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 P2₁2₁2₁ with two mol­ecules in the asymmetric unit, while tetra­gonal form (It) crystallizes in the space group P4₁2₁2 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.