5- Thio sugars anomeric effects

Figure 3.21 Time courses of the hydrolysis of an isotopic quasi-racemate in two acid-catalysed hydrolyses of glycosides. Both figures, taken from Ref. 107, by kind permission of Prof. A. J. Bennet and the American Chemical Society, refer to a mixture of labelled d-sugar and unlabelled l-sugar (10mgmL each), show the least-squares best fit and display only every fourth data point for clarity, (a) Ring effect for methyl (3-xylopy-ranoside (b) anomeric effect for methyl 5-thio-oc-xylopyranoside. See Table 3.3 for numerical values of the effects, which are in opposite senses.

Several carbohydrate applications have been included in a review of micro-wave-assisted organic reactions, and a survey of chemical reagents in photo-aflinity labelling included the role of various base-labelled azido- and thio-nucleosides, sugar azide derivatives and p-benzoylbenzoate esters of nucleosides. A further review intitled Reverse Anomeric Effect Fact or Fiction refers to early evidence based on studies of glycosyl pyridinium and imidazolium salts. It concludes that its origin as an electronic effect is not supported by theory or experimental results. ... 

Tatsuta and Toshima [73] designed conformationally rigid 2,6-anhydro-2-thio sugars for the stereocontrolled synthesis of 2,6-dideoxy-a- and -P-glycosides which are very commonly found in bioactive natural products. These new donors have a very rigid structure (boat conformation) and the stereoselectivity of the glycosylation should not be affected by the anomeric effect in the same manner as the more usual chair conformers. 

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