Chemical transformation of protoberberines. XIV. Acid-catalyzed cleavage of 8-alkyl-8,14-cycloberbines. A simple method for the preparation of N-unsubstituted spirobenzylisoquinolines.
On treatment with an acid, 8-alkyl-8, 14-cycloberbines (9) afforded the N-unsubstituted spirobenzylisoquinolines (10, 11, and 12) through regioselective C8-N bond cleavage in contrast to the 8-unsubstituted 8, 14-cycloberbine (9d), which gave the benzindenoazepine (19, R = H) through regioselective C14-N bond cleavage. Reduction of 9 with NaBH4 or LiAlH (OBut) 3 yielded stereoselectively the alcohol (20 or 21, respectively) as the main product. Acidic treatment of the isomeric alcohols (20 and 21) effected regioselective C8-N bond cleavage, resulting in the N-unsubstituted spirobenzylisoquinolines (22-26).