAP
which can catalyze the kinetic resolution of secondary alcohols [20], the asymmetric
synthesis of amino acids [21], and the asymmetric hydrogenation of dehydroarnino acids
[22]. The catalytic asymmetric aldol reaction is a very useful type of organic reactions

effecting enantioselective C-C bond formation [23].

Chiral auxiliaries. The use of auxiliaries is an alternative way towards chirality. One .

common class of chiral auxiliaries is chiral oxazolidinones (Evans’ chiral auxiliaries).
They have been used as auxiliaries for a wide range of asymmetric transformations [24].
The use of chiral auxiliaries is different from catalytic reactions in one fundamental way.
At least stoichiometric quantities are used. Chiral auxiliaries modify the properties of the
reactants such that preferably one diastereomeric product which give an enantiomerically

Q

pure entry on removal of the auxiliary is formed. An auxiliary must be inexpensive, easily

:5;
Us

:2"

\ ‘ 0
ill.

I.
, ‘1

 

removed at the end of the reaction. There are a few chiral auxiliaries that can meet the
above four necessary requirements. Even so, the methodology has been highly successful
in the stereoselective construction of a number of natural products, antibiotics and other

compounds.

Biocatalysis, fermentation, enzyme catalyzed synthesis. A third general strategy for
obtaining optically pure compounds is by biocatalysis. This includes enzymatic
transformations and microbial assimilation. The use of microorganisms in the preparation
of optically active compounds is a promising field with several advantages. One of these
advantages is the availability of a diversity of inexpensive raw materials that can be used
as substrates for microbial transformations . These range from petroleum hydrocarbons
to agricultural waste products. Generally, very high regio and stereo selectivity is
obtained. The major limitation to this method is the low product concentration and the

large amount of waste biomass produced. The productivity of fermentation methods is
considerably low compared to chemical or enzymatic processes. Enzymatic
transformations are also a very promising area. Enzymes are very efficient catalysts and
no organic solvents are required. The reaction conditions are usually very mild. Most
enzymes Show high substrate selectivity and very high chemo-, regio- and
stereoselectivity. Two widely developed families of enzymes that are used in asymmetric
organic synthesis are aldolases and lipases [25-28]. The former one can catalyze
asymmetric C-C bond forming reactions and the latter can catalyze the enantioselective

of hydrolysis of esters to achieve racemate resolution. Enzymes have been used more and

10

more in organic synthesis but their availability and stability are still problems. Because
of the specificity, they lack general utility in a broad range of organic transformations.
These drawbacks limit their applications in industry synthesis of optically pure

compounds.

Chiral pool approaches. The easiest and most straightforward approach to the synthesis
of optically active compounds is to use an optically active raw material available from the
chiral pool. The most abundant source of optically pure compounds in nature is
carbohydrates which generally have the D-configuration. Other candidates of the chiral
pool are amino acids and or- or 9- hydroxy acids such as lactic acid and malic acid.
Although carbohydrates have been widely used as chiral building blocks in organic '
synthesis, very few methods have found industrial applications. This is mainly due to the
fact that carbohydrates are over-functionalized with many hydroxy] groups with Sirmlar
chemical reactivities and many chiral centers, thus making it difficult to use them for
synthesis. Natural amino acids are the most important class of compounds in the chiral
pool [29]. They have a relative simple structure with one or two chiral centers amenable
to variety of chemical transformations. They are readily available in bulk from
fermentation and other process. L-Glutamic acid is the least expensive of all the amino
acids, and both L and D forms of it is available. Because of the versatility as chiral
synthons and its availability, it is the most useful one of the amino acids. L-phenylalanine _
and L-glutamine are readily available starting material for many organic synthesis as chiral

synthons.

ll

O/\(\ OH

0 O
W OH % OH ([3/\/ CI W Cl K O
(R)'1 ( S)"1 (R)-2 (S)-2
Glycidol Epichlorohydrin . (R)-3
o o o o
ONE/Um /\:_/U\OH ()th OH $0
0 O O X0
X X HO
(R)-4 (R)-5 (S)-5 (S)-6

Figure 1.1. The structures of some 3-carbon chiral synthons.

There is an increasing interest in the use of 3-carbon chiral molecules (C3-chiral synthons)

that have an asymmetric center at the central carbon atom (Figure 1.1.) for the synthesis

of optically active, biologically important compounds. Protected glycerol, glycidol and

glyceraldehyde are versatile building blocks to a variety of synthetic transformations.

There are many known applications in drug synthesis for R-isopropylidene glycerol. It can

be used for the preparation of L-carnitine, S-beta-blockers, phospholipids, antiviral agents

etc. Because of its high price, several of these routes are not economically viable. Scheme

1.1 shows some examples of the synthesis of these C3-chiral synthons. One elegant method

for [30] for obtaining C3-synthons is from protected D-mannitol. Cleavage between the

12

D-mannitol NaOCl.
97%l RuCt3
+0 0
O 1
R5
0
o>< O
O 0 H202
— 0
HO OH 3
COO-l
L-asoorbicacid

D-ascorbicacid—-> R-4

Scheme 1.1. The synthesis of C-3 chiral synthons in literatures.

13

‘l

"s

91H
.4..-

C-3 and C- 4 carbons affords two identical chiral molecules-glyceric acid (5). However,
the reaction require at least stoichiometric quantities of sodium periodate or lead
tetraacetate [31] to give the R-IPG. Another way to chiral C3 synthon from the chiral pool '
is the oxidation of protected d-or L—ascorbic acids to D-or L- protected glyceraldehyde (4).
The method has some potential for replacing the above conventional methods but it also
has some limitations. The overall yield is moderate (around 60%), it utilizes a Ru
catalyst and the reduction of the aldehyde involves the use of either Ru/C or Pd/C
catalysts. This approach is inferior to enantiospecific microbial oxidation 0 other enzymatic
transformation methods. Another approach is to degrade unprotected carbohydrates
directly to unprotected C-3 chiral synthons. The degradation of glucose to D- glyceric acid
in basic media, by sodium anthraquinone-Z-sulfonate (AMS) in three steps is an elegant

method. The overall yield is about 60% [32].

Though there is abundant research in this area, little of which is applicable to large scale
industry synthesis because either the reagents are toxic or expensive, reaction conditions
are not mild or the overall yield is too low. In recent years, Hollingsworth has developed
a route to obtain chiral 4-carbon synthons from raw carbohydrate feed stock. This is a one
step degradation of maltose, lactose, or other 1,4-1inked sugar to give optically pure (S)-3-
hydroxy butyrolactone (6) in high yield [33, 34]. The lactone is a very valuable 4-carbon
chiral building blocks, leading to a variety of important chiral intermediates. This
approach (Scheme 1.2) has many advantages over the other methods discussed above and

it has been adopted by industry on the multi ton scale. The advantages include the low

14

OH o

0 H O , NaOH
“0% 2 2
HO OH ‘---—> 0
OH

>85% HO

Sta h
m 99.9% e.e 8'6

OH OH

R0 0H R0 OH
Hog 83‘ H OH HO ‘3
OH .OH OH OH
o o
HO

——>
HO Y) OH
OH 0

HO' H o

\./’

l

0 OH OH
0 H202 H+ OH fig;
' +— OH
HO .+ ““' OH HO
“r0” 0 ° 0

HO + OH

0
RO
HO
OH
0'

Scheme 1.2. The direct conversion of carbohydrate to S-3-hydroxy-y-butyrolactone.

15

In '
(FD
r 1.

film

(’7

’aJ

l

‘ l.
\_ ‘4“
.“4

price of reagents, and the carbohydrate raw materials, ease of implementation and

simplicity of isolation of the products.

1.3. Chiral 3 and 4-carbon building blocks and their applications.

There is an increasing interest in chiral 3-carbon molecules such as glycidol (1),
epichlorohydrin (2), glycerol protected as the 1,2-O-isopropylidene acetal (3) because
they are the key intermediates for the synthesis of many biological important compounds
such as carnitine, y-amino-B-hydroxybutyric acid (GABOB), beta-blockers, antiviral
agents etc (Figure 1.2). Many methods have been developed for the synthesis of these
small chiral molecules. The uses of chiral glycidols were reviewed in 1991 by Hanson
[35]. The uses of (R) and (S)-2,3-O-isopropylidene glyceraldehyde (4) in stereoselective
organic synthesis were reviewed a few years earlier [36]. The C3-synthons can be
obtained by oxidative degradation of mannitol, by transformation of serine and also by
catalytic asymmetric reactions. Many of the current methods to these 3— carbon
compounds currently being practiced require the use of expensive catalysts containing
metals such as cobalt, platinum, rhodium or manganese. These metals are either toxic or -
expensive or both. The two leading technologies were developed in the United States at
Scripps (37) and Harvard (38, 39) Both technologies use heavy metals that are harmful

to the environment and their use involves expensive metal recovery and / or clean up.

C-3 chiral synthons such as 3-amino-l,2-dihydroxy propane, glycidyl chloride, l-chloro-

2,3-dihydroxy propane have been used in several routes to camitine. L-camitine is a

16

.r'

OH

grik‘ggggne (S)-beta-Blocker.

(general structue)

NH2 NH2
“91“» “fl
tN' NY 04..

.....OCH2l:(Ol-l)2

O
OH OH
antiviral agent

antiviral agent Cidofovir

0‘
. o

CwHarO/YV-fi—Cma (I j/
OAC O O

CH2NR2

 

Platelet aggregation factor alpha-Adrenergic antagonist
NHCHQCHZNHCON O
{HZ} °M°H
/\.\OMe
. . _ Xamoterol,
Thromboxane synthase '"h'b'tor betal-Adrenoceptor partial agonist

Figure 1.2. The important compounds that can be derived from chiral C-3 synthons.

l7

naturally occurring carrier molecule that transports fatty acids into cellular mitochondria .
for their conversion into cellular energy. It is also used for the shuttling of acyl groups
that are potentially toxic to the cell. Diseases resulting fiom lack of carnitine lead to severe
neurological disorders. Carnitine and its derivatives are therefore very important drugs.
Carnitine is also used as a nutrient for life enhancement. The naturally occurring (correct)
(L) form is difficult to synthesize. Gamma amino-B-hydroxybutyrate (GABOB) has a very
similar structure to carnitine and itself is an important neural transmitter. Many routes to
L- carnitine, GABOB and B-blockers are related as they all involve the same C3 synthons
as key building blocks. Interestingly, many methods towards the synthesis of L-carnitine
start with carbohydrates. For example, there are syntheses from D-ascorbic acid (vitamin
C) [40], D—arabinose [41], D-galactono- l ,4-lactone [42]. Other chiral pool methods use .
B-pinene [43], D—malic acids [44]. These routes are not very efiicient. They are lengthy
and also the starting material is expensive usually despite some improvement over recent
years. As noted earlier, many catalytic asymmetric methods have been developed for the
preparation of C3-Synthons. They include asymmetric epoxidation, selective
dihydroxylation of alkenes ( Scheme 1.3 ) [37], kinetically-controlled enantioselective
opening of epoxide rings (Jacobson’s methods) and asymmetric hydrogenation of a-amino
ketone derivative ( Scheme 1.4 ) [45]. There are some other methods involving resolution
of precursors of R-carnitine such as resolution of epichlorohydrin by Jocobsen’s catalyst
[46] and resolution of trimethyl ammonium chloro alcohol intermediate [47]. The '
Sharpless asymmetric epoxidation is the best known of these methods and provides a

viable route to glycidols, but the product optical purity is usually low and recrystallization

l8

K3Fe(CN)6, K2003, OH i. NeC(OMe)3. PTSA QAC
Br . $ 5 % Cl ’ Br
W Dal-{303, Ligand1, HOW Br ii. Me3SiCl \/\/

 

 

K20502(OH)4 ”93% KCN, MeOH
61-74% 70-82%
OH
Q“ i. Cone HCI OH aCI- Mes“ CI CN
N+ c '*———Me3N+ ’ CN<~— \/\/
M63 W 02 ii.Amberlite CI'W 51% 72%) ee
IRA410 >95%ee l 49%
93%
OH
ODHQD H3NW002'
Ph N
L' nd1 I I N 90% Be
I98 I N
Ph \N ’
ODHQD

DHQD= Dihydroquinidine

Scheme 1.3. The synthesis of L-carnitine and R-GABOB by asymmetric dihydroxylation.

[48] or HPLC methods [49] are usually necessary for fmther purification of the product.
dichloropropanol and the other S-dichloropropanol as the single source of carbon. If the

Daiso Co. developed routes to afford optically active 3-carbon molecules such as

epichlorohydrin, glycidol, 2,3-dichloro-1-propanol etc by bioassimilation methods '

(Scheme 1.5). They use two types of bacteria one of which preferentially assimilates R-
dichloropropanol and the other S-dichloropropanol as the single source of carbon. If the
bacteria are fed racemic chlorodiols, they utilize one and leave the other stereoisomer as

an enantiopure compound [50, 51]. The optical purity is usually very high. This approach

19

 

 

NaH.
(2R,4R)—MCCXM-Rh(l)
\n/\N< (EtO)ZCO EDWN< > Etc N(
o o o ””33, ”2' ”9°“. 0 HO H
l crawler
-0 ,/
(R)-camitine WM:—
0 HO
“WV Na“ *9 DMF NaBr02,AcOHO
arzgoon
91%B
51% (CH3)2CHNH2
HCI,

°M mark

. H2, [Rh(COD)C|]2, (28,48)-MCCPM,
OH H ‘L O '1'
00 Et3N, MeOH, 98% (90.8%ee) 00
recrystallization, 92%, 100% ee

 

(S)-propanolol
CVZP CyzFB,
2?» Pth l > """ \ PAr2
l
éON'lCHa CON-ICH3
Cy= cyclohexyl
(28,4S)-MCCPM (2R,4R)-MCCXM

Scheme 1.4 The synthesis of L-Carnitine and S-beta-Blockers by asymmetric
hydroxylation of a-amino ketone derivative.

20

Pseudomonas sp.H \CI
OS-K-29 WC. base V7\ H
HO/Y\ Cl
CI

\
2,3-Chloro-1-propanol Alcaligenes sp. HO/;:\C|__, base V/\CI

DS-K838 H Cl

(R) (S)
Epichlorohydri

Pseudomonas sp. bas
os-K-2o1 “CD/mm A (o7 ‘H0

(R)

-Chloro—1,2-propanedlol HOY Cl bass V/JOH
HO H O

Alcaligenes sp.
08-8-76
(8) ' (R)

Glycidol
Chemical purity>98%,
optical purity > 98% ee

Scheme 1.5. The stereoselective bio-assimilation of C3 building units.

21

based on microbial resolution has much promise [52]. Despite the number of routes to L-
camitine that have been explored, currently, there are only two commercial routes to L-
carnitine [53]. Although many C3- chiral synthons are suitable precmsors for R- carnitine,
none of them can compete with the existing commercial routes yet. The reasons are either
the synthetic route is too lengthy, low optical purity, low yield and or involve the use of
toxic heavy metal and the chiral synthons are too expensive. So the production of R-
camitine is dominated by classical resolution and fermentation route. One route is owned
by an Italian company called Sigma Tau (Scheme 1.6). Another route is owned by a
Swedish concern called Lonza (Scheme 1.6). As the major producer of L-carnitine, Sigma '
Tau uses racemic epichlorohydrin as the starting material and a classical resolution of
racemic camitinarnide as the key step. This key step is very efficient, however it produces
equal amounts of the D-enantiomer as a waste material. They have developed some
methods for recycling the D-carnitine, including dehydration of the D-carnitine to give
intermediate alkene and then carnitine hydrolyse mediated hydroxylation of the
intermediate to give the correct product, and also by chemical synthesis to invert the
stereocenter in D-carnitine through a B-lactone intermediate to give the correct isomer
[54,55].However these need more steps and are obviously not very economic.
Alternatively, Lonza commercialized a route based on microbial oxidation of .
butyrobetaine. There is only one step in the transformation, but the problem they have is
the low productivity of the microorganism though they devoted much effort to improving

it [56].

22

 

 

OH 0
resolution
\ \ NHz
,N"
Lamitine (R)
+
\ N'lz
Sigma Tau ’TW
OH o
(S)
o . . . .
\ lTICl'Oblal oxrdatron
\
Jill/Var > IWWO
' OH o
Lonza

Scheme 1.6. The commercial synthetic routes to L-carnitine, top is by Sigma Tau, bottom
one is by Lonza.

23

HOHH

AIO N’ R I | <
O”. 0 0..
HZNCOCHZ

S-beta-Blocker (S)-atenolol
General structure

/_\0
0M NH)\ :NNINUO
5“ S OVN/Q
00 H OH H
(S)-Propanol (S)-Timo|ol

0H )_ ,
Q\/l\/NH >—NH OH H

O

beta-3 adrenergic receptor
(S)-Metroprolol for treatment of obesity, diabetes

Figure 1.3. The structures of some important beta-blockers.

24

\n

r».

(.J'

l

Beta-blockers are a type of drug for the treatment of hypertension and heart diseases. They ‘
have the general structure shown in Figure 1.3. They reduce the symptoms connected with
hypertension, cardiac arrhythmias, migranine headaches, and other disorders related to the
sympathetic nervous system. Well recognized trade names are atenolol, metroprolol and
propranolol (Figure 1.3). These are currently used as a mixture of both forms (D and L) but
there has always been a great interest in the development of methods for preparing only
one form. It has been found that the S-form is more than 100 times more effective than the
R-forrn of the drug. The biological active form is the S-form which can be derived from
the corresponding 3-carbon synthon. Although many routes [57] have been developed for
chiral intermediates of beta-blockers (which are Similar as those involved in the synthesis .
of carnitine) an efficient and economic one is elusive. They are not competitive enough to
allow the replacement of the racemate product. The synthesis of racemic beta-blockers
is by treatment of racemic epichlorohydrin with substituted aromatic phenoxide. At the
beginning, people tried the same route as the racemate process by using optically pure
epichlorohydrin, but it didn’t give optical pure product because of the activity of
epichlorohydrin. There are two paths to opening the epoxide ring leading to a mixture of
enantiomer. Certain modifications to the epichlorohydrin had to be made. The chiral
intermediates leading to optical pure beta-blockers can be obtained via circuitous routes
fi'om D-mannitol [58] and many enzymatic processes including benzylpenicillin acylase
and Baker’s yeast [59,60] and other routes including biochemical and chemical synthesis
[61-63]. Many studies have been devoted to obtain optical pure S-beta-blockers. These

include lipase catalyzed reactions [64-72], catalytic asymmetric epoxidation [73, 74] and

25

kin

r.-

the

//

kinetic resolution [75], catalytic asymmetric hydrogenation [45, 76], microbial oxidation
[77], microbial assimilation [78] etc. Some examples are shown in scheme 1.7. Despite
the enormous effort have been devoted by scientists in this area, none of the route to
optically pure beta-blockers are competitive enough to replace the racemate product. A
better route which is short, efficient and economic is yet to be defined, although
asymmetric synthesis or enzymatic kinetic resolution appear to be on the edge, C3-chirons

via carbohydrates conversion may offer hope to this matter.

 

WC... “(0004. DIP; (l)/\/ OH T501 0,,“ OTs 40%
2eq cumene 33" W 94% ee
hydroperoxide

ArOH, NaH
DMF

iPrNH2

OH H O. l
+— OAr
O\/'\/ NY H20 l/\/

60%, 70% after recrystallization

Scheme 1.7. Synthesis of S—Propranolol by Sharpless epoxidation method.

26

Antiviral agents are another class of drugs that can be derived from these three carbon
molecules. These are used for treating a number of viral diseases including AIDS. They
include drugs such as acyclovir and cidofovir (Figure 1.4). (S)-1,2-Dihydroxy-3-
arninopropane is an important substructure in the drug cidofovir [79]. This is an
established antiviral agent that is used for the treatment of herpes and is now being
promoted for some AIDS indications. Besides S-l-[3-hydroxy-2-
(phosphonylmethoxy)propyl]cytosine (cidofovir or HPMPC) other nucleotide analogues
[80] also have shown important antiviral activity. S-9—[3-hydroxy-2-
(phosphonylmethoxy)propyl]adenine (HPMPA) has been found to be active in vitro against
a broad spectrum of DNA virals [81, 82] . These antivirals are usually synthesized [83-
88] by condensation of nucleic bases such as cytosine, adenine, guanine etc. with the
dihydroxychloropropane or other three carbon units ( Figure 1.4). A straightforward and
cost effective route to the aminodiol will lead to a more efficient route to this and related
drugs. It will also enable the development of other analogs as the older drugs lose ‘

effectiveness because of build up of resistance.

Besides the above application, 3-carbon chiral compounds are used in the development
and manufacture of other drugs. These include platelet activation factors (PAFs), molecules
that control the interaction between red cells and stimulate processes such as clotting.
They play an important role in the cardiovascular system. PAF or
1-O-alkyl-2-O-acetyl-sn-glycerol-3-phosphorylcholine has emerged as one of the most

important lipid mediators known [89, 90]. Thromboxane synthase inhibitors are another

27

4‘

n4

class. These molecules are involved in the biosynthesis of prostaglandins [91]. Three
carbon chiral synthons are also used in the preparation of lipid analogs [92]. The structures '

of these important compounds are shown in Figure 1.2.

NH2 NH2 NH2
N/ / N
N N
O
0” OH OH
antiviral agent antiviral agent Cidofovir HPM P A
HPMPC
NHCOPh NHCOPh NH2
V7(\OTI'
'- + N’ NaH, DMFN /
O H J?! —-—> OA/IN ——>, i l
O N O N
H 4steps V
_....OH ‘\|\----OCHzfi(OH)2
O
OH OH

Figure 1.4. The strictures of some antiviral agents and their synthesis from C-3 synthons.

28

1T.

CD

(I

1.4. Introduction of cis-l-Amino-Z-indanol

The importance of preparing enantiopure materials is well recognized by the chemical
community, and many research groups have devoted a lot of efforts to new asymmetric
synthesis methods. However, to find a general useful building block is still challenging.
Chiral amino alcohols are important classes of molecules for drug development [93]. One
particular molecule of this family is cis-a-amino-Z-indanol. This is an important subunit
in HIV protease inhibitors and is also for some other applications. This aminoindanol
substructure seems to play a crucial role in biological systems and is an important target
in the field of asymmetric synthesis [94, 95]. The molecule is a building block for .
important drugs [96-98] (Figure 1.5). In the synthesis area, it has been used as a chiral
resolving agent, asymmetric catalyst and as a chiral auxiliary (Figure 1.5). The
aminoindanol derived oxazolidinone (12) has been used as a chiral auxiliary in
asymmetric syn-aldol reactions [99]. The aminoindanol derivative indane-bis (oxazoline)
(14, 15) can catalyze the Diels—Alder reaction [100]. The Sepracor group has shown that
optically pure cis-l-arnino-Z-indanol (7) is an extremely effective enantioselective
catalyst for the borane reduction of some important ct-halo ketones [101]. It was found that
besides the anrino alcohol functionalities, the constrained indane platform is essential for

obtaining stereoselectivity [102].

Because of the importance of cis-l-amino-Z-indanol in biological systems and in
asymmetric synthesis, many method have been explored for synthesizing enantiopure cis

l-arnino-Z-indanol. Surprisingly, practical syntheses have been elusive until recently when

29

H

[NH2 -
'- OH
----- 0H 063R vvvv °

1S,2R -cis-1-amino-2-indanol 1R 23 ent-7 3 R=H
9. R: Me

7
R M Ph
I e / N OH H
“‘3’ max/Er“ 0”
0. 0%,... 0 OH.

10. R=H . . . . .
11_ R=Me Mercks CrIXIvan- IndInaVIr
i‘
OH
----- 0 Q .00
OH OH
12

Eli Lilly's PKC\~ inhibitor
R

 

Figure 1.5. The structures of Cis-l-amino-Z-indanols, important HIV-protease inhibitors,
chiral catalysts derived fi'om the substructure of cis-aminoindanol.

30

the

l

the Merck and Sepracor groups independently developed two relatively practical l-
amino-Z-indanol. Surprisingly, practical syntheses have been elusive until recently when
the Merck and Sepracor groups independently developed two relatively practical .
processes for the preparation of chiral cis -l-amino-2-indanol. The general synthetic
approaches to cis-l-amino-Z-indanol are Shown in Scheme 1.8. An earlier synthesis [103]
by the Merck group is shown in ( Scheme 1.9). Racemic 2-bromoindanol 16 was first
treated with concentrated ammonium hydroxide to give the trans-aminoindanol 17, which
was then converted to the oxazoline 18. The oxazoline was hydrolyzed to racemic cis-
aminoindanol. Resolution of the racemic compounds was carried out by reaction with
BOC-phenylalanine followed by removal of the Boo-group and chromatographic
separation of the corresponding phenylalanine amide. Hydrolysis of the amide gave
optically pure cis-aminoindanol 7. Many enzymes such as Baker’s yeast [104] and lipases
[105,106] have also been used toward the enantioselective synthesis of cis-l-amino-2- '

indanol.

An example of chemoenzymatic synthesis of cis- aminoindanol is shown in Scheme 1.10.
The starting bromoindan 19 was oxidized to enantiopure cis-2-bromo-l-indanol 20,
which was then converted to the optically pure cis-aminoindanol via a Ritter type reaction
[107] with sulfuric acid in acetonitrile followed by basic hydrolysis. Senanayake and co-
workers have shOwn that chiral indan-1,2-diols also undergo Ritter -type reactions leading
to cis l-aminoindianol in high yield. In a recent synthesis, indene was converted to the

racemic epoxide, this was subjected to with ring opening to give racemic trans-

31

asymmetric epoxidation
(AE)

Blological or chemical
Vmetric dihydroxylation (AD)

OH

..... p
= - O ------ 0H

Selective amination\s .NH2 ,/Selective amination
----- ..

