Glycosylation reactions stereoselectivity

As far as the stereochemistry concerns, when the cyclization occurs through a Michael reaction, the thermodynamic product is generally obtained, but some exceptions have been observed such as that of A-acetyl glucosamine protected as 4,6-benzylidene, the reaction of which with [(ethoxycarbonyl)methylene]triphenylphosphorane afforded the ot-C-glycosyl compound stereoselectively.28... 

This synthesis demonstrated that the neighboring-group participation effect on the stereoselectivity of glycosylation reactions can be extended to sohd-phase processes. In this case, milder and more practical cleavage conditions than previously discussed were established. The use of /V- bromosuccinimide as the thiophilic reagent in acetone/water or tetrahydrofuran/methanol permitted the release of oligosaccharides in form of lactols or 1-0-Me glycosides, respectively. The tetrasaccharide derivative was isolated in 34% yield from thiol resin 3 (80% yield per step). 

The synthesis and application of glycosyl dimethylthiophosphate donors has also been reported.52 These donors were prepared from the reducing sugars in moderate to good yield and exhibit considerable shelf stability. Glycosylation reactions with a variety of primary and secondary glycosyl acceptors suffered from poor stereoselectivity and required silver triflate as promoter to achieve reasonable coupling yields (Scheme 6.16). 

Besides stereoselective synthesis of various monosaccharides, stereoselective reaction for the preparation of glycosides is an important problem in the synthetic field of carbohydrate chemistry. However, the classical methods, which require the assistance of heavy metal salts or drastic reaction conditions, are still employed by and large in the synthesis of such compounds. Taking these disadvantages into consideration, new glycosylation reactions, which proceed under mild reaction conditions with high selectivity, have been developed and exploited. 

In C-glycosylation reactions a new stereocenter is formed, and in general only one of the two stereoisomers is desired. Therefore, it is quite important to develop stereoselective C-glycosylation procedures, in which the stereochemical outcome can be predicted... 

Carbohydrate chemistry is engaged in the synthesis and variation of deoxy sugar chains, where a wide set of protective groups and stereoselective glycosylation techniques are required. This contribution centers on stereoselective syntheses of mono-and oligosaccharides in the field of 2,6-dideoxy- and, in particular cases, branched-chain sugars, and summarizes modem synthetic glycosylation reactions which have been developed for this special kind of carbohydrate chemistry. 

The various ways in which functionalities in both the glycosyl donors and glycosyl acceptors contribute to the stereoselectivity of the glycosylation reaction are further described in Chapter 4. 

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