Synthesis of tryptanthrin (couroupitine) derivatives by reaction of substituted isatins with phosphoryl chloride

Reaction of 5-methyl-, 5-bromo-, and 5-iodoisatins with phosphoryl chloride gave the corresponding 2,8-disubstituted indolo[2,1- b ]quinazoline-6,12-diones in moderate yield. 5,7-Dichloroisatin failed to react with POCl 3 . Treatment of an equimolar mixture of isatin and 5-bromoisatin with POCl 3 afforded indolo[2,1- b ]-quinazoline-6,12-dione (tryptanthrin), 2,8-dibromoindolo[2,1- b ]quinazoline-6,12-dione, and two isomeric monobromo-substituted tryptanthrin derivatives, the 2-bromo isomer prevailing.