Direct coupling of the pyridylsulfone of N-acetylgalactosamine with aldehydes or ketones is promoted by samarium diiodide unexpectedly giving the corresponding alpha-C-glycosides.
A General Approach to 1,2-trans-C-Glycosides via Glycosyl Samarium(III) Compounds
or aldehydes under Barbier conditions led unexpectedly to the stereoselective synthesis of alpha-C-galactosamine derivatives in good yields. With carbonyl substrates, alpha:beta selectivities ranged from 20:1 to 5:1, and with aldehydes a stereoselectivity of approximately 5:1 was observed at C7 in favor of the S-isomer. The stereochemical preference of these C-glycosylation reactions is explained by