Intermolecular Anomeric Radical Addition

was generated photochemically and coupled to the nitronate derivative of the C6 functionalized glucoside 112. Tin hydride-promoted denitration then afforded the desired C-dimer. 

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C-Glycoside synthesis via the intermolecular addition of anomeric radicals to activated alkenes has been pioneered by Giese and co-workers [120]. A wide variety of acceptors have been used in this area and both simple [131,132] and more complex (a C-disaccharide 290) [133] examples are shown (Scheme 77). 

In addition to intermolecular reactions, C-glycosides can also be synthesized by intramolecular sequences. A radical cyclization is a very fast reaction, in particular 5-exo-trig cyclizations. Thus, intermediate anomeric radicals have only a short time window of reactivity before undergoing the desired cyclization. 

Intermolecular addition of C-, S-, or / -centered radicals to the exocyclic unsaturated C-C bond of 2,6-anhydro-hept-l-enitols leads to anomeric radicals that, after subsequent hydrogen quenching, can produce C-glycosyl compounds or... 

Another competing reaction is the intermolecular replacement of bromine by hydrogen in the radical translocating group. Additional approaches capitalizing on stereospecific hydrogen translocation to the anomeric radical have been described by Crich [186, 187]. Despite excellent stereoselectivity, the radical approach is rendered impractical by the circuitous methods needed for generation of the key anomeric radical [51]. 

A major strategy for the preparation of branched-chain sugars is the intermolecular addition of nonanomeric carbohydrate-based radicals to a 7t-system. Since no special electronic factors, such as radical anomeric effects, are present, the stereoselectivity is controlled sterically, and the stereochemistry of the substituents adjacent to the radical center mainly governs the stereochemical control. Representative results are summarized in Scheme 23. 

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