ALPHA-HYDROXY ESTERS AS CHIRAL AUXILIARIES IN ASYMMETRIC CYCLOPROPANATIONS BY RHODIUM(II)-STABILIZED VINYLCARBENOIDS

The use of several alpha-hydroxy esters as chiral auxiliaries for asymmetric cyclopropanation with rhodium-(II)-stabilized vinylcarbenoids is presented. Use of either (R)-pantolactone or (S)-lactate allowed entry into both series of enantiomeric vinylcyclopropanes with predictable absolute stereochemistry. Steric and electronic modifications of the chiral auxiliary as well as catalyst structure were shown to have major effects on the asymmetric induction. These results were rationalized on the basis of an interaction between the carbonyl oxygen of the chiral auxiliary and the carbenoid carbon. By combining the asymmetric cyclopropanation with a subsequent Cope rearrangement, an enantioselective entry into hydroazulenes was achieved. The potential of the asymmetric cyclopropanation was illustrated by a short synthesis of (1R,2R)-2-phenylcyclopropane amino acid 5.