Glycosyl sulfenate

A range of other sulfur(IV) and (VI) derivatives formally obtained by the oxidation of thioglycosides have been prepared, but they are apparently not employed to date in O-glycosylation reactions. These include glycosyl sulfenamides and sulfonamides, as well as sulfinates and sulfonates [289]. S-Glycosyl sulfenic acids have been prepared as transients by the syn-elimination of S-(2-cyanoethyl) glycosyl sulfoxides [311]. 

In attempts to improve the yields of problematic glycosylation reactions, it has become clear that the mechanism is much more complex than Scheme 3.5 suggests. In essence, the oxycarbenium ion(V) is susceptible to attack by nucleophiles other than the glycosyl acceptor (Scheme 3.6). Two intermediates for which there is experimental evidence are glycosyl triflates(VII)17 and glycosyl sulfenates(IX).18... 

Other literature methods not described in detail here, utilise glycosyl sulfenate [41], thiopseudourea [42], xanthate [42, 43], thiocyanate [44, 45] and dithioacetal [44, 46] intermediates or rely on radical addition to thiohemiacetals [47, 48] or thermal decomposition of diazo compounds prepared by reacting 1-thioaldoses and aryl diazonium salts [44]. Some of these procedures have specific advantage in some specific cases, e.g. the dithioacetal to thioglycoside route is valuable for preparing 1-thio-hexofuranosides [46, 49]. 

Scheme 3-6 Triflate and sulfenate intermediates in the sulfoxide glycosylation.

Gildersleeve and coworkers have suggested that the formation of a sulfenate(IX) can be competitive with glycoside formation this has been confirmed by characterization of reaction products generated at low temperatures (-78°C).18 The sulfenate itself becomes a glycosylating agent at higher temperatures (-20-20°C). This is consistent with earlier reports that the activated sulfoxide appears to be extremely reactive at -78°C (oxycarbenium ion, V) and yet stable at room temperature (sulfenate, IX).1... 

Y. Ito and T. Ogawa, Sulfenate esters as glycosyl acceptors A novel approach to the synthesis of 2-deoxyglycosides, Tetrahedron Lett. 28 2123 (1987). 

Ito, Y, Ogawa, T, SuHenate esters as glycosyl acceptors a novel approach to O-glycosides from thioglycosides and sulfenate esters. Tetrahedron Lett., 28, 4701-4704, 1987. 

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