Synthesis and Opiate Activity of Pseudo-Tetrapeptides Containing Chiral Piperazin-2-one and Piperazine Derivatives.

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Abstract
Enantiomeric piperazin-2-one derivatives, N, N'-ethylene-bridged alanylphenylalanines (1a or 1b), were synthesized using (S)- or (R)-alanine and phenylalanine as starting materials, and were inserted into the second and third positions of enantiomeric pseudo-tetrapeptides (P1a- or P1b-OEt). The corresponding piperazine derivatives (1a-or 1b-sRed) obtained by selective BH3 reduction of the amide carbonyl groups of 1a or 1b were similarly inserted into the same positions of tetrapeptides (P1a- and P1b-sRed). Enantiomeric N, N'-ethylene-bridged tyrosyltyrosine derivatives (2a or 2b) obtained from (S)- or (R)-tyrosine were also inserted into the first and second positions of two pairs of enantiomeric tetrapeptides (P2a- and P2b-OEt or P'2a- and P'2b-OEt). The opiate activities of the eight peptides thus obtained were studied by use of the mouse vas deferens and the guinea pig ileum assays in order to elucidate the structure-activity relationships of these peptides, especially with respect to stereochemistry.

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