Glycosyl sulfones

Chen et al. [21] have studied the Bamberg-Backlund reaction on carbohydrate anomeric sulfones and observed that the a-p ratio of unreacted sulfones 64a and 64p recovered from the basic reaction mixture was not the same as the starting ratio with the amount of a-anomer enhanced (Fig. 8.19). 

This observation could be explained in two ways (1) the p-isomer was reactive, whereas the a-isomer was not, and (2) the p-isomer was isomerizing to the a-isomer. 

Starting material T (°C) Time Ratio (trans/cis) AG° kcal/mol 

Comparison of these rates in 76, 77, and 78 shows that the presence of a P-synclinal oxygen atom accelerates the reaction by a factor of 200 another synclinal oxygen on the same carbon, as in 80, leads to a further 20-fold rate increase. An antiperiplanar oxygen, however, as in 74 and 75 increases the rate more than lO times relative to 77 and 78. Alternatively, if 74 and 75 are compared with 76, changing of the oxygen atom from the S3mclinal to the antiperiplanar orientation increases the rate by 71- and 95-fold. That this 

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Glycosyl sulfones, normally of low reactivity, have been reported as glycosyl donors, activated by magnesium bromide etherate.188... 

Glycosyl residues, nomenclature, 125 Glycosyl sulfones, as glycosyl donors, 199— 200... 

Scheme 4.55 Glycosylation with a 2,3-unsaturated glycosyl sulfone.

General Procedure for the Preparation of Diazirines from Glycosyl Sulfonates... 

Similarly, starting from 2,3 5,6-di-0-isopropylidene-D-mannofuranose 72, a one-pot procedure that implies the synthesis of glycosyl sulfones by Horner-Wadsworth-Emmons olefination with phosphonate 73, followed by subsequent Michael addition and Ramberg-Backlund reaction92 gives compound 74 in 78% yield93 (Scheme 24). 

In conclusion, the Ramberg- Backlund reaction of glycosyl sulfones is a useful and general route to a variety exo glycals. Simple conversions of the exo-glycals make available a variety of C-glycosides. 

An alternative to the above methods is the activation of hemiacetals via glycosyl sulfonates 185 (Scheme 4.36). 

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