Piliostigmin, a 2-phenoxychromone, and C-methylflavonols from Piliostigma thonningii

Piliostigmin, a 2-phenoxychromone, and three C-methylflavonols, 6,8-di-C-methylquercetin 3-methyl ether, 6-C-methylquercetin 3,7-dimethyl ether and 6,8-di-C-methylquercetin 3,7-dimethyl ether, were isolated from the leaves of Piliostigma thonningii, together with the known compounds quercetin, quercitrin, 6-C-methylquercetin 3-methyl ether, 6-C-methylquercetin 3,7,3′-trimethyl ether, 6,8-di-C-methylkaempferol 3-methyl ether and 6,8-di-C-methylkaempferol 3,7-dimethyl ether. The structures of the new compounds were established by spectral methods, especially 2D NMR. Complete 13C assignments using HMBC experiments are reported for the four latter C-methylflavonols.