A new route to the chromene ring system has been developed which is the reaction of an α,β-unsaturated Fischer carbene complex of chromium with a propargyl ether bearing an alkenyl group on the propargylic carbon. This transformation involves a cascade of reactions that begins with a benzannulation reaction and is followed by the formation of an o-quinone methide, and finally results in the emergence of a chromene upon an electrocyclization in yields up to 95%. This reaction was extended to provide access to naphthopyrans by employing an aryl carbene complex. This constitutes the first synthesis of chromenes in which both rings of the chromene system are generated in a single step. The success of this method is highlighted in the synthesis of lapachenole and vitamin E. The asymmetric version of benzannulation/o-quinone methide formation/electrocyclization cascade was also explored for the synthesis of chiral chromenes using optically active enynes. Enantiomeric excesses up to 60% could be obtained. Interestingly, it was observed that the double bond geometry in the chiral enyne controlled the absolute stereochemistry in the final chromene product. A mechanistic investigation was carried out to have a better understanding of the reaction pathway.
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