Glycosidation Using Glycosyl Phosphites

Glycosyl phosphites were introduced by Schmidt [451-453], Wong [454-457] and Watanabe [458,459], A variety of dialkyl and cyclic phosphites has proved suitable for the reaction [459,460], In fact, diethyl and dibenzyl phosphites are the most frequently used. Glycosyl phosphites react with alcohols at low temperamres in the presence of catalytic amounts of Lewis acid promoters such as TMSOTf, BF3-Et20 and Sn(OTf)2, thus creating glycosides. Phosphites are also activated by other promoters, which are summarized in Table 4.9. 

Besides the common organic solvents used for glycosylations, syntheses of glycosides from glycosyl phosphites in ionic liquids were recently reported [463]. Glycosyl phosphites are frequently used for sialylations [451,454] and a typical example is shown in Scheme 4.44. As illustrated, reaction of the (3-neuraminyl phosphite 205 with the lactose diol 206 afforded the 3-(9-neuraminyllactose derivative 207 in a 55% yield [460]. 

Nagai H, Sasaki K, Matsumura S, Toshima K. Enviromnen-tally benign 3-stereoselective glycosidations of glycosyl phosphites using a reusable heterogeneous solid acid, mont-morillonite K-10. Carbohydr. Res. 2005 340 337-353. 

Full details have appeared on the use of 0-protected glycosyl phosphites for Lewis acid-catalysed glycosidation (cf. Synlett, 1993, 115). Again anomeric mixtures were produced, but reasonable yields resulted - especially when O-benzyl protection was employed with glucosyl, galactosyl and mannosyl donors and with 0-acetylated sialic acid donor. 

Trichloroacetimidates and glycosyl sulfoxides are now commonly used for coupling (95, 96). Other coupling functional groups include phosphites, phosphates, thioglycosides, and pentenyl glycosides (97-99). 

Several direct and stereoselective glycosylation methods using 2-deoxyglycosyl phosphates, phosphoramidites," and phosphites have been reported. An alternative approach to construct 2-deoxy-p-glycosides was developed by Hashimoto et al. (Scheme 38.3). In their smdy, the gly-cosylations of several 2-deoxyglycosyl diethyl phosphites with several glycosyl-acceptors including primary and... 

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