LANTHIONINE CHEMISTRY .4. SYNTHESIS OF DIASTEREOISOMERIC CYCLOLANTHIONYL DERIVATIVES

The direct synthesis of monomeric cyclo-L-lanthionyl (2a) and cyclo-(D→L)-lanthionyl (2b) derivatives has been achieved using derivatives of L- or meso-lanthionine differentially protected at the amino and the carboxyl groups, e.g. Nα-benzyloxycarbonyl-N α′-trityl-L- (or D-H-)-lanthionine α′-methyl ester (6a, b). The specific rotation of L-lanthionine in acidic or alkaline solutions is markedly temperature-dependent; it increases in the former and decreases in the latter as the temperature increases. Previous investigations on the reversibility of the specific rotation change in acidic solutions have been confirmed and expanded. After heating acidic solutions of L-lanthionine above 96 °C, or at this temperature for >5 h, an irreversible change of the specific rotation was observed. Racemization calculated from the observed decrease of the specific rotation includes transformation to both DL- and meso-lanthionine. The meso-stereoisomer was determined by Moore and Stein amino-acid analysis. After acidic hydrolysis the cyclo-L-lanthionyl compound (2a) exhibited a similar temperature- and time-dependent formation of DL-and meso-lanthionine. A change of the configuration was also observed when either meso-lanthionine or cyclo-(D→L)-lanthionyl derivative (2b) were treated under acidic conditions.