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C-Glycosidation of Unprotected Aldopentoses with Ketones Using Proline-Triethylamine as Catalyst

Reactions of unprotected aldopentoses with ketones catalyzed by proline and triethylamine that afford C-glycoside derivatives are described. C-Glycoside derivatives are important as bioactives, probes, and other functional molecules. C-Glycosidation reactions have often been performed on preactivated forms of carbohydrates with protected hydroxy groups or on specific precursors bearing functional groups for the bond-formation at the anomeric carbons. Direct C-glycosidation reactions of unprotected carbohydrates without the need for protection of hydroxy groups are more preferable than are reactions that require protection and deprotection steps and/or that require special precursors. Whereas direct C-glycosidation reactions of some aldoses with acetylacetone to provide acetone-derived C-glycosides have been performed since 2000, these reactions require basic conditions and high temperature. In these reactions, a deacylation step is needed to afford simple ketone-attached C-glycosides. Recently, C-glycosidation reactions of unprotected carbohydrates with ketones other than β-diketones have been developed. For example, we have reported direct C-glycosidation reactions of unprotected diand trisaccharides with ketones that afford functionalized C-glycoside ketones using pyrrolidine-boric acid catalysis under mild conditions. We have also reported C-glycosidation reactions of 2-N-acyl-aldohexoses with ketones using amine-based catalyst systems. Despite this progress, the scope of C-glycosidation reactions of unprotected carbohydrates with ketones is limited. Catalysts developed for C-glycosidation of certain types of carbohydrates with ketones do not always catalyze C-glycosidation reactions of other types of carbohydrates. Here we report C-glycosidation reactions of unprotected aldopentoses with ketones. First, catalysts and conditions were evaluated in the reactions of several aldopentoses with acetone to provide C-glycoside products (Supporting Information). Previously published results indicate that carbohydrate stereochemistry, carbon chain length (for example, aldohexose, aldopentose), and HETEROCYCLES, Vol. 99, No. 1, 2019 633