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[摘要]：The development of a new glycosylation method for the stereoselective synthesis of b-glycosides in the absence of the traditional C(2)-ester neighboring group effect is described. This process relies on the ability of the cationic palladium catalyst, Pd(PhCN)2(OTf)2 generated in situ from Pd(PhCN)2Cl2 and AgOTf, to direct b-selectivity. The new glycosylation reaction is highly b-selective and proceeds under mild conditions with 1-2 mol % of catalyst loading. This b-glycosylation method has been applied to a no. of glucose donors with benzyl, allyl, and p-methoxybenzyl groups incorporated at the C(2)-position as well as tribenzylated xylose and quinovose donors to prep. various disaccharides and trisaccharides with good to excellent b-selectivity. Mechanistic studies suggest that the major operative pathway is likely to proceed via a seven-membered ring intermediate, wherein the cationic palladium complex coordinates to both the C(1)-imidate nitrogen and C(2)-oxygen of the trichloroacetimidate donor. Formation of this seven-membered ring intermediate directs the selectivity, leading to the formation of b-glycosides. |
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