REACTIVITY AND STEREOSELECTIVITY IN GLYCOSYLATION: EMPHASIS ON STRUCTURE OF GLYCOSYL ACCEPTOR AND CONCENTRATION OF REAGENTS

摘要

Considerable influence of concentration of reagents on the outcome of glycosylation has been reported in a number of cases, both the product yield and stereoselectivity as well as the reactivity of the glycosyl donor being affected by dilution of the reaction mixture. In most publications, results of the glycosylations at only two concentrations - "dilute" (0.001—0.005 mol/L) and "regular" (0.05 mol/L) -were usually compared. A more detailed analysis of the influence of concentration on glycosylation outcome, performed in a wide concentration range [1-3], revealed that concentration changes may affect, for example, stereoselectivity in a rather complicated manner. Depending on a system, stereoselectivity may dramatically increase upon dilution [2], experience a maximum [3] or depend on concentration of reagents in a more complex way [1]. Worthy of note is recently disclosed bimodal concentration-dependent reactivity pattern of a glycosyl donor [3], which is a special case of a more general phenomenon of existence of clearly defined concentration ranges, separated by critical concentrations,[4] where the solute has distinct chemical properties [1-7] (see the review [8]).