Complexation of vinylcyclopropanes with zirconocene-1-butene complex: Application to the stereocontrolled synthesis of steroidal side chains

Reactions of vinylcyclopropane derivatives with a zirconocene-1-butene complex (''Cp2Zr'') caused regioselective cleavage of the cyclopropyl bond to give eta(3) pi-allylic and/or eta(1) sigma-allylic complexes. The regioselectivity of the bond cleavage and the formation of a eta(3) pi-allylic or eta(1) sigma-allylic complex depend on the bulkiness of the substituents on the cyclopropyl group and the presence of leaving functionality. A catalytic use of ''Cp2Zr'' in the presence of excess Grignard reagent also caused a ring opening of the vinylcyclopropane derivatives with the same sense of regiochemical selectivity. The reaction of the thermally equilibrated ''Cp2Zr''-propenylcyclopropane complex with acetone indicated the possibility for the synthesis of the steroidal side chain in either natural or unnatural forms. The synthetic utility of the present ''Cp2Zr''-vinylcyclopropane chemistry was ascertained by the stereocontrolled preparation of the C-20, C-24 dimethylated steroidal side chain which is identical to the active metabolite of vitamin D-2.