Concise Synthesis of the Core Structures of Saundersiosides

A divergent synthesis of three core pentacyclic lactones of nine rearranged cholestane sapogenins, saundersiosides A–H (1–8) and candicanoside A (9), is reported. Key features include a one-flask CBS reduction/Brown hydroboration–oxidation, a SmI2-mediated intramolecular Reformatskii reaction, and an intramolecular transesterification. This synthesis provides a general strategy and key precursors for the collective synthesis of natural and designed saundersiosides. An efficient formal synthesis of candicanoside A is also achieved.