Total Synthesis of Caesalpinnone A and Caesalpinflavan B: Evolution of a Concise Strategy

The total syntheses of caesalpinnone A (1) and its putative biosynthetic precursor caesalpinflavan B (3) are described. Herein, we describe the evolution of a synthetic strategy toward 1 and 3, which entails a convergent Barluenga coupling that quickly delivers a heavily functionalized benzopyran containing the core carbon framework and exploration of two distinct synthetic routes for forging the flavanoid C-ring by reducing a sterically encumbered embedded alkene: one via a stepwise approach and a second, more direct and atom-economical, enabled by a Shenvi-HAT hydrogenation. The latter strategy allowed access to caesalpinflavan B in 6 steps after Pd-mediated deallylation. A late-stage dearomative phenolic oxidation and deallylation/oxa-Michael cascade was implemented to access caesalpinnone A (1) in 7 steps. We also describe an enantioselective total synthesis and stereochemical revision of (-)-caesalpinflavan B, as well as a formal enantioselective synthesis of (-)-caesalpinnone A, by implementing an enantioselective Pd-catalyzed conjugate addition developed by Stoltz.