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

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 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.