Total synthesis of two novel 5,6,7,8-tetrahydroindolizine alkaloids, polygonatines A and B

The structures of polygonatines A and B, two simple but structurally novel alkaloids, have been substantiated by synthesis. Cyclisation of 4-(1H-pyrrol-1-yl)butanoic acid or its ethyl ester produced 6,7-dihydroindolizin-8(5H)-one 10, formylation of which at C-3 followed by reduction afforded polygonatine A 7. Acetylation of this alkaloid followed by displacement of the acetate with ethanol yielded polygonatine B 5, possibly via an azafulvenium cation.