IMINIUM SALTS FROM ALPHA-AMINO-ACID DECARBONYLATION - APPLICATION TO SYNTHESIS OF BERBINES

Berbines are synthesized from α-(tertiary amino) acids in high yields through decarbonylation to regiospecific iminium salts followed by an acid-catalyzed cyclization reaction. Syntheses of the α-(tertiary amino) acids from various phenylalanines which involve as the key step alkylation of a 1,2,3,4-tetrahydro-3-isoquinolinecarboxylic acid with a 2-phenylethyl bromide are described. The synthesis of isopropyl 1,2,3,4-tetrahydro-7,8-dimethoxy-3-isoquinolinecarboxylate (15), obligatory to the synthesis of 9,10-dimethoxyberbines by the above method, is described. It utilizes a metalation to align four contiguous substituents on the aromatic nucleus followed by a difficult selective reduction of an amide α to an ester. © 1978, American Chemical Society. All rights reserved.