Anomeric sulfoxides

P-Mannosides are difficult to introduce because the axial C-2 substituent of a mannosyl donor sterically and electronically disfavors nucleophilic attack from the P-face. P-Mannosides have been obtained by the direct substitution of a-glycosyl triflates, which are conveniently prepared by the treatment of an anomeric sulfoxide with triflic anhydride (Tf20) or thioglycosides with NIS (Scheme 4.3a)... 

Scheme 4.11 Chemoselective glycosidations of anomeric sulfoxides by a one-pot procedure.

Sulfoxides have also been used in the synthesis of nucleoside analogs (Scheme 3.2). Chanteloup and Beau reported the synthesis of ribofuranosyl sulfoxide 13 and its use in the glycosylation of a series of silylated pyrimidine and purine bases.7 Although 16 is not an anomeric sulfoxide, its reaction with cytosine derivative 17 is conceptually related.8... 

Scheme 5.1 Mechanism of thioglycoside activation (a) by thiophiles X1 such as /V-bromosuccinimicle (NBS), 11,12 methyl triflate,13 dimethyl(methylthio)sulfonium triflate (DMTST),14 phenylselenyl triflate (PhSeOTf),17,18 iV-iodosuccinimide/triflic acid (MS/TfOH),19 20 and iodonium di-sym-collidine perchlorate (IDCP)21 (b) by tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA,+)25 and (c) via anomeric sulfoxides.26 The stereochemical outcome of these glycosylations follows the same general trends as with many other glycosyl donor/promoter combinations (m-CPBA = mcta-chloroperbenzoic acid).

Fig. 5. Clycosylation with anomeric sulfoxides as donors for an efficient and stereocontrolled synthesis.

Wipf, P, Reeves, J T, Glycosylation via Cp2ZrCl2/AgC104-mediated activation of anomeric sulfoxides, J. Org. Chem., 66, 7910-7914, 2001. 

The use of anomeric sulfoxides as glycosylating reagents was reported by Kahne and coworkers in 1989 [51]. Upon activation with triflic anhydride at low temperatures, anomeric sulfoxides are transformed into extremely reactive glycosyl donors, which can glycosylate very hindered acceptors. The excellent reactivity of the sulfoxide donors coupled with the selectivity achieved by use of a participating group in the C2 position prompted their use in the synthesis of oligosaccharides on the solid support [52]. 

Yan L, Taylor CM, Goodnow R, Kahne D. Glycosylation on the merrifield resin using anomeric sulfoxides. J Am Chem Soc 1994 116 6953-6954. 

The use of p-methoxybenzyl (PMB) groups in place of benzyl ethers was required for final deprotection of ciclamycin 0, but was found to decrease the yield of the one-step coupling reaction. Therefore, a stepwise approach to the trisaccharide was finally performed with noticeable improvements in glycosylation steps [109]. Activation of an anomeric sulfoxide with triflic anhydride releases phenylsulfenyl tri-... 

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