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A series of three homologous amino acids derivatives:  N -acetyl- N -methyl-α,β–dehydroalanine  N ′-methylamide ( 1 ),  N -acetyl- N -methyl-L-alanine  N ′-methylamide ( 2 ), and  N -acetyl- N -methyl-DL-alanine  N ′-methylamide have been synthesised. The racemic species undergoes spontaneous separation into L and D-enantiomers. From these two chiral forms, the structure of L-enantiomer ( 3 ) was analysed. The molecules of  1 – 3  adopt the  cis  arrangement of the N-terminal amide bond. The molecular conformations are similar for  1  (φ, ψ = 94.6(1)°, −1.7(1)°) and  3  (φ, ψ = 111.5(1)°, −23.8(1)°), and also  2  (φ, ψ = −114.8(2)°, 29.5(2)°), if inversion through the chiral C2 carbon is considered. They are stabilised by intramolecular N—H…N hydrogen bond. In the case of  1 , the π—electron conjugation resulting from the planar arrangement of the C2—C21 double bond and C-terminal amide group is another stabilising force of this conformation. Thus, the  N -methyldehydroalanine residue should promotes  trans-cis  isomerisation to a greater extend than the saturated analogue. The compounds  1 – 3  reveal similar associative pattern and form centrosymmetric dimers linked by the intermolecular N—H…O hydrogen bonds.

The conformation cis of N -acetyl- N -methyl-α,β-dehydroalanine N ′-methylamide and saturated analogues