N‐Acyl‐diketopiperazines

As shown by the values of dCα (0.487 and 0.442 ÅA) and by the β angle of folding (41°), N‐(pyruvoyl)‐cyclo(‐L‐Phe‐D‐Pro‐) (I) in the crystals adopts a boat conformation whose high degree of buckling approaches the values found in proline containing homochiral diketopiperazines. Only one peptide bond is slightly non planar with ω2= ‐4.7°. The imide ‐CO‐N‐CO‐ group is practically planar and not conjugated with the ketonic carbonyl; the angle between the planes containing the two groups being 79.7°. The proline ring is in a β‐envelope conformation. The axial benzylic side chain of the Phe residue in the crystal adopts an unexpected extended conformation (X11= ‐60.9°) which is not retained in chloroform solution, where the folded conformation is preferred. Crystals: space group P21 with a = 9.039(7), b = 14.472(6), c = 6.215(6)ÅA, β= 106.24(6)° and Z = 2. The final R and Rw are 0.049 and 0.068, respectively.