Improved Synthesis of Cyclic α-Hydrazino Acids of Five- to Nine-Membered Rings and Optical Resolution of 5,6,7-Membered Ring Hydrazino Acids

Abstract

Synthesis of cyclic α-hydrazino acids of five- to nine-membered rings has been described. Di-tert-butyl or dibenzyl malonate was used as starting materials instead of diethyl malonate, which was used in our first report. Deprotection of tert-butyl or benzyl ester of the final compounds was much easier than that of ethyl or methyl esters. Overall yield of these acids were 39, 50, 47, 52, and 51%, respectively. These acids were then converted to the diastereomers either via the formation of peptides with L-phenylalanine methyl ester or via the formation of esters (for five- to seven-membered rings) with L-2-phenylalaninol. All diastereomers were separated except the nine-membered ring by flash chromatography. Hydrolysis of diastereomeric esters generated the optically pure five-, six- and seven-membered cyclic α-hydrazino acids. In this process, both the enantiomers have been isolated and the chiral auxiliary L-2-phenylalaninol was recovered. Absolute stereochemistry was determined from x-ray crystallographic analysis.