1972 Volume 45 Issue 7 Pages 2015-2018
The methyl and ethyl esters of α-L-aspartyl-L-phenylalanine (I and II), -L-tyrosine (III and IV), -L-(β-cyclohexyl)alanine (V and VI), and -L-[β-(4-hydroxycyclohexyl)]alanine (VII and VIII) have been found to react with ketones (acetone, methyl ethyl ketone, and cyclohexanone), in the absence of any catalysts, to give the corresponding 4-imidazolidinone derivatives (XVII–XXX). The NMR spectra of the imidazolidinones with an aromatic side chain (XVII–XXVIII) have indicated that these compounds exist in the folded conformation (in which the aromatic ring faces the imidazolidinone ring) in the solution. The imidazolidinones (XVII–XXX) were hydrolyzed with hot water to give the peptide esters (I–VIII), all in their original optical purity. On the other hand, the β-isomers of the peptide esters (IX–XVI) did not react with these ketones under the same conditions. These characteristics were used successfully in the separation of the two structural isomers of these peptide esters.
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