Two diastereomers of 9-(2,6-dichlorophenyl)-4a-hydroxy-3,3,6,6-tetramethyl-1,2,3,4,4a,5,6,7,8,9a-decahydroxanthene-1,8-dione in the same crystal

Searching for stereoisomerism in crystallographic databases: algorithm, analysis and chiral curiosities

The automated identification of chiral centres in molecular residues is a non-trivial task. Current tools that allow the user to analyze crystallographic data entries do not identify chiral centres in some of the more complex ring structures, or lack the possibility to determine and compare the chirality of multiple structures. This article presents an approach to identify asymmetric C atoms, which is based on the atomic walk count algorithm presented by Rücker & Rücker [(1993), J. Chem. Inf. Comput. Sci. 33, 683-695]. The algorithm, which we implemented in a computer program named ChiChi, is able to compare isomeric residues based on the chiral centres that were identified. This allows for discrimination between enantiomers, diastereomers and constitutional isomers that are present in crystallographic databases. ChiChi was used to process 254 354 organic entries from the Cambridge Structural Database (CSD). A thorough analysis of stereoisomerism in the CSD is presented accompanied by a collection of chiral curiosities that illustrate the strength and versatility of this approach.

Crystal structure of 10a-hy-droxy-9-(3-nitro-phen-yl)-3,6-diphenyl-3,4,5,6,7,8a,9,10a-octa-hydro-1H-xanthene-1,8(2H)-dione

In the octa-hydroxanthenedione unit of the title compound, C31H27NO6, the central di-hydro-pyran ring shows an envelope conformation, while the bilateral cyclo-hexene and cyclo-hexane rings adopt a half-boat conformation and a chair conformation, respectively. The nitro-benzene ring is twisted with respect to the two benzene rings, making dihedral angles of 63.1 (1) and 63.0 (1)°. In the crystal, O-H⋯O hydrogen bonds link the mol-ecules into supra-molecular chains propagating along the a-axis direction.