Reaction conditions, stereoselectivity glycosylations

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. 

The reaction conditions also significantly influence the stereoselectivity of glycosylations. Solvents are particularly important in this respect. To achieve stereoselectivity, reactions should... 

The importance of the choice of the glycosylation catalysts and reaction conditions is illustrated by an example in Scheme 4.6. Reaction of the glycosyl donor 40 with the same glycosyl acceptor 41 performed under different conditions produced excellent yields of either the a-hnked disaccharide 42a or the 3-linked isomer 42p in a highly stereoselective manner [45]. 

SCHEME 4.6 The effect of the promoter and the reaction conditions on the stereoselectivity of glycosylations. 

Reviewed was the use of our newly developed Pd-catalyzed glycosylation reaction for the incorporation of rare and unnatural sugars on unique biologically relevant structures. This de novo approach for the introduction of carbohydrate structures, under non-Lewis acidic conditions, into preexisting structural motifs is unique. To date, there simply does not exist an equivalent chemical or biochemical method for the stereoselective glycosylation of under-fimctionalized D- and L-sugars, which can be elaborated into a myriad of carbohydrate analogues. Not to mention this method also allows for complete a/(3-stereocontrol. The power and uniqueness of this approach is probably best... 

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