Total Synthesis of (+)-Hyacinthacine A₂ Based on SmI₂-Induced Nitrone Umpolung

A concise total synthesis of (+)-hyacinthacine A₂, a polyhydroxylated pyrrolizidine alkaloid, is described using our recently discovered inversion of the C−N bond polarity in nitrones. In the key step, the diastereoselective reductive coupling of a l-xylose-derived cyclic nitrone with ethyl acrylate allowed the assembly of the bicyclic core of the target molecule, by way of a tandem formation of the C−C and C−N bonds. The method opens a novel, short, and general route for the synthesis of other pyrrolizidine alkaloids.