Characterization of benzodiazepinooxazoles: crystal and molecular structure of trans and cis isomers of oxazolam.

The stereochemistry of the trans and cis isomers of 2(S)- and 2(R)-oxazolam is described. A total of four molecular structures of the 11b(R) and 11b(S) diastereomers of 2(S)- and 2(R)-oxazolam have been revealed by X-ray diffraction techniques. 2(S)-Oxazolam crystallizes in space group P2(1)/c, with cell dimensions a = 16.180(2) A, b = 8.791(2) A, c = 13.574(2) A, and beta = 102.07(1) degrees. The molecule in an asymmetric unit exhibited disorder at the 2-carbon (C2) atom, and the structures for trans and cis isomers have been resolved crystallographically. 2(S)-Oxazolam forms polymeric chains with intermediation of ethanol through hydrogen bonding. 2(R)-Oxazolam crystallizes in space group P1, with cell dimensions a = 7.754(1) A, b = 8.654(1) A, c = 15.795(2) A, alpha = 104.09(1) degrees, beta = 91.19(1) degrees, and gamma = 114.00(1) degrees. A similar disorder at the C2 position has been elucidated by a mixture of trans- and cis-2(R)-oxazolams. The two 2(R)-oxazolam molecules form a dimer with a hydrogen bond net. The structural details, focusing on the conformation and molecular complex formation of both oxazolams, have been discussed in connection with the oxazolidine ring stability and the crystal polymorph.