An access to (Z)-ethylenic pseudodipeptides based on ring-closing metathesis

A new access to enantiopure (Z)-ethylenic pseudopeptides, starting from the chiral pool of amino acids and enantiopure 2-substituted-but-3-enoic acids is proposed and illustrated by the syntheses of the (Z)-ethylenic pseudopeptidic analogs of l-Phe-l-Phe, l-Phe-d-Phe, l-Phe-l-Val, l-Phe-d-Val and racemic (ll,dd) and (ld,dl) (phenyl)Gly-Phe. The key-steps of these syntheses are a ring-closing metathesis, catalysed by Grubbs’ ruthenium alkykidene complexes, on diethylenic amides and the hydrolytic cleavage of the resulting dihydropyridones under mild conditions through intermediate formation of cyclic imidates.