A flexible approach for the asymmetric syntheses of hyacinthacines A2, A3 and structural confirmation of hyacinthacine A3.

A concise and flexible approach for the asymmetric syntheses of polyhydroxylated pyrrolizidine alkaloids hyacinthacines A(2) and A(3) has been developed using iterative reductive alkylation of O,O'-dibenzyltartarimide (5) as key steps. The ambiguity about the structure of synthetic hyacinthacine A(3) due to the differences in the NMR data of the synthetic material (2) and the natural product (hyacinthacine A(3)) was eliminated jointly by comprehensive 1D and 2D-NMR studies, and by analysis of the (1)H and (13)C NMR spectra of a mixed synthetic product and natural hyacinthacine A(3). The latter method also allowed a confirmation of the structure of the natural hyacinthacine A(3), and may be useful for structural confirmation of other hydroxylated alkaloids.