2-Pyridyl sulfones, anomeric

We have noted exceptional behavior of this type with anomeric pyridyl sulfone 46 and similar mannosyl and galactosyl sulfones (Fig. 27). ... 

Another intramolecular approach to C-linked disaccharides has been described by Skrydstrup and Beau [116] using 2-pyridyl sulfones to generate anomeric radicals. Tethering sulfone 280 with chlorosilane 279 provides silyl ether 281 (O Scheme 60). 

Other extensions of this C-glycosylation methodology have been reported for the preparation of two C-linked 1,6-disaccharides. Reaction of the pyridyl sulfone of N-acetylglucosamine 54 with aldehyde 55 produced mainly the a-C- saccharide 56 (Scheme 13) [35]. The stereoselectivity at the anomeric center is explained in this case by internal coordination of the fV-acetyl group with the metal ion of the intermediate glycosyl samarium species 57, retarding anomerization to the ther-... 

The standard tin hydride conditions for generating an anomeric radical can be replaced by the divalent samarium-induced reductions of glycosyl aryl sulfones [59, 60]. In one instance the Sinay group employed the 2-pyridyl sulfone group, introduced by Skrydstrup and Beau in 1994 [30, 31], which was rapidly reduced by Sml2 at room temperature (Scheme 31). In this way, a significant increase in the cyclization yield of 118 was achieved [60]. 

Predictable stereochemistry at Cl would be expected when applying 5-exo-cyclizations in silicon-tethered radical reactions for the construction of C-disaccharides. A sole example has been provided by Skrydstrup and Beau using glycosyl pyridyl sulfones for the generation of the anomeric radical [31]. This approach has been successfully exploited in the synthesis of peracetylated methyl C-isomaltoside 137 (Scheme 37). 

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