Anomeric radicals, rearrangement

Scheme 34. Generation of anomeric 2-deoxy-a-glycosyl phosphate donors by a radical rearrangement.

An anomeric 2-deoxy-a-glycosyl phosphate 150 is generated by a radieal 2 1 migration of the phosphate ester group, which occurs when bromide 148 is irradiated in the presence of tributyltin hydride [100]. The intermediate anomeric radical 149 (in a boat conformation) rearranges into a C-2 radical in an usual Ci (d) chair conformation an hydrogen atom is then trapped mostly from the less shielded above face (Scheme 34). 

The influence of the classical anomeric effect and quasi-anomeric effect on the reactivity of various radicals has been probed. The isomer distribution for the deu-teriation of radical (48) was found to be selective whereas allylation was non-selective (Scheme 37). The results were explained by invoking a later transition state in the allylation, thus increasing the significance of thermodynamic control in the later reactions. Radical addition to a range of o -(arylsulfonyl)enones has been reported to give unexpected Pummerer rearrangement products (49) (Scheme 38).A mechanism has been postulated proceeding via the boron enolate followed by elimination of EtaBO anion. 

The stability of the radicals involved ensure that rearrangements of glucosides involving migration of the anomeric centre are particularly effective.89 For example, 123 gives 124 in 40% yield and as a single diastereoisomer. 

Figure 7.6 Rearrangement of 2-acyloxy and related glycopyranosyl radicals (a) tetra-O-acetylglucopyranosyl, (b) tetra-O-acetylmannopyranosyl, (c) a phosphatoxy migration note how the a-face of the sugar is (d) reverse migration where the anomeric centre is stabilised, (e) failure of migration of a 5-thio radical.

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