Synthesis of 1,12b-didehydrolycoran (α-anhydrodihydrocaranine) and 12bα-lycoran (γ-lycoran)via photocyclisation of an enamide-ketone

Abstract

A novel stereoselective synthesis of lycorine-type alkaloids from a key intermediate obtained by photocyclisation of an enamido-ketone is described. Birch reduction of 6-methoxyindoline, prepared by a benzyne reaction of 3-chloro-4-methoxyphenethylamine, gave 3,3a,4,5-tetrahydro-6-methoxy-2H-indole (18) which reacted with benzoyl and 3,4-methylenedioxybenzoyl chloride to form the corresponding N-acylindol-6-one derivatives (20) and (21). Irradiation of both enamido-ketones yielded, respectively, the pyrrolo[3,2,1-de]phenanthridine derivatives (22) and (23). The latter (23), on treatment with lithium aluminium hydride, was stereoselectively converted into (±)-1,12b-didehydrolycoran (3), which was then hydrogenated to (±)-12bα-lycoran (5).