Synthesis and opiate activity of pseudo-tetrapeptides containing chiral piperazin-2-one and piperazine derivatives

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 ilcum assays in order to elucidate the structure-activity relationships of these peptides, especially with respect to stereochemistry.