Stereochemical considerations on the stereoselective cyclopropanation reactions of 3-aryl-2-phosphonoacrylates induced by the (−)-8-phenylmenthyl group as a chiral auxiliary

Abstract

The cyclopropanation of (−)-8-phenylmenthyl (E)-3-aryl-2-phosphonoacrylates E-3a–d with dimethyloxosulfonium methylide and diazomethane affords the corresponding trans cyclopropane derivatives as the major diastereomers with high diastereoselectivity. On the other hand, the cyclopropanation of (−)-8-phenylmenthyl (Z)-3-aryl-2-phosphonoacrylates Z-3a–d and (−)-menthyl derivative 4 gives mixtures of cis and trans cyclopropane derivatives with low diastereoselectivity. The high diastereoselectivity in the cyclopropanation of (E)-acrylates E-3a–d can be rationalized by considering the π–π interaction between the phenyl ring of the chiral auxiliary and the acrylate moiety in the s-cis conformer. The low selectivity in the cyclopropanation of (Z)-acrylates (Z-3a–d and 4) can be attributed to a low-lying conformer with the acrylate moiety twisted out of conjugation.