Lythraceae alkaloids possess the unique structures having a trans- or cis-quinolizidine ring with a biphenyl or biphenyl ether and a twelve- or fourteen-membered lactone. The Mannich reaction of isopelletierine (1) with arylaldehydes under alkaline condition afforded cis-4-arylquinolizidin-2-ones, which then isomerized into the corresponding trans-quinolizidines. The stereoselectivity of this reaction was found to depend on solvents, solubilities of the starting aldehydes and the products, and reaction time. Based on the above observation, the first total synthesis of one of these alkaloids, decaline (17) having a trans-quinolizidine ring, was accomplished starting from 6-bromoisovanillin (8). (±)-Vertaline (26) having a cis-quinolizidine ring, (±)-desmethyl-decaline (34), and (±)-methyldecinine (45) having a biphenyl linkage, were also synthesized. The proposed structure (58) for lagerine required revision because the synthetic racemic compound (58) was found not to be identical with lagerine. The total synthesis of (±)-methyllagerine established its revised structure (69).
