Synthetic Methodology for the Construction of Structurally Diverse Cyclopropanes

Abstract

Practical and efficient routes for the stereoselective conversion of homoallylic alchols to diastereomerically pure cis-, trans-1,2-disubstituted, and 1,2,3-trisubstituted cyclopropanes have been developed. The routes are highlighted by olefin metathesis strategies and the stabilization of an intermediate cyclopropylcarbinyl cation by the β-silicon effect. The stereospecificity of the key cyclization step has been rationalized by transition-state models in which the important determinants include (i) a minimization of the steric interactions about the forming cyclopropane bond and (ii) an inversion of stereochemistry at the activated homoallylic alcohol position. The cyclopropane product chirality is ultimately controlled by the choice of homoallylic alcohol starting material. Through this method nonracemic, diasteromerically pure homoallylic alcohols can be converted in two steps to nonracemic, diasteromerically pure cyclopropane structural units. The scope and limitations of this versatile methodology have also been investigated.