An organocatalytic strategy for the stereoselective synthesis of C-galactosides with fluorine at the pseudoanomeric carbon.

The α-fluorination of α- and β-C-ethanals of galactose using Jørgensen catalysts and NFSI was investigated. The crude reaction products were transformed to their primary alcohol or methylenated derivatives, which are versatile precursors to biologically interesting fluorinated glycomimetics. The α-C-glycoside substrate gave moderate to high yields of fluorinated α-C-glycosides with minor amounts of β-C-glycoside analogues. The reactions on the β-C-glycoside were lower yielding but gave exclusively fluorinated β-C-glycosides. For both α- and β-C-glycoside substrates (R) and (S) catalyst showed complementary stereoselectivity. The preparation of difluorinated materials required the use of racemic catalyst as enantiomerically pure catalyst gave intractable mixtures of products. These results are in line with the results for simple achiral aldehydes, and suggest that stereochemistry in the reactions of these chiral, highly substituted, carbohydrate-derived aldehydes are controlled primarily by the chirality in the catalyst.