REACTIONS OF ALKOXYARYLOXYCARBENES WITH TETHERED TRIPLE BONDS - A NEWSYNTHESIS OF SUBSTITUTED BENZOFURANS

Cyclic 3-alkoxy-3-aryloxyvinylcarbenes can be generated by a highly ex o-selective intramolecular cyclization of a dioxycarbene onto a tether ed triple bond. Like their dialkoxy counterparts, the 3-alkoxy-3-arylo xyvinylcarbene intermediates are capable of interesting reactions. In contrast to 3,3-dialkoxyvinylcarbenes, which undergo a formal [3+2] cy cloaddition with highly electron-deficient olefins such as benzylidene malononitrile, 3-alkoxy-3-aryloxyvinylcarbenes undergo a highly diaste reoselective [1+2] cycloaddition with benzylidenemalononitrile to give cyclopropylketene acetals. At high temperatures, those cyclopropylket ene acetals undergo a clean vinylcyclopropane rearrangement in which t he stereochemical integrity of the cyclopropane is retained.