SnCl4- and TiCl4-catalyzed anomerization of acylated O- and S-Glycosides: analysis of factors that lead to higher alpha:beta anomer ratios and reaction rates

Abstract

The quantification of factors that influence both rates and stereoselectivity of anomerization reactions catalyzed by SnCl4 and TiCl4 and how this has informed the synthesis of alpha-O- and alpha-S-glycolipids is discussed The SnCl4-catalyzed anomerization reactions of beta-S- and beta-O-glycosides or 18 substrates followed fist older equilibrium kinetics and k(t) + k(r) values were obtained. where k(r) is the rate constant for the forward reaction (beta -> alpha) and k(r) is the late constant for the reverse reaction (alpha -> beta) Comparison of the k(t) + k(r) values showed that reactions or glucuronic acid or galacturonic acid derivatives were 10 to 3000 times faster than those or related glucoside and galactopyranoside counterparts and alpha beta ratios were generally also higher Stereoelectronic effects contributed from galacto-configured compounds were up to 2-fold faster than those of corresponding glucosides The introduction of groups, including protecting groups. which are increasingly electron releasing generally led to rate enhancements. The anomerization of S-glycosides was consistently faster than that of corresponding O-glycosides. Reactions were generally faster for reactions with TiCl4 than those with SnCl4 Anomeric ratios depended on the Lewis acid. the number equivalents of the Lewis acid temperature. and substrate very high ratios of alpha-products for both O- and S-glucuronides were observed for reactions promoted by TiCl4 for these substrates TiCl4 was superior to SnCl4 Anomeric ratios from anomerization of S-glucosides were higher with SnCl4 than with TiCl4 The dependence of equilibrium ratio on Lewis acid and die number of equivalents of Lewis acid indicated that the equilibrium ratio is determined by a complex of the saccharide residue bound to the Lewis acid and not the flee glycoside The high alpha beta ratios observed for anomerization of both O- and S-glyeuronic acids can be explained by cool (filiation of the C-1 heteroatom and C-6 carbonyl group of the product to the Lewis acid, which would enhance the anomeric effect by increasing the electron-withdrawing ability of the anomeric substituent find lead to an increase in the proportion of the alpha-anomer Such an observation would argue against the existence or a reverse anomeric effect Support for a chelation-induced endocyclic cleavage mechanism lot the anomerization is provided by the trapping or a key intermediate The data herein will help predict the tendency of beta-glycosides to under go anomerization, this includes cases where 1,2-trans glycosides are initial products or glycosidation reactions catalyzed by TiCl4 or SnCl4