Scheme 1.8. The synthetic approaches to cis-l-amino-2-indanol.

 

 

 

Ph
OH N§r
NH HO
0.3 ..... 4O __,~,OH 1 PhCOCI, 0H0 ..... O
69% 2. 8002 70%
18
'aCem'c 1. SN H2804
16 2. BOC—Phe,
74% EDC, HOBt
3.TFA
.NH? P“ H HO
5 1. Separation N- i
..... OH 4 H2 . isomer
2. NaOEt, EtOH O .

7 0

Scheme 1.9. Synthesis of cis-l-amino-Z-indanol by Merck’s early method.

32

azidoindanol. A lipase was used to resolve the racemic product. The resulting desired azido
acetate was then converted to the desired product by standard transformations.
Unfortunately, these methods are either lengthy, low in productivity, have low yields or

they suffer some other problems.

 

 

,0 1. CH3CN,
O'Br PseudomonasO H23040 ...... OH
rPutida UV4 0‘ r2 KOH
19 35% IS. 2R
0 1. MsCl ”30%
80 /0 021.0. 2. KOH
0.1 CHacN, H280
o ..

2. KOH
TR, 23
70-85%

Scheme 1.10. The synthesis of cis-l-amino-Z-indanol by chemoenzymatic method.

The current most efficient existing routes (scheme 1.11) were developed by Merck and -
Sepracor [108-110]. Both of these groups rely on Jacobsen’s catalyst (chiral Mn-salen
complexes) to generate enantiopure epoxide intermediate in the key step [111,112].
Sepracor prepared the indene oxide in 83% yield and 84% ee. The optically active indene
oxide was then treated with ammonia to give trans aminoindanol which was transformed

to optically pure benzarnide 48 by crystallization. The benzarnide was transformed to the

33

 

 

Merck Process]

(S,S)-MnLCI(S,S-22)~:p H2804, so, H
A “" to ----- °

 

 

PhCl, NaOCI/P3NO

89% 88% 8.6. l 1. H20

%% a o
50% overall yield 2- tartanc acud

...... 0H

40% overall T H20

“(RRrMnLCKRR-ZZ) O 1. NH3/MeOH .N=(
6 t: . 5 . ..... O
NaOCl/CHZCIZ “O 2- PhC(O)CI. NaOH

3. 80% st04

 

 

83% 84% ea.

 

 

Sepracor Process]

 

8.8-(salen)Mn(III)C| R,R-(salen)Mn(lll)Cl
s,s-22 R,R-22

Scheme 1.11. The current most efficient synthetic routes to cis- l -amino-2-indanol.

34

cis amino-indanol. The overall yield from indene is 40%. Merck’s approach is similar to

Sepracor’s, except that they used J acobsen’s catalyst in a combination with the Ritter type

reaction. Although these two methods are the most efficient routes currently, it is not the
end of the battle. The yield and optical purity of the key-asymmetric epoxidation step are
moderate. Right now the requirement of Crixivan by AIDS patients have not been met
because of the difficulty in producing the drug. There is still great need for a more

economic route that is more environmentally fi'iendly and more efficient.

35

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41

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42

The D
Carbl

Chapter 2

The Direct Conversion of (S)-3-Hydroxy-y-Butyrolactone to Chiral 3-
Carbon Building Blocks and Facilitation of Synthetic Routes to L-
Carnitine, (R)-GABOB and (S)-beta-Blockers

43

Abstract
(S)-3-Hydroxy-y-butyrolactone is a very valuable 4-carbon chiral synthon for which a
simple, l-step, high-yield route starting from starch, maltose, lactose, maltodextrins and
other 4-linked D-pyranoses exists. Because of its ready availability, it is desirable to develop
an efficient way of performing a simple chain degradation reaction in which the acyl carbon
is lost. This would provide an entry route into chiral 3-carbon synthons via a method that
does not involve heavy metal catalysts and utilizes renewable resources. This was
accomplished by performing a Hoffman reaction on the isopropylidene acetal of the amide
formed by treating the lactone with ammonia. This extends the use of carbohydrates as chiral
raw materials for the preparation of important glycerol derivatives. These are important
building blocks in the synthesis and development of several classes of drugs ranging from
0-blockers to antiviral agents, phospholipids, platelet activation factors and thromboxane
synthase inhibitors. A straightforward conversion of the lactone to various chiral 3-carbon
building blocks will be discussed. Two routes for converting the lactone to other chiral
intermediates which will facilitate the synthesis of L-carnitine, (R)-GABOB and (S)-beta-'

blockers will also be discussed too.

2.1. Introduction

Many pharmaceutical compounds contain 3-carbon chiral substructures the development of
routes to which is often the most difficult aspect of their large scale synthesis. Because of
this, there is a constant ongoing effort to identify synthetic routes to these 3-carbon chiral
building blocks. Such building blocks include chiral glycidols, halo diols and amino diols.
Three key 3-cat’oon building blocks are (R)-glycidol (1), (R)-1-bromo-2,3-dihydroxypropane
(2) and (S)-3-amino-l,2-dihydroxypropane (3). Current routes to (R)-glycidol (1) and its
(S)-isomer include catalytic oxidations with peroxides and chiral transition metal complexes
"2, kinetic resolutions by salen Co complex3' 4, enzymatic resolutions of racemic esters by
lipases to selectively deacylate one enantiomer 5‘7 or using glycerol kinase to selectively
phosphorylate one isomer 3. Another common method is to treat a chiral 1,2-propane diol
with a leaving group such as a halide or tosylate ester in the 3-position with base 9’ 3. The.
chernistries of compound 1 and 2 are interconnected. The availability of an easy route to 2
constitutes a straightforward route to 1 since this transformation is easily effected by
treatment of 2 with a mild base such as silver oxide or potassium carbonate. The aminodiol
3 is a substructure that appears in a large number of classes of important drugs. These include
the B-blockers such as Propanalol (4) Atenolol (5), antiviral agents such as (6) and the
thromboxane synthase inhibitor (7). Despite the existence of these methods, there is a need
for others that might have certain advantages. For instance, they might have the potential to
yield some compounds more directly, give better results at larger scale, do not require high
pressures or hydrogen, give superior optical purity, offer some potential price advantages or
do not produce certain waste materials especially those containing heavy metals.

45

O I:O)_/Br HO NHz
HO} HO HO
R-1 13-2 s-3
OH H OH H
. O N
O\/'\/ N\)/ AU \r
\o
5
4
NHz N I
N / N [- 3 0

6
7 .
0 9H 9“
x50 Me3N\/\/ 002- H3NW 002'
HO
8 9 10

Figure 2.1. The structures of some chiral building blocks and their derivatives.

46

HE

511

31‘.

20}

"3‘3"

3:3.

51‘»

P731

Another strategy for chiral drug synthesis, the so-called “chiral pool” approach, is to identify
natural sources containing these substructures and carve them out to provide the
substructures in a form that could be integrated easily into a drug synthesis scheme. Because
of the high degree of oxygenation in the desired 3-carbon fragments, carbohydrates are a
logical choice for this strategy. They have the highest density of chiral functional groups
of all naturally occurring materials. Many of them, such as starch and lactose are readily
available and are very cheap, renewable feedstock. It is, however, extremely difficult to.
convert carbohydrate raw materials into chiral small molecules in good yield by a simple
process. This is because several steps of protection and functionalization typically have to
be followed. In addition to this, carbohydrates are appreciably soluble only in water. This has
proven to be a serious limitation to the chemical transformations they can be subjected to.
They are also very sensitive both to acids and bases and therefore pH conditions are also a
major constraint. Despite these difficulties, there are some applications where carbohydrates
have been used as a source of small chiral building blocks. One of these methods with some
practical value is outlined in Scheme 2.1. In this process, fi'uctose or mannose is reduced to
mannitol by catalytic hydrogenation”. This is then converted to a di-O-isopropylidene acetal-
which can be oxidized to yield two molecules of D-isopropylidene glyceraldehyde, a
valuable 3-carbon synthon, by treatment with sodium periodate' '° ‘2 or lead tetraacetate”. The
drawback of this approach is that the carbohydrate has to be modified and protected before
the actual reaction to generate the building block is carried out. In addition to this, lead is
toxic and periodate is relatively expensive and large quantities are required because of its
high molecular weight and the overall yield is unsatisfactory. An alternative set of reagents

47

for carrying out this oxidation is sodium hypochlorite and ruthenium III chloride (Scheme

2.1). In this case, the protected glyceric acid is the product”.

0“ 9H 0 OH OH OH %
i I H2/cat acetone, H+ OH O
___, z 0

OH 5H CH 55% O
OH OH OH AFC OH

11

+0 36
o ”3°C" Pb(0Ac>4 9 o
OH RUCI3 _____} H '
<—— " EtOAc W

0 97% 76% o
13 12

Scheme 2.1. The conversion of carbohydrate to C3-chiral building blocks in literature.

There is one significant advance in the use of unprotected carbohydrates as a raw material
for producing a chiral building block that could be used to gain access to compounds such
as 1-3. This is the development of a route to (S)-3-Hydroxy-y-butyrolactone (8), a very
valuable 4-carbon chiral synthon for which a simple, l-step, hi gh-yield route starting from
starch, maltose, lactose, maltodextrins and other 4-linked D-pyranoses has only recently been
introduced into the literature '5”. (S)-3-Hydroxy-'y-butyrolactone readily undergoes a variety
of chemical transformations to yield chiral substituted tetrahydrofurans, amides, lactams,
nitriles and epoxides. This high degree of flexibility as a chiral intermediate makes it a very

48

valuable synthon for the pharmaceutical industry. A simple l-carbon degradation method
should provide the desired transformation of this intermediate to a 3-carbon structure thus
providing ways of obtaining the building blocks of interest. This will provide a
straightforward conversion to the Chiral 3 carbon building blocks directly from carbohydrate
starting material and it has the potential to reduce the price of the current C3-synthons thus

make it more available in large quantities.

(R)-3-Hydroxy-4-trimethylaminobutyric acid (L-carnitine) 9 and (R)-4-amino-3-hydroxy-
butyric acid (GABOB) 10 are two compounds with a very high level of medical significance.
L-Carnitine is a very important intermediate in lipid biosynthesis. It functions as a canier for
transporting fatty acids into mitochondria for oxidation. Since fatty acid oxidation is a.
critical step by which cells derive energy, carnitine is important for cellular energetics.
Deficiencies in the biosynthesis of carnitine leads to severe neurological problems. The two
major uses of carnitine are in sports medicine and infant nutrition. Supplemental R-carnitine
is beneficial to heart patients and also it is effective in treating systematic an myopathic

deficiencies. There are several medical indications for which carnitine can be prescribed ‘8’”.

(R)-4-Amino-3-hydroxy-butyric acid is a well known drug substance that functions as an
agonist of gamma amino butyric acid (GABA). It has been demonstrated to be effective in
managing a variety of clinical conditions including schizophrenia and other character based-
illnesses epilepsy and other illnesses that result in severe convulsions ”'25 and its use for the
correction of clinical conditions observed in children being treated for DOWN syndrome

49

has also been explored 2".

Despite the importance of L-carnitine and (R)-GABOB in medical science and the
simplicity of the structure of these molecules there are not many straightforward synthetic
routes to them. Although many preparative routes including kinetic resolution, enzymatic
transformations and asymmetric synthesis have appeared in the literature for the synthesis
these molecules, the current existing commercial routes are dominated by kinetic resolution
by Sigma-tau27 and microbial oxidation by Lonza 2" . The drawback of the kinetic resolution-
is that in the process, half of the product produced is treated as waste, the method using the
bacteria is their low productivity. Asymmetric synthesis routes are usually lengthy, and
involve the use of expensive and or toxic reagents such as heavy metal catalysts. It is
desirable to define an alternative route to L-carnitine by a simple economic asymmetric

synthesis starting with the readily available (S)—3-Hydroxy-y-butyrolactone 8 '5'”.

The functionalities present in this molecule make it potentially easily amenable to conversion
to carnitine and GABOB by placing a trimethylammonium group in the 4—position after ring
opening the lactone with hydrogen bromide to form 4-halo acid and then displacing the halo
group with trimethylamine. However, the stereochernistry at the 3-position is not the desired.
one. It leads to the undesired D-carnitine, which is thought to be a competitive inhibitor of
the L-camitine (Scheme 2.2). Synthesizing these molecules with the correct stereochemistry

from (S)-3-Hydroxy-y-butyrolactone requires inversion of the 3-hydroxyl group or some

50

O

0
OH
xdo HBr OH 1%: M N+\/'\/CO-
9
HO HOAC HO ‘ 3 2
Br
s-9
1%,
OH
H3N+\/'\/Coz'
s-1o

Scheme 2.2. The synthesis of S-carnitine from S-3-hydroxy-y-butyrolactone.

equivalent transformation. Because of its position relative to the carbonyl group, attempts
at inverting the 3-hydroxyl group by activation and displacement readily leads to elimination
to yield 2-(5H) firranone. Some other inversion is required. One possibility is to switch the
priorities of the l and 4 positions in the 4-carbon intermediate represented by (S)-3-hydroxy-
y-butyrolactone. This would require removal of the l-carbon and addition of a new high-
priority carbon at position 4. We accomplish this here by carrying out a Hoffman reaction
on a suitably protected butyramide to generate a chiral 3-carbon molecule that can readily

be converted to the desired products. In another approach, the l-carbon extension and the

Curtius or Hoffmann reaction are performed in that order.

51

Beta-blockers are a large group of medications that act to block specific receptors in the
nervous system. The current drugs are racemic. The effect of beta blockers results in
slowing of the heart rate, reduction in blood pressure, and reduced anxiety, they are.
important drugs for heart diseases and for hypertension and other diseases ”‘33. Though the
biological inactive forms are not known to have bad side effects, there is great interest in
providing only the biological active S-form of the drug. It is desirable to provide only the
effective half form both from the interest of patients (since the dosage is reduced) and of
the chemist. Recent development of a family of beta-blockers at Merck which can treat
obesity, and a range of other diseases are chiral, many of them also contain the same C3
synthons”. Despite numerous synthetic effort 354° on this area, the asymmetric synthesis of
beta-blockers are still not good enough to supplant the use of the racemic form. The 3-C
synthons mentioned above are suitable precursors for synthesis of S-beta-blockers, but they-
are too expensive and some of them are not available on large scale. The possibility of
defining a new synthesis of beta-blockers which may have potential commercial value will

be discussed.

2.2. Results and discussions

2.2.1. Preparation of chiral 3-carbon synthons.

In principle, the one carbon deradation of (S)-3-hydroxy-y-butyrolactone by any one of
the classical routes such as the Hofiinan, Curtius, Lossen or Schmidt reactions. These
involve rearrangement of acyl species with migration to an electron-deficient nitrogen and
lost of the acyl carbon. Of these, the Hoffman reaction 4" ‘2 seemed the most attractive

52

because the starting compound is a primary amide. This primary amide can easily be
obtained by treating the lactone with ammonia. There are potential complications however.
Some of these are peculiar to this starting material and include B-elimination and the
participation of the y-hydroxy group instead of migration to reform the lactone. There are
also a myriad of side reactions, such as dialkyl urea formation, that normally attend the
Hoffmann reaction. Protection of the hydroxy groups was necessary and the use of an
isopropylidene function was explored because of the ease of installation and removal. In

addition, confining the B-alkoxy fragment to a dioxolane ring could also restrict the group

0 O

NH3 (OHahCO/ +
O NH2 dlmethoxypropane /H O NH2
HO HQ >
OH *0
14

15

NH2
00F / 0H O/Q NaN02 / H+/Br’ ”0 Br
———> _ ,

*0 HO
‘HCI/ 16

\NaNOz / H“ / cr
2
H0 or

 

 

HO NH

”of “of

3

Scheme 2.3. The synthesis of 3-carbon chiral molecules fiom S-3-hydroxy-y-butyrolactone

53

sufficiently to limit access to the conformation required for elimination. The reaction

sequence is shown in Scheme 2.3.

The lactone '7 was first converted to a protected amide 15. The first product of the Hoflinann
reaction on the protected amide was the protected aminodiol (16). This was formed with
virtually quantitative conversion as judged by NMR spectroscopy analysis of the crude
reaction mixture although some product was lost on extraction and concentration. The.
isopropylidene acetal 16 is a very usefirl form of 3 since it allows modification of the amino
group without interference from the hydroxyl functions. It also has a much lower boiling
point than 3 and this makes purification by distillation possible even with very modest
vacuums. The acetal group was readily removed by treatment of 16 with slightly more than
an equivalent of acid to give 3 as a salt. The protected aminodiol (16) could be easily
converted to the bromodiol 2 by treating it with nitrous acid in the presence of bromide ion.

The corresponding chloro compound (17) was prepared in good yield by using hydrochloric

acid and sodium chloride instead.

2.2.2. Synthesis of L-camitine and R-GABOB intermediates

There are several routes for the conversion of (S)-2,3-dihydroxypropyl trimethylammonium
compounds (18) in good yields to L-carnitine (Scheme 2.4) 43’ 4“. There has also been
significant patent activity around the preparation of these salts 45“”. The challenge then is to

devise a means of transforming (S)-3-hydroxy-y-butyrolactone to (S)-2,3-dihydroxypropyl

54

0 0
NH 2 ,2-dimethoxy-
3 propane
O ——-> NH2 —i/O1; H
HO HO
OH

10H- / OCI-
HOH +310 3H CH3| or #0 H
' N\ ‘_ ,I +— 0
HO N\ (CH3)2304 NH2
18 l
17 16

HOAc/HBr AcO _.H h, A00 H '+/
17 ———> \>'\)\ N30“, N\

Br NC
19

lH+lH20

H \+
0H0; N/

HO

Scheme 2.4. The direct conversion of S-3-hydroxy-y-butyrolactone to R-carnitine.

55

trimethylammonium compounds. One way of performing this transformation would be to
convert the lactone to the 3,4-dihydroxybutyramide and carry out a Hoffmann reaction to
yield the amino-propanediol followed by quatemization of the nitrogen by treatment with
a methylating agent. This simple scenario is complicated by two factors. The first is the less
than reliable nature of the Hoffmann reaction and the second is the potential for interference
by the hydroxyl group in the 4-position. The Hoffmann reaction usually fails on compounds
containing a y-hydroxyl group because of participation to lead to the formation of the
starting lactone instead of the migration of the 2-carbon onto the electron—deficient nitrogen.
One way around the latter problem would be to protect the diol function in the amide as an
isopropylidene acetal. The successful transformation of the resulting 2,2-dimethyl-l,3-
dioxolane acetamide followed by methylation of the amino group would result in the
formation of (S)-(2,2-dimethyl- 1 ,3-diolan-4-ylmethyl) trimethylammonium salts (17). The
acetal group can be readily and quantitatively removed by mild acid treatment. This would

constitute a formal synthesis of L-carnitine from (S)-3-Hydroxy-y-butyrolactone.

The trimethylamino diol 18 has been employed in two routes to L-carnitine. The use of this
intermediate is not of industrial significance, however, because of the substantial cost of the
key optically pure 3-carbon starting materials, epichlorohydrin and chloro-dihydroxy
propane. In the first synthesis, (S)—epichlorohydrin is treated with trimethylamine to form
(S)-3-chloro-2-hydroxypropyl trimethylammonium sulfate. This is then converted to a nitrile
by displacement of the chloro group with cyanide. Hydrolysis yields L-carnitine45' 46. In the
other synthesis, (R)-1-chloro-2,3-dihydroxypropane is converted to 18 by treatment with

56

trimethylamine. The diol 18 is then converted to (S)-3-chloro-2-hydroxypropyl
trimethylammonium chloride by treatment with thionyl chloride. This is then hydrolysed to
yield L-carnitine‘". This reaction sequence described here has several advantages over the
other two. Firstly, it avoids the use of optically pure epichlorohydrin which is very costly.
Secondly, gaseous trimethylamine is not used. This synthesis departs from (S)—3-hydroxy-y-
butyrolactone which is available easily, in high yield with high optical purity from starch,
lactose and a variety of readily available and cheap carbohydrate raw materials”. The (S)-
2,3-dihydroxypropyl trimethylammonium salt is obtained cleanly and efficiently from the-
amide 15 which is, in turn, obtained quantitatively from (S)-3-hydroxy-y-butyrolactone. This
route should therefore constitute an economically viable path to the commercial production

of L-carnitine.

In the alternative route to L-carnitine, the lactone is transformed to an (R) 4-cyano-3-
hydroxybutyric acid ester which is then transformed to the corresponding amide by treatment
with ammonia in methanol. The Hofman reactions using simple reagent such as sodium
hypochlorite or sodium hypobromite were carried out on this amide, however the product
was not the expected 3-cyano-2-hydroxy amine 22, instead it gave the 3-hydroxy-
pentanedioic acid (23) (scheme 2.5). The amide has been synthesized from the
corresponding dinitrile48 and its conversion to to be converted to R-carnitine by other
Hofman rearrangement reagents such as I, I-bis-trifluoroacetyloxy-iodobenzene ‘9 has been
demonstrated 5°. The reagents for this transformation are expensive and this method is
therefore not very practical. It is our interest to search for some very efficient and economic

57

NH3
Et
MeOH HO ’

 

NaOH ‘ OH
1CN 22
, OH O
ethyl Vinyl NaOCI,
21 $9911., CH o>< ———->HO o
3 NaOH 23
l MOMCI
O NaOCI,
——->
NH2 NaOH 23
H300H'2
CN
O
“”2 NaOCI. O OH O
——>
HO NaOH HO OH
CN 23
21
NH (O
HO"
-—> “c? —->
CN

 

Scheme 2.5. The exploration of an alternative route to S-carnitine via Hoffman reaction.

58

synthetic routes. A simple process using a simple reagent such as bleach is desirable and
will have much economic value. The free hydroxy group was therefore protected and a.
variety of functionalities were tried including acetyl group, methoxymethyl ether, and
iopropyl ether. All of these trails gave the diacid as the product. The protecting groups were
not able to survive the strong oxidizing conditions. Still bearing our objective in mind,
another one carbon degradation reaction, -the Curtius reactions, gave the desired product in

a short simple sequence (Scheme 2.6). The conversion was carried out by treating the cyano

O
HBr fiOEté NaOH OEt NaCN OE t
HOAC HO EtOH HO

HOJ:Ié CN
24
O
O
H2NNH2
CN 22
20 25

Scheme 2.6. The synthesis of R-carnitine and GABOB via Curtius reaction.

59

ester with hydrazine and then the resulting hydrazide (25) was treated with sodium nitrite
and sulfiuic acid at room temperature for 24 hours or at 60°C for 10 hours. The reaction was
followed by ' H NMR spectroscopy. The conversion from the hydrazide to the amine is
nearly quantitative (> 95%). The resulting cyano amine can be converted to GABOB by
refluxing it with an acid and also to carnitine by methylation followed by hydrolysis of the
cyano group5"52. These conversions are straightforward and well documented in the
literature. The process just described there for provides another general route to L-Carnitine

or (R)-GABOB.

2.2.3. Exploration of new practical routes to chiral S-beta-blockers

In the effort of trying to prepare optically pure S-beta-blockers, several routes for converting
the (S)-3-hydroxy-y-butyrolactone to these products have been explored, one route is shown
in Scheme 2.7 . The lactone was treated with ammonia, to give the amide and then a direct
coupling reaction between the diol and 4-methoxy phenol was carried out under the
Mitsunobu wndifion553. Ideally this should give the product 26, which can be converted to
the corresponding amine by Hoffman degradation. The amine can be transformed into 5-
beta blockers by reductive amination with acetone. The problem with this reaction is that the
amide 14 is not soluble in most organic solvents. Solubility in an organic solvent is a
requirement for the Mitunobu reaction”. The amide is only soluble in DMF. Another
complication arises from the presence of the primary amide function. Under the conditions
of the reactions (triphenyl phoshphine and DEAD) the reaction product is mostly the starting
lactone instead of the coupling product (qu , Scheme 2.7). The mechanism of this reaction

60

O

 

o O
NH3 ' NH
”0 “0 DMF, ArOH OAr
OH 26
Hofrnman
H Rearrangement
l
N—< NH2
Acetone ’Q
<———
HO NaCNBH3 HO 0A
OAr r
S-beta-blockers
o o O
NH DEAD, PPh3 NH
HO 2 4' 0 + H0 2 Eg1
DMF, ArOH H0 OAr
26 major minor
0 O o
OEt ArOH, OEt
—> OEt
”0 cho3 \ + £12
Br HO OAr
OAr

~ 0%

Scheme 2.7. Synthetic explorations to S-B-blockers from S-3-hydroxy-y-butyrolactone.

61

OH acetone

OH
HO NH ——-—>
\/'\/ 2 NaCNBH3 HC)\/K/ NH‘<

3.3 27
q i
s\ R
o R OH R o
/ soc: '
own ‘ 2 Ho\/l\/H‘< —-> l>\/N—<
so CH2Cl2 23 29
1.ArO' SOCIz, HBr 1.ArO'
2. H“ 2. H*

OH H OH R OH H
ArO\/'\/N_< Br\/k/N N_<E__*1Ar0 ArO\/l\/N N‘<

S- beta- blockers

OAc
HBr
s-s __, Br\/'\/NHAC A’OH __>Ar0\{:|):/NHA
HOAc 32 K2003

H20, OH‘
l acetone, NaBH3CN

OH
ArO\/'\/ NH—<

Scheme 2.8. Synthetic routes to S-beta-blockers fi'om C-3 synthons.

