α- and β-Lipomycin: total syntheses by sequential stille couplings and assignment of the absolute configuration of all stereogenic centers.

40 years ago spectroscopy, derivatization, and degradation revealed the structures of α-lipomycin and its aglycon β-lipomycin except for the configurations of their side-chain stereocenters. We synthesized all relevant β-lipomycin candidates: the (12R,13S) isomer has the same specific rotational value as the natural product. By the same criterion the (12R,13S)-configured D-digitoxide is identical to α-lipomycin. We double-checked our assignments by degrading α- and β-lipomycin to the diesters 33 and 34 and proving their 3D structures synthetically.