Molecular and crystalline structures of three (S)-4-alkoxycarbonyl-2-azetidinones containing long alkyl side chains from synchrotron X-ray powder diffraction data

The (S)‐4‐alkoxo‐2‐azetidinecarboxylic acids are optically active β‐lactam derivatives of aspartic acid, which are used as precursors of carbapenem‐type antibiotics and poly‐β‐aspartates. The crystal structures of three (S)‐4‐alkoxo‐2‐azetidinecarboxylic acids with alkyl chains with 10, 12 and 16 C atoms were solved using parallel tempering and refined against the X‐ray powder diffraction data using the Rietveld method. The azetidinone rings in the three compounds display a pattern of asymmetrical bond distances and an almost planar conformation; these characteristics are compared with periodic solid‐state, gas‐phase density‐functional theory (DFT) calculations and MOGUL average bond distances and angles from the CSD. The compounds pack along [001] as corrugated sheets separated by approximately 4.40 Å and connected by hydrogen bonds of the type N—H...O.