Radicals glycosyl

The bridged 1,4-dioxepanones 112 (R = TBMS X = H, Me) were prepared, unexpectedly, in moderate yield by radical deoxygenation of capuramycin derivatives. These products were rationalised in terms of a novel intramolecular radical glycosylation-lactonization reaction <00H(52)133>. 

With all the anomeric activating groups useful in the generation of sugar-derived free radicals, glycosyl halides have been the most exploited. In the remainder of this section, various reactions involving glycosyl halides will be discussed. Additionally, the stereochemical consequences of these reactions will be analyzed. 

Junker, H.-D. and Fessner, W.-D., Diastereoselective synthesis of C-glycosylphosphonates via free-radical glycosylation. Tetrahedron Lett., 39, 269, 1988. 

Bivalent and tervalent residues 2-Carb-32. Radicals, cations and anions 2-Carb-33. Glycosides and glycosyl compounds... 

Tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA) differs from the other promoters in that its cation is a radical, and as such produces radical cationic sulfonium ions as glycosylating species from thioglycosides.85 The use of this promoter arose from earlier work on the electrochemical generation of 5-glycosyl radical cations as glycosylating species. 

Hunt, J.V., Dean, R.T. and Wolff, S. (1988). Hydroxyl radical production and autoxidative glycosylation. Glucose oxidation as the cause of protein damage in the experimental glycation model of diabetes mellitus and ageing. Biochem. J. 256, 205-212. 

Under certain conditions glucose molecules can induce free-radical production (see section on non-enzymatic glycosylation of protein). 

Glycosylation of LDL may involve free-radical reactions and itself lead to oxidative damage (Steinbrecher et al., 1984 Esterbauer etal., 1992). However, it appears that glycated LDL is poorly recognized by the classical LDL receptor, but is preferentially recognized by a high-... 

Hunt, J.V., