Total synthesis of benzo[ c]phenanthridine alkaloids based on a microwave-assisted electrocyclic reaction of the aza 6π-electron system and structural revision of broussonpapyrine

Abstract

Total syntheses of the des- N-methyl (nor) type of benzo[ c]phenanthridine alkaloids 1a– f and 19 and benzo[ c]phenanthridine alkaloids, chelerythrine ( 2d), and broussonpapyrine ( 2f) were achieved. The key step was the construction of tetracyclic 10,11-dihydrobenzo[ c]phenanthridines using a microwave-assisted electrocyclic reaction of the 2-cycloalkenylbenzaldoxime methyl ether 4 as an aza 6π-electron system, which was derived in two steps from a Suzuki–Miyaura cross-coupling reaction of 2-bromobenzaldehyde 6 with 2-(3,4-dihydro-6,7-methylenedioxynaphthyl)boronic acid pinacol ester 7. In addition, the exact structure of broussonpapyrine ( 2f) (2,3,9,10-tetraoxygenated type) was determined to be chelerythrine ( 2d).