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.
GRAPHICAL ABSTRACT
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ACKNOWLEDGMENT
We are thankful to the Inorganic Chemistry Department of IACS for their help in crystal structure determination.
Notes
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.