62

to lactone instead of coupling product by the phenol with the primary hydroxy group is.
probably due the activation of the primary amide by the TTP and DEAD. The primary
hydroxy group then attack the activated amide function to give the ester. In another
experiment, the coupling of phenoxide with the bromoester 24 made by opening the lactone
with HBr and acetic acid lead to elimination products instead of displacement of the bromo
group with the phenoxide group (Eq2, Scheme2.7). It seems therefore that to install the
aromatic system prior to the Hoffman rearrangement might not be viable. A reasonable
route is proposed in scheme 2. 8. The amino diol can be converted to the S-beta-blockers
by several steps. Reductive amination on the S-3 will give intermediate 27 on which the
amine needs to be protected by a t-Boc or similar protecting group such as Cbz.
Alternatively, the secondary OH together with the amine can be protected by an oxazolidone.
function“ to give compound 28, which can then be converted to the bromo compound 31,
the epoxide 29, or the cyclic sulfite40 30. These intermediates 29, 30, 31 can be converted
to beta-blockers with the correct stereochemistry. Or the aminodiol can be converted to the
direct intermediate bromo compound 32 which can couple with various aromatic phenol to
give the precursor of S-beta blockers 33 which upon hydrolysis of the amide and reductive

amination will lead to the final products S-beta-blockers.

2.3. Conclusions

A general route for converting the (S)-3 -hydroxy-y-butyrolactone to other useful chiral three
carbon building blocks including ( S)-3-amino- l ,2-dihydroxypropane as its isopropylidene
acetal, (R)- 1 -bromo-2,3-dihydroxypropane and the corresponding chloro compound has been

63

deve10ped. The latter two compounds are glycidol equivalents. The chemical yields are high
and the enantiomeric purities are extremely high. The propensity for the y-hydroxyl group
to participate has been removed by tying it up in a cyclic acetal. No B-elimination was
observed. The same reaction schemes can be used to obtain the other enantiomers of the
above 3-carbon building blocks by using (R)-3 -hydroxy-y-butyrolactone as starting material.
Two general and efficient routes toward the synthesis of biological important compounds
such as R-carnitine, GABOB are also developed, the first route is related with the synthesis
of three carbon synthons at which the trimethyl ammonia substituted intermediate was the
precursor of R-carnitine, the latter by using a Curtius reaction on the extended 5 carbon
cyanoester derived from the lactone to give the cyano hydroxy amine which is the direct
precursor of carnitine and GABOB. The chiral C-3 synthons are suitable precursors to (S)-
beta-blockers. The study provides an entry route fi'om carbohydrate raw materials to chiral
C3-synthons, in which the reactions are simple, conditions are mild andno heavy metal
waste or other undesirable waste is produced. The process is simple straightforward and
efficient. The high optical purity of the lactone was retained through to the final products.
This chemistry may prove to be commercially viable paths to the synthesis of C-3 chiral
build blocks, R-carnitine, and GABOB. It offers an opportunity to replace the current.
existing methods or to facilitate the replacement of racemic beta-blockers with chiral beta-

blockers.

2.4. Experimental Section
(SH-(2,2-dimethyl)—1,3-dioxolane acetamide (14). (S)-3-hydroxy-y-butyrolactone (204g,

64

2 mol) was converted to the amide by treatment at room temperature for 14 hours with 440
ml of 30% ammonium hydroxide (3.4 mol). The solution was then concentrated to a syrup
at ~ 50° C under reduced pressure until no more water could be removed. Acetone (2 liters)
and 2,2-dimethoxypropane (420 g, 2 mol) was added. Sulfuric acid (6 mL) was then added
and the mixture protected from moisture with a calcium chloride drying tube, heated at 60.
°C for 30 mins and stirred at room temperature for 12 hours. Sodium carbonate (50g) was
added and the mixture stirred for 1 hour. Methanol (400 ml) was then added and the mixture
filtered and concentrated to dryness. The amide (15) crystallized on concentrating. The
crystals were washed with hexane and then acetone and was used without further
purification. Conversion was essentially quantitative. A small amount when recrystallized
from acetone gave white crystals mp, 98-100 °C. [a]23539 = -15.4 (CHCl3, c =1), 1H- NMR
(CDC13, 300MHz) 5 ppm 6.10 (s, 1H), 5.65 (s, 1H), 4.43 (m, 1H), 4.14 (dd, 1H, J=8.1 and
6.3 Hz) 3.63 (dd, 1H, J=8.1 and 6.8 Hz) 2.55 (dd, 1H, J=15.3 and 7.5Hz), 2.46 (dd, 1H,
J=15.3 and 4.8Hz), 1.42 (s, 3H), 1.35 (s,3H) '3C-NMR (CDCl3, 75MHz) oppm 172.86,

109.50, 72.21, 69.05, 40.07, 26.90, 25.50.

(S)-C-(2,2-dimethyl)—1,3-dioxolan-4-yl-methylamine (16) The amide (15) (79.5 g, 0.5mol)
was treated with 10-12% sodium hypochlorite solution (500 ml) and the mixture stirred until
all of the solid had dissolved (~ 5 mins). Sodium hydroxide (80 grams dissolved in 500 ml
water) was added to the mixture and the solution was warmed to 50-60 'C and the kept there
for 24 hours by which time conversion to amine 16 completed. lH-NMR spectroscopy
indicated 100% conversion of 15 to 16. The amine 16 was isolated as a light yellow liquid

65

by extraction of the mixture with ether which upon standing give colorless crystals mp 54-56
°C. The yield was 56.5g (86.3%). This was reported55 to be liquid, bp 62-65 ' C, 15 torr,
probably because it was not isolated in as pure a state as we have here. [a]23589= +0.9 (CHC13,
c=l), 'H-NMR (CDC13, 300 MHz) 5ppm 4.13 (m, 1 H), 4.00 (dd, 1 H, J=8.1 and 6.6Hz),
3.67 (dd, 1H, J=8.l and 6.3Hz), 2.85 (dd, 1H, J=l3.2 and 4.2Hz), 2.78 (dd, 1H, J=13.2 and
6.0Hz), 1.40 (s, 3H), l.34(s, 3H), 1.3 1(s, 2H). l3c-NMR (CDCl3, 75MHz) 5 ppm 109.10,

77.36, 66.90, 44.71, 26.81, 25.31.

(S)-3-Amino-1,2-propanediol (3) lg of compound 16 was treated with 2 ml concentrated
hydrochloric acid and 2 ml of water, heated on steam bath for 30 minutes, the solvent was
removed by rotatory evaporation, a light yellow syrup left in the round bottom flask, which
upon cooling to room temperature give white crystals of the hydrochloride salt. The.
conversion was quantitative. [a]23589=-23.3 (H20, c=1). lH-NMR (D20, 3.00 MHz) 6ppm
3.76 (m, 1 H), 3.47 (dd, 1 H, J=12Hz and 4.8Hz), 3.41 (dd, 1H, J=12 and 5.7Hz), 3.00 (dd,
1H, J=13.2 and 3.0Hz), 2.78 (dd, 1H, J=l3.2 and 9.3Hz). '3C-NMR (D20, 75MHz) 5ppm

63.32, 58.64, 37.15.

(R)-3-Bromo-l ,2-propanediol (2) The amine 16 (10g) was dissolved in 400 ml water. HBr
solution (50 ml, 47% aqueous solution) and 52 g of sodium bromide were added to the
solution which was then cooled to 10°C. Sodium nitrite (70g) was added to the mixture and
it was stirred at room temperature for 20 hours after which time NMR spectroscopy indicated-

complete conversion of the aminodiol to the bromo diol. It was neutralized by sodium

66

bicarbonate, then most water was removed by rotary evaporation and the residue was taken
up in chloroform. The chloroform phase was dried over sodium sulfate and removal of the
solvent gave the bromo diol 2 as a yellow liquid. The yield was 10.3 g (87.1%). [0L]23589= 4.00
(CHCl3, c=1). lH-NMR (CDCl3, 300 MHz) 5 ppm 3.93 (m, 1H), 3.77 (dd, 1H, J=1 1.4 and
3.6Hz), 3.66 (dd, 1H, J=1 1.4 and 6.0 Hz), 3.85-3.46 (m, 2H). l3C-NMR (CDC13,75MHz)

6ppm 71.44, 64.27, 34.62.

(R)-3-Chloro—l,2-propanediol (17) The procedure is similar as preparation of bromodiol.
The amine 16 2.62g (0.02mol) was dissolved in 10 ml water, sodium chloride 8.78g'
(0.15mol), concentrated hydrochloric acid 20 ml (0.2 mol) dilute with 10 ml water were
added to the mixtures. Sodium nitrite 10.4g (0.15mol) was added to the reaction mixtures in
a period of 10 minutes. Then it was stirred for 24 hours, the reaction checking by NMR
spectroscopy indicated complete conversion to the chlorodiol. The mixture was
concentrated to dryness, the product was extracted by chloroform 3 or 4 times. The extracts
were combined and dried with sodium sulfate, removal of solvent give chlorodiol 16 as a
light yellow liquid 1.81g (81.9%). [a]23589= -7.l6 (H20, c=5) 1H NMR (D20, 300 MHz)
6ppm 3.78 (m, 1H), 3.56-3.38(m, 4H). l3C-NMR (D20, 75MHz) 6ppm 66.44, 57.77, 41.08.

All products were > 99.5 % optically pure by chiral G.C.

(RH-Cyano-3-hydroxy butyramide ( 21). The cyano ester 15.7 grams (0.10 mol), stirred
with 30% ammonia hydroxide 21 g (0.18 mol) and 20 ml methanol for 10 hours, after which
the reaction is essentially completed, the possible byproduct of the reaction is the acid

67

instead of the amide at the l-position, this and other ions were removed by passing through
a mix-bed resin by methanol and water as the eluting solvent after removal of the solvent
give the amide as a yellow crystalline solid. Yield 10.6g (82.8%) lH NMR (D20, 300 MHz)
bppm, 4.25ppm(m, l H); 2.71 (dd, J=4.8, 1H 20.4Hz), 2.60(dd, J=6.6, 1H17.1Hz), 2.36 (d,

2H, J=6.6Hz). l3C-NMR (D20, 75MHz) 5ppm 170.72, 1 14.12, 59.34, 36.79, 20.42.

(RH-cyano-3-hydroxy butyrohydrazide (25) 5.2 grams (0.033mol) of the cyano ester.
20 was dissolved in absolute ethanol (10ml) and the mixture was added to 1.6g (0.05mol)
of anhydrous hydrazine, in absolute ethanol (10ml). The mixture was left stirring for 2 hours
over which time a white solid precipitated. The white solid was filtered by vacuum filtration
and washed twice with 5m] ethanol. yield: 4.25g (90%), mp. 1 l9.0-120.0°C, 'H NMR (D20,
300 MHz) oppm 4.24(m, 1H), 2.70(dd, 1H, J=4.5, 17.1Hz), 2.58(dd,lH, J=6.3, 17.1Hz)2.36

(m, 2H) ”C-NMR (D20, 75MHz) 6ppm 166.72, 114.17, 59.36, 3567,2044.

(R)-4- amino-3-hydroxybutyrocyanide ( 22) . 1.43g (0.01mol) the hydrazide 25 was
dissolved in 10ml water. The mixture was stirring and then 1.2g concentrated sulfuric acid'
diluted in 10 ml water, was added to the hydrazide solution. The mixture was cooled in an
ice bath and then 1.36g (0.02mol) of NaNO2 was added. It was left stirring at room
temperature for 24 hours or at 60 °C for 10 hours, after which time the reaction was
essentially completed as determined by 1H NMR spectroscopy. The reaction mixture was
then concentrated to dryness and then taken up in tetrahydrofuran. It was stirred for 1 hour,
filtered to remove salts and other insoluble material and the THF layer was dried with

68

500i

353

163

(0.

sodium sulfate. The solvent was then removed by rotatory evaporation to give the product
as a brownish semi-solid 0.92g (92%). 1H NMR (D20, 300 MHz) 4.90 (m, 1H), 3.73 (dd,
J=9.0, 9.9Hz), 3.30(dd, 1H, J=5.7, 9.9Hz), 2.94(dd, 1H, J=4.2,17.4Hz), 2.84 (dd, 1H, J=5.7,

17.4Hz) 6ppm ”C-NMR (13,0, 75MHz) 5ppm 112.78, 67.56, 39.98,l8.37.

Trimethyl ammoniium salt (17). The preparation was by the same method used to
prepare the protected amine 16 from the lactone, except that at the end of the Hofi‘rnan
reaction, the amine was converted to the trimethyl ammonium propane diol 17. Amide (3)
(0.08g,0.005mol) was treated with 10-12% sodium hypochlorite solution (5 ml) and the
mixture stirred until all of the solid had dissolved (~ 5 mins). Sodium hydroxide (0.8 grams
dissolved in 5m] water) was added to the mixture and the solution was warmed to 50-60 °
C and kept there for 24 hours by which time conversion to amine 16 was complete. 1H-
NMR spectroscopy indicated 100% conversion of 15 to 16. The amine 16 was not isolated
but was directly converted to the trimethylamino derivative 17 by adding dimethyl sulfate
(6 equivalents), sodium hydroxide (0.85g, 0.021 moles) and 2 ml of methanol and stining
for a further (12 hours). Proton NMR spectroscopy (figure 1) clearly indicated complete
conversion to the required (2,2-dimethyl-l,3-diolan-4-ylmethyl) trimethylammonium
compound. The methine proton signal appeared as a triplet (J= ~ 8H2) at 4.12 ppm, the
methylene protons adjacent to the trimethylamino group was a doublet (J= ~ 8H2) at 3.4 ppm
and the signals for the methylene group on the dioxolane ring were partially obscured by the
one for the methylsulfate anion at 3.59 ppm. 'H NMR (D20, 300 MHz) 4.14(dd, 1H, J=6.9,
8.7Hz) 3.64-3.58(m, 5H), 3.44-3.38(m, 2H), 3.08(s, 9H), l.34(s, 3H), l.27(s, 3H).

69

S-Bromo diacetyl amino alcohol ( 32). 6.3g (0.068mol) aminodiol in 250ml round bottom"
flask was cooled in ice bath, 3 equivalent (48g) of 30% HBr-Acetic acid was added to the
round bottom flask at 0°C in a 15 minutes period, it was then left for stirring at room
temperature for 30 minutes. 200ml water was then added to the reaction mixture, sodium
carbonate was added to neutralize the mixture to pH=7, then the product was extracted with
ether 3 times. The ether layer was combined and dried over sodium sulphate overnight,
removal of solvent give deep yellow product as a semisolid. Yield was 15.4 g, 95%. 'H NMR
(CDC13, 300 MHz) 5ppm (s, 1H), 4.99(m, 1H), 3.30-3.60 (m, 4H), 2.10 (s, 3H), 2.06 (s, 3H).

13C-NMR (CDC13,75MHZ) 5ppm, 177.8, 177.1, 158.1, 81.8, 54.3, 31.1, 28.6, 27.3.

70

References:

10

ll.

12.

13.

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1987, 109, 1283-1285.

Fuganti, C.; Grasselli, P.; Servi, S.; Lazzarini, A.; Casati, P. Tetrahedron 1988, 44,
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Chenault, H. K.; Chafin, L. F.; Liehr, S. J. Org. Chem. 1998, 63, 4039-4045.
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Baldwin, J. J.; McClure, D. E.; Gross, D. M.; Williams, M.; J. Med. Chem. 1982, 25,
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Makkee, M., Kieboom, A.P.G.; van Bekkum, H. 1985 Carbohydr. Res. 138, 237-245.
Jackson, D.Y., Synth. Commun 1988. 18, 337-341.

Kawakami, Y.; Asai, T.; Umeyama, K.; Yamashita, Y. J. Org. Chem. 1982, 42,
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Baer, E.; Fischer, H.O.L.; J. Am. Chem. Soc. 1948, 70, 609.

Emons, C.H.H.; Kuster, B.F.M., Vekemans, J .A.J .M.; Sheldon, R.A. Tetrahedron
Asymmetry 1991, 2, 359-362.

Hollingsworth, R. I. Biotech. Ann. Rev. 1996, 2, 281-287.

Huang, 6.; Hollingsworth, R. I. Tetrahedron 1998, 54, 1355-1360.
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Goa, K. L.; Brogden, R. N. Drugs 1987, 34, 1-24.

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19.

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37.

Guarnieri, G.; Ranieri, F .; Toigo, G.; Vasile, A.; Cinarn, M.; Rizzoli, V.;
Morachiello, M.; Campanacci, L. Amer. J. Clin. Nutr. 1980, 33, 1489-1492.

Tomsen, J. H.; Sug, A. L.; Yap, V. U.; Patel, A. K.; Karrase, T. J.; De Flice, S. L.
Amer. J. Cardiol. 1979, 33, 300-309.

Chapoy. P. R.; Angelini, C.; Brown, W. J.; Stiff, J. 13.; Shug, A. L.; Cederbaum, S. _
D. New Engl. J. Med. 1980, 303, 1389-1394.

Takano, S.; Yanase, M.; Sekiguchi, Y.; Ogasawara, K. Tetrahederon Lett. 1987, 28,
1783.

Pinelli, P. Farmaco, Ed. Sci. 1970, 25, 187.
DeMaio, D.; Madeddu, A.; Faggioli, L.; Acta Neurol. 1961, 16, 366.
Buscaino, G. A.; Frrari, E. Acta Neurol. 1961, 16, 748.

Serra clinics, Treatment of Down Syndrome, web site available at
http://members.tripod.com/~hipodcl/sierra/down—syndr0me.htm

Jung, H.; Jung, K.; Kieber, H. P.; European Patent, 1989 320,460 1989. to Sigma-
”$31121, H. G. Chimia, 1991, 45, 81-85.

Ikram, H; Fitzpatrick, D; Crozier, I. G. Am. J. Cardiol. 1993;71:54C-60C.
Eichhom, J. 13., Am. J. Med. 1992, 92, 527-38.

Bristow, M. R. Am. J .Cardiol. 1993, 71, 12C-22C.

Krukemyer, J. J. Clin Pharm 1990, 9, 853-863.

Wadworth, A. N. Murdoch, D.; Brogden, R. N. Drugs, 1991, 42, 468-510.

Ann E. Weber, Ann. Report in Medicinal Chemsitry, Chapter 19. [33 Adrenergic
Receptor Agonists for the Treatment of Obesity, 1998, 193-202,

Klunder, J. M.; K0, S. Y.; Sharpless, K. B. J. Org. Chem. 1986, 51, 3710-3712.

Miyano, 8.; Lu, L. D.-L.; Viti, S. M.; Sharpless, K. B. J. Org. Chem. 1985, 50,
4350-4360.

Katsuki, T. Tetrahedron Lett. 1984, 26, 2821-2822.
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38.

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55.

Pellegata, R.; Dosi., 1.; Villa, M.; Lesma, G.; Palmisano, G. Tetrahedron, 1985, 41,.
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Matsuo, N.; Ohno, N. Tetrahedron Lett. 1985, 26, 5533-5534.
Carlsen, P. H. J .; Aase, K. Acta Chem. Scand. 1993, 4 7,5 617-619.
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Sy A. 0.; Raksis, J. W. Tetrahedron Lett. 1980, 21, 2223-2226.
Triggle, D. J.; Belleau, 8.; Can, J. Chem. 1962, 40, 1201-1215.

Jaeger, D. A.; Jarnrozik, J .; Golich, T. G.; Clennan, M. W.; Mohebalian, J. J. Am.
Chem. Soc. 1989, 8, 3001-3006.

Reirnschuessel et al, Process for the preparation of anhydrous N-3-chloro-2-
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Kimbrell et al, Preparation of halohydroxypropyl-trialkylammonium, US. patent
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Katayama et a], Process for preparing 3-halogen-2-hydroxypropy1trimethyl
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Preuilles, P.; Leclerc, R.; Uguen, D. Tetrahedron Lett. 1994, 35, 1401-1404.

Almond, M. R.; Stimmel, J. B.; Thompson, E. A.; Loudon, G. M. Organic Syntheses
1993, 8, 132-136.

Leclerc, R.; Uguen, D. Tetrahedron Lett. 1994, 35, 1999-2002.

Fuganti, C.; Grasselli, P.; Seneci, P. F.; Servi, S. Tetrahedron Iett. 1986, 27, 2061-
2062.

Jung, M. E.; Shaw, T. J. J. Am. Chem. Soc., 1980, 102, 6304-6311.

Mitsunobu, 0. Synthesis, 1981, 1-28.

Tsuda, Y.; Yoshimoto, K.; Nishikawa, T. Chem.Pharm. Bull. 1981, 29, 3593-3600. .
Danklmaier, J.; Hoenig, H. Liebigs Ann. Chem. 1988;1149-1154.

73

Chapter 3

The Exploration of New Synthetic Routes to Important Chiral
Intermediates from Amino Acids

74

Abstract
Amino acids are very good candidates in the chiral pool for constructing industrially chiral
molecules because they are readily available and have a relatively simple structure with only
one or two stereocenters. The two major functional groups (amino and carboxylic) are also
very amenable to transformations to other fimctionalities. Among the family of amino acids,
glutamic acid and phenylalanine are two compounds of special interests due largely to their
availability and special aspects of their structure. Glutamic acid is the least expensive amino
acid available, from this molecule, removal of one carbon is expected to give a series of 4
carbon chiral building blocks which are of great interest for pharmaceutical industry.
Phenylalanine is of special interest because it contains an unsubstituted phenyl group. It is
also available in very large quantities because it is a component of the popular sweetener
aspartame. Many important chiral building blocks can be derived fi'om this molecule by
simple transformations. Aminoindanols and arninotetralins are two such types of chiral
molecules. In this chapter, the exploration of these two amino acids as the chiral pool

starting materials will be described.

75

3.1. Introduction:

Amino acids are excellent candidates from the chiral pool[1]. They can be converted to a
variety of other chiral building blocks because they are readily available and have a.
relatively simple structure with only one or two stereocenters. The amino and carboxylic
acid functions are also easily transformed. Among the family of amino acids, glutamic acid
and phenylalanine are two compounds of special interests because of their structure and

ready availability.

Ho ? ?
mm WWW

1. L-Glutamic Acid 2. L -Glutarrine
OQCOOH chooH QCHZOH
H H
3 4 5

Glutamic acid 1 is a proteogenic amino acid with two carboxyl groups of which only one
forms a peptide bond and the other gives an acid character to the peptide. Glutamic acid is
involved in the transport of potassium ions in the brain and also detoxifies ammonia in the
brain by forming glutamine 2, which can cross the blood-brain barrier. Both the D and L

forms of 1 are commercially available. L-glutamic acid is the least expensive of all the amino

76

acids
gluta
man:

WISE

acids and it is also the most useful amino acid as chiral synthon. Many synthetic routes using
glutamic acid as starting material have been developed, it is a versatile chiral molecule for
many other important chiral compounds. The cyclized form the L-pyroglutamic acid 3 is a
versatile chiral synthon too. It can be obtained by refluxing the aqueous solution of 1 with
retention of stereochemistry [2]. L-glutamic acid reacts with nitrous acid in aqueous solution
to give the a-butyrolactone—a-carboxylic acid 4 with complete retention of configuration. D-
prolinol 5 can be obtained by reduction of 4 by NaBH4 [3,4] or borane-dimethylsulfide[ 4,
5 ]. The stereocenter in the molecule can be inverted without disturbing the lactone ring and
without racemization [6]. This family of compounds are useful building blocks for many

biological interesting compounds including pheromones, steroids, terpenes, and drugs [7-9]:

One desirable transformation fiom these 5 carbon chiral molecules is to remove one carbon
from the molecule to give a family of 4-carbon multifunctional chiral building units. Some
of these such as y-amino butyric acid derivatives are of biologically interest themselves and
they are also intermediates for many other interesting compounds some of which lack chiral
centers. For instance, the compounds 6-8 are useful drugs for treatment of human cognitive
disorders [10]. The 2-pyrrolidinones such as oxiracetam 6 and aniracetam7 are non otropic
agents that have low toxicity. They have been shown to enhance resistance to brain insults.
The dimeric amino acid derivative 8 SUAMM 1221 possesses antiarnnesic properties and.
ability to strongly inhibit mammalian prolyl endopeptidase [11]. Compounds with the
common structure 9 are nonsteroidal anti-inflarnmatory drugs (N SAIDs). They are used in

the treatment of a number of arthritic conditions [12]. The N-hydroxy form of 3-amino-2-

77

pyrrolidione is a glycine antagonist and occurs in some potent sedatives[l3,14]. All these

compounds possess the core pyrrolidinone structure.

N
OCH

3

°“ 90
O
O N
0%? N )\-——0(CH2)3Ph
Kr

5- Oxiracetam 7. Aniracetam 8. SUAMM1221

 

 

HO / HN 5 OH
\ / N‘R

R=H, OCH3, CH3, etc.

10. 3S-3-h dro -2- rrolidinone
9. NSAle y xy py

Given the importance of these 5 membered ring compounds, it is of great interest to
synthesize the core structure 10 because all the others can be derived from it via standard
transformations. The S-3-hydroxy-2-pyrrolidinone 10 has been synthesized by many
different routes. It has been synthesized from gamma-butyrolactone [15] , 2-hydroxy-4-
phthalirnidobutyric acid [16], 2-bromo-gamma-butyrolactone[l7], trimethylsilyl-Z-
pyrrolidinone[ l 8], S-malic acid [10 ], and by enzymatic methods [19 ]. A straightforward

way of doing so is by the Hoffman rearrangement to convert glutamine to (S)-2,4-diamino-

78

butyric acid. The 2-amino group can be easily converted to other functional groups (Scheme
3.1). An efficient and straightforward route from glutamine to these 4 carbon building
blocks starting from the commercially available compound t-Boc protected glutamine by

Hoffinan reaction using sodium hypochlorite is discussed.

