The stereospecific synthesis of methyl α-C-mannobioside: a potential inhibitor of M. tuberculosis binding to human macrophages
摘要:
The synthesis of the C-glycoside analogue of the mycobacteria capping disaccharide, Man alpha 1-2Man, employing a glycosyl organosamarium is described.
or aldehydes under Barbier conditions led unexpectedly to the stereoselective synthesis of alpha-C-galactosamine derivatives in good yields. With carbonyl substrates, alpha:beta selectivities ranged from 20:1 to 5:1, and with aldehydes a stereoselectivity of approximately 5:1 was observed at C7 in favor of the S-isomer. The stereochemical preference of these C-glycosylation reactions is explained by
Mannosyl pyridylsulfones with varying C2OH protecting groups were reacted with cyclohexanone in the presence of SmI2. With SiMe2tBu and Bn, high yields of an α-C-mannoside were obtained. In the former case no β-elimination was observed. The relative configuration of the major diastereomer obtained upon coupling with aldehydes was determined.
在SmI 2的存在下,具有不同C2 = OH保护基的甘露糖基吡啶基砜与环己酮反应。使用SiMe 2 t Bu和Bn,可获得高收率的α - C-甘露糖苷。在前一种情况下,未观察到β-消除。确定了与醛偶联后获得的主要非对映异构体的相对构型。
Samarium Diiodide PromotedC-Glycosylation: An Application to the Stereospecific Synthesis ofα-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 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.
A General Approach to 1,2-trans-C-Glycosides via Glycosyl Samarium(III) Compounds
The synthesis of the C-glycoside analogue of the mycobacteria capping disaccharide, Man alpha 1-2Man, employing a glycosyl organosamarium is described.