Acid zeolites as efficient catalysts for O- and S-glycosylation

We report herein the first regioselective synthesis of beta-galactofuranosides by Fischer glycosidation of GalNAc with methanol catalyzed by HY, HZSM-5 and HBEA acid zeolites. The zeolite HY (Si/Al ratio 3.1) was the most efficient catalyst, leading to the highest yield of methyl beta-galactofuranoside, isolated as its acetylated or isopropylidene derivatives, indicating that with large pore zeolites, the reaction efficiency depends upon the concentration of the zeolite acid sites and its hydrophilicity. However, the best regioselectivity for beta-galactofuranoside versus beta-galactopyranoside was obtained with the medium pore zeolite HZSM-5, which also led to the lowest starting material conversion, suggesting that both the zeolite pore size and topology are determinant for the obtained results. Furthermore, these acid zeolites proved to be efficient catalysts to transform 1,5-anhydro-3,4,6-tri-O-benzyl-2-deoxy-D-arabino-hex-1-enitol (6) (3,4,6-tri-O-benzy1-D-glucal) exclusively in the 2,3 -unsaturated-O- and -S-alpha-D-glycosides by Ferrier rearrangement, in moderate yield. For this reaction, the number of acid sites was the key factor for the reaction yield, being HY (Si/Al ratio 3.1) also the most effective zeolite. (c) 2007 Elsevier B.V. All rights reserved.