A GENERAL-APPROACH TO 1,2-TRANS-C-GLYCOSIDES VIA GLYCOSYL SAMARIUM(III) COMPOUNDS

The samarium diiodide reduction of glycosyl pyridyl sulfones with keto nes or aldehydes under Barbier conditions leads to the instantaneous a nd stereospecific formation of 1,2-trans-C-glycosides in good to accep table yields. Mannosyl pyridyl sulfones 5a-c,h produce alpha-C-glycosi des 7-10, 12, 13, 15-17 and 57 in yields up to 86% with minimal beta e limination. In contrast, glucosyl pyridyl sulfones 19a and 19b lead to the corresponding beta-C-glycosides 20-22 in yields up to 56% with in creased beta elimination. Similarly, galactosyl pyridyl sulfones 23a a nd 23b afford beta-C-galactoside 24. The stereochemical discrepancies between these reactions are probably based on the intermediacy of a co mmon alpha-anomeric glycosyl samarium(III) compound (kinetic product) with an axially oriented C1-Sm bond after reduction of the pyridyl sul fone group. The thermodynamically more stable anomeric organosamarium with an equatorially oriented C1-Sm bond may then be obtained by a lea st energy pathway in the form of either a conformational ring-flip (in the manno series) or as a configurational change (in the gluco or gal acto series). The tendency towards beta elimination can be explained b y the preference of glycosyl organosamarium compounds to undergo an un precedented syn-elimination mechanism more easily achieved in the gluc o and galacto series. C2-Unsubstituted 2-deoxy sugars display little o r no stereoselectivity at C1 upon C-glycosylation.