Photocyclisation of enamides. Part VII. A new synthesis of the protoberberine alkaloids

Abstract

N-Acylation of the 3,4-dihydro-1-methylisoquinolines (Ia and b) occurred smoothly to afford the enamides (IIa-k), which underwent ready photocyclisation to give the berbin-8-ones (IVa-d) and (Va-m). Reduction of the berbinones (IVc) and (Vd), (Vh), and (Vj) gave (±)-xylopinine (VIb), (±)-tetrahydropalmatine (VIc), and (±)-sinactine (VId), respectively.