81H? 0
1 OH H2N....
HOW ——- ‘ o
o
13
14
NHBOC T
H2N ? OH NaOCI @HBOC l
_——> ' OH 0
o o NaOH WNW P...
12 O - o
11
o l 17 R=Br
R 0 18 R=O
H2N.,,_
dNH ‘— 1:1:NH
16 R=Br, 15
1o R=OH

Scheme 3.]. The conversion to various 4-carbon synthons from t-Boc glutamine.

79

Phenylalanine ( 19 ) is another useful amino acid that has been explored for chiral synthesis.
Many studies have been carried out starting from phenylalanine to give other important
chiral molecules such as isoquinoline derivatives [20]. Chiral indanones can be created from
phenylalanine by intramolecular Friedel-Craits acylation (Scheme3.2). The product 21 is

produced in 55-75% yield and is greater then 98% enatiomerically pure [21].

1 PCI5
NHcone 3H3 NHCOzMe
NHCOC:Me2 ~an

1. SiHClg, TEA
To 0 l UAW l 2. HCI
0H 6.. 9H
NH2 "wow ~~NH2
L-phenyl analine 22

19

Scheme3.2. The synthesis of chiral indanone fiom phenylalanine in the literature.

1 R, 2S-czs-l-amino-2-indanol 23 and its antipode 24 have gained much attention in recent
years [22, 23]. They have been used in the preparation of a series of potent HIV-l protease
inhibitory peptides (25, 26) and are also useful in asymmetric synthesis by acting as chiral
inducing agents, chiral resolving agents, asymmetric catalysts and as a chiral auxiliary in
asymmetric syn-aldol reactions [24]. The aminoindanol derivative indane-bis(oxazoline) can
catalyze the Diels-Alder reaction [25]. The Sepracor group has shown that optically pure cis--

l-amino-Z-indanol 23 is an extremely effective enantioselective catalyst for the borane

80

reduction of some important a-halo ketones [26]. It was found that besides the amino
alcohol functionalities, the constrained indane platform is essential for obtaining

stereoselectivity [27].

----- ..
1S,2R -cis-1-amino-2-indanol %TH

23 25 Merck's Cn'xivan-lndinavir

NH2

00°“

1R, 28
24

 

26 Eli Lilly's PKC inhibitor

Because of the importance of cis- l -amino-2-indanol in biological systems and in asymmetric
synthesis, many methods have been explored for synthesizing it in an enantiopure form
including enzymatic resolution[28-30]. Many chiral intermediates such as cis-2-bromo-1-'
indanol, and chiral indan-l,2-diols have been converted to the optically pure cis-
aminoindanol via a Ritter type reaction [31] with sulfuric acid in acetonitrile followed by

basic hydrolysis. The current most efficient existing routes (Scheme 3.3) were developed

81

 

Merck Process]

 

 

 

Me
(S,S)-MnLCl(S,S-27) ---- ,0 sto,,so3, .N=(
PhCl, NaOCl/P3NO CH3CN '-
890/0 88% 8.6. 56% 1' H20
50% overall 2. tartaric acid
NHz

...... 0,,

40% overall 1 H20

Ph
~(R,R)—MnLCI(R,R-27) O 1. NH 3/MeOH .5 Oi
NaOCI/CH2012 , 2. PhC(O)C|. NaBH

3. 80% st04

 

 

33% 84% ea.

 

Sepracor Process]

 

 

Scheme 3.3. The current most efficient routes to the synthesis of cis- l -amino-2-indanol by
Merck and Sepracor.

82

 

S, S-(salen)Mn(l|l)Cl R, R-(salen)Mn(ll|)Cl
3.3.27 Fifi-27
by the Merck and Sepracor’s group [32-34]. Both of these groups rely on Jacobsen’s
catalyst chiral Mn-salen complexes) (27) to generate enantiopure epoxide intermediate in
the key step [35, 36]. Sepracor prepared the indene oxide in 83% yield and 84% ee. The
optically active indene oxide was then treated with ammonia to give cis aminoindanol which
was transformed to the optically pure benzamide by crystallization. The benzamide was
transformed to the cis amino-indanol. The overall yield from indene is 40%. Merck’s
approach is similar to Sepracor’s, except that they used J acobsen’s catalyst in a combination
with the Ritter type reaction. Although these two methods are the most efficient asymmetric
routes. Thus far they are too inefficient to be used on a commercial scale. The yields and
optical purity of the key-asymmetric epoxidation steps are moderate. Right now the
requirement for Crixivan by AIDS patients have not been met because of the difficulty in
producing the drug which is till manufactured by chiral resolution. There is a dire need for
a more economic route that is more environmental fiiendly and more efficient. Another
approach starting from phenylalanine to cis-1-amino-2-indanol will be discussed. The

reaction routes are outlined in Scheme 3.4.

83

Chi.

0 o
O OH NaNOz- “3'0:8\1j\OH (COCI)2
NH2 NaBr

19
. lmcraac

NH2 0
NaOAc
oi ----- .. -— o ., <— .
EtOH
29
pH
1R1 ""20 23
.. _. O B, —- .- ——-

 

R
in 3, NH
OH . . .- 2
Rutter type reaction -'
e .. . ----- ..
CH3CN, 97%HZSO4
NH2

°~ -->——» 0. ..

 

D-Phenylalanine 24

Scheme 3.4. A new synthetic route from phenylalanine to chiral aminoindanols and other.
chiral indanol skeletons.

84

Rla/NHz ..... OH *OH q/NHz

33Aminotetralins 34 35' 36

 

39 Pentazocine

Aminotetralins (33) and tetralinols skeletons are very useful pharmaceutical intermediates.
They are used in a variety of important compounds. Aminotetralins are an important class
of molecules that are known to have potency in treatment of central nervous system
disorders such as Parkinson disease and other brain disorders [37-39]. The aminotetralin
skeleton is also found in the very important class of drugs called the morphines. Morphine
37 is the most well studied and most used clinically among all of members of the morphine
family. It is used for pain relief acting as powerful analgesic. The problem with morphines
or other opioid analgesics is that dependency soon develops. The morphines are very
addictive. They are also very toxic. It is of great interest to develop morphine analogs that
are devoid of or reduced side effects. Intermediates such as compounds 34-36 are very

useful in building the morphine skeleton. The synthesis of these amino or hydroxy

85

O NaNOz, O

O OH m OR BH3-THF OH
———>
NH2 Br 0 Br

L-phenylalanine 28 R=H ' 41

19 4o R=CH3 lt-BUOK

THF
0“ H+ NaCNO
*— .
./\/\U/+—_ +__©/\/\ CN baseO O
fiat?! 42

45

_> ”.. ”N...

36

 

D-phenylalanine

Scheme 3.5. The synthesis of arninotetralins fi'om phenylalanine.

tetralones are very difficult and a general preparation on large scale is highly desirable. In
the literature, the syntheses of tetralins usually employ the aryl ketones which are subjected
to reductive amination to convert the carbonyl group directly to an amino group[40-42]. In

this way, or-aryl ketones are converted to B-amino phenyl compounds[43]. The commercial

86

routes to these molecules are not well known. In this paper, a new strategy which has the
potential to be used in large scale synthesis and with high chiral integrity will be explored
The routes is shown in Scheme 3.5, starting from phenylalanine. In this route the length of
the side chain is increase by one carbon, giving the hydroxy acid 44 , which upon F riedel-
Crafts cyclization gives the key intermediate 35. This can be easily converted to the
opposite stereochemistry (compound 34 ). Starting with D-phenylalanine and using the same
scheme also leads to 34. The hydroxy group can be converted to an amino group by
displacement reaction to give compounds 46 and 36. These are direct intermediates in the

preparation of morphine type compounds.

3.2. Results and discussion:

The Hoffman reaction had been carried out starting with S-glutamine, the amino group was
protected by p—toluenesulphonyl chloride, and sodium hypobrorrrite was used to yield the N-
tosyl-2,4—diaminobutyric acid [44-47]. Deprotection gave the hydrochloride salt. The overall
yield for these steps was 63%. The problem with this route is that the tosyl protection is not
very easy to remove(treatment with Zn/acetic acid for several days). An alternative route to
the four carbon building blocks was therefore explored. The reaction scheme is outlined in
scheme3.1, the starting material is commercial available t-Boc protected glutamine. Using
sodium hypochlorite (10% solution,) the t-Boc protected amino acid 12 was obtained in 75
% conversion based on NMR analysis. The transformation is still being optimized and the.
yield is expected to be further improved. The resulting compound can be converted to

various other building blocks very easily. The t-Boc group has much greater ease of removal

87

and also are stable to other transformations.

In the synthesis of aminoindanol, the starting material used is L—phenylalanine (scheme 3.4).
In this route, the amino group is converted to a bromo group with retention of
stereochemistry. This was then converted to the acid chloride which was then cyclized
to give the key intermediate compound with the indanol skeleton. In the attempt at obtaining
the bromo amine by reductive amination with sodium cyanoborohydride and ammonium
acetate, the bromo indanone was converted to cis-bromoindanol exclusively. The
stereochemistry of this compound was confirmed from the 'H NMR spectroscopy coupling
constant between the two methine protons J=3.6Hz, corresponding to the cis-
configuration[48]. Treatment of the bromoindanol with base such as potassium carbonate
and sodium carbonate, potassium acetate etc, in ethanol or ethanol water led to predominant
elimination of hydrogen bromide. The resulting product was the l-indanone with a very
small amount of trans dihydroxyindanol. An attempt to convert the carbonyl group to an
amino group was made by converting the carbonyl group first to an oxime, followed by
reduction with sodium borohydride, or sodium cyanoborohydride. The intermediate bromo
amine was expected to be obtained. The displacement of the bromo group should give the
cis-l-amino-Z-indanol as the final product. The synthetic sequence is short, and quite.
efficient. The intermediates are interconvertible and they can be converted to the product by
standard transformations[49-56]. The intermediate cis or trans bromoindanol has been
converted to the cis-aminoindanol by a Ritter type reaction [31, 49]. The conversion of

bromo amine 28 to the stereocherrrically correct product has many advantages if the reaction

88

can be carried out with good yields. Further study is being devoted towards these later
stages of the synthesis. The strategy shown here is a straightforward and general route to
aminoindanol skeleton. The opposite stereochemistry can be obtained by using the other

enantiomer , D-phenylalanine, which is also commercial available.

The synthesis of tetralin type of compounds is shown in scheme 3. 5, the phenylalanine was.
converted to the bromo acid 28, then it was reduced by borane to 41 which can be converted
to the epoxide 42 by treating with base, the one carbon chain elongation was realized by
opening the epoxide with cyanide to give 43, which can be hydrolyzed to hydroxy acid 44.
The acid 44 itself is a very valuable compound. It can be readily converted to the tetralone
skeleton 35 which in turn can be converted to the amino aryl ketone 46. This is a new
compound that can be used in the building of morphine analogs and many other important
structures. The other isomer can be readily obtained by starting either form D-phenyl alanine

or by converting 35 to the compound with opposite stereochemistry 34, then to 36.

3.3. Conclusions:
This chapter described the exploration of new synthetic routes to important pharmaceutical
intermediates including chiral 4 carbons and aromatic amino alcohols from readily available
amino acids. The chiral diaminobutyric acid was produced by Hoffman rearrangement for
m glutamine. The aminotetrolins and aminoindanols are synthesized from phenyl alanine.
The approaches explored here have many advantages over the existing methods, including

the availability of starting raw material, the reagents used are common and inexpensive, the

89

reaction sequence is short and with high chiral integrity, the routes are accessible to scale up

in industry.

3.4. Experimental Section:

3.4.1. Preparation of C—4 carbon intermediates.

Preparation of compound 12. L-tert-butoxylcarbonyl glutamine 4.92g (0.02mol), 22ml of 10-
13% commercial sodium hypochlorite solution, was mixed and cooled by ice, 3.2 grams of
sodium hydroxide in 20ml of water was added to the mixture, it was stirred for 10 minutes,
then it was heated at 65 °C for 2 hours. The mixture was neutralized with 3 N hydrochloride
acid to pH=3, solvent was removed, the product was isolated by extracting with THF. The
yield was: 72% ‘ H NMR, 6ppm, 4.10(m, 1H) 3.00(t, 2H, J=7.8Hz), 2.1 1(m, 1H), l.94(m,

1H), 1.3l(s, 9H). '3 C (D20) NMR, 170.4, 153.0, 77.2, 63.3, 46.8, 32.0, 24.0, 23.1.

Preparation of Compound 15 The t-Boc-diamino acid can be converted to the methyl ester
by treating with acetyl chloride and methanol under refluxing condition for 6 hours or so,
removal of the solvent give the diamino acid methyl ester, the t-Boc group was removed
during this process. The resulting ester. 'H NMR (D20, 300MHz) bppm, 4.09(m, 1H),
3.66(s, 3H), 3.04(m, 2H), 2.10(m, 2H), '3 C NMR(D20, 75MHz), éppm 164.6, 49.4, 45.7,

31.3, 22.8.

The cyclization of the diamino acid can be achieved by treating the above ester with sodium

carbonate solution, the free amine react with the ester in situ to give the ring closure form

90

amino pyrrolidone[44]. ' H ( D20, 300MHz), 6ppm, 4.09(m, 1H), 3.20(m, 2H), 2.32(m,

1H), l.82(m, 1H). ‘3 C NMR(DZO, 75MHz), 161.6, 48.2, 34.5, 23.9.

3.4. 2. Preparation of aminoindanol precursors

Phenylbromoacid 28 50 g(0.297mol) L-Phenylalanine 19 was dissolved in 500ml of water
and 90g (3 eq) sodium nitrite, concentrated hydrobromic acid 360ml were added to the
solution. The rrristure was stirred at 5 °C in an ice/water bath. 43g (2eq) of sodium nitrite as
added over a 15minutes period. The reaction was stirred at 5 °C for another 1 hour and then
at room temperature for 5 hours. Over this time , a light yellow solid settled out, The mixture.
was extracted with toluene. The toluene layer was concentrated to yield 64 grams (0.279mol)
of 28 as a yellow oil. Yield 94%. ' H NMR (300 MHz, CDC13), 6ppm, 7.14-7.38(m, 5H),
4.4l(t, 2H, J=7.6Hz), 3.45(dd, 1H, J=8.4 and 14.1Hz), 3.23(dd, 1H, J=7.5 Hz, 14.1Hz). ‘3

C NMR (300 MHz, CDC13) 5ppm, 175.0, 136.3, 129.1, 128.8, 127.5, 44.8, 40.7.

The brom-indanone 29 20 grams (0.087mol)of above product 28, treated with 40 grams of
oxalyl chloride (after cooling with ice) the mixture was left at room temperature for 24 hours,
then it was concentrated, taken up in dichloromethane, cooled in ice and treated with12
grams of anhydrous AlCl3. It was stirred in ice/water for 30 minutes and then at room
temperature for 12 hours. Another 300ml of dichloromethane was added, diluted with water,
and phase separated, the organic phase was filtered through celite and dried over sodium
sulphate, removal of solvent give 16.8 grams of bromoindanone as a dark liquid, upon

cooling gave dark brown crystals, no firrther purification was needed . Yield was 91%.

91

' H NMR (300 MHz, CDC13), 6ppm, 7.83(d, 1H, J=7.5Hz), 7.65(t, 1H, J=7.5Hz), 7.42(t,
1H, J=7.5Hz), 7.25(m, 1H), 4.64(dd, 1H, J=3.4, 7.5Hz), 3.84(dd, 1H, J=7.5Hz, 12.1Hz), 3.40
(dd, 1H, J=3.4, 18.3Hz). ‘3 C NMR (300 MHz, CDC13) 6ppm, 199.6, 151.1, 136.0, 133.5,

128.3, 126.4, 125.1, 44.0, 38.0.

Cis-2-bromo-l-indanol 32: In all attempt to do the reductive amination[57,58] on the
bromoindanone give the cis-bromoalchol 32 as the major product, with small percentage of
the bromo amine. The conditions have been tried are, 2.11g(0.01mol)bromo indanone,
7.7g ammonium acetate (10 mol), 1 eq of sodium caynoborohydride, at pH=4 (by adding
acetic acid), and pH=7-8. The mixture was stirred for 24 hours at room temperature, after
which the reaction mixture was acidified to pH=2 by concentrated hydrochloric acid, the
solvent was removed by rotatory evaporation under reduced pressure. Ether was added to
extract most of the organic material in the mixture, after drying the solvent give 1.59g of cis-

bromoalcohol 32.

The direct reduction of the bromoindanone by sodium borohydride give the cis-
bromoindanol in excellent yield. 1.05 g(0.005mol) of bromoindanone in 100ml round bottom
flask was cooled in ice bath, then 0.25g sodium borohydride was added in a 5 minutes
period. It was left stirring at room temperature for two hours at which time the reduction is
completed indicated by NMR spectroscopy. The reaction mixture was quenched with 1N
hydrochloric acid , extracted with diethyl ether. The ether layer was dried and the removal

of solvent give the cis-brom-indanol 1.05g as a yellow solid(needle crystalline). Yield was

92

99%. ' H NMR (300 MHz, CDCl_,), 5ppm, 7.20-7.50 (m, 4H), 4.90-5.05(m, 2H, CH-Br, CH-
OH), 3.45(dd, 1H, J=5.0, 17.0 ), 3.36 (dd, 1H, J=3.2, 17.0), ‘3 C NMR (300 MHz, CDC13)

5ppm, 141.6, 139.2, 128.8, 127.4,125.0, 124.6, 76.2, 60.7, 40.2

The acetyl derivative of cis-2-bromo-l-indanol. It was obtained by treating the bromo
compound with acetic anhydride, pyridine. ' H NMR (300 MHz, CDC13), 5ppm, 7.10-7.45
(m, 4H), 6.02(d, 1H, J=4.8Hz), 4.86 ( m, 1H ), 3.48(dd,lH, J=6.3, 16.5Hz), 3.38(dd, 1H,
J=5.4, 16.5) ‘3 C NMR (300 MHz, CDC13) 6ppm, 170.5, 140.3, 138.2, 129.3, 127.4, 125.1,

124.7, 76.7, 51.4, 40.9, 20.9.

Cis l-amino-Z-indanol. The bromoindanol can be converted to the cis amino indanol in one
step by Ritter reaction. 2.13 g of the cis-2-bromol-indanol was dissolved in acetonitrile, it
was cooled in acetone-dry ice bath, 3.2 g of firming sulfuric acid was added to the mixture
dropwise. It was left stirring for overnight, 20 ml of water was added, the solvent was.
removed under rotatory evaporation then the mixture was heated at 60 °C for 4 hours, after
which the mixture was concentrated and water removed, the residue was treated with 25%
sodium hydroxide solution, the crystals was collected by filtration, and the solid was taken
up in methanol or chloroform to remove any salt in the solid, the liquid phase was dried, and
give l.35g cis-l-amino-2-indanol' as a white solid (yield: 90% ) A small amount of the
sample was recrystallized from CH2C12zhexane, m.p. 119.5-120.5°C. ‘ H NMR (300 MHz,
CDC13), bppm, 7.21-7.30(m, 4H), 4.36(m, 1H), 4.28(m, 1H), 3.07(dd, 1H, J=l6.5, 5.5Hz),

2.92(dd, J: 16.5, 2.7Hz), 2.48(s, 3H, exchange with D20). ' H NMR (300 MHz, c0301»,

93

appm, 7.38(m, 1H), 7.20(m, 3H), 4.41(dt, J=3.3Hz, 5.1Hz), 4.15(d, J=5.1Hz), 3.07(dd, 1H,
J=5.4Hz and 16.2Hz), 2.89(dd, 1H, J=3.3Hz, 16.2Hz).
‘3 c NMR (75 MHz, CDC13) 6ppm, 143.9, 140.8, 127.9, 126.9, 125.4, 123.9, 72.7, 85.5,

39.3.

3. 4. 3. Synthetic routes of aminotetralin precursors.

Synthesis of the bromoalchohol 41. The bromoalchol can be synthesized by reducing the
bromoacid 28 by borane-THF complex or by reducing the methyl ester of the 28 by
LiBH4[59]. The procedure: 2.43g benzyl bromacid 28 (0.01mol), 14ml of 3H,-THF 1M
solution was added to the acid at 0° C in 10 minutes period, then the mixture was stirred at
room temperature for 10 hours. At which time proton NMR indicated the reduction of the.
acid is essentially completed. (The reaction was followed by ' H, NMR and TLC, at 1 hour,
about 50% reduction, at 5 hours about 80% reduction, 10 hours, the reduction is completed.
About 0.5m] of sample was taken each time for the analysis. Quench with 1N HCI, extract
with ether, remove the ether give the white solid for the NMR) The reaction mixture was
quenched with 1N HCl, extracted with ether, the ether layer was dried by sodium sulphate,
removal of the solvent give the bromoalchohol 41 as a white solid, yield 2.03 g (94%) . ‘ H
NMR (300 MHz, CDCl,), bppm, 7.40-7.20 (m, 5H), 4.34(m, 1H), 3.84 (dd, 1H, J=3.9, 12
Hz), 3.75(dd, 1H, J=6.0 andlZHz), 3.28(dd, 1H, J=7.2 and 14.1Hz), 3.18(dd, 1H, J=7.5 and
14.1Hz). ‘3 C NMR (75 MHz, CDC13) 6ppm, 139.0(1C), 129.2(2C), 128.5(2C), 127.0(1C),

66.0, 58.6, 41.3.

94

The epoxide 42: 0.10g bromoalcohol 41 and, 1.5eq of potassium tert-butoxide 0.08g, and
4ml of tetrahydofuran was mixed and stirred at room temperature for 12 hours, after which
the mixture was filtered and solvent was removed give the epoxide with good purity and
yield . ' H NMR (300 MHz, CDCl_,), 6ppm, 7.38-7.18 (m, ~5H), 3.18(m, 1H), 2.92(dd, 1H,
J=14.4, 4.8Hz), 2.80(dd, 1H, J= 11.7 and 4.8Hz), 2.77(m, 1H), 2.54(dd, 1H, J=4.8 and
2.7Hz). This is same as the literature vale[60]. '3 C NMR (300 MHz, CDC13) 6ppm,

137.1(1C), 129.0(2C), 128.5(2C), 126.6(1C), 52.4, 46.8, 38.7.

95

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Evans, D. A.; Murry, J. A.; cvon Matt, P.; Norcross, R. D.; Miller, S. J. Angew. Chem.
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Senanayake, C. H.; Roberts, F. E.; DiMichele, L. M.; Ryan, K. M.; Liu, J.; Fredenburgh,
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98

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99

Chapter 4

Introduction for Advanced Materials Based on Self-Assembling
Lamellar Supramolecular Arrays and Two Dimensional Polymers

100

Abstract
In recent years, the design and fabrication of novel multifunctional advanced materials have-
gained increasing attention. However, there has been difficulty in finding a simple reliable
technique for arranging functional groups in molecules into ordered arrays. Self-assembly
which is learned from nature is becoming a key factor and important concept for this type
of application. In this approach, the units are made with structures and connecting sites that
recognize adjacent units and come together in the correct configuration to form the desired
supramolecular (very large) system. In this way, the laborious task of sequentially arranging
and fixing each bond is avoided. The information for forming the overall supramolecular
structure is built into the individual units. Units may self-assemble to form sheets or tubes
or spheres or any other kind of regular object. These molecular assemblies have specific
properties that are encoded by chemical functionalities in the constituent units. They are
referred to as “smart materials”. Nature has her unique and amazing way of organizing living
systems. The cell membrane for instance is a perfect example of a natural self assembled
system and studies are based upon copying it. Major foci are to design molecules that can
assemble themselves into much larger objects to fabricate oriented thin films and other
lamellar structures such as stabilized vesicles. The lamellar vesicles are important as models
for biomembranes, fabricating biocompatible surfaces, for drug encapsulation, gene delivery
and as vehicles for delivering various agents used in cancer therapy. The controlled oriented
thin film is useful in developing molecular devices, electronic and photonics materials and
other advanced materials which are pivotal for the revolution in communication, information
transmission and storage.

101

4.1. Membrane lipid structure and properties.

Cell membranes are critical components of the cell surface of living organisms and are

responsible for maintaining cellular integrity and communication with the environment and

with other cells or organisms. Biomembranes are composed primarily of lipids and protein.

The protein is spreaded as an extended sheet over the ionic heads of the phospholipid bilayer

or are buried in the hydrophobic core of the bilayer'. The lipid membrane provide a matrix

for membrane proteins and the structure of the membrane is very important for the

biofunctions of cell. There are four main groups of lipids in cell membranes; phospholipids,

sphingolipids, glycolipids, and sterols. Some of the examples are shown in Figure 4.1.-

Phospholipids are the most abundant of the biological membrane lipids.

