PURPOSE: To obtain a tricyclic benzodiazepine compound which is a central nervous system depressant with low toxicity readily in high yield, by oxidizing a novel oxazolobenzodiazepine derivative prepared by various methods.

CONSTITUTION: A compound of formula II (R1 and R2 are H, NO2 group, halogen, etc.; R3 is H or lower alkyl group; R5 is H, lower alkyl group, etc.) is reacted with a compound of formula III (R4 is H or lower alkyl) to give a novel compound of formula I, which is then oxidized with chromic acid or manganese oxide in water or an organic solvent at room temperature to form a compound of formula IV. The compound of formula I is not only an intermediate for the compound of formula IV but also useful as a sedative agent or a tranquilizer per se.