Highly alpha- and beta-selective radical C-glycosylation reactions using a controlling anomeric effect based on the conformational restriction strategy. A study on the conformation-anomeric effect-stereoselectivity relationship in anomeric radical reactions.

Abstract

We hypothesized that, because the stereoselectivity of anomeric radical reactions was significantly influenced by the anomeric effect, which can be controlled by restricting the conformation of the radical intermediate, the proper conformational restriction of the pyranose ring of the substrates would therefore make highly alpha- and beta-stereoselective anomeric radical reactions possible. Thus, the conformationally restricted 1-phenylseleno-D-xylose derivatives 9 and 10, restricted in a (4)C(1)-conformation, and 11 and 12, restricted in a (1)C(4)-conformation, were designed and synthesized by introducing the proper protecting groups on the hydroxyl groups on the pyranose ring as model substrates for the anomeric radical reactions. The radical deuterations with Bu(3)SnD and the C-glycosylation with Bu(3)SnCH(2)CH [double bond] CH(2) or CH(2) [double bond] CHCN, using the (4)C(1)-restricted substrates 9 and 10, afforded the corresponding alpha-products (alpha/beta = 97:3-85:15) highly stereoselectively, whereas the (1)C(4)-restricted substrates 11 and 12 selectively gave the beta-products (alpha/beta = 1:99-0:100). Thus, stereoselectivity was significantly increased by conformational restriction and was completely inverted by changing the substrate conformation from the (4)C(1)-form into the (1)C(4)-form. Ab initio calculations suggested that the radical intermediates produced from these substrates possessed the typical (4)C(1)- or (1)C(4)-conformation, which was similar to that of the substrates, and that the anomeric effect in these conformations would be the factor controlling the transition state of the reaction. Therefore, the highly alpha- and beta-selective reactions would occur because of the anomeric effect, which could be manipulated by conformational restriction of the substrates, as expected. This would be the first radical C-glycosylation reaction to provide both alpha- and beta-C-glycosides highly stereoselectively.