Wittig rearrangements of silyl allylic cyclic ethers and [1,2] carbon to carbon silyl migration of alpha-hydroxy allyl silanes

The [1,2]- and [1,4]-Wittig rearrangements of cyclic α/γ-silyl allyl ethers have been studied. These rearrangements are based on the ability of the silicon atom to stabilize an α-anion. Treatment of diastereomeric 2-silyl-6-aryl-5,6-dihydro-2H-pyrans with n-butyllithium (sec-butyllithium for cis diastereomer) result in stereoconvergent ring contraction to corresponding α-silylcyclopentenols and/or (β-cyclopropyl)acylsilanes via [1,2]- and [1,4]-Wittig rearrangements respectively. Studies on expanded ring size, 2-silyl-7-aryl-2,5,6,7-tetrahydrooxepins, led to the formation of αsilylcyclohexenols and (β-cyclobutyl)acylsilanes. On the other hand, moving the silyl group to the 4-position of the dihydropyran moiety led predominantly to [1,4]-Wittig rearrangement.Concurrently, 2-silyl-3-hydroxy-5-arylcyclopentan-1-one and 2-silyl-3-hydrox-6- arylcycloyhexan-1-one can be accessed by subjecting 1-silyl-5-arylcyclopent-2-en-1-ol and 1-silyl6-arylcyclohex-2-en-1-ol, respectively, to m-CPBA and NaHCO3. In this transformation epoxidation triggers a [1,2]-carbon-to-carbon silyl migration. The stereochemical orientation of the resulting product is such that the α-silyl and β-hydroxy groups are trans to one another. Furthermore, the hydroxy group in the starting silane directs the epoxide formation, which in turn dictates the stereochemical outcome of the resulting β-hydroxy making this reaction stereospecific in nature. Lastly, the synthesis of 2-alkyl-2-silylalkanals can be achieved by conditions that effect spontaneous [1,2]-carbon-to-carbon silyl migration in 1-silyl-2-alkyl-2-alken-1-ols. These conditions are independent of the substituents on silicon but require an alkyl substituent at the olefin position next to the sp3 carbon bearing the silyl group.