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.