Selective Formation of β-O-Aryl Glycosides in the Absence of the C(2)-Ester Neighboring Group

The development of a general and practical method for the stereoselective synthesis of beta-O-aryl glycosides that exploits the nature of a cationic palladium(II) catalyst, instead of a C(2)-ester directing group. to control the beta-selectivity is described. This beta-glycosylation reaction is highly diastereoselective and requires 2-3 mol % of Pd(CH3CN)(4)(BF4)(2) to activate glycosyl trichloroacetimidate donors at room temperature. The current method has been applied to D-glucose, D-galactose, and D-xylose donors with a nondirecting group incorporated at the C(2)-position to provide the O-aryl glycosides with good to excellent beta-selectivity. In addition, its application is widespread to electron-donating, electron-withdrawing, and hindered phenols. The reaction is likely to proceed through a seven-membered ring intermediate, wherein the palladium catalyst coordinates to both C(1)-trichloroacetimidate nitrogen and C(2)-oxygen of the donor, blocking the alpha-face. As a result, the phenol nucleophile preferentially approaches to the top face of the activated donor, leading to formation of the beta-O-aryl glycoside.