DIASTEREOSELECTIVITY IN THE CYCLOPROPANATION OF 3,3-BIMETALLIC ALLYLIC ALCOHOLS - PREPARATION OF DIASTEREOMERIC CYCLOPROPYL CARBINOLS VIA A SIMPLE OXIDATION-REDUCTION SEQUENCE

A highly diastereoselective cyclopropanation of allylic alcohols, containing a silyl and/or stannyl group using samarium/dihalomethane, provides a variety of bimetallic cyclopropane carbinols in good yield. A comparison with the more traditional Simmons-Smith conditions and its variants is made. The diastereoselectivity varies from 1:10 to >50:1 and depends on the substituents on the carbinol side chain (R-group) and on the substituents on the alkene portion of the allylic alcohol. Excellent diastereoselectivity (>50:1) was always obtained whenever a eis-substituent was present. Moreover, the minor diastereomer from the cyclopropanation can easily be obtained via a simple oxidation of the major cyclopropyl carbinol, followed by a selective reduction of the corresponding cyclopropyl ketone. Using LiAlH4 at 0 degrees C, facial selectivities of 15 to 20:1 are obtained for this reduction, while slightly higher selectivities up to (29:1) can be obtained using DIBAL-H. The combination of the cyclopropanation and oxidation/selective reduction sequence provides access to both diastereomeric bimetallic cyclopropanes.