Studies in protoberberine alkaloids. Part 12. Novel transformation of some 1-(2-bromo-α-methylbenzyl)-1,2,3,4-tetrahydroisoquinolines to isoquinobenzoxazepines during Mannich reactions

Abstract

1-(2-Bromo-α-methyl-4,5-methylenedioxybenzyl)-1,2,3,4-tetrahydro-6,7-dimethoxyisoquinoline (7a) and its analogues (26a) and (26b) were prepared by well known routes. When they were treated with formaldehyde and hydrochloric acid the expected Mannich reaction to give the 12-bromo-13-methyltetrahydroprotoberberines did not take place. A novel rearrangement occurred to give the unexpected isoquinobenzoxazepines (16a), (27a), and (27b), respectively, the structures of which were assigned by i.r., u.v., n.m.r., and mass spectroscopy and confirmed by comparison of some of their degradation products with compounds synthesised by standard methods. Unequivocal evidence for the structure and stereochemistry of these compounds were obtained by an X-ray crystallographic study on compound (27a). Some aspects of the scope and mechanism of this novel reaction have been explored.