Conformation of the oxalamide group in retro-bispeptides. Three crystal structures.

We have determined the structures of a range of peptides having the oxalamide (-NH-CO-CO-NH-) unit located at their center. The oxalamide group has the trans conformation in two retropeptides and an approximately orthogonal conformation in the peptide with Pro residues. Torsional angles about the CO-CO bond are 180 degrees in MeO-Aib-CO-CO-Aib-OMe (1), 175 degrees in MeO-L-Leu-CO-CO-L-Leu-OMe (2), and -108 and -106 degrees for the two independent molecules in the crystal of MeO-L-Pro-CO-CO-L-Pro-OMe (3), owing to steric hindrance between CO and the pyrolidine ring. Crystal data are: (1) C12H20N2O6, triclinic, space group P1, a = 6.190(1), b = 10.044(2), c = 11.989(2) A, alpha = 86.13(12), beta = 83.13(2), gamma = 80.16(2) degrees, R = 0.057 for 1646 observed reflections [magnitude of Fo > 3 sigma (magnitude of Fo)]; (2) C16H28N2O6, tetragonal, space group P4(1), a = b = 11.121(3), c = 15.775(6) A, R = 0.058 for 1216 observed reflections [magnitude of Fo > 3 sigma (magnitude of Fo)]; (3) C14H20N2O6, monoclinic space group P2(1), a = 9.556(2), b = 17.864(3), c = 9.618(2) A, beta = 104.35(1) degree; R = 0.051 for 2100 observed reflections [magnitude of Fo > 3 sigma (magnitude of Fo)].