摘要:
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 employing an approach which closely parallels O-glycoside synthesis. The key step included the samarium diiodide reduction of mannosyl pyridylsulfone 18 in the presence of the C2-formyl branched mannoside derivative 17a to furnish the C-disaccharide derivative 19a in high yield. An intramolecular formyl group transfer reaction by means of 5-exo radical cyclization and concomitant fragmentation yielded aldehyde 17a stereospecifically. We also present a potentially viable alternative for the deoxygenation of sterically encumbered secondary alcohols. Attempts to extend this procedure to the synthesis of the C-trisaccharide of Man(alpha 1-->2)Man(alpha 1-->2)Man were frustrated by the inability of the disaccharide, pyridylsulfone derivative 43, to undergo coupling with carbonyl substrates upon treatment with SmI2, possibly owing to the sterically bulky C2 substituent.