Synthesis and in vitro activities of new tryptophan-modified and thiomethylene-containing pseudopeptide antagonists of the neurokinins

A series of pseudopeptide analogs of the substance P-like hexapeptide Ava-Phe-Phe-Gly-Leu-Met-NH₂ was produced by N_α-protection, introduction of the thiomethylene bond, of d - and non-proteinogenic amino acids, and alteration of the side chain of tryptophan. Synthesis of the pseudopeptides on a solid phase was successfully improved by direct formation of the CH₂—S bond on the resin. However, while thiomethylene formation between leucine and norleucine led to the expected SS diastereoisomer, the major product of the similar coupling between two phenylalanines was the SR isomer. An improved resistance of the analogs to proteolysis was observed, which could be related to the structural changes. Interestingly, these modifications led to three water-soluble and potent neurokinin antagonists on classical in vitro bioassays.