Radical-initiated cyclization as a key step for the synthesis of oxoprotoberberine alkaloids

The oxoprotoberberine alkaloids 1a–d have been synthesized efficiently from the enamide derivatives 2a–d by a radical-initiated cyclization reaction utilizing n-Bu3SnH/AIBN and CuCl. The enamide derivatives 2a–d were prepared from phenylethylamine analogues 5a–b, followed by acylation with acetic anhydride, Bischler–Napieralski cyclization with POCl3 and benzoylation with the corresponding bromobenzoyl chloride, respectively.