Abstract
Bis (phenylmethyl)-2,5-piperazinedione, CIsH~sN202, Mr=294.3, orthorhombic, P22~21, a =6.181(1), b=10.380(3), c=23.795(4)A, V= 1526.8 (6) A 3, z = 4, D x = 1.28 g cm -3, 2(Cu Ka) = 1.54178 A, # = 6-0 cm -~, F(000) = 624, room tem- perature, R = 0.056 for 959 observed reflections and 176 variables. The 2,5-piperazinedione ring has a flattened boat conformation with the C~ atoms in pseudoaxial positions. One of the phenylalanyl residues faces the 2,5-piperazinedione ring (X~ = 69.0 (6)°), the other is in an extended conformation (X~ = -61.9 (6)°). The 09 torsion angles in both cis peptide units are small and equal to -0.9 (7) and 0.1 (7) °. Introduction. The preference for an aromatic part of an amino-acid residue to fold over the 2,5-piperazinedione (hereafter DKP) skeleton is a well known phenomenon. The folded conformation has been observed both in the solid state and in solution. However, for cis cyclic dipeptides built up of two aromatic amino-acid residues, steric repulsions prevent the molecule from adopting a conformation in which both aromatic rings fold over
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