Thermal effect in β-selective glycosylation reactions using glycosyl iodides

The unique reactivity of glycosyl iodides and the fact that they react under neutral conditions makes them the donors of choice in our glycosylation strategies. Glycosyl iodides are generated in situ from either the anomeric acetate or the anomeric silylated derivative yielding the alpha-iodide. In the reported glycosylation reactions, protected glucosyl, galactosyl, and mannosyl iodides were reacted with trimethylene oxide as the acceptor, yielding the beta-anomer as the major product. In the absence of neighboring group participation, beta-selectivity is thought to arise from nucleophilic displacement of the alpha-iodide in an S(N)2-like mechanism, while the alpha-product is the result of nucleophilic attack on the beta-iodide. In this study, increased beta-selectivity using an inverse thermal effect is demonstrated. (c) 2005 Elsevier Ltd. All rights reserved.