Many membrane lipids can form the basic bilayer structure. The phospholipids can self
assemble into spherical liposomes or lipid vesicles, which contain aqueous solution within
an enclosing lipid bilayer. The self organization arises from the inherent properties of lipid
molecules and intermolecular interactions between lipids and water. The interactions include
hydrophobic, hydrophilic, dipolar, hydrogen-bond, and electrostatics. The combination of
these interactions produce the unique characteristics of the assembled bilayers. The ability
of lipids to form bilayer structures is mostly due to their arnphipathic character. Lipids-
have a polar or hydrophilic head group region and a nonpolar or hydrophobic part. Such
molecules will naturally orient themselves to ensure that the polar head groups associate
with water molecules and the hydrophobic tails interact with each other. If the molecule is
roughly cylindrical in dimension and have no net charge then the biomolecular planar

102

PHOSPHOLIPIDS

R1—COO—CISH2
Rz- coo-cH (”3
CH2— o— If— o—x

O
— OH Phosphatidic acid (PA)

—CH20H2N*(CH3)3 Phosphatidylcholine(PC)

—CH20H2NH2 Phosphatidylethanolamine(PE)
— CH ZC'IHC 02' Phosphatidylserine(PS)
NH2
—- O— CH 2— C|3H- OH Phosphatidylglycerol( PG)
OH
— o
O®OH PhosphatydylinositoKPI)
OH
R1— CH: CH— cH- OH
Rz-CO- HN- 9H 9 Sphingolipid
CH 2— o— P— o— x
0'
R1— co— o— (I3H2
Rz-CO— o—cH CHZOH
CH2-0 0 0” Glycolipid
OH
OH

Figure 4.1. The general structures of some phospholipids.

103

leaflets will be the most stable configuration in aqueous solution. If they are charged, the
presence of an appropriate counter ion will also stabilize the bilayer structure. Besides the
planar bilayer structure, some membrane lipids tend to adopt other ordered systems such-
as micelles and hexagonal II phases 2 ( Figure 4.2). The hexagonal packing provide good
permeability barriers with the vast bulk of the lipid arranged in a bilayer in vivo. A micelle
is a spherical aggregate of lipids and is usually small. Micelles are not as important as
lamellar bilayers are in biological systems and for biomaterial based applications. They are,
however, important in some other areas such as acting as surfactant in water to dissolve
insoluble materials. The bilayer structure is the basis of biological membranes and is very
important for cell firnction. The tendency to form a particular which type of packing
structure depends on the structure of the lipids. Among the most important phospholipids,
phosphatidylcholine usually forms a bilayer structure, phosphatidyl ethanolamine tends to:

form micelles or inverted hexagonal structures because of its small head group.

Phospholipids serve as the prototypical materials for the design of thin film materials
because of their extremely high packing densities and propensity to form lamellar systems.
These 2-dimensional lamellar systems find applications in several areas of research and
technology including drug encapsulation [3, 4], gene delivery [5,6], biocompatibility [ 7,8],
bacteriacides [9,10], biosensors [l l] and specialized receptor surfaces [12-14] and
advanced biomaterials [15-20]. In all of these applications, it is desirable to form a self-
assembled lamellar system with some desired functionality on one surface. In the drug
encapsulation and gene delivery areas, it is a requirement that the lamellar system have an

104

Lipid type Shape of the lipid Structures formed

Monoacylphospholipids
Detergents a

Phosphatidylcholine
Sphingomyelin
Phosphatidylserine
Phosphatidylinositol
Phosphatidylglycerol
Phosphatic acid
Diphosphatidylglycerol
Di-sugar diacylglycerols

 

Unsaturated phosphatidylethanolomime {%JO/Lfl x
Phosphatidic acid+Ca2+
Diphosphatidylglycerol + Ca2+ 1‘ r/J/Oqiql‘er

Monosugar diacylglycerols , g

Figure 4.2. Lipid shapes and their packing characteristics (adapted fiom reference 2).

105

overall spherical topography to form hollow structures (vesicles or liposomes). Because of
its high propensity to form bilayer structures or vesicles, phosphatidylcholine (lecithin) is
the most studied phospholipid [21-24]. Potential uses include encapsulating agents in drug
delivery [21, 22] and extracting toxic heavy metal from waste water [25-27]. However, the
uses of lipid membranes have been limited by difficulties in their fabrication and their poor

stability.

4.2. Liposomes, Stabilized Phospholipid Analogs and Their Biological Properties

One of the new and exciting avenues of research that has evolved over the past few years is
the preparation of large molecular systems with shapes that allow them to be used as
containers for the purpose of trapping and transporting substances such as drugs. These
containers (called vesicles or liposomes) are interesting because they are not of the size that

can be observed with the naked eye but are of molecular dimension. .

They have a multilameller or unilarnelar structure and various sizes fi'om 25 run up to 10
um (Figure 4.3) [28]. Because liposomes contain both hydrophobic layers and hydrophilic.
layers alternatively, hydrophobic solutes can be contained within the bilayer and water
soluble materials within the aqueous compartments. Highly nonpolar drugs are dissolved
in the hydrocarbon core of the micelle, whereas more polar substances are located in the
polar head groups near the surface. Inside a liposome, a drug is protected from the cellular

enzymes that would destroy it. This increases the lifetime of the drug in the body and,

106

Multilamellar vesicle (MLV)

A

up to
10,000 nm.

 

 

lipid bilayer aqueous compartment

Small unilamellar vesicle (SUV)

20 - 50 nm.

aqueous compartment

  
 

aqueous

compartment 50 - >10,000 nm.

Figure 4.3. The structure and dimensions of unilamellar (SUV and LUV) and
multilamellar(MLV) liposome (adapted from reference 28)

107

therefore, reduces the dosage required to obtain a desired effect. Researchers are developing-
targeted liposome drug delivery systems. These should be very useful in cancer therapy,
because such liposomes should selectively localize anti-cancer drugs at the tumor site thus
reducing the toxicity of the drugs to normal cells and improving their therapeutic activity
because of the higher drug levels being delivered to the tumor. Liposomes or lipid-based
carriers are also being explored as vehicles for carrying DNA into cells for gene therapy. In
this therapeutic technique, a defective gene (stretch of DNA) is replaced by a new stretch
without defects. Inside of a liposome, the polar DNA molecule can traverse the protective

membrane of the target cell. It is also protected from nucleases, enzymes that degrade DNA.

A vast of technology has been developed using liposomes since their discovery by Bangham
[29]. Many applications of liposomes as delivery systems have been developed though few
of them have reached commercial stage. These include enzyme [targeted delivery of
anticancer drugs [30-32] bacteriacide [33] , and moisturizers and antiflarrunatory agents [34]
for skin. Cationic liposomes are widely explored for gene therapy [3 5, 6, 5 ]. The potential
of liposomal delivery seems very promising. But there are some problems with
conventional liposomes. Although the idea of using them as drug delivery systems has
been around since the early 70$, conventional liposomes have proven not to be good
carriers for drugs because they are readily broken by contact with other surfaces (very much
like soap bubbles) and because they are easily degraded by cellular enzymes. They can also
be easily disrupted by changes in pH, temperature or salt strength. The medical utility of the
conventional liposomes is limited by their rapid uptake by phagocytic cells of the immune

108

 

108

011‘

C0111

system, predominantly in the liver and spleen. This uptake is due to the characteristic
nonspecific reactivity of the liposomes, which results in their largely uncontrollable

properties upon administration in vivo.

Researchers have sought many ways of stabilizing liposomes and bilayer membranes.
These include the preparation of sterically stabilized [36-39] (stealth) or polymorphic
liposomes. Synthetic polymers are used for steric stabilization. Another approach is cross-
linking membrane components covalently, or the polymerization of polymerizable lipids
[40, 41]. Supported self assemble lipid bilayers [13] are also used as self-assemble
monolayer stabilized microbubbles [42]. Stealth liposomes contain a phospholipid with
polyethylene glycol (PEG) attached to prevent the liposomes from sticking to each other and
to the blood cells or vascular walls. They are invisible to the immune system and have
shown encouraging results in cancer therapy. Coating liposomal vesicles with a hydrophilic
polymer such as PEG reduces uptake by the liver. They can therefore remain in circulation
longer than conventional liposomes. Stereo stabilized liposomes have been created in which
the lipid bilayer contains glycolipids or are conjugated with ethylene glycol. This can
provide a steric barrier outside the membrane. Sterically stabilized liposomes can stay in the
blood up to 100 times longer than conventional liposomes thus they can increase the
pharmacological efficacy of encapsulated agents. Furthermore they have revived the
possibility of ligand-dependent targeting to specific cells by incorporating targeting ligands
on their surface, because they are much less subject to nonspecific uptake than are the
conventional liposomes [43].

109

The practical use of self -assembled systems in technological applications requires that the
supramolecular assembly to survive and function over a great range. One strategy for
obtaining stability is polymerization of the microstructure following self-assembly. There
have been intensive studies in this area [44]. Fatty acids have been used to prepare
polymerizable vesicles [45-47]. Phospholipids containing diacetylenes in the fatty acyl-
chains [48-52] and diacetylenic lipids with ammonium or glutamate-based head groups [53]
have been studied. Other polymerizable groups that have been explored include butadienes
[54 ], terminal vinyl and methacryloyl functionities [8, 55] and acrylates [56]. These
materials have been investigated for producing stabilized microstructures for biosensors and

encapsulation applications.

One other methods for forming stabilized systems that has been explored is supported self
assembling monolayers and bilayers [57-60] in combination with the Langrnuir-Blodgett
technique [61, 62]. Supported membranes on solid supports have been widely studied as
theoretical models of cell membranes and used as a way of biofunctionalization of inorganic
surfaces. These are potential useful for biocompatible surfaces, opto-electronic devices,
biosensors and immobilization of proteins. In some cases, hybrid bilayer membrane systems
containing both phospholipids and alkanethiol components formed by self assembly
supported on a metal surface has been studied and the stability potential applications as
sensors and in bioelectronics etc. has been evaluated [63-65]. Self-assembled monolayers

are now being used for the oriented immobilization of biomolecules, for frmctional and

110

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structural studies of proteins in immobilized biomembranes and on micro-to nanostructured

surfaces for biosensor and nanotechnology applications [66].

As noted earlier, one general drawback to the use of phospholipids in the applications
discussed above is the general instability of the supramolecular structures they form. Hence
vesicles tend to lyse too easily and leak the materials that are trapped therein. Planar lamellar
systems on solid supports often lack sufficient intermolecular cohesive power and are too
easily perturbed. Some special device such as a Langrnuir-Blodgett trough is often required
to form them in the first place. It is therefore important to design and synthesize
phospholipid variant structures that contain some extra stabilizing features. Sarcina
ventriculi is a Gram-positive bacterium which is tolerant to extremes of pH (2.0 to 10) as
well as moderately high temperatures, and the presence of organic solvents[67, 68]. In.
recent years, the mechanism of the extreme stability in these bacteria have been established
by Hollingsworth etc [69]. The membranes contain very long bifunctional fatty acids
spanning the cell membrane and they are synthesized by the tail-to-tail joining of membrane
lipid chains between the bilayers. Such organisms live in extremes of environmental
conditions such as low pH, high pH and high temperature in addition to the pressure of high
concentrations of drugs and organic solvents. These organisms survive by synthesizing
membrane lipids that contain very long fatty acid chains that go through the entire membrane
instead of just from the middle of the membrane to the outside [70-72]. The fatty acids are
very long ( 28 to 36 carbons ) a, (o- dicarboxylic acids that are esterified to a glycerol.
molecule on both ends. The head groups are phosphatidyl glycerol, monoglucosyl diacyl

lll

glycerols or any of the common functionalities found in the typical membranes [figure 4.4.].
It is clear that much of the stability of the membranes of such extrernophilic organisms stems

from the presence of the transmembrane fatty acyl group.

The archaebacteria lipids have been used to prepare liposomes, they form single lipid
membranes [73] rather than the normal bilayer type because they too contain very long
chain lipids twice the length of normal lipids [Figure 4.5]. Membrane lipids extracted from
these bacteria have also been used to form liposomes. Physical studies show positive results
such as extra stability towards high temperature, high or low pH, and other harsh conditions-
[ 74-78]. Because isolation from bacteria only yield material in very small quantities, it is
impractical to use them for applications such as targeting drug delivery system, and in the
manufacturing of biocompatible surfaces. In recent years due to the importance of these
special types of membrane lipids, many synthetic efforts have been made in this area. The
total synthesis of these archaebacteria lipids is generally difficult, long winded and not

scalable to practical applications [79-82].

Some bolaforrn amphiphilic models of archaebacteria membrane lipids [83-87] which
contain transmembrane alkyl chains and ether linkage to the head groups have been
prepared. These tetra ether bipolar lipids have the advantage of forming lamellar systems
that are stable to extreme pH [88], high temperatures [84, 89] and high ionic strengths [90].
These models have ether instead of ester linkages connecting the hydrocarbon chains to the
head group and lack many of the structural features of biologically important lipids. Another

112

OH
HO
O
OH
HO
HO

HO-P=O OH
O
HO OH
O
OWl/WWWO
O O
O _ - O
OH 0 OH

Figure 4.4. The membrane lipids from Sarcina Ventriculi

113

 

Archaebacterial Lipids Type B
i? .
P: —l|3-OCH2CH2N (CH3)3

O
X=H, sugars, phosphoethanolamine, and phosphosugars.

Figure 4.5. The structures of some archaebacteria membrane lipid models.

114

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potential problem of using ether linkage is that they are stable to lipases, so release of
encapsulated substances by the action of phospholipase A-type activities (esterases) is not
possible. This might limit the practicality of using such liposome systems in applications

where host enzymes mediate the release of the trapped substance.

4.3. Mutifunctional Self Assembling Thin Films

It is becoming more and more important to develop synthetic methods for arranging
substructures in space for the purpose of preparing new functionalized organic materials
where groups with specific structures are organized and arranged to achieve some desired
physical effect. These effects can be electrical, Optical or mechanical or even a combination.
of these (electro-optical). Such materials should have broad applications in optical and
electronic devices and biosensors. For instance, it is proposed that the can be used as
components in pivotal optical devices used in optical computing based on photo-switching
and nonlinear optical memory effects. Other areas of potential use are the development of
sensors capable of signal amplification, electronic devices such as non-linear resistant
components, self-regulating devices, and components in artificial organs. A new generation
of intelligent materials with high-level functions is often imagined. It is hoped that such
materials can deform or change properties radically by applying external stimuli such as
temperature changes, pH changes or electric fields. They can be used as biomimetic'
membranes which have stimuli-responsive firnction or as self-oscillation membranes

simulating information acceptance and conversion functions, as observed in sensory-organ

115

cells and neurons. It is hoped that attributes such as perception, judgment, movement, and

recognition will eventually be built into such materials.

Molecular self assembly is a recognized strategy for the above applications. Though it is
still at too elementary stage to mimic even the most simple process which occurs in
biological systems, some success has been made by self assembly on surfaces according to
Whitesides[9l]. Self-assembled monolayers (SAMs) are well characterized and studied. .
They are usually prepared by immersing a substrate (gold, silver etc) in the solution
containing a ligand (alkanethiol etc.) that is reactive toward the surface or by exposing the
substrate to the vapour of the reactive species[92]. Alkanethiol SAMs on gold and silver
are usually stable, highly organized and electrically insulating [93, 94] they have been
explored for nanofabrication of molecular scale electronic devices [95]. The thickness of
SAMs along the dimension perpendicular to the plane of the monolayer can be controlled
by varying the structures of the molecules making up the monolayer. They can provide
tailorable functions by changing the structures of the organic molecules in straightforward
ways. Micropatterrring is a powerful method for controlling surface properties, with
applications from cell biology to electronics [96-98]. Self assembled monolayers (SAMs)
of alkanethiolates on gold and silver [99-101]- the structures most widely used for preparing
organic films with specific surface properties are usually patterned by partitioning the
surface into regions formed from different thiols [102-105]. The patterning required in
microfabrication is usually carried out with photolithography and soft lithography. These
techniques have potential applications in the fabrication of microelectronic devices.

116

 

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bu

Although the alkanethiol SAMs have shown enormous potential in nanofabrication, there
are some problems associated with it too. The most promising avenues for self-assembly are
presently those based on organic compounds but they are usually electrical insulators. Thus
many ideas for information processing and electrical/mechanical transduction will require
either fundamental redesign in going from the macroscopic systems presently used to
self-assembled systems or the development of new types of organic molecules that show
appropriate properties. There are also some fundamental problems with SAMs on metals.
For instances, the microscopic roughness of gold and silver surfaces and the presence of
steps and kinks in the lattice generate defects in the SAMs. Another problem is the
influence of grain boundaries and other defects in the SAMs itself. SAMs of alkanethiols
on gold and silver are trans-extended, but tilted respectively by 30° and 1 1 ° from the normal
to the surface. This tilt results in domains in SAMs even on atomically flat surfaces. The
packing of the chains at the domain boundaries is critical [102]. In recent studies, mercury
has been used as the substrate[102, 106]. It also has high affinity for thiols and the SAMs'
on a Hg surface with the thiols chain perpendicular to the surface are reported to be free
from domain boundaries. However since mercury is liquid, the surface thus formed will have

some fluidity and other means are needed to fix it.

SAMs represent one type of structure that used in molecular self-assembly to build structure
and function on the nanometer scale. They are not a general solution to the problem of
building functional nanostructures, but the lessons learned from them will be valuable in
building more versatile systems.

117

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There is an increasing amount of interest in the design of planar lamellar systems containing
conjugated polydiacetylene functions. The polydiacetylenes (PDAs) have very fascinating
properties and great potential for applications in materials science, medicinal science, etc.
These systems are known to display several interesting properties that could lend themselves
to the fabrication of a variety of devices [107,108]. One phenomenon observed in PDAs
is the blue to red color transition. These color changes occur in polydiacetylene single
crystals, cast films, solutions, and Langrnuir-Blodgett films. The transition can be triggered
by a variety of environmental perturbations including temperature (therrnochromism) and
mechanical stress (mechanochromism). They display mechano-optical effects in which
compressing the polydiacetylene layers lead to a change in color of the films [109,1 10]. It
has also been observed that biomernetic polydiacetylenes incorporating carbohydrate ligand.
change color fiom blue to red upon specific binding of a biological target (biochromism)
[107, 11 1-1 17]. The liposomes contain diacetylene fatty acids or thin films formed by long
chain polydiacetylene containing a receptor binding ligand have been used to detect the
binding of influenza virus to surfaces [111, 112, 114] ( Figure 4.6), in this case the head
group of the fatty acid chain is sialic acid. In another study the molecular recognition and
colorimetric detection of chlora toxin has been investigated by polydiacetylene liposomes
incorporating Gml ganglioside [113]. The charge induced chromatic transition of amino
acid derivatized polydiacetylene liposomes was also observed [118]. Polydiacetylene
layers also demonstrate color changes in response to alterations in temperature [1 19-122] ,

pH [118, 123] and on exposure to some solvents [124-125].

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Polydiacetylenes have much potential as new organic materials with potential applications
in electrical conductivity, biosensors, non-linear optical applications and near-infrared active
applications [126-131]. The fabrication of 2-dimensional systems in which polydiacetylene-
functions occur as a band within the system are especially of interest [132-135].
Polydiacetylene is well recognized for its high electrical conductivity and high third order
non linear optical property due to their extended systems [130, 136—143]. Many studies of
the nonlinear optical properties of polydiacetylenes have been done to establish a
combination of suitable parameters for useful devices. Materials which do not possess a
center of inversion and have oriented dipoles can show so-called second-order nonlinearities
like second-harmonic generation or electro optical effects. Third-order nonlinearities which
lead to third-harmonic generation or to an intensity-dependent refractive index can appear
in every material and do not have the symmetry constraints like the second-order effects. The
nonlinear optical response depends on the magnitude of the corresponding material I
parameters called second- and third-order nonlinear optical susceptibilities respectively [
144, 145]. Besides the high third order non linear susceptibility, the second order nonlinear
susceptibility of polydiacetylene systems are also being explored. Some aromatic substituted
asymmetric PDAs have been prepared and they exhibit second harmonic generation [127,
146 -148]. A variety of systems containing PDAs have been studied, and the possibilities

of using them as optical electronic materials and biosensors has been explored.

Despite the vast amount of literature in this area, there are still some fundamental problems
associated with polydiacetylene systems. The chromic transition of PDAs is well known'

121

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PDA backbone structures based on the ab initio calculation. The average value from the
molecular mechanics calculation is shown in parentheses. (Adapted from reference 154).

122

and the origin of it has been studied by a number of investigators and attributed to many
different phenomena. However, the mechanisms of these phenomenon are not fully
established and there are many conflicting reports in this area. General principles have yet
to be established [149, 150, 154]. The second issue is the mode of polymerization. There
are two types of polymerization modes that currently have been proposed (Figure 4.7). The
most common one is the acetylenic form, the triple double bond alternating mode, the other

one is the butatriene type, depending on the side chain structure [152-154, 119].

Besides the lack of accurate theoretical explanation of the chromism, the most problematic
issue in the application of the polydiacetylene system is that a good method of fabricating
them is lacking. The current existing systems are not quite useful for applications such as
advanced opto-electronic devices and other materials. The intractability of typical
polyethylenes make them difficult to orient. They are insoluble in most solvents, form hard
masses or bundles of fibers that are impossible to transform to flat (2D) systems, and they
are very sensitive to oxygen and radical species which react with the double bonds and
reduce their conductivity. In fact, films can become resistive if exposed to oxygen for a.
short period. Also the polydiacetylene tend to form fibers instead of films. This makes it
unsuitable for many applications especially when lamellar systems are desirable. Under
favorable circumstances, the use of amphiphilic molecules with acetylene groups in the
hydrocarbon chains leads to film formation [156-157]. However such molecules often

contain only one hydrocarbon chain and they have a tendency to form micellar systems.

123

The alignment of long chain diacetylenes to effect polymerization with the resulting
formation of these highly conjugated polydiacetylenes layers or bands is one of the most
difficult aspects of the fabrication of these materials. The acetylene functions need to be
within 0.5 A of each other [155]. Several strategies have been tried to obtain good alignment
of the diacetylene chains. The most common strategy in recent years is based on the same
idea of anchoring alkanethiols on metal surfaces. Thiolated diacetylene chains are anchored
on a flat support such as a highly polished gold surface or other metal surfaces [156, 157,
158-162]. However, there are some problems associated with obtaining long range order
of alkyl chain containing diacetylenes by this method to. The most severe problem being
that, given the large atomic radius of gold compared to carbon, variations in the surface of
only a few gold atoms put the acetylene fimctions in the aligned chains out of register thus
terminating polymerization (Figure 4.8). It is therefore very difficult to obtain any high
degree of long scale polymerization. It is also an extremely labor-intensive approach. There
is also the tendency to form disulfide groups. When this happens, the chains are not aligned
and polymerization is not possible [156]. The SAMs of n-alkylthiol-gold show no
significant variation in chain crystallinity or long range order with substrate preparation,
but for diacetylene containing monolayers the long range order is hard to establish even
though a study has show that on special treatment of the gold surface or on a high
temperature evaporated gold surface, favorable alignment occurs thus allowing

polymerization of the diacetylene units.

124

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.8. Schematic diagram of the structure and polymerization of diacetylene
monolayers on gold or other surfaces with a high ration of step to terrace sites. The
polymerization requires strict spatial arrangement of the diacetylene groups. a=4.7-5.2A,.
b=3.4-4.0 A. Polymerization across step sites is greatly hindered because the spatial
tolerances are exceeded.(c and d are too large). (Adapted from reference 156).

125

Another approach for aligning diacetylene chains is to generate monolayers using Langmuir
Blodgett technology [163-168]. Although it is a good method to prepare highly ordered thin
films, it is a labor-intensive method and LB films are not stable enough to remain intact
under a variety of chemical or physical conditions. This restricts their utility. This method

is also not suitable for large scale preparations.

Because phospholipids readily form stable lamellar systems or other highly ordered
structures, another possible approach is to synthesize diacetylene lipids that are
biomembrane mimics. The inclusion of conjugated diacetylenic groups at the same position-
in each acyl chain of a phospholipid chain [39, 40, 169-170] (Figure 4.9) should give ideal
self-assembling units which can be polymerized to form highly organized, stable, 2-
dimensional systems containing a conducting polydiacetylene layer. Such phospholipids
have been prepared and the physical properties characterized. These phospholipids
containing diacetylene function in the middle of the chains were reported to form tubules
under certain conditions [171-172]. Another approach that has been tried is to use
microorganisms to carry out the integration of fatty acids containing diacetylenic functions
into phospholipids. Using this strategy, as much as 90% integration of diacetylenic fatty
acids into microbial phospholipids was obtained [17 3]. There are some problems with this
approach however. Because the membrane is only two molecules thick and just surrounds
the cell, the actual amount of material recovered per gram of cell mass is extremely small.
In addition to this, the lipid species made by any one microorganism are extremely diverse

and may include neutral, and negatively charged headgroups with different structures. It is

126

a challenge to separate species with only one type of headgroup and, even then, there is a
tremendous amount of diversity in the fatty acid species that are derived from the normal
microbial metabolism. Also microorganisms contain a myriad of membrane-associated
enzymatic activities that can reduce or oxidize the diacetylenic functions. The problems with
diacetylenic phosphatidyl choline are not only that they are difficult but that are also it has
been found that these DAPC have problems on photopolymerization. Some other double.
chained diacetylenic lipids with a fairly flexible head group such as glutamate [132, 174,
175] and amidobutyl-nitrilotriacetic acid derivatized chelator lipid [134, 176] (Figure 4.9)
are also being synthesized and characterized. The photopolymerization of diacetylenes is
acutely sensitive to the molecular order of crystals and supramolecular assemblies. It was
found that diacetylenic fatty acids are polymerizable only in close-packed solid-like
monolayers [177-178]. Bilayer membranes of diacetylene lipids are neither
photopolymerizable above the lipid phase transition temperature of the membrane nor in
small sonicated vesicles where the lipid chain packing is disordered by the sharp radius of
curvature of the membrane. The absorption maximas of PDAs are indicative of the length
of the polymer chain and the order of the polymer structure. Longer conjugation and higher.
ordered PDAs exhibit absorption maxima at 650 nm, while shorter and less ordered PDAs

such as phosphatidylcholine diacetylene bilayers show absorption at 540 nm.

