An orthorhombic polymorph of a cyclization product of perindopril.

Low-temperature X-ray diffraction experiments were employed to investigate the crystal structures of an orthorhombic polymorph of the intramolecular cyclization product of perindopril, a popular angiotensive-converting enzyme (ACE) inhibitor, namely ethyl (2S)-2-[(3S,5aS,9aS,10aS)-3-methyl-1,4-dioxo-5a,6,7,8,9,9a,10,10a-octahydro-3H-pyrazino[1,2-a]indol-2-yl]pentanoate, C19H30N2O4, (Io), and its tetragonal equivalent, (It), which was previously reported at ambient temperature [Bojarska et al. (2013). J. Chil. Chem. Soc. 58, 1415-1417]. Polymorph (Io) crystallizes in the orthorhombic space group P2(1)2(1)2(1) with two molecules in the asymmetric unit, while tetragonal form (It) crystallizes in the space group P4(1)2(1)2 with one molecule in the asymmetric unit. The geometric parameters of (Io) are very similar to those of (It). The six-membered rings in both polymorphs adopt a slightly deformed chair conformation and the piperazinedione rings are in a boat conformation. However, the proline rings adopt an envelope conformation in (Io), while in (It) the ring exists in a slightly deformed half-chair conformation. The most significant difference between the two structures is the orientation of the ethyl pentanoate chain. Molecules associate in pairs in a head-to-tail manner forming infinite columns. In (Io), molecules are related by a twofold screw axis forming identical columns, while in (It), molecules in successive neighbouring columns are related by alternating twofold screw axes and fourfold screw axes. In both cases, the crystal packing is stabilized by weak intermolecular C-H···O interactions only.