Quinolizidines. VIII. Structure and Synthesis of the Alangium Alkaloid Alangicine: Syntheses of (+)- and (+)-Alangicines

Abstract

The first total synthesis of alangicine (3), an Alangium lamarckii alkaloid, has been achieved in the form of a racemic modification by means of an initial alkaline hydrolysis of the (±)-tricyclic ester 6 and succeeding steps proceeding through the intermediates (±)-7, (±)-10, and (±)-9. A parallel synthetic route starting with the (—)-tricyclic ester 6, derived from (+)-cincholoipon ethyl ester (8), produced the chiral target molecule (+)-3 via the intermediates (—)-7, (—)-10, and 9. The identity of the synthetic (+)-3 with alangicine unequivocally established the structure and absolute stereochemistry of this alkaloid. The 13C nuclear magnetic resonance spectra of (+)-alangicine (3) and the ipecac and Alangium alkaloid psychotrine (18) confirmed their endocyclic double bond structures in the dihydroisoquinoline moiety. Catalytic reductions of 11, (±)-12, and 15 using hydrogen and Pd-C were investigated, and the results have shown that hydrogenolysis of the benzyloxy group proceeds much faster than saturation of the endocyclic C=N bond. © 1983, The Pharmaceutical Society of Japan. All rights reserved.