Crystal structure of dehydromonopetide, (Z)‐N‐acetyl‐α,β‐dehydrophenylalanine methylamide

The crystal structure of (Z)‐acetyl‐α,β‐dehydrophenylalanine methylamide (monoclinic Cc, a = 10.241(1), b = 15.252(1), c = 8.643(1) Å, β = 120.98(1)°, Z = 4) has been solved by x‐ray diffraction to an R‐factor = 0.148, and compared to that of the homologous L‐phenylalanine dervative. Molecules are intermolecularly hydrogen‐bonded to four neighboring molecules in a three‐dimensional network with alternating layers of interacting amide bonds and orthogonally arranged phenyl rings. The existence of the Cα = Cβ double bond results in a phenyl orientation that is forbidden for phenylananine (χ1 = −7,8°), and in shorter Cα − Cβ and Cβ − Cγ distances. The geometrical paramenters of the peptide backbone are not drastically modified by α,β‐unsaturation. However, the N‐Cα‐C′ angle is increased by nearly 5°, and the dimensions, and therefore probably the electronic conjugation, of the N‐terminal amide group to be affected by the occurrence of the vicinal Cα = Cβ double bond.