Abstract
Abstract During the course of synthesis of both a linear and a cyclic enkephal in pseudopeptide containing a ψ[CH2S] amide bond replacement, two isomeric products were produced in each case in virtually equimolar quantities. RP-HPLC was used to: 1) isolate and characterize both pairs of products; 2) confirm their sulfide content by partial oxidation to their ψ[CH2SO] equivalents, with two new pairs of diastereomeric sulfoxides formed in each case, and 3) confirm that the isomers were formed by epimerization of the C-terminal alpha carbon of the pseudopeptide, H-Tyr-D-Ala-Gly-Pheψ[CH2S]Leu-OH and its cyclic counterpart in an early synthetic step. Both the presence and the absolute configurations of the new epimeric center were further established by RP-HPLC. This involved acid catalyzed hydrolysis and comparison of the resulting HPLC-isolated pseudodipeptides with authentic species of Pheψ[CH2S]Leu and Pheψ[CH2S]D-Leu prepared by controlled stereochemical routes.
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