Convergent synthesis of (+)-carambolaflavone A, an antidiabetic agent using a bismuth triflate-catalyzed C -aryl glycosylation.

We describe a convergent total synthesis of carambolaflavone A, a natural flavonoid C -aryl glycoside with significant antihyperglycemic properties. The synthesis features a bismuth triflate-catalyzed stereoselective C -aryl glycosylation of a flavan derivative and an appropriately protected D-fucose derivative as the key step. Inexpensive and non-toxic bismuth triflate provided the best results among various other Lewis acids screened for this C -aryl glycosylation. The method can be utilized for the synthesis of other bioactive C -glycosyl flavonoids. The glycosylation partners were synthesized from commercially available (±)-naringenin and D-(+)-galactose, respectively. An oxidative bromination and elimination reaction sequence was utilized to construct the flavone. The natural product is obtained in 13 steps (longest linear sequence) from D-(+)-galactose.