Glycosyl sulfoxides

VIII. Glycosyl Sulfoxides, Sulfones, and Selenoglycosides as Glycosyl Donors. 198... 

Although the aim of this chapter is specifically to survey the use of thioglycosides in glycosylation reactions, the presentation is incomplete without some information concerning glycosyl sulfoxides and selenoglycosides in glycosylation... 

Scheme 8.19. Glycosylation reactions with a glycosyl sulfoxide.

In addition to the standard mCPBA, many oxidants have been applied in the conversion of thioglycosides into glycosyl sulfoxides, including sodium metaperio-... 

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]. 

The most common activator for the glycosyl sulfoxides is trifluoromethanesulfonic anhydride (triflic anhydride), which, in the absence of nucleophiles, rapidly and cleanly converts most sulfoxides into the corresponding glycosyl triflates in a matter of minutes at —78 °C in dichloromethane solution [86,280,315,316]. In the more extensively studied mannopyranose series, only the a-mannosyl triflate is observed by low-temperature NMR spectroscopy (Scheme 4.35) [280]. In the glucopyranose series, mixtures of a- and (1-triflates are observed, in which the a-anomer nevertheless predominates (Scheme 4.36) [280],... 

When triflic anhydride is added to a preformed mixture of glycosyl sulfoxide and acceptor alcohol, it seems apparent that the first formed oxacarbenium ion is directly trapped by the alcohol, without the need for the implication of glycosyl triflates [75,280,323],... 

Although most glycosyl sulfoxides are cleanly and rapidly activated by triflic anhydride at — 78 °C, occasional exceptions are found. Examples include the 2,3-anhydro lyxo- and ribo-pentofuranosyl sulfoxides 1 and 2 that were demonstrated by NMR spectroscopy to not proceed to completion below —40°C [324]. 

The combination of dicyclopentadienylzirconium dichloride and silver perchlorate activates armed glycosyl sulfoxides in dichloromethane between -20 °C and room temperature, but only very simple acceptors were studied [335]. Other Lewis and Bronsted acids studied include the environmentally benign europium, lanthanum and ytterbium triflates [336], certain polyoxometallates [337], sulfated zirconia [338] and Nafion H [338]. 

Intriguing approaches have been developed using mannosyl fluorides,30,31 phosphites,32 and thioglycosides (Scheme 7.8).33 However, their generalities, particularly in terms of applicability to complex and biologically relevant systems, are yet to be clarified. Quite probably, the most successful donor for direct P-mannosylation is the sulfoxide developed by Crich and coworkers (Scheme 7.9).34-37 Glycosyl sulfoxides were originally developed by Kahne and have proved... 

Scheme 7.9 Glycosyl sulfoxide for highly selective p-mannosylation.

M. Casillas, A. M. Gomez, J. C. Lopez, and S. Valverde, A simple entry to pyranoid glycals reaction of anomeric glycosyl sulfoxides with organolithiums, Synlett, (1996) 628-630. 

SCHEME 3.15 Synthesis of disaccharide 33 using glycosyl sulfoxide. Piv, pivaloyl. 

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