Samarium Diiodide PromotedC-Glycosylation: An Application to the Stereospecific Synthesis ofα-1,2-C-Mannobioside and Its Derivatives
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.
The synthesis of the C-glycoside analogue of the mycobacteria capping disaccharide, Man alpha 1-2Man, employing a glycosyl organosamarium is described.