Asymmetric Synthesis of Protected 2-Substituted Cyclopropane Amino Acids

The cyclopropanation reactions of (1a) with ethyl and t-butyl (dimethylsulfuranylidene)acetate proceeded with good diastereoselectivity and resulted in the formation of three diastereoisomeric products. The major diastereoisomeric product ((2a) and (2b), respectively) could be isolated in pure form by simple recrystallization. The stereochemistry of the major cyclopropane product (2b) has been determined by single-crystal X-ray structural analysis. These cyclopropanation products were susceptible to ring opening of the cyclopropane ring upon reduction with sodium borohydride or acid hydrolysis. The reaction of (1b) with ethyl (dimethylsulfuranylidene)acetate gave a mixture of four diastereoisomeric cyclopropanation products. The reactions of (1a) and (1b) with the sulfur ylide derived from 3-methoxycarbonylallyldimethylsulfonium bromide were less successful in terms of product diastereoselectivities.