Samarium diiodide promoted C-glycosylation: An application to the stereospecific synthesis of alpha-1,2-C-mannobioside and its derivatives

The synthesis of the C-glycoside analogue of the disaccharide Man(alpha 1-- >2)Man has been achieved in a highly stereoselective and efficient manner e mploying an approach which closely parallels O-glycoside synthesis. The key step included the samarium diiodide reduction of mannosyl pyridylsulfone 1 8 in the presence of the C2-formyl branched mannoside derivative 17a to fur nish the C-disaccharide derivative 19a in high yield. An intramolecular for myl group transfer reaction by means of 5-exo radical cyclization and conco mitant fragmentation yielded aldehyde 17a stereospecifically. We also prese nt a potentially viable alternative for the deoxygenation of sterically enc umbered secondary alcohols. Attempts to extend this procedure to the synthe sis of the C-trisaccharide of Man(alpha 1-->2)Man(alpha 1-->2)Man were frus trated by the inability of the disaccharide, pyridylsulfone derivative 43, to undergo coupling with carbonyl substrates upon treatment with SmI2, poss ibly owing to the sterically bulky C2 substituent.