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■ Synthesis and Conformational Analysis of Hexahydroisoquino[3,2-b][3]benzazepines (iso-B-Homoberbines)

Darius Paul Zlotos and Werner Meise*

*Pharmaceutical Institute Poppelsdorf, University of Bonn, Kreuzbergweg 26, D-53115 Bonn, Germany

Abstract

A new simple synthetic approach to the hexahydroisoquino[3,2-b][3]benzazepines has been developed. 3-Benzyl-2-tetralones (3a-c) were prepared in two different ways from 2-tetralone and 6-methoxy-2-tetralone. Regioselective methoxycarbonylation with dimethyl carbonate or with magnesium methyl carbonate gave the known β-keto esteres (1) or (1’), respectively, which could be alkylated to benzyl derivatives (2) and (2’), respectively. Demethoxycarbonylation using lithium iodide afforded the desired 3-benzyl-2-tetralones (3a-c), which furnished the lactams (4a-c) by submitting to the Schmidt reaction. Reduction usign borane and a final Pictet-Spengler cyclization gave the title compounds (6a-c). NMR measurements indicated that iso-B-homoberbines exist as equilibrium mixture of the cis and trans conformers in CDCl₃ solution at room temperature, whereas the hydrochloride salt of 6b adopts a cis B-fused conformation.