DIASTEREOSELECTIVITY IN THE CYCLOPROPANATIN OF 3,3-BIMETALLIC ALLYLICALCOHOLS - PREPARATION OF DIASTEREOMERIC CYCLOPROPYL CARBINOLS VIA A SIMPLE OXIDATION-REDUCTION SEQUENCE

A highly diastereoselective cyclopropanation of allylic alcohols, cont aining a silyl and/or stannyl group using samarium/dihalomethane, prov ides a variety of bimetallic cyclopropane carbinols in good yield. A c omparison with the more traditional Simmons-Smith conditions and its v ariants is made. The diastereoselectivity varies from 1:10 to >50:1 an d depends on the substituents on the carbinol side chain (R-group) and on the substituents on the alkene portion of the allylic alcohol. Exc ellent diastereoselectivity (>50:1) was always obtained whenever a eis -substituent was present. Moreover, the minor diastereomer from the cy clopropanation can easily be obtained via a simple oxidation of the ma jor cyclopropyl carbinol, followed by a selective reduction of the cor responding cyclopropyl ketone. Using LiAlH4 at 0 degrees C, facial sel ectivities of 15 to 20:1 are obtained for this reduction, while slight ly higher selectivities up to (29:1) can be obtained using DIBAL-H. Th e combination of the cyclopropanation and oxidation/selective reductio n sequence provides access to both diastereomeric bimetallic cycloprop anes.