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.
Herein, the first total synthesis of (+)-alterbrassicicene C (2) is described. Key features of the synthesis include an oxiranium mediated ether ring expansion, an oxa-Michael/retro-oxa-Michael cascade, and installation of a vinyl methoxy ether moiety via Stille coupling. can be used to access additional congeners and, given the potent activity of many related fusicoccanes, assayed for biological activity.
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