Crystal structure of N-(tert-butoxycarbonyl)phenylalanyldehydroalanine isopropyl ester (Boc–Phe–ΔAla–OiPr)

In the title compound, the dehydrodipeptide (Boc–Phe–ΔAla–OiPr, C20H28N2O5), the molecule has a trans conformation of the N-methylamide group. The geometry of the dehydroalanine moiety is to some extent different from those usually found in simple peptides, indicating conjugation between the H2C=C group and the peptide bond. The bond angles around dehydroalanine have unusually high values due to the steric hindrance, the same interaction influencing the slight distortion from planarity of the dehydroalanine. The molecule is stabilized by intramolecular interactions between the isopropyl group and the N atoms of the peptide main chain. In the crystal, an N—H...O hydrogen bond links the molecules into ribbons, giving a herringbone head-to-head packing arrangement extending along the [100] direction. In the stacks, the molecules are linked by weak C—H...O hydrogen-bonding associations