Enantioselective autocatalysis. III. Configurational and conformational studies on a 1,4-benzodiazepinooxazole derivative.

We report here on configurational and conformational studies undertaken on the bromofluoro-1,4-benzodiazepinooxazole derivative I, which has previously been found capable of undergoing total spontaneous resolution under racemizing conditions. Due to its bridgehead N-atom I may exist in any of four diastereomeric forms, CRNR (or CSNS) and CRNS (or CSNR). Molecular mechanics calculations revealed that in their lowest energy conformations the CRNR (or CSNS) diastereomers were favored over the CRNS (or CSNR) diastereomers by some 3.42 kcal/mol, while semi-empirical quantum mechanical calculations indicated heats of formation also favoring the CRNR (or CSNS) diastereomers by 3.83 kcal/mol. The 1H NMR spectra of the three crystalline modifications (alpha, beta and gamma) of I were examined in acetone, and found to be identical. These data, combined with the results of the above calculations, suggest that I exists in methanol or methanol-acetone solutions as the CRNR (or CSNS) enantiomer prior to its racemization.