The reactions of Fischer carbene complexes with enynes were initially carried out to probe whether the carbene complex would react first with the alkyne or with the alkene.  We found that the alkyne function was much more reactive toward the carbene complex and that the overall reaction lead to the formation of a cyclobutanone product [1]. Since the alkyne and alkene function are tethered in the starting enyne the products were bicyclic compounds in which one of the rings was a cyclobutanone.The structure of the product was also found to depend on the substitution pattern on the alkene.  For most olefins such as that in enyne 109 a bicyclo-[3.2.0]-heptanone product of the type 110 was produced .  If the terminus of the alkene was disubstituted as in enyne 112, then a bicyclo-[3.1.1]-heptanone of the type 113 was formed [2].The mechanism of the formation of this product is thought to be loss of a CO ligand from the carbene complex followed by addition of the alkyne to give the vinyl carbene complexed intermediate 115. Carbon monoxide insertion then would give the ketene complex 116 or 117.  A [2 + 2] cycloaddition of the ketene and the tethered alkene in 116 would lead to the bicyclo-[3.2.0]-heptanone 110.  The same process for ketene complex 117 proceeds in a crossed manner to give the bicyclo-[3.1.1]-heptanone 113. This switch in the regiochemistry of addition of the alkene to the ketene presumably occurs because the more electron-rich end of the double-bond is switched on

going from the alkene in 109 to that in 112. The reaction has been found to be stereoselective as indicated in the reaction of the enyne 118, which gives a 92 : 8 preference for the isomer 119, and the enyne 121, which gives 122 with high stereoselectivity.  The source of the stereoselectivity of these reactions is currently under investigation as well as the applications of these reactions to the total synthesis of natural products.