C-Glycosidation of Unprotected Di- and Trisaccharide Aldopyranoses with Ketones Using Pyrrolidine-Boric Acid Catalysis

C-Glycoside derivatives are found in pharmaceuticals, glycoconjugates, probes, and other functional molecules. Thus, C-glycosidation of unprotected carbohydrates is of interest. Here the development of C-glycosidation reactions of unprotected di- and trisaccharide aldopyranoses with various ketones is reported. The reactions were performed using catalyst systems composed of pyrrolidine and boric acid under mild conditions. Carbohydrates used for the C-glycosidation included lactose, maltose, cellobiose, 3′-sialyllactose, 6′-sialyllactose, and maltotriose. Using ketones with functional groups, C-glycosides ketones bearing the functional groups were obtained. The pyrolidine-boric acid catalysis conditions did not alter the stereochemistry of non-C–C bond formation positions of the carbohydrates and led to the formation of the C-glycosidation products with high diastereoselectivity. For the C-glycosidation of the carbohydrates under the pyrrolidine-boric acid-catalysis, the hydroxy group at the 6-position of the reacting aldopyranose was necessary to afford the product. Our analyses suggest that the carbohydrates form iminium ions with pyrrolidine and that boric acid forms B–O covalent bonds with the carbohydrates during the catalysis to forward the C–C bond formation

C‑Glycosidation of Unprotected Di- and Trisaccharide Aldopyranoses with Ketones Using Pyrrolidine-Boric Acid Catalysis

C-Glycoside derivatives are found in pharmaceuticals, glycoconjugates, probes, and other functional molecules. Thus, C-glycosidation of unprotected carbohydrates is of interest. Here the development of C-glycosidation reactions of unprotected di- and trisaccharide aldopyranoses with various ketones is reported. The reactions were performed using catalyst systems composed of pyrrolidine and boric acid under mild conditions. Carbohydrates used for the C-glycosidation included lactose, maltose, cellobiose, 3′-sialyllactose, 6′-sialyllactose, and maltotriose. Using ketones with functional groups, C-glycosides ketones bearing the functional groups were obtained. The pyrolidine-boric acid catalysis conditions did not alter the stereochemistry of non-C–C bond formation positions of the carbohydrates and led to the formation of the C-glycosidation products with high diastereoselectivity. For the C-glycosidation of the carbohydrates under the pyrrolidine-boric acid-catalysis, the hydroxy group at the 6-position of the reacting aldopyranose was necessary to afford the product. Our analyses suggest that the carbohydrates form iminium ions with pyrrolidine and that boric acid forms B–O covalent bonds with the carbohydrates during the catalysis to forward the C–C bond formation

Regioselective Oxidative Trifluoromethylation of Imidazoheterocycles via C(sp²)–H Bond Functionalization

Catalytic oxidative trifluoromethylation of imidazopyridines has been carried out at room temperature through the functionalization of the sp² C–H bond employing Langlois reagent under ambient air. A library of 3-(trifluoromethyl)­imidazo­[1,2-<i>a</i>]­pyridines with broad functionalities have been synthesized regioselectively. This methodology is also applicable to imidazo­[2,1-<i>b</i>]­thiazole and benzo­[<i>d</i>]­imidazo­[2,1-<i>b</i>]­thiazole