It is unclear why the DAPC systems could not afford extensive polymerization [179]. It

probably has something to do with the way that they were prepared. It might also be related

127

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Figure 4.9. The structures of three types of diacetylene containing lipids in literature.

128

to the structure of the head group. In any event, this approach of using biomembrane mimic
lipids to achieve long range order in polydiacetylene systems is very promising. Some other
systems are with simple head group but which possess the self assembly properties of

phospholipids are desirable.

In summary, polydiacetylenes are very promising conductive polymers for many
applications, including biosensors, optical electronic devices, mutifunctional materials, etc.
However, many problems need to be solved before they can reach the actual application

stage. The major one is to obtain high long range order of the alkyl diacetylene systems.

129

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139

Chapter 5

Synthesis and Properties of a Bipolar, Bis-Phosphatidyl Ethanolamine
that Forms Stable 2-Dimensional Self-Assembled Bilayer Systems and
Liposomes

140

Abstract
Phosphatidyl ethanolamine is one of the most common naturally occurring phospholipids.
The presence of an amino group also makes it one of the most straightforward to'
functionalize. Unfortunately however, it tends to form micelles and hexagonal clusters
instead of forming lamellar systems such as sheets and vesicles (liposome), key
supramolecular structures for a variety of biomedical and other technical applications. This
has been overcome by synthesizing a dimeric phosphatidyl ethanolamine molecule in which
the acyl chains at the 2-position of glycerol are joined at their termini by a carbon-carbon
bond thus resulting in a trans-membrane type fatty acyl linkage. Such tail-to-tail bipolar
transmembrane lipids are found in the membranes of thermophilic and other extremophilic
bacteria. The dimeric phosphatidyl ethanolamine readily forms very uniformly flat self
assembled lamellar supramolecular arrays and liposome that are stable at temperatures up.
to 80°C. The planar systems formed by the new lipid were characterized by atomic force
microscopy and the uniformity of the vesicles were observed by laser scanning confocal
microscopy. The thermal stability of the phospholipid and the vesicles formed was examined
by NMR spectroscopy including proton Tl measurements as a function of temperature.
Fourier transform infrared spectroscopy indicated a much greater order of the alkyl chain
in the dimer than in phosphatidyl ethanolamine. This extremely stable and readily
functionalizable dimeric phospholipid has potential uses in the fabrication of biomaterials,

stable membrane models and liposome drug delivery systems.

141

5.1. Introduction

There has recently been much interest in the use of phospholipids, phospholipid analogs and
other systems that form 2-dimensiona1 lamellar systems in several areas of research and
technology including drug encapsulation" 2, gene delivery 3", biocompatibilty“, sensors and
specialized receptor surfaces 7' 8 and advanced biomaterials 9'”. Phospholipids serve as the
prototypical materials for the design of such surfaces because of their extremely high packing
densities and propensity to form lamellar systems. In all of these applications, it is desirable
to form a self-assembled lamellar system with some desired functionality on one surface. In
the drug encapsulation and gene delivery areas, it is a requirement that the lamellar system'
have an overall spherical topography to form hollow structures called vesicles or liposome.
From the standpoint of chemical functionality, of all of the naturally occurring phospholipid
systems, none is more suited for such applications than phosphatidyl ethanolamine 1. This
is because of the presence of the easily derivatizable primary amino group. Unfortunately,
because of the small size of its head group, phosphatidyl ethanolamine tends not to form
lamellar systems but forms micelles or hexagonal phases instead”. It is therefore not

possible to make pre-forrned lamellar systems and then functionalize them.

9(CH2)2N+R3
'o- $=o
0
PM
0W
O 1. R=H
3. R=CH3

One general drawback to the use of phospholipids in the applications discussed above is the
general instability of the supramolecular structures they form. Hence vesicles tend to lyse too
easily and leak the materials that are trapped therein. Planar lamellar systems on solid
supports often lack sufficient intermolecular cohesive power and are too easily perturbed.
Some special device such as a Langrnuir-Blodgett trough is ofien required to form them in
the first place. It is therefore important to design and synthesize phospholipid variant.
structures that contain some extra stabilizing features. The preparation and use of such

materials is of special significance to several very different fields.

There are good clues as to how to stabilize the lamellar bilayer structure of phospholipids if
one examines the structures of biomembranes of organisms from habitats where extremes
of temperature, pH or the presence of deleterious chemical substances persist. Such
organisms can flourish at temperatures in excess of 100 °C in geothermally active sites at the
bottom of the ocean. For such organisms, one common structural feature of their membrane
lipids is the presence of transmembrane fatty acyl components'f‘g. These lipids contain very
long (28 carbons or more) woo-dicarboxylic acids formed by tail-to-tail joining of lipid fatty.
acyl chains between the two leaflets of the bilayer'8"9. The synthesis of lipids such as 2 with
the phosphoethanolamine head groups is an excellent target. Such molecules are not known
but would combine the good thermal and chemical stability associated with the presence of
the very long cum-dicarboxylic acid group with the ease of modification of the primary
amine group. In addition, any tendency to form non-lamellar systems such as micelles would
be suppressed. Because of the stabilization by ionic forces at both ends and the extra

143

stabilization by the very long hydrocarbon chains, it should not be necessary to compress
films of these lipids molecules into compact layers using Langrnuir-Blodgett troughs. This
approach has been used with synthetic bolaforrn amphiphile models of archaebacteria
membrane lipidszo'z" which also contain transmembrane alkyl chains except that they are
ether linked to the head groups. These tetra ether bipolar lipids have the advantage of
forming lamellar systems that are stable to extreme pH”, high temperatures“: 26 and high
ionic strengths”. However, these models have ether instead of ester linkages connecting the.
hydrocarbon chains to the head group and lack many of the structural features of biologically
important lipids. This might be critical for medical applications. We describe herein the
preparation and characterization of the transmembrane stabilized triacotanedioic acid -
containing phosphatidyl ethanolamine 2. The lipid contains normal phosphatidyl
ethanolamine head groups and ester linkages. Since archaebacteria lipids contain ether
linkages between the hydrocarbon chains and the head groups, release of encapsulated
substances by the action of phospholipase A-type activities(esterases) is not possible. This
may limit the practicality of using such liposome systems in applications where host

enzymes mediate the release of the trapped substance.

144

O

 

 

O
O 1. TMSCI, Nal, CH3CN ’U\/\/\/\/\/\/\/\
$ CHaO I
2. CH3OH .
11 4
O 0
DMSO’ NaHCOS /u\/\/\/\/\/\/\/lk
> CH30 H
150°C 5
O
...... W
4 + PPR; ——> crao ppmq
reflux, 24 6
Cl'bOle, DMF
5
Cl‘bOmCH2(CH2)12CH= CHCH2)1 2CH2COCX3 H3
1. PdlC, H2 7
O 2. NaOH, H20
HOJK/VW\/\/\/\/\/\/\/W\/\NOH
O
8
l (COCI)2, CH2C12
O
C|/U\/\/\/\/\/\/\/\/\/\/W\/\/\n/Cl
9 0

Scheme 5.1 The synthesis of triacotanedioic acid 8.

145

(IXO'leHZ

 

HO- 1?: o O(CH2)2NHBOC
0 HO- P=O
0H 1800120. 513'“ °
DMAP, CHaoI-T 0”
O (CH2)12CH3
Y o
0 W
O
10 11
9
ridine
('XCthNHBOC z: I 9(CH2)2NHBOC
HO- P=O '20 2 0:13-014
o o o
z—OWWVWWS 0 K
0 /\/\/\/\/\/\/IL
0W O
O
12
CFacmH
0(0H212NH2 “202 0(CH2)2NH2
HO- 13:0 0: 15- OH
0 o o
%OMWWVM O K
O /\/\/\/\/\/\/u\
OWWWV O
O 2

Scheme 5.2 The synthesis of dimeric phosphatidyl ethanolamine 2.

146

5.2. Results and discussion

Synthesis of the transmembrane phospholipid analog 2. The synthesis of 2 involved
firstly the preparation of triacotanedioic acid followed by its coupling to a protected lyso-
lipid. Triacotanedioic acid was prepared from pentadecalactone according to Scheme 5.]:
The lactone was converted to the 15-iodoacid 4 and a portion of the acid oxidized to the
aldehyde 5 . The aldehyde and iodoacid (protected as esters) were joined by Witti g reaction
to form triacotan-lS-ene dioic acid dimethyl ester which was converted to triacotanedioic
acid dimethyl ester by catalytic hydrogenation. Saponification and acidification yielded the
diacid which was converted to the acyl chloride 9 with oxalyl chloride. The transmembrane
stabilized phospholipid 2 was then constructed as shown in scheme 5.2. Two molecules (11)
of arnino-protected (t-butyloxycarbonyl) lyso-phosphatidyl ethanolamine with a
tetradecanoyl group in the l-position were joined by one molecule of the acid chloride of
triacotanedioic acid. The desired compound 2 was obtained after removal of the t-

butyloxycarbonyl group with trifluoro acetic acid.

Supramolecular structure, chain packing, and film forming properties. The fully
hydrated films formed by compound 2 and a typical phospholipid that forms lamellar
systems and vesicles easily (phosphatidyl choline 3) were studied by laser confocal scanning
microscopy. The images were observed using polarized light, dark field, and phase contrast
optics. The results are shown in Figure 5.1. The lipid 2 exhibited very different behavior
compare to the normal phospholipid 3. Whereas the latter formed sheets, ribbons and

vesicles with a large variation in size, lipid 2 formed only vesicles with a very narrow

147

 

Figure 5.1. Laser scanning confocal microscopy images of hydrated compound 2 and 3. The
images to the left A and B are phase-contrast pictures of 2. The circular structures are
liposomes. Note the uniformity of their size. Image D is a dark field image of compound 2
showing the spherical outline of the liposomes. The image C is a phase contrast picture of
3. Note the variation in size of the liposomes as well as the presence of several other lamellar
structures corresponding to sheets and ribbons.

148

5.6511011 1"..111;1l,r'.\13

 

Figure 5.2. The atomic force micrographs of the thin film formed by compound 2. (a) is
the section analysis over an 800 nm range, (b) is a top view of the same area from which
the section analysis was taken and (c) is the surface plot of the same area as above two
images.

149

distribution in size.

The atomic force micrographs (Figure 5.2) of thin films formed by compound 2 on mica
plates demonstrated that they were extremely flat and uniform with a maximum surface
variation of only 5.8 nm over a distance of 800 nm (0.7%). This ability to form 2-
dimensional systems with such low surface variation has much significance for the potential
use of lipid 2 in a variety of applications. The presence of the fiee primary amino head group
should allow surfaces modified with this lipid to be further functionalized with a variety of
reagents especially peptides and proteins with specific biological functions. The amino
groups can also be functionalized with various optical probes and a myriad of other
structures. Such surfaces have potential value in nano-fabrication, surface patterning,

advanced material science and the construction of biological membrane mimics.

Information about intermolecular chain packing and intramolecular chain conformation of
phospholipids and other membrane forming amphiphiles can be obtained by vibrational
spectroscopy methods such as Raman and infi'a-red spectroscopy28'3'. It is known that for
hydrocarbon chains, the intensity ratio of the methylene asymmetric (2918cm“) over
symmetric (2850cm") vibrations is sensitive to the intermolecular chain packing order and
the ratio increases with increasing order". Fourier transform infrared spectroscopy
experiments were carried out on lipid 2 and on di myristoyl phosphatidyl ethanolamine l in
both the dry and the hydrated states. The peaks were deconvoluted and quantified by Fourier.

analysis in conjunction with a curve-fitting routine using a mixed Laurentian and Gaussian

150

line shape and a peak width parameter that could be adjusted to give optimum fitting.
Analysis on the dry films of di myristoyl phosphatidyl ethanolamine gave an area ratio
(asymmetric to symmetric) of 1.83 while the value obtained for the dimer was 2.58. The
hydrated samples (prepared by forming the films from water by slow evaporation onto a
calcium fluoride window) gave a ratio of 1.37 for di myristoyl phosphatidyl ethanolamine
and 2.33 for compound 2. These results demonstrated a substantially higher ordering of the
alkyl chains in the dimeric phospholipid compared to di myristoyl phosphatidyl
ethanolamine in both dry and hydrated states. In the hydrated state, both lipids showed a'
small decrease in order but the dimer was still significantly better packed than the normal

phospholipid.

Self assembly properties, stability, and liposome formation. The structural order and
stability of the supramolecular systems formed by lipid 2 were probed by two different
nuclear magnetic resonance (NMR) spectroscopy experiments. Nuclear magnetic resonance
spectroscopy is a very useful tool for studying biomembrane systems '6' '7' 2°. The motional
freedom of specific groups in the lipid can be determined by measuring the parameter
referred to as T1 or the spin lattice relaxation time. It is also known as the longitudinal.
relaxation time. This parameter describes the rate of magnetization transfer from a nucleus
at its high energy state to its environment via a dipolar coupling mechanism. If the system
is rigid and / or ordered, this transfer is effective and TI is short. When the system is less
rigid or structured, the nucleus may not be close to any other one for long enough (on

average) to facilitate good transfer and the value of TI is then larger. If T1 is evaluated as a

151

function of temperature, a sudden, simultaneous change in its value at any given temperature
for more than one signal would indicate a change in phase of the system. Tl relaxation times
of 2 in deuterated DMSO at various temperature were measured. The curves for Tl

dependence on temperatures ( l/T) are shown in Figure 5.3. As expected, the terminal

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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a— __ 2.84ppm, T1
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I .3— 2.27ppm, T1
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o d
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.5 3 _._ 1.50ppm, T1
.5 2. \.
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2.8 2.9 3 3.1 3.2 3.3 3.4

1/T(x0.001), T=temperature, K

Figure 5.3. The relationship of T, relaxation time versus the reciprocal of temperature( 1/T)
for bulk methylene (1.22ppm), methyl (0.83ppm), methylenes next to amino group
(2.84ppm), methylene a to carbonyl groups (2.27ppm and 2.16ppm) and methylene [J to
carbonyl groups (1.55ppm).

152

methyl groups displayed the largest T, values among all the groups present in the lipid. In
general, the value of T, increased with temperature. There was a discontinuity in the plot of
T, vs temperature for the methyl signals at 70°C indicating a phase transition. No
discontinuity was observed for the curve corresponding to the bulk methylene groups up to
a temperature of 75 °C. Two signals corresponding to the methylene protons adjacent to the
ester carbonyl groups were observed. One appeared at 2.16 and the other at 2.27 ppm. The
nuclei giving rise to the first signal had a significantly larger T, value indicating that it was
most likely due to the protons on the more mobile 14 carbon chain. A discontinuity was
observed for both signals at 70°C. In the head group region, the signals for the methylene
group adjacent to the amino group displayed a larger T, value indicating that it was more
mobile than the other methylene group. A discontinuity at 70°C was also observed. The
concerted abrupt change in T, at 70°C is indicative of a significant change in phase at this
temperature. It indicates that there is a high degree of supramolecular organization in lipid
2 even in dissociating solvents and at temperatures way abovethose that normal lipids

function .

Liposome or vesicles have been explored for drug delivery models because of their ability
to encapsulate various component inside. Because of their instability, liposome made from
typical lipids are not good drug carriers and many methods have been developed to improve
the stability of liposome 32' 33. Lipids isolated fi'om thermophilic bacteria were used to prepare
liposome which proved to have increased resistance to leakage’“? In order to explore the

stability of liposome formed from the stabilized phospholipid dimer 2 as a drug delivery

153

system, the ability to trap water (H20) and ferric ions (paramagnetic relaxing agents) inside
such liposome was studied by proton-NMR spectroscopy. The spectra of a suspension of
these liposome were obtained in D20 with increasing temperature with the expectation that
when the temperature was high enough to cause leakage, the ferric ions would leak out and
cause a sudden increase in line width of the bulk external water line. The liposome were
prepared according to literature methods 374° with modifications as described in the
experimental section. There was one striking feature of the NMR spectra (Figure 5.4). The
signal for the water line at 20°C was broad and triangular in shape indicating that the water
was restricted in motion and confined to a region with significant chemical shift anisotropy.’
This was very reminiscent of the peak shape for phosphorous signals in phospholipids 41. 42
where the shape stems from the same cause. Closer inspection of the peak revealed that there
were several envelopes superimposed on each other. These signals were due to trapped water
in different environments inside of the vesicles. As the temperature was increased, the
envelopes became more discreet as the individual peaks narrowed because of increased
mobility of the water molecules. As the temperature was increased, there was also a gradual
exchange of H20 from inside for DZO from outside of the vesicles. This was indicated by
a gradual disappearance of some of the signals. The presence of multiple water peaks even
at 80°C indicated how resistant to breakage and leakage the liposome were even at that.
temperature. Breakage would have led to an immediate reduction of all of the signals to one

broad signal. It was clear from the experiments that the ferric ions were still trapped inside

of the vesicles even at this temperature.

154

 

 

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155

5.3. Conclusion

In these studies, we have synthesized a new membrane-spanning dimeric phosphatidyl
ethanolamine molecule that readily forms stable vesicles of uniform size as well as flat thin
lamellar film with very small variation in surface features. The motion of the lipids in these
systems is considerably more restricted and the alkyl chains are more ordered than those in
typical phospholipid systems in both the dry and hydrated states. The lipid has good thermal
stability and high phase transition temperature. The vesicles fabricated from this tail-to-tail
dimer lipid are extremely uniform in size and very stable to high temperatures. The results.
we obtained here are important because phosphatidyl ethanolamine does not form vesicles
on its own. The synthesis of phospholipids in general is difficult and the synthesis of ether-
containing archaebacteria-type lipids even more so. Lipids from archaebacteria are not
generally available in significant quantities. Naturally occurring phospholipids are available
in substantial quantities and the strategy we adopt here can be employed if the sn-2 fatty acyl
groups were removed with a phospholipase A enzyme. The approachwe describe here is
generally applicable to other lyso- phospholipids provided that the head groups can be
protected. These ultra-stable membranes can be further developed for fabricating stabilized
drug delivery systems. They also have potential for the preparation of biocompatible surfaces
and the fabrication of molecular devices through modification of the primary amino group
perhaps with metal-binding functional groups. There are also possibilities for other surface

chemistry applications in material science and biotechnology.

156

5.4. Experimental section

5. 4. 1. Characterization of physical properties

Atomic force microscopy. These analyses were performed using a Nanoscope III instrument
operating in contact mode. For these measurements, the Compound 2 was dissolved in
chloroforrn- (0.5-1% solutions) and ~ 10 11L used to coat freshly cleaved mica plates

spinning at 200 rpm to effect uniform coating.

Laser scanning confocal light microscopy. These experiments were performed on a Zeiss
210 instrument with a 488 nm laser. Images were obtained in the dark-field, phase contrast
and polarization modes. For the polarizing mode experiments, an analyzing cross-polarizer
was placed on the objective lens and rotated until light cancellation. For film preparation,
The compound was dissolved in chloroform, and deposited on a microslide, after the
solvent evaporated, a drop of water was added on top of the film, the film was hydrated for
two-three days. Before the measurements, a drop of water was added on top of the film
which was then covered with a round cover glass. For comparison, a sample using PC'

(compound 3) was prepared and observed by the same methods.

Vesicle stability experiment. The vesicles were prepared according to methods in the
literature’“0 with modifications. About 1-2 mg of compound 2 was suspended in 0.4 ml
phosphate buffer solution (PBS) (Dulbecco, pH=7) in a small vial, about 50 micro liter of
0.1M MgSO4 solution and 3-4 mg of ferric citrate were added to the vial. The mixture was

then sonicated and vortexed altematingly for about 45 minutes. It was then allowed to stand

157

at room temperature for 20 hours before being dialyzed against PBS. Just before the NMR
spectroscopy measurements, the water was exchanged with PBS made up in deuterated
water. About 50 11L of this liposome suspension was added to 0.6 ml of PBS in deuterated

oxide for the NMR study.

FT-IR experiment. The FT-IR experiments were performed on Nicolet 710 spectrometer.
For the dry state measurements, the sample was dissolved in chloroform and the solution
was transferred to a sodium chloride window and the solvent allowed to evaporate. For
measurements in the hydrated state, the sample was dissolved in water which was sonicated
to form a suspension, a sample of which was placed on a calcium fluoride window. The
suspension was air dried for 3-4 hours to remove the excess water. Heating and reduced
pressure is generally required to remove the hydration water. The deconvolution of the'
methylene symmetrical and asymmetrical absorptions and the calculation of peak area were

performed using Galacitic Peaksolve program by Galactic Industries Corporation.

T, relaxation experiments. Proton T, measurements were performed on a Varian 300
MHz NMR spectrometer. Measurements were made at temperatures of 25, 30, 35, 40, 45,
50, 60, 70, 75°C. The parameters for each temperature were the same. 14 points were
collected for each relaxation curve at each temperature. The pulse sequence used was rt-r-

n/2, pw 90:12.7 msec. The solvent used was deuterated-DMSO.

158

5. 4. 2. Synthesis
lS-iodo-pentadecanoic acid methyl ester 4. To a 1000 ml round bottom flask was added
pentadecalactone 22.0 g (0.915mol), trimethyl silyl chloride 23.2 ml (0.183mol), sodium
iodide 41.2 g(0.275 mol), and 220 ml of acetonitrile. The miXture was left stirring at 50 °C
for 14 hours, 220 ml of methanol was added to the reaction mixture, the heat was turned off
and the mixture left for another 3 hours. The dark brown mixture was filtered and the liquid
was concentrated by rotatory evaporation to remove all solvents. The residue was dissolved
in chloroform, washed with 50 ml of saturated sodium thiosulfate and the organic layer dried
with sodium sulfate. Upon removal of solvent, a white to light yellow solid was obtained.
34.6 g (99%). Mp. 44-45 °C. 1H NMR( CDC13, 300 MHz), 8 3.64 (S, 3H ), 5 3.17 (t, 2 H,
J=6.9-7.2 Hz) , 6 2.28 (t, 2 H, J=7.5-8.4 Hz), 5 1.80 (q, 2H, J=7.2 Hz), 8 1.59 (q, 2H, J=7 .2
Hz). 13C NMR (CDCl,, 75 MHz), 5 174.29, 51.40, 34.09, 33.55, 30.49, 29.56, 29.55, 29.51,
29.4l,29.39, 29.23,,29.13,28.52, 25.72, 24.93, 7.27. IR (NaCl, CHC13, wavenurnber, cm“),

2916.7, 2851.2, 1734.2, 1473.8, 1457.2, 1250.0, 1202.2, 1169.0, 756.2.

15-oxo-pentadecanoic acid methyl ester 5. A mixture of 4 g sodium hydrogen carbonate
and 60 ml of dry dimethyl sulfide was heated to 150°C under dry nitrogen. To this mixture
5.32 g (13.9 mmol) of compound 4 was added and heating and stirring continued for 4
minutes. The flask was cooled quickly and the mixture was poured into water which was.
stirred for several hours. The white solid that formed (the aldehyde) was filtered out or
recovered by extraction 4 times with ether. The yield of aldehyde 5 was 3.70g ( 98.6%).

The aldehyde can be purified by chromotography (solvent Hexane: acetate =7: 1) The yield

159

after purification is 3.20 g (86.5%). 1H NMR( CDCl,, 300 MHz), 5 9.74 (t, 1H, J=2.0 Hz),
5 2.40 ( d't, 2 H, J=7.2 Hz, 2.0Hz), 2.28 (t, 2H,7.5Hz). 13C NMR (CDCl,, 75 MHZ), 5ppm

202.84, 174.28, 51.36, 43.86, 34.06, 29.52, 29.37, 29.29, 29.19, 29.09, 24.90, 22.04.

Synthesis of the triacontanedioic acid 8. Preparation of the triphenyl phosphine salt 6 :
The methyl ester iodide 4 7.64g (0.02 mol) and 10.48 g (0.004 mol) triphenyl phosphine
were stirred in benzene under refluxing condition for 24 hours after which the solvent was
removed by vacuum distillation. The residue was taken up in diethyl ether and the solution
stirred vigorously for at least one hour. The mixture was filtered to remove the triphenyl
phosphine in ether and the white solid was collected and washed with ether again to ensure
all the excess triphenyl phosphine to be removed. The product 6 was dried under vacuum
oven for 24 hours and the yield was 11.6 g (90%). The salt 6 ( 6.4 g) and anhydrous
dimethylsforrnamide 20 ml were mixed in a round bottom flask. Sodium methoxide (0.6g)
was added very quickly to the solution which was stirred and cooled in an ice bath for 10
minutes. After this time, then 0.45 g of freshly prepared aldehyde 5 dissolved in 10 ml of
dry DMF was added to the mixture in a dropwise fashion under dry nitrogen. The
temperature was maintained at 0-5°C during the addition. The reaction mixture was stirred
for 24 hours, after which TLC analysis indicated that the reaction was essentially completed.
The reaction mixture was diluted with water and extracted with hexane several times. The
crude product in the hexane extract was purified by flash column chromatography (hexane:
ethyl acetate= 9:1). The product was a white solid. Yield: 0.74 g (87%). 'H NMR( CDCl,,

300 MHz), 5.40-5.28ppm (m, 2H ), 3.66ppm (s, 3H), 2.30ppm (t, 4H, J=7.5Hz), 2.07-

160

1.91(m, 4H) 1.68-1.54ppm (m, 4H), 1.25(s, broad, 40H). 13C NMR (CDCl,, 75 MHz),
174.32, 129.85, 51.41, 34.08, 29.75, 29.63, 29.56, 29.43, 29.29, 29.24, 29.12, 27.17, 24.93.
The ester was hydrogenated (50-200 psi) using pd-C (10%) in ethanol to give the saturated
compound, which upon soaponification and acidification of the sodium salt, gave the diacid.
8 (0.56 g) as a white solid. The overall yield was 80% for these two steps. 1H NMR (DMSO,
300MHz) 2.149(t, 4H , J=7.2Hz), 1.52-1.36(m, 4H ), 1.32-l.12(s, broad, 48H). 13C NMR
(DMSO, 75MHz), 175.65, 34.50, 29.70, 29.52(m), 29.32, 25.30. IR(CHC13, NaCl)
3046(broad), 2915, 2848, 1694, 1471, 1462, 1434,1408, 1284, 1109, 897.6, 718.5. MS Cale

0,011,804 482 ES(+) 505 ( M+Na+).

Preparation of compound 2. The Boc protection of lipid 9. 0.55g (1.29 mmol)
phospholipid 1-myristoyl-2-hydroxy-sn-glycero-3-phosphoethanolamine l 1 dissolve in 100
ml of a 1:1 chloroform and methanol mixture. (Boc)20 0.6 g and 0.02 g of dimethylamino-
pyridine, 0.3 ml of triethylamine was added to the reaction mixture, stirred overnight. The
solvent was removed, the colorless liquid was washed with 0.1N HCl once , saturated
sodium bicarbonate solution once, then by brine. The organic layer was dried overnight by
sodium sulfate. After removal of the solvent, and dried in vacuum oven for 24 hours yielded
a light yellow semi-solid 0.65 g ( 95 %). 1H NMR( CD3OD, 300 MHz), 8 4.07ppm ( dd,
1 H, J=4.5 Hz, 11.7Hz), 3.99 ppm (dd, 1H, J=5.7Hz,1 1.7Hz), 3.85 (m, 1H) (3,01-3.89),
3.72-3.80 ppm (m, 4H), 3.16 ppm (2H, t, J=5.4Hz) 2.24ppm(2H, t, 7.5Hz), 1.44-1.57ppm

(2H, m), 1.32ppm (s, 9H), 1.18(s, broad, 20H), 0.79 (t, 3H, 6.6Hz).

161

Dicarboxylate acid (0.12g, 0.25mmol) 8 in 10 ml of oxalyl chloride and 5 ml of
dichloromethane, was stirred for 12 hours. The solvent was removed quickly and the
resulting acid chloride 9 was dissolved in 5 m1 dry dichloromethane and added to a round
bottom flask which contained phospholipid 11 (0.32 g, 0.61 mol), 5 m1 dry pyridine and
10 ml dichloromethane. The mixture was stirred for 24 hours protected from moisture by
a drying tube. The solvent was removed, the crude product was taken up in chloroform and
washed with sodium bicarbonate solution. The organic layer was dried and the solvent was
removed. The crude product was purified by flash column chromatography. (
chloroform:methanol=7:3). The purified compound 12 was obtained as a white solid 0.24g
(65%)- 'H NMR( CD301), CHC13= 5:1) ppm , 5.22 (s, 2H), 4.25-4.10(m, 4H), 4.10-
3.82(m, 8H), 3.40-3.25(4H, overlap with D-methanol absorption), 2.35(t, 8H, J=7.5Hz),-
1.61(m, 8H), 1.44(s, 9H), 1.29(s, 88H), 0.90(t, 6.8Hz). ”C NMR(CDC13, DEPT) 68.8, 66.9,
64.5, 41.0, 34.0, 31.9, 29.7, 29.5, 29.4, 29.2, 28.9, 28.4, 24.8, 22.7, 14.1. IR(CHCl3, NaCl)

3380, 2929, 2856, 1743, 1670, 1524, 1464, 1365, 1246, 1173, 1106, 1067, 954.1.

A sample of 12 (~50mg) was deprotected by treatment of a solution in 3ml of
dichloromethane with 3 ml of trifluoroacetic acid. This solution was stirred at room
temperature for 4 hours and the solvent and other volatile material formed during the
reaction was removed on the rotatory evaporator. Compound 2 was obtained as an off-
white solid. The removal of the protecting group was completed as judged by NMR-
spectroscopy. 1H NMR( CD3OD, CHC13= 5:1) 5.24 (m, 2H), 4.30-3.68(m, 12H), 2.40-2.24

(m, 8H), l.61(m, 8H), l.27(s, 88H), 0.88(t, 6H, 6.6Hz). l3c NMR(CDC13, DEPT) 33.9, 31.9,

162

29.7, 29.3, 29.2, 24.8, 22.6, 14.0. IR(CHC13, NaCl) 3397, 3240, 2919, 2851, 1738, 1682,
1468, 1207, 1140, 1078, 1024, 800.0, 771.8, 723.4. MS: Calc. MW for C68H,,4N2PZO,6 1297

(ES-) 1296(M-l)

Acknowledgment. This work was supported by grants FG-02-89ER14029 fi'om the U.

S. Department of Energy and IBN 9507189 fiom the National Science Foundation to R.I.H.

163

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166

 

Chapter 6

Synthesis and Properties of Chiral Self-Assembling Lamellar
Polydiacetylene Systems with Very Long Range Order

167

 

Abstract
Two new, very accessible, chiral, self-assembling phospholipid analogs containing
diacetylenic units in the middle of their acyl chains were prepared by very simple and highly
efficient routes. Fihns were cast and then polymerized by activation with UV light or doped
with iodine. X-ray diffraction, NMR spectroscopy measurements and molecular mechanics
analyses using the MM3 force field indicated that the acyl chains are closely packed in a
parallel fashion. The new molecules and the polymerized films derived from them were
characterized by a variety of physical methods including confocal polarized light
microscopy, atomic force microscopy, and near infrared spectroscopy. These analytical.
methods showed that the un-polymerized molecules are able to form lamellar systems with
very long range order. They formed very uniform, extremely flat, thin films which are readily
polymerized to give extensively conjugated systems which absorbed well out into the near
infiared region unlike typical polydiacetylenes. The propensity of the new molecules to form
such well-packed structures arises from their resemblance to diacyl glycero- lipid with the
rt-stacking interaction fiom the diacetylene units giving extra stabilization. The results
indicate that this is an excellent approach for preparing ordered polydiacetylene systems for

use in designing advanced materials.

168

 

6.1 . INTRODUCTION

There is an increasing amount of interest in the design of planar lamellar systems containing
conjugated polydiacetylene functions. These systems are known to display several interesting
properties that could lend themselves to the fabrication .of a variety of devices“ 2. For
instance, they display mechano-optical effects in which compressing the polydiacetylene
layers lead to a change in color of the films3' 4. In other experiments, attaching a
carbohydrate molecule to a polydiacetylene layer resulted in a change in color when viral
particles bound to the carbohydrate“. Polydiacetylene layers also demonstrate color changes
in response to alterations in temperatures” , pH'1 and on exposure to some solvents”: ”.
Polydiacetylene is well recognized for its high electrical conductivity and high third order
non linear optical property. Unfortunately, it forms fibers not films and its high insolubility
and general physical intractability makes it unsuitable for many applications especially when
lamellar systems are desirable. Under favorable circumstances, the use of amphiphilic
molecules with acetylene groups in the hydrocarbon chains leads to film formation" ”.
However such molecules often contain only one hydrocarbon chain and they have a tendency
to form micellar systems. In order to obtain suitable films, the chains then have to be.
anchored to surfaces ”"9 or Langmuir-Blodgett troughs have to be employed 20‘2". The
problem with the ordering of acetylenic thiols on gold and other metal surfaces is the
difficulty in ensuring that the chains are aligned so that the alkyne groups are in a proper
orientation and close enough to allow the polymerization process . This is difficult because
imperfections on the metal surface of only a few atoms in dimension force adjacent chains
to be at different heights thus separating the acetylenic groups by too great a distance. A

169

 

substrate-independent way of ordering alkyl chains with diacetylenic functions is therefore

highly desirable. Molecular self-assembly has much promise in this area.

('30'420142 N+(CH3)3
O'-P=O

KW- .W

 

 

Figure 6.1. The structure of a typical phospholipid (phosphatidyl choline) containing
diacetylenic fatty acyl groups.

Phospholipids readily form stable lamellar systems or other highly ordered structures. The
inclusion of conjugated diacetylenic groups at the same position in each acyl chain of a
phospholipid chain”: 2° (figure 1) should give ideal self-assembling units which can be.
polymerized to form highly organized, stable, 2-dimensional systems containing a
conducting polydiacetylene layer. Such phospholids have been prepared and the physical
properties characterized. However, the synthesis of phospholipids is extremely laborious.
These phospholipids containing diacetylene fimction in the middle of the chains were
reported to form tubules under certain conditions”: 28. Another approach that has been tried
is to use microorganisms to carry out the integration of fatty acids containing diacetylenic
functions into phospholipids. Using this strategy, as much as 90% integration of diacetylenic
fatty acids into microbial phospholipids was obtained 29. There are some problems with this
approach however. Because the membrane is only two molecules thick and just surrounds,

170

 

the cell, the actual amount of material recovered per gram of cell mass is extremely small.
In addition to this, the lipid species made by any one microorganism are extremely diverse
and may include neutral, and negatively charged headgroups with different structures. It is
a challenge to separate species with only one type of headgroup and, even then, there is a
tremendous amount of diversity in the fatty acid species that are derived fi'om the normal
microbial metabolism. Also microorganisms contain a myriad of membrane-associated

enzymatic activities that can reduce or oxidize the diacetylenic functions.

Based on the above discussion, it is clear that only synthetic approaches have the potential
for producing phospholipids or phospholipid analogs containing diacetylenic functions in.
the fatty acyl chains and which have a high degree of chemical integrity. Because of the
difficulty in preparing phospholipids and hard to prepare on large scale, simpler analogs
which still contain the critical structural elements of phospholipids, a chiral 1,2-diacyl
moiety and a polar headgroup, are desirable. Even more desirable are phospholipid analogs
that have the general structure of the lipids found in bacteria that inhabit environments with
extremely high temperatures or extremes of pH. The lipids of such organisms contain two
transmembrane hydrocarbon chains that are linked to a headgroup at either end. In some
bacteria the linkages are ether linkages but in others 30' 3‘, they are ester functions (figure 2).
Such molecules should self-assemble to form extremely stable lamellar systems without the
aid of devices such as Langmuir-Blodgett troughs. They would be excellent targets for the
preparation of planar polydiacetylenic systems. We therefore embarked on the synthesis and
characterization of compounds 1 and 2. Like typical phospholipids, they have two long acyl

171

 

HO
O
OH

O
HO
OH
O O O
Y - -- .
WWW/Yo
O
O 0

Figure 6.2. The structure of a membrane lipid from a thermotropic bacterium. I

chains attached to a chiral 1,2-diol. They also have a dimethylaminoethane group similar to

 

the one found in phospholipids such as N, N-dirnethylphosphatidyl ethanolamine. When the L
diacetylenic units polymerize, chiral two dimensional polymers will be obtained and they are
expected to be stabilized by polymerization and self organization. (S)-3-Hydroxy-y-
butyrolactone was the source of chirality for compounds 1 and 2. The synthetic routes are

outlined in schemes 1 and 2 respectively.

 

 

 

 

172

0 /
N
é 11a 1 i
0 +NHZCH20H2N(CH3)2 L9: NHW \

 

 

 

 

 

 

H0 100% HO OH
4 5
(COCI)2
HOOC(CHz);CEC-CEC(CH2)3000H , CDC(CH2)30§C-CEC(CH2)3COCI
CH2C|2 6
N<CH3>2 0

(CH3)2N\/\ NH

..... CW 0
E 2 W0

0 O 0

W E : W
O 1 0
Scheme 6.1. The synthesis of bilpolar molecule 1.
(COC|)2
CH3(CH2)11CEC-CEC(CH2)8000H CH—T CH3(CH2)11CEC-CEC(CH2)800CI
2 7
NH’\/N(CH3)2
: : \/\/\/\/\/\
Oxfc'D/V\/\/\ — —
'= = \/\/\/\/\/\
0

Scheme 6.2. The synthesis of lipid analog 2.

173

 

6.2. RESULTS AND DISCUSSION

Preparation of compounds 1 and 2. The chiral, diacetylenic phospholipid analogs 1 and.
2 were obtained in very good yield . In the case of 2 the preparation necessitated only three
steps all of which proceeded in good yield. The first step to form the N, N-
dimethylaminoethyl dihydroxybutyramide was essentially quantitative. In the case of
compound 1 the only possible complication was the formation of the isomeric structures in
which the acyl chain was linked to the primary hydroxyl group on one side and to the
secondary group on the other. This did not occur since only the intermediate species (3) in
which a single chain was linked to the primary position of an N-alkyl dihydroxybutyramide
on each side was detected under conditions in which the reaction was only partially
complete. The identity of the intermediate species as 3 was readily determined by lH-NMR.
spectroscopy. The signals for the methylene protons adjacent to oxygen appeared at 4.30 and
4.10 ppm, substantially downfield from their original positions in the starting diol. The
methine proton signal was not shifted from 4.06 ppm. There were no signals corresponding
to un-esterified primary alcohols in the spectrum although a substantial degree of under-

esterification of the secondary position was evident from the spectra of the total mixture.

Supramolecular Structure, Stability and Long-Range Order. The parallel orientation of
the two acyl chains of both 1 and 2 was quite evident by NMR spectroscopy from the
coupling constants of the methylene protons adjacent to the acyloxy group. The coupling
constants between these protons and the neighboring methine proton are indicative of the
relative orientation of the acyloxy groups on these two carbons. The splittings were similar

174

 

to those observed in a similar molecule where the acyl chains bore pyrenyl substituents at
their termini and were known to be parallel by virtue of the fact that the pyrenyl groups
displayed excirner emission”. For compound 2, the signals for these methylene protons were
two mutually coupled doublet of doublets . One appeared at 4.30 ppm (3.6Hz and 12.0 Hz)
and the other at 4.12 ppm (5.8 Hz and 12 Hz). These values were also similar to those
observed for the coupling constants for the l and 1' protons with the 2-proton of the glyceryl
moiety of diacyl glycerols. Similar results were observed for 1 although the signals were
considerably broadened in this case because of slower rotational averaging in this latter-
system. The parallel nature of the acyl chains was also confirmed by X-ray diffraction
analysis of the lipid systems in a water/ alcohol system. Reflections at 4.0 Angstroms in the
case of lipid 2 and 3.4 Angstroms in the case of 1 were observed. These correspond to alkyl
chain separations and the smaller value for 1 is expected because in this molecule, the chains
are held together at both ends. These results in conjunction with molecular mechanics

calculations supported the conformation of the molecules as shown in Figure 6.3.

The long-range order of the 2-dimensional systems formed by 1 and 2 was examined by a
combination of atomic force microscopy and laser scanning confocal microscopy using
phase-contrast, dark field and polarizing optics. As was mentioned earlier, X-ray analysis of
the firlly hydrated systems indicated that the hydrocarbon chains were arranged in a stacked
parallel order as is expected in lamellar systems. A reflection at 60 angstroms corresponding
to slightly less than twice the width (34 A) of a monolayer as measured from the molecular
models was observed. This indicated that compound 2 formed slightly interdigitated bilayers.

175

 

 

Figure 6.3. Space filling models showing the conformations of compounds 1(left) and 2
(right) as indicated by NMR spectroscopy and molecular mechanics calculation. Red is
oxygen, blue is nitrogen, gray is carbon and light blue is hydrogen.

176

 

Sex; 111:1". 1311911, .323

 

Figure 6.4. AF M images of a film formed by compound 1 on a fi'eshly-cleaned mica
surface. A. Section analysis over a 4 pm length. B. The top view of the same area as in A.
C. Surface plot of the same area.

177

39.011511 54:11:11,313

   

Figure 6.5. AF M images of a film formed by compound 2 on a freshly-cleaned mica
surface. A. Section analysis over a 2 run length. B. The top view of the same area as in A.
C. Surface plot of the same area.

178

Atomic force micrographs (Figures 6.4 and 6.5) of layers of compounds 1 and 2 respectively
prepared on mica plates demonstrated they formed flat films with a surface variation of only
9.85nm over a distance of 311m (0.3%) for 1, and 12nm over al.51rm range (0.8%) for layer

2.

Information on the order and supramolecular organization of the two systems could also be
obtained by analyzing images obtained from optical laser scanning confocal microscopy. The
most useful information is obtained from images using polarizing optics. If the layers are
ordered relative to the plane of the'slide and the polarized laser light is blocked by a cross
polarizer after going through the layer then only a black background would be observed. If,
however, there are regions of disorder in the film, domains within the layer where the
molecules are oriented differently or the layers buckle, then the plane of polarization of the
laser light is rotated and is no longer canceled by the cross polarizing filter. Areas of
brightness are then observed in these defect regions. As can be seen in figure 6, the polarized
light micrographs fiom both systems (6a, 60) indicated a very high degree of order with only
a few point defects. The dark field images (6b, 6d) show the film edges and the topological
properties of the films. Significant defects were only observed at the edge of the layers where

there was a discontinuity or curvature as the film bent to contact the glass.

Based on the 3-dimensional structures proposed for 1 and 2, a packing model which is
consistent with their long-range stability and high packing densities of these systems could
be derived. The model is one that maximizes the contact between the hydrocarbon chains ,

179

 

 

Figure 6.6. Laser scanning confocal micrograph of hydrated films of compound 1 (top, a,
b) and compound 2 (bottom, c, d). The images on the left (a, c) were acquired using cross
polarizers and the images to the right (b, d) were obtained using dark field optics. The
films are the areas to the right of the fields.

180

 

H.” 1v 42mm. {1.51060 cc). figg
. 4. arc/r3: ... a. so. re... 3.. fig
«40¢.éfisboo (it:
.... 1. é... s. a. 4..
and .4 Ere. x9. Boo
sod J El» 51 boo
«on. .d flag yr boo
no 3 42:12:3- ... .5. so.
we Aw 311. hr 5 o

1% o

 

181

Figure 6.7. Proposed packing models from compound 1 (top) and 2 (bottom) based on X-

ray powder diffraction information and molecular modeling.

such that the rt-interaction is maximized and the alkane chains are in contact along their
entire length (Figure 6.7). The n-overlap is expected to be one of the dominant forces
between the chains of these molecules. Based on the enthalpies of vaporization of a series of
alkanes and conjugated and unconjugated diacetylenes. 33. 3° , this component could be
estimated at 5.1 kcal/mol for chains just at the point of separating into the gas phase. This is
a lower limit and a much higher value is expected for closely packed chains. The dispersion
forces between methylene groups in hydrocarbons is an area that has obtained much attention
Of special importance has been problem of calculating the dispersion energy between
extended hydrocarbon chains in isolated molecules and large extended arrays” 3°. The van
der Waals interaction energy between alkane chains scales linearly with chain length and
varies as the inverse fifth power of chain separation”. In the case of compound 1 where there
are 16 methylene groups in contact and the chains are separated by 3.42 Angstroms (based
on the X-ray data) the total interaction energy is 45.2 kcal/ mol. In the case of compound 2
there are 20 methylene groups in contact, but at a greater separation, this energy is expected
to be 31.5 kcal/ mol. This gives a lower limit for overall inter-chain interaction energies for
1 and 2 of 50.3 and 36.6 kcal/mol respectively. These are substantially higher than the
interaction energies of lipid molecules in biomembranes of comparable acyl chain length. In
the case of 1, since the hydrocarbon chains are tethered at both ends, the average inter-chain
separation is even less. The attraction between the hydrocarbon chains in this case is even
greater since it is known to increaseby two fold if the two chains move one Angstrom closer
from a distance of 5 Angstroms”. This dramatic increase in van der Waals energy with

reduced chain separation will also be true for the n-stacking component which is likely to be

182

 

severely underestimated here.

Formation, Characterization and Properties of Polydiacetylene Films. Two methods
were used to prepare materials with the desired optical and spectroscopic properties that are
characteristic of conductive polydiacetylene layers. Such systems exhibit intense electronic
transitions at long wavelengths. These go beyond the visible spectrum and well into the
infiared region. The optical properties of the polydiacetyenic systems were characterized by
near IR spectroscopy experiments. The samples were prepared as described in the
experimental section and spectra were measured over two ranges, fi'om 600-1050 nm and
900-1700 nm. The results shown in Figure 6. 8 demonstrate that the optical behavior of the
two compounds polymerized by the same method are similar. Both formed blue films on
exposure to UV radiation. In the case of compound 1, there was a strong absorption at 848.3-
nm, and this extended to 1700 nm. The maximum for UV- irradiated compound 2 appeared
at 817.8 nm. Both spectra displayed a small maximum at ~ 700 nm. These polymerized blue
films turned red on exposure to solvents such as chloroform. Films that were treated with
iodine, were bright orange. The absorption maximum for iodine doped 1 was appeared at
823.1 nm and the maximum for 2 was at 772.8 nm. Again there was a small maximum at
~700 nm in both spectra. The occurence of such intense maxima in the region of 800 nm and
beyond with significant absorbance over 1600 nm in the films we have prepared by this
method is very unique. Typically, no significant absorbance beyond 600—680 nm is observed
in these systems" 20* 22' 37. This might indicate either an exceptionally long statistical length

for the chain that makes up an exciton unit, an alternative mode of polymerization to the

183

 

0.27012
0.20002
0.20712
0.20003
0.20413
0.2084
0.20114
0.20fl0
0.20010
0.25000
0.20010

0.40203
0.40009
0.40130
0.44001
0.44000
0.43034
0.43000
0.42400
0.41932
0.41090
0.40004

  

000.2 041.0 000.9 732.3 777.7 023.1 000.0 913.0
0

0.92027
0.91700
0.91490
0.91221
0.90903
0.90004
0.90410
0.N147
0.09079
0.09010
0.09341

000.0 000.0 700.0 750.0 000.0 000.0 ”0.0 900.0 1000.01000.0

 

1.0040
1.0400
1.0200
1.0120
0.9900
0.“
0.9704
0.9004
0.9424
0.9204
0.9144

000.0 000.0 700.0 700.0 000.0 000.0 900.0 900.0 1000.01000.0

 

000.0 000.0 700.0 750.0 000.0 050.0 900.0 900.01000.01000.0

  

0.40944
0.40000
0.40220
0.09007
0.39000
0.39149
0.30790
0.30430
0.30071
0.3771 2
0.37303

002.0 90401040711203 210.0 292.! 37454000 030.41 020.3 702.3

   

 

1 .0030

1 .0007
1.0410

1.0140
1.0001

1.0723
1.0004

 

002.0

1.0279
1.3110

 

.2902
002.0 904.01040.7| 120.0 21 0.. 292.! 3745400.. 030.. 020.3 702.3
I!

Figure 6.8. Near infrared spectra of films formed from 1 (top 4) and 2 ( bottom 4). In a, b,
e, f, the film was polymerized by UV irradiation and in c, d, g, h, the films were iodine
doped. Spectra on the left were aquired in the range of 600-1050rrm, and those to the right

were acquired between 880 and 1700 nm.

184

typical 1,4-addition mode or a local excimer or charge transfer fi'om juxtaposition of

conjugated systems.

6.3. Conclusions

In summary, the method we describe here allows the assembly of hydrocarbon chains
containing diacetylene fimctions with the packing densities, orientation and long-range order
necessary for forming highly conjugated 2-dimensional polymer systems. Our strategy was
to prepare molecules which are analogs of bio-membrane lipids and use their self-assembling
propensity to direct and achieve the regularity of packing and the high 2-dimensional order
that is required. The syntheses are characterized by brevity and very high efficiency. A series
of instrumental analyses indicated that these membrane lipid mimics have the desired
properties. X-ray diffraction, laser confocal polarized light microscopy and molecular
modeling indicate that they form well oriented lamellar films. Atomic force microscopy
experiments prove that they are able to form very flat thin films. Near IR study confirms that
the UV polymerized or doped films have the desired optical properties. The conjugation is
exceptionally high with electronic transitions occurring far into the infiared spectrum. These
membrane mimics are excellent building units for east films. There is no need for an external
boundary to confine monomers, the behavior of the system is built into the structure of the'
monomer. What we have described here is a simple way of generating 2-dimensional
molecular networks containing diacetylene fimction in the system. This should prove to be
of important utility in the design of advanced materials such as conducting materials and drug

delivery systems and electro-optical devices.

185

 

 

6.4. EXPERIMENTAL

Near IR experiments. The compounds 1 or 2 were dissolved in chloroform to give
approximately 1% solutions. A few drops of each clear solution were transferred by a Pasteur
pipette onto clean microscope slides; the liquid spread and evaporated to form a thin film.
Films were either polymerized by irradiation with UV light (254 nm, 6 Watts, 350 uWatts
per cm2 for 3 minutes) or the layers were dOped by holding the slide in a horizontal position
about 10-15 cm above some iodine crystals at room temperature. In the UV polymerization
experiment, the light yellow to colorless film turned to blue afler UV irradiation. The films
turned to yellow-orange afier doping with iodine. Near IR experiments were performed on
diode array spectrometers (CD1 SPEC) from Control Data (South Bend, Indiana). The films

were irradiated using a tungsten source and the slides were placed directly in the light path.

Atomic force microscopy. These analyses were performed using a Nanoscope III instrument
operating in contact mode. For these measurements, compounds 1 and 2 were dissolved in
chloroform-methanol (0.5-1% solutions) and ~ 10 uL transferred to freshly cleaved mica

plates spinning at 200 rpm. This facilitated even film spreading and evaporation.

Laser scanning confocal light microscopy. These experiments were performed on a Zeiss
210 instrument with a 488 nm laser. Images were obtained in the bright field, dark-field,
phase contrast and polarization modes. For the polarizing mode experiments, an analyzing
cross-polarizer was placed on the objective lens and rotated until light cancellation. For film

preparation, the compounds were dissolved in a 4:1 ethanol : water mixture, and a few drops

186

 

of solution were deposited on clean glass slides which were left in a horizontal position at

30-40° C for two hours to allow the solvent to evaporate.

X-ray diffraction. These studies were performed on a Rigaku instrument with a Rotaflex
rotating copper anode operating at 45 kV with a current of 100 mA. The X-ray beam was
collimated with a 1/6 slit and the Km line was selected. The compounds were dissolved in a
minimum volume of ethanol and 1/4 to 1/3 the volume of water added so that an overall 20%
cloudy but uniform dispersion of sample was obtained. The samples were sonicated and
vortexed several time to ensure uniformity of distribution and then sealed in glass capillaries

and diffraction data obtained.

Molecular Mechanics Calculations. These were performed using the MM3 forcefield38 as
implemented in the program Alchemy (Tripos Inc. St. Louis, MO 63144 USA)
Minimizations were performed using the conjugate gradient method. The parameters were
used without modification since the MM3 forcefield is parametrized to very accurately

reproduce the geometries and heats of formation of hydrocarbons.

Synthesis. (S) I-N,N-Dimethylaminoethyl-3,4-dihydroxybutyramide (5). The synthetic route
utilizes the stereocenter in (S)—3-hydroxybutyrolactone as the source of chirality. The
preparation of the lactone has been described earlier and the steps described here have been
demonstrated to preserve the stereocenter”. (S)-3-hydroxybutyrolactone 51g (0.5mol), N,

N—dimethylethylenediamine 44g (0.5mol), and 100 ml absolute ethanol were mixed and

187

 

 

stirred for 24 hours. The solvent was removed by rotatory evaporation under reduced
pressure. The product was dried by vacuum oven for 24 hours to yield a heavy brown syrup
95 g (100% ) .‘H NMR ( 300 MHZ, CDC13) 5 6.19 ( s (broad), 1H), 4.05 (m, 1H), 3.65 (dd,
1H, J=11.4, 3.9 Hz), 3.52 (dd, 1H, J=11.4, 5.1Hz), 3.49-3.39 (m, 1H), 3.33-3.21(m, 1H),
2.45-2.35 (m, 4H), 2.23 (s, 6H). '3 C NMR (75 MHZ, CDC13) 172.34, 69.12, 65.89, 57.83,
44.95, 39.60, 36.58. IR (NaCl window, CHCl3 as solvent), 3297, 3090, 2944, 2865, 2824,

2780, 1647, 1555, 1462, 1190,1040 cm"

Preparation of compound 1. All reactions and workups relating to diacetylenic compounds
were conducted with exclusion of light. Amberized glassware was used and samples covered
with aluminum foil. A photography safe light was used for illumination when conducting
chromatographic separations. 10,12-Docosadiynedioic acid 1.81 g (0.005 mol), oxalyl
chloride 15ml (0.172mol), and dry dicholoromethane 10ml, were mixed and stirred under a
dry atmosphere overnight at room temperature. The solvent and exceSs oxalyl chloride were
quickly removed under reduced pressure by rotatory evaporation. The product was taken up
in 5m] hexane and rotatory evaporated to dryness to remove the last traces of oxalyl chloride.
The crude, freshly prepared amide 5 were used directly for the next reaction step. A mixture
of dried amide 5 (0.95g, 0.005mol), 5 ml dry pyridine and 5 m1 of dry dichloromethane was
cooled to 0 °C in an ice bath under dry nitrogen. The acid chloride 6 (0.005mol) dissolved
in 5 ml of dry dichloromethane was added to the mixture with a dropping funnel over a 10
minute period. The reaction mixture was then stirred for 24 hours. The solvent was

removed by rotary evaporation, and the residue was taken up in chloroform, washed

188

 

 

sequentially with 0.1 N HCl, saturated sodium bicarbonate solution, and then brine and the

 

organic phase was separated and dried with anhydrous sodium sulfate. Removal of solvent
yielded product 1, 2.3 g (89%)as a red solid which was homogenous by thin layer
chromatography. 1H NMR ( 300 MHZ, CDC13) 6.41 (s (broad), 2H), 5.40 (m, 2H), 4.31 (m,
2H), 4.13 (m, 2H), 3.31 (m, 4H), 2.47 (m, 4H), 2.35-2.15 (m , 32H), 1.64-1.40 (m, 16H),
1.27 (b, 32H) l3C NMR ( 75 MHZ, CDC13) 173.25, 172.73, 168.63, 68.54, 65.20, 64.27,
57.53, 44.88, 37.82, 36.51, 34.17, 33.98, 29.01, 28.86, 28.73, 28.24, 24.76, 19.12 IR (NaCl,
CHC13) 2932, 2855, 1738, 1653, 1547, 1462, 1159, 621.2 Fast atom bombardment mass

spectrometry (FAB MS) 1033.9 (C60H96N4Om, MH“)

 

Preparation of compound 2. The method was essentially the same as described above for
1 with a slight difference in the procedure. 10,12-Pentacosadiynoic acid (2.24g, 0.006 mol),
10 m1 oxalyl chloride (0.115mol), and dry dicholoromethane (10 ml) were stirred overnight
at room temperature. The workup was the same as above. For the esterification, the amide
5 0.47 5g (0.0025mol) was used for the reaction. The crude product was purified by flash
column chromatography on silica gel, ( chloroform, acetone, methanol; 1:1:1) Product 2 was
obtained as a purple solid 1.97g(87%). ‘ H NMR, ( 300 MHZ, CDC13), 6.92 (s, 1H), 5.39
(m, 1H), 4.30 (dd, 1H, J=3.6, 12.0 Hz) 4.12 (dd, 1H, J=5.8, 12Hz) 5.35 (m, 2H), 2.55-2.44
(m, 2H), 2.32-2.18 (m, 24H), 1.64-1.42 (m, 12H), 1.40-1.10 (m, 2H), 0.85 (t, J=6.6 Hz). 13C
NMR( 75 MHZ, CDC13), 173.30, 172.78, 168.90, 68.58, 65.27,65.18, 64.38, 57.52, 44.50,
37.75, 36.20, 34.23, 34.03, 31.89, 29.61, 29.46, 29.33, 29.09, 28.91, 28.85, 28.78, 28.33,

24.80, 22.67, 19.18, 14.11. IR (NaCl, CHCl3) 2919, 2851, 1734, 1653, 1470, 1246, 1175,

189

718. cm". FAB MS 903.7 (MH+, cnggsNzos). Because oftheir high liability to oxygen and

instability to light, combustion analyses could not be obtained on these materials.

Acknowledgments: This work was supported by the Michigan State University Research

Excellence Fund.

 

190

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Rubner, M.F.; Sandman, D.J.; Velazquez, C. Macromolecules 1987, 20, 1296-1300.
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Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Haussling, L.; Ringsdorf, H.; Wolf,
H. J. Am. Chem. Soc. 1994, 116, 1050-1053.

Kim, T.; Crooks, R. M.; Tsen, M.; Sun, L. J. Am. Chem. Soc. 1995, 116, 3963-3967.

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193

L‘s-JEL-

 

 

Chapter 7

 

Preparation of 2-Dimensional Polydiacetylenes by Monomer Chain
Alignment Along a Polar Polyamide Backbone: A General Strategy for
Preparing Highly Ordered, Stabilized 2- Dimensional Systems

194

 

Abstract
A new approach to obtain long range order and perfect alignment of long chain
polydiacetylene by anchoring them along a polymer backbone is introduced. Both the
oligomer and the polymer which arrange the long alkyl chain containing diacetylene
function in the middle of the chain were prepared and characterized. In addition to the

optical properties and chromism of typical polydiacetylenes, they exhibit some fascinating

properties such as the absorption spectra are different fiom all current existing systems, they.

absorb into the near IR range. The IR spectra and X-ray powder diffi‘actions suggest a novel
mechanism of polymerization occurring in these systems. The polymer can self assemble and
have a bilayer packing, the system has very high order, high packing density and it is a new
method to fabricate 2-dimensional ordered functional materials and expect to be used in

optical-electronic devices, chemosensors and new multifunctional smart materials.

195

 

 

 

i1

7.1. Introduction:

One of the newer and more exciting goals of modern organic chemistry is the fabrication of
large regular molecular arrays that approach nanoscale dimensions in size and that have
special optical, electronic or other physical properties. Such materials, it is hoped, will-
form the basis of an emerging molecular technology that will facilitate the construction of
devices such as optical switches, light emitting diodes, light dependent conductive or
resistive devices and frequency doublers. (1-5). The construction of such large molecular
systems and ensembles requires a method for accurately and reproducibly aligning the
molecular units in close proximity with proper position of the fimctional groups from which
the desired physical properties are desired. A general , reliable way of doing this has
tremendous implications for the new field of advanced materials. We demonstrate such a
method here. It should allow the precise alignment of groups that interact by dipoles, charge
transfer, electron transfer and that can be subjected to chemical polymerization. All of these

yield materials with exciting new physical properties.

The system of polydiacetylenes are a case in point. These have much potential as new
organic materials with possible applications in electrical conductivity, non-linear optical
applications and near-infrared active applications (6-11). The fabrication of 2-dimensional
aligned hydrocarbon systems in which the polydiacetylene functions occur as a band or layer
that is sandwiched between two application layers are especially of interest (12-l7).
Unfortunately, the alignment of long chain diacetylenes to effect polymerization thus
forming of these highly conjugated polydiacetylenes layers or bands is one of the most

196

 

 

difficult aspects of the fabrication of these advanced materials. The acetylene functions need
to be within 0.5 Angstroms of each other (18). Several strategies have been tried the most
common one being to align the chains on a flat support such as a highly polished gold
surface (19-22). The molecules are generally anchored to the surface via thiol groups. Unlike
the case for self assembling monolayers (SAMS) formed by saturated alkanethiols (23, 24)
, there are some problems associated with this method for obtaining long range order of
alkyl chains containing diacetylenes functions that are suitably positioned for
polymerization. The most severe of these is that, because of the large atomic radius of gold
compared to carbon, variations in the surface of only a few gold atoms put the acetylene
functions in the aligned chains out of register thus terminating or inhibiting polymerization.
It is also an extremely labor-intensive approach. There is also the tendency to form disulfide
groups. When this happens, the chains are not aligned and polymerization is not possible
(19). Another approach to solving the problem is to generate monolayers of hydrocarbons
containing polydiacetylenes using Langmuir Blodgett technology (25-29). Although it is a
good method for preparing highly ordered thin films, it is a labor-intensive one. In
addition to this, LB films are not stable enough to remain intact under a variety of chemical

or physical conditions. They are also not suitable for large scale preparations.

One possible approach is to align the long chain alkyl groups containing diacetylene
functions along the backbone of a polymer to form a comb structure.This approach has
spawned several strategies for the preparation of side-chain polymers(30-33).The problem
here is to arrange for there to be a group attached to each monomer unit in the chain. This

197

 

 

 

 

 

 

 

 

1

Scheme 7.]. The formation of the polymer by reaction of the long chain amine with 2(5H)-
furanone.

198

 

 

 

is generally not a straightforward goal if the polymer is pre-formed because of steric
reasons. It is, however, possible if the monomer contains the desired alkyl chain with the
diacetylene function. Such side-chain polymers have not been synthesized as yet. We
demonstrate here that such a monomer can be easily prepared by a Michael-type addition of
a primary amine to an ene-lactone such as 2(5H)-furanone. This generates a B-alkylamino-y-
butyrolactone which immediately polymerizes to form a material with the desired properties-
(scheme7.1). If the alkyl chains are aligned in a parallel fashion, the overall superstructure
is one in which the actual system is a 2-dimensional, chemically connected monolayer
system with polar amide and hydroxyl groups on one side and hydrophobic hydrocarbon
groups on the other. This arrangement is highly desirable for forming monolayers. One
important advantage is that such monolayers do not have to be mechanically compressed
because the units are held together at very close range and in uniform register by covalent

bonds in the polyamide chain region.

 

 

 

(COC|)2
CH3(CH2)11CEC-CEC—(CH2)8COOH ——-> CH3(CH2)11CEC-CEC-(CH2)3COCI
0+1sz 3
NH2(CH2)3NH2
H CH2C12
H2N\/\/N\n/W\/\ : : W
O 2
H +
N\n/\/\/\/\ — —
— -— W
H
CN 0 — — W
0 4

Scheme 7.2. The synthesis of the primary long chain amine containing diacetylene group.

199

 

 

 

 

 

 

 

 

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11111 1111 11.1.). .111111l 0-... . I}! \11 111111111. 1.111111 111111... ‘11....111111
., «3 /\

 

 

 

 

201

7.2. Results and discussion:

Synthesis of the polymer .Primary long chain amines with conjugated diacetylene groups
were not readily available but an appropriate surrogate could be easily prepared from a
commercially available acid and a diamine (Scheme 7.2). This amine readily reacted on
gentle heating with 1 equivalent of 2(5H)-fi1ranone in chloroform / ethanol at 50 0C to give
oligoarnides. NMR spectra at this stage showed signals for unchanged furanone and for the
intermediate D-alkylamino-y-butyrolactone (Figure 7.1. ).This material could be further
polymerized by heating in the absence of solvent at 80-90°C after which all of the firranone
signals had disappeared and the resonances for the polyamide backbone and diamine linker
were broad and ill-defined as is typical for polymers (Figure 7.2.). Preliminary analysis
using matrix assisted desorption mass spectroscopy (MALDI-MS) indicated that the degree
of polymerization was well in excess of 100 repeating units (>50,000 Daltons) but an upper

limit could not be established .

Optical properties The oligomers and polymers both exhibited the typical therrno-and
solvatochromatic properties of polydiacetylenes. The green-blue oligomer product (Figure
7.3) was colorless when placed on glass slide as a thin film and it turned to deep blue on-
irradiation with UV. light and red on dissolution in chloroform, treatment with acid or on
heating (Figures 7.4). The blue color was restored on cooling or on removal of the solvent.
If the material was polymerized completely by heating at 90°C to form the high molecular
weight polyamide, a bright red product was formed which turned to an amber solid on
cooling. This product turned to a bright golden color (Figure 7. 5) on doping with iodine.

202

 

 

 

 

 

 

 

Figure 7.3. The photograph of the oligomer solid after UV irradiation of 10 minutes.

203

Figure 7.4. The film formed by the oligomer amide by dissolving in chloroform, left
(colorless strip) is the film prepared by casting the material on to the glass slide by
solvent evaporation; the middle (blue) is the film prepared as the left one being exposed
to UV light for 7 minutes, the right (red) is the blue fihn prepared as the middle one then
was heated at 80 °C for 1 minute.

204

 

 

 

 

 

Figure 7.5. The photograph of the polymer upon treating with UV for 10 minutes and
then doped with iodine.

205

0.67366
0.66906
0.66622
0.66246
0.66676
0.66602 '
0.661 26
0.64766
0.64361
0.64006

0.53535 «- f . '
500.0 550.0 700.0 750.0 000.0 050.0 000.0 050.0 1000.0 1050.0

Murmurs

  
  
  

Absorbent» Units (AU)

0.0202

0.0066

0.691 1

0.6765
0.6620
0.6474
0.6320
0.6164
0.6036
0.7603

0.7747
662.6 064.6 1046.71

Absorblnco Units (AU)

Mammal":

0.3300
0.61 1 6
0.2036
0.2755
0.257 3
0.2302
0.2210
0.2029
0.1647
0.1 666
0.1464

Abundance Units (AU)

 

600.0 658.7 712.0 768.5 824.4 880.2 038.1 002.0 31103.8
Nanomun

0.06356
0.06210
0.06062
0.07914
0.07766
0.07616
0.07471
0.07323
0.071 76
0.07027

  
  

Absorbanco um (AU)

000.0 060.0

Monomer-rs

Figure 7.6. The near IR spectra of the oligomer cross linked by UV 1ight(A, B) and the Near
IR spectra of the polymer cross linked by UV light (C, D).

206

 

 

 

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’fx.» ‘ .v" 3.x

7"Ear-11","
“"31“ng ””fv- I):

 

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Figure 7 7 The proposed model of packing structure of the polymer

207

Both the doped and un-doped polymers absorbed strongly in the near infrared region of the
electromagnetic spectrum indicating a new mode conjugation by the polydiacetylene cross
linking. The oligomer also show strong absorption in the near IR region (Figure 7.6 a, b).
Material that was polymerized by heating for 10 hours at 90 oC and UV cross-linked
displayed an absorption maximum at 1038 nm. The beginning of another absorption band
with a tail at 1700nm was also evident (Figure 6 c, (1). Absorption in this region of the
spectrum is generally not observed and certainly have not been reported for typical

polydiacetylenes. This indicates that this method of chain alignment makes some other

mode of polymerization in addition to the typical 1,4 addition mode possible. Bands that are

normally attributed to the 1,4-mode were also visible at 587 nm and 640 nm.

High Long Range Order Both single crystal X-ray diffiaction and powder diffraction data
experiments indicated the high degree of order of these polymer systems. The X-ray
PoWder diffraction data was consistent with a model of the polymer as shown in Figure 7.7.
There was a strong reflection at 26 Corresponding to 58.9 Angstroms, a distance equivalent
to the height of the 2-D system. There were also reflections in the wide angle region at 4.4,
4.3, 3.9, 3.7 Angstroms corresponding to four different inter-chain separations of alkyl

chains. These are significantly shorter than the alkyl chain separation of 4.9 A in the

classical 1,4 addition ene-yne mode , and also for that of the butatriene mode which is 4.7-

A (34, 35). The most likely mechanism is by l, 2 addition. This gives a polyene backbone
with pendant acetylene groups (36). The pending acetylenic group further polymerize to
give the polyacene type structure.

208

71
.1

 

 

   

Figure 7.8. The laser scanning confocal micrograph of the oligomer. A, by polarized
light; B, by phase contrast method, C, by fluorescence.

209

   

Figure 7.9. The atomic force microscopy of the oligomer. A, section analysis; B. The top
view of the cross section area the same with A, C. the surface contour plot of the same
area as above.

210

Characterization of the thin film formed by the oligomer. The laser scanning confocal
images of the oligomer shows that they formed very uniform flat thin film (Figure 7.8), by
the polarized light microscopy. This is also further demonstrated from the atomic force
microscopy (AF M) measurement (Figure 7.9). The AF M images show that they formed.
extremely flat, highly ordered thin film. The surface variation is only 11 nm over a range
of 4000 nm (0.3%). These experimental results indicated that the oligomer can self

assemble and form highly organized supramolecular structure.

7.3. Conclusion

A new method of obtaining high long range arrangement of long polydiacetylene containing
alkyl chain was described. The two dimensional (or 3-D) comb polymer show some
remarkable new properties in addition to the well known properties of polydiacetylene
systems. The oligomer can self assemble into highly ordered extremely flat thin films, both
the oliogrner and the polymer are readily polymerized. The absorption spectra of the two.
systems are fiom the UV till way out in the near IR region. This might suggest very high
level of conjugation and or a new mechanism of polymerization occurred in this type of
system. This method offers a solution to the problem of arranging long chain
polydiacetylene in perfect order, provides the material obtained and the approach applied
here are expected to be useful for fabricating advanced multifunctional material such as

optic-electronics and chemosensors.

211

 

 

 

7.4. Experimental Section

Near IR experiments. The green oligomer and the further connected polyamide were
dissolved in chloroform to give approximately 1% solutions. A few drops of each clear
solution were transferred by a Pasteur pipette onto clean microscope slides; the liquid spread
and evaporated to form a thin film. Films were either polymerized by irradiation with UV
light (254 nm, 6 Watts, 350 uWatts per cm2 for 3 minutes) or the layers were doped by
holding the slide in a horizontal position about 10-15 cm above some iodine crystals at room
temperature. In the UV polymerization experiment, the colorless oligomer film turned to
blue after UV irradiation afier 30 seconds and it turned deeper with longer irradiation time.
The polyamide thin film was brownish at room temperature it turned to different color as the
normal blue, upon doping with iodine, it turned to golden metalic color. Near IR experiments
were performed on diode array spectrometers (CDI SPEC) fiom Control Data (South Bend,
Indiana). The films were irradiated using a tungsten source and the slides were placed

directly in the light path.

Atomic force microscopy. These analyses were performed using a Nanoscope III instrument
operating in contact mode. For these measurements, the oligomer was dissolved in
chloroform-methanol (0.5-1% solutions) and ~ 10 11L transferred to freshly cleaved mica

plates spinning at 200 rpm. This facilitated even film spreading and evaporation.

Laser scanning confocal light microscopy. These experiments were performed on a Zeiss
210 instrument with a 488 nm laser. Images were obtained in the bright field, dark-field,

212

 

 

 

phase contrast and polarization modes. For the polarizing mode experiments, an analyzing
cross-polarizer was placed on the objective lens and rotated until light cancellation. For film
preparation, the compounds were dissolved in a 4:1 ethanol : water mixture, and a few drops
of solution were deposited on clean glass slides which Were left in a horizontal position at

30—40° C for two hours to allow the solvent to evaporate.

X-ray diffraction. This study was performed on a Rigaku instrument with a Rotaflex
rotating copper anode operating at 45 kV with a current of 100 mA. The X-ray beam was
collimated with a 1/6 slit and the K01 line was selected. The green oligomer solid was grinded
to fine powder and suspended in water ethanol (10:1 mixture), then the slurry was transferred
to the glass slide, air dry for one day, the solid deposited on the slide was used for the
measurement. After the measurement the part under x-ray beam turned to black, the part

without X-ray irradiation is still green.

Molecular Mechanics Calculations. This was performed using as implemented in the

program Biograf. Minimizations were performed using the conjugate gradient method.

Synthesis of polyamide

Preparation of compound 2. All reactions and workups relating to diacetylenic compounds
were conducted with exclusion of light. Amberized glassware was used and samples covered
with aluminum foil. A photography safe light was used for illumination when conducting
chromatographic separations. 10,12-Docosadiynedioic acid 0.90 g (0.0025 mol), oxalyl

213

 

 

 

chloride 5m] (0.054 mol), and dry~ dicholoromethane 10ml, were mixed and stirred under
a dry atmosphere overnight at room temperature. The solvent and excess oxalyl chloride
were quickly removed under reduced pressure by rotatory evaporation. The product was

taken up in 5m] hexane and rotatory evaporated to dryness to remove the last traces of oxalyl

chloride. The crude, freshly prepared amide 5 were used directly for the next reaction step.

To the mixture of Diaminopropane (0.6g, 3 eq) in dichloromethane, the acid chloride 3
(0.0025mol) dissolved in 5 ml of dry dichloromethane was added to the mixture with a
dropping funnel over a 10 minute period. The reaction mixture was then stirred for 24 hours
protected from moisture. The residue was taken up in chloroform, washed sequentially
with 0.1 N HCl, saturated sodium bicarbonate solution, and then brine and the organic phase
was separated and dried with anhydrous sodium sulfate. Removal of solvent yielded product
2 with a small portion of byproduct 4 . The desired product 2 can be separated from the
dimer by product 4 by flash column chromatography (eluting solvent: methanol:

chloroform=l :3. Product 2 was obtained as white powder yield 0.86g (80%), 1H NMR ( 300

MHz, CDC13) 6.22 (s (broad), 1H), 3.34 (q, J= 6.0Hz, 2H), 2.78 (t, J=6.3Hz, 2H), 2.24-210

(m, 6H) , 1.64-1.20 (m, 34H), 0.86 (m, 3H) IR (NaCl, CHC13) 3296, 2918, 2849, (2176,
2140 weak), 1700, 1639, 1553, 1463, 722 cm" . Compound 4, l H NMR ( 300 MHZ,
CDC13), 6.18(s, 2H), 3.25 (q, 4H, J=6.0Hz), 2.26-2.10 (m, 12H), 1.60 (m, 4h), l.47(m, 8H),
1.40-1.18(m, 66H), 0.86(m, 6H). IR (NaCl, CHC13). 3345, 2918, 2850, 2716(w), 2140(w),

1728, 1636, 1525, 1471, 1423, 1255, 1192, 956, 717 , 660 cm“.

Preparation of compound 1. The amine 2 0.59 g was added to 0.12g furanone, 3ml

214

 

 

 

ethanol, 7m1 chloroform, the mixture was stirred for 12 hours at room temperature, then at
50 °C for another 6 hours until all the solvents were evaporated, the oligoamide was formed
as a green crystalline solid. The charaterization of the compound, ' H NMR, ( 300 MHz,
CDCl3), the signals are broadened, 4.38(m), 4.10(m), 3.70(m), 3.40-3.20(m), 2.80-2.60(m),
2.24-2.10(m), l.80-1.40(m), 1.40-1.18(m), O,86(m). IR (NaCl, CHC13) 3309, 3090, 2819,
2849, 2178(w), 2141(w), 1772, 1638, 1544, 1470, 1418, 1065 cm". The polymer after
heating the oliogamide for 10 hours: ' H NMR, ( 300 MHz, CDC13), the signals are
significantly broadened, and the terminal functional groups’ signals disappeared. 3.25ppm
(m, broad), 2.40-2.20(m), 1,59(m), 1.49(m), 1.23(m), 8,86(m). IR (NaCl, CHC13), 3310,
3082, 2919, 2850, 1776, 1645, 1544, 1467, 1620, 720. Because of their high liability to
oxygen and instability to light, combustion analyses could not be obtained on these

materials.

215

 

 

 

 

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