(1S,2E,6R,7aR)-1,6-Dihydroxy-2-(4-nitrobenzylidene)-2,3,5,6,7,7a-hexahydro-1H-pyrrolizin-3-one

The crystal structure of the title compound, C14H14N2O5, contains two distinct conformers in the asymmetric unit. The compound has three defined stereocenters, two of them contiguous, and a C=C double bond with an E conformation. The stereocenters exhibit the same chirality in both conformers, with significant differences in the conformation of the five-membered rings of the pyrrolizine unit (both either in a twist or in an envelope form) and in the dihedral angles between the corresponding mean planes and the benzene rings. A prominent feature is a change from almost coplanar rings in one conformer to a new conformation in the second conformer, in which the mean plane of a five-membered ring is almost perpendicular to the benzene ring, with a dihedral angle 87.19 (8)degrees; the corresponding angle in the first conformer is 14.02 (10)degrees. In the crystal, molecules are linked by O-H center dot center dot center dot O and C-H center dot center dot center dot O hydrogen bonds. Crystallographic data were essential to confirm the configuration of the double bond, which was unclear from the available two-dimensional NMR data. In addition, reliable Flack and Hooft parameters were obtained, allowing for the correct absolute structure to be determined.