Atropisomeric Properties of 9-Methyl-1,4-benzodiazepin-2-ones

The atropisomeric and conformational properties of 1,4-benzodiazepin-2-ones were investigated by freezing the conformation with a methyl group at the C9 of 1,4-benzodiazepine. It was revealed that 1,4-benzodiazepin-2-ones exist only as a pair of enantiomers [(a1 R, a2 S) and (a1 S, a2 R)], which was confirmed by X-ray analysis. The absolute configuration of each atropisomer was deduced by comparing the [α]D and CD data with those of (–)-N-methoxycarbonylmethylated 9-methyl-5-phenyl-1,4-benzodiaepin-2-one derivative. It was elucidated that the corresponding N-methylated derivative showed similar CD spectra, although the rotational direction of [α]D was opposite to that of others.