Reductive Aldol Approach to Natural Products: Bioinspired Synthesis of abeo -11(12 → 13)-Oleanane Triterpenoids.

A synthesis of alstoscholarinoid B ( 1 ) and 3 β -acetoxy-11 α -hydroxy-11(12 → 13) abeo oleanan-12-al ( 2 ) has been accomplished in 7-9 steps and 10%-16% overall yield from oleanolic acid. This synthesis featured a bioinspired SmI 2 -mediated reductive aldol reaction to establish the abeo -11(12 → 13)-oleanane framework of both 1 and 2 and a retro -aldol/aldol/lactonization cascade to fully construct the skeleton of 1 . Moreover, the investigation of the bioinspired aldol reaction also sheds light on the potential biogenesis of natural products.