Photocyclisation of enamides. Part X. Total syntheses of the yohimbine skeleton: demethoxycarbonyldihydrogambirtannine, angustidine, nauclétine, (±)-angustoline, and related compounds

N-Acylation of the pyrido[3,4-b]indole derivative harmalan (I) occurred smoothly to afford the enamides (II) and (Va—c), which underwent ready photocyclisation to give the yohimbine-type compounds (III), angustidine (VIa), nauclétine (VIb), (VId), (VIIa, b, d, and e), and (±)-angustoline (VIc)(after reduction).