Reactions of alkoxyaryloxycarbenes with tethered triple bonds: a new synthesis of substituted benzofurans

Abstract

Cyclic 3-alkoxy-3-aryloxyvinylcarbenes can be generated by a highly exo-selective intramolecular cyclization of a dioxycarbene onto a tethered triple bond. Like their dialkoxy counterparts, the 3-alkoxy-3-aryloxyvinylcarbene intermediates are capable of interesting reactions. In contrast to 3,3-dialkoxyvinylcarbenes, which undergo a formal [3 + 2] cycloaddition with highly electron-deficient olefins such as benzylidenemalononitrile, 3-alkoxy-3-aryloxyvinylcarbenes undergo a highly diastereoselective [1 + 2] cycloaddition with benzylidenemalononitrile to give cyclopropylketene acetals. At high temperatures, those cyclopropylketene acetals undergo a clean vinylcyclopropane rearrangement in which the stereochemical integrity of the cyclopropane is retained. Keywords: carbene, alkoxyaryloxy, [1 + 2] cycloaddition, ketene acetal, vinylcyclopropane rearrangement, oxadiazoline.