Chiral Synthesis of Erythrina Alkaloids. I Total Synthesis of (+)-Erysotrine via Asymmetric Diels-Alder Reaction under High Pressure.

(S)-(+)-3, 4-Dimethoxyphenylalanine methyl ester (1b) was converted, in 3 steps, into (5S)-(-)-8, 9-dimethoxy-1, 5-dimethoxycarbonyl-2, 3-dioxo-2, 3, 5, 6-tetrahydropyrrolo[2, 1-a]isoquinoline (2b). Diels-Alder reaction of 2b with 1-methoxy-3-trimethylsilyloxybutadiene under extremely high pressure (10 kbar), followed by desilylation, gave moderate yields of erythrinan derivatives (11), which were converted, in several steps, into (+)-erysotramidine (23b) and (+)-erysotrine (4), thus accomplishing the first total synthesis of natural Erythrina alkaloids in chiral forms.