A rhodium-catalyzed C–H activation/cyclization approach toward the total syntheses of cassiarin C and 8-O-methylcassiarin A from a common intermediate†

Three short and efficient total syntheses of cassiarin C are reported, from a chromanone common key intermediate. A C–H activation strategy, under rhodium catalysis on its pivaloyl oxime, enabled the installation of the pyridine ring. Dehydrogenation of 8-O-methylcassiarin C afforded 8-O-methylcassiarin A. A kinetic experiment and DFT calculations of the intermediates helped to gain insight into the unusual site- and stereo-specific H/D exchange of cassiarin C in CD₃OD.