Enantiomerization of 3-carbethoxy-1,4-benzodiazepin-2-one: Combined chiral HPLC and spectroscopic study

Recently developed chiral HPLC columns CHIRIS AD1 and CHIRIS AD2 have been demonstrated to separate racemic, configurationally unstable ethyl-7-chloro-2-oxo-5-phenyl-2,3-dihydro-1H-1,4-benzodiazepine-3-carboxylate (1) and its 3-methyl congener 2; fast on-column enantiomerization of configurationally unstable 1 was observed, however. Addition of 0.1% of AcOH to the eluting mixture inhibits enantiomerization, whereas the same percentage of Et3N completely precludes enantioseparation, suggesting base-catalysis by free beta -aminoethyl groups, present in low percentage in chiral stationary phase (CSP). When both CSPs were prepared under conditions of nonexhaustive acylation by N-DNB-alpha -aminoacids, no separation of 1 was observed. The rate of enantiomerization on CHIRIS AD2 was determined at 25 degreesC, the mechanism is discussed, and experimental results correlated with calculated relative stabilities of the tautomers la-c. Absolute (3S) configuration of (+) enantiomers of 1 and 2 was determined by comparison of their eluation profile to that of (+/-)-3 and (3S)-(+)-3, taking into account relative (psia or psie) configuration of the prevailing conformer in solution. (C) 2002 Wiley-Liss, Inc.