Crystal structure and pseudosymmetry analysis of the triclinic prodrug cloxazolam (Z' = 4)

The prodrug cloxazolam [systematic name: 13-chloro-2-(2-chlorophenyl)-3-oxa-6,9-diazatricyclo[8.4.0.0^2,6]tetradeca-1(10),11,13-trien-8-one], C₁₇H₁₄Cl₂N₂O₂, crystallizes in the triclinic space group P-1, with four chemically identical independent molecules in the asymmetric unit. However, in order to facilitate the analysis of the striking pseudosymmetry relating the four independent molecules, the structure has been analysed and reported in the nonconventional centred B-1 space-group setting. Pseudosymmetry is an eminently local property, valid only in the realm of the unit-cell boundary and not propagating to the whole crystal structure. It has been analyzed using the MP procedure described in the preceding article [Baggio (2019). Acta Cryst. C75, 837-850]. The molecules consist of a rigid core made up of three rings (five-, six- and seven-membered) and an extra six-membered ring joined to the latter group by a single C-C bond, together with a clamping intramolecular C-H...O interaction preventing free rotation and providing additional rigidity. The four molecules in the asymmetric unit pair into dimers with almost exact twofold pseudosymmetry, further linked into (001) slabs as the building bricks of the structure. Interpenetration of slabs finally leads to a three-dimensional structure of unusual compactness for an organic structure, with a Kitaigorodskii packing index of ca 0.71.