The oxazolidone-fused benzodiazepine derivative (1) was prepared by treatment of chlordiazepoxide (2) with sodium hydride and ethyl chloroformate followed by the N-oxide rearrangement of the resultant carbamate (3) with acetic anhydride. The first reaction was found to be accompanied by a multi-step rearrangement of the initially formed 3 as a side reaction giving rise to the indolenine derivative (4). The structure of 4 was determined by X-ray crystallographic analysis.