Abstract:The samarium diiodide reduction of glycosyl pyridyl sulfones with ketones or aldehydes under Barbier conditions leads to the instantaneous and stereospecific formation of 1,2-trans-C-glycosides in good to acceptable yields. Mannosyl pyridyl sulfones 5 a ± c,h produce a-C-glycosides 7 ± 10, 12, 13, 15 ± 17, and 57 in yields up to 86 % with minimal b elimination. In contrast, glucosyl pyridyl sulfones 19 a and 19 b lead to the corresponding b-C-glycosides 20 ± 22 in yields up to 56 % with increased b elimination. Similarly, galactosyl pyridyl sulfones 23 a and 23 b afford b-C-galactoside 24. The stereochemical discrepancies between these reactions are probably based on the intermediacy of a common a-anomeric glycosyl samarium(iii) compound (kinetic product) with an axially oriented C1 ± Sm bond after reduction of the pyridyl sulfone group. The thermodynamically more stable anomeric organosamarium with an equatorially oriented C1 ± Sm bond may then be obtained by a least energy pathway in the form of either a conformational ring-flip (in the manno series) or as a configurational change (in the gluco or galacto series). The tendency towards b elimination can be explained by the preference of glycosyl organosamarium compounds to undergo an unprecedented syn-elimination mechanism more easily achieved in the gluco and galacto series. C2-Unsubstituted 2-deoxy sugars display little or no stereoselectivity at C1 upon C-glycosylation.

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