title: (1S,2E,6R,7aR)-1,6-Dihy-droxy-2-(4-nitro-benzyl-idene)-2,3,5,6,7,7a-hexa-hydro-1H-pyrrolizin-3-one.
abstract: The crystal structure of the title compound, C(14)H(14)N(2)O(5), 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)°; the corresponding angle in the first conformer is 14.02 (10)°. In the crystal, molecules are linked by O-H⋯O and C-H⋯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.
authors.forename: F L
authors.initials: FL
authors.lastname: Oliveira
authors.forename: K R L
authors.initials: KR
authors.lastname: Freire
authors.forename: R
authors.initials: R
authors.lastname: Aparicio
authors.forename: F
authors.initials: F
authors.lastname: Coelho