Anomeric phenyl sulfones

A similar process is possible using samarium diiodide. With this reagent carbohydrate radicals are generated through a one-electron reduction of, for example, anomeric phenyl sulfones. The so-induced cyclization of the tethered sugar 59 gives compound 60, which has been transformed to a C-disaccharide [31]. 

An attractive flexibility in using anomeric phenyl sulfones is that a stereodivergent synthesis of C-glycosides is available by alkylation before the reductive desulfonylation event. Thus, a one-pot four-step sequence of sulfone deprotonation-electrophile quenching-reductive lithiation-methanol quenching on sulfone 14 provides stereoselectively P-C-glycosides 19 as shown with aldehyde 16 (Fig. 

Reductive denitration was also investigated in the furanose series,45,252 using diethyl (2,3 5,6-di-0-isopropylidene-l-nitro-a-D-mannofuranosyl)phosphonate (158), an anomeric mixture of the analogous phenyl sulfone (159), (4R)-4,7-anhydro-2,3,4-trideoxy-5,6 8,9-di-0-isopropylidene-4-C-nitro-D-manno-nono-... 

Our strategy for the Smij-promoted synthesis of l,2-cis-C-glycosides is illustrated in Figure 3. It was assumed that one-electron transfer to the LUMO of the aryl sulfone group in A would result in concomitant cleavage of the C1 -S bond liberating an anomeric radical intermediate, as previously observed in the reductive lithiation of glycosyl phenyl sulfones." If 5-exo cyclization onto the silicon-tethered alkene/alkyne is substantially favored over a second electron transfer from Smij, then the exocyclic carbon radical B would ultimately be formed. On the other hand, reduction of the Cl-radical by... 

As expected, fast addition of 2.2 equiv of Sml2 to sulfone 46 provides exclusively glycal 48 by elimination (rf an anomeric or-ganosamarium. In the presence of 16 equiv of HMPA, dimers 47 become the major products. This behavior of anomeric sulfones is unique in that, under the same circumstances, phenyl sulfone 50 or bromide 49 only leads to elimination. Previously, dimerization of anomeric radicals was only observed by photolysis (or electrolysis ) of the corresponding bromides or phenyl selenides. For example, irradiation of bromide 51 in benzene at 50 °C in the presence of 1 equiv of hexamethylditin yields 32% of dimers 53 (Fig. 28). ... 

This method has also been applied to a high-yielding stereoselective synthesis of C-linked IV-acetylneuraminic acid (Neu5Ac)-containing disaccharides [92] starting from either the 2-p)ridyl (phenyl) sulfones 189 and 190 or the anomeric chloride 191 (O Scheme 40). 

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