Dehydroamino acids with the methylated N-terminal peptide group occur in natural small cyclic peptides. The structural analysis was used to investigate the cis-trans isomerization of the N-terminal tertiary amide group of diamides: Ac-(Z)-Δ(Me)Abu-NHMe (1), Ac-(Z)-Δ(Me)Phe-NHMe (2), Ac-(E)-Δ(Me)Phe-NHMe (3), Ac-Δ(Me)Ala-NHMe (4), and Ac-(Me)Ala-NHMe (5). The compounds were analyzed in the solid state by an X-ray crystallography (1-3), and in the solution by FTIR (MeCN and CHCl(3) ) and NMR (DMSO-d6 and CDCl(3) ) methods (1-5). In the solid state, the studied compounds adopt the cis configuration of N-terminal amide. In solution, this configuration also prevails for the dehydroamino acids 1-4, in contrast to the saturated analog 5. The results indicate that N-methyldehydroamino acids present a promising tool to induce the cis configuration of the amide bond.

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Dehydroamino acids / N-methylation / trans-cis Isomerisation / Single crystal structure analysis / X-ray diffraction Abstract. A series of three homologous amino acids derivatives: N-acetyl-N-methyl-a,b-dehydroalanine N 0 -methylamide (1), N-acetyl-N-methyl-l-alanine N 0 -methylamide (2), and N-acetyl-N-methyl-dl-alanine N 0 -methylamide have been synthesised. The racemic species undergoes spontaneous separation into l and d-enantiomers. From these two chiral forms, the structure of d-enantiomer (3) was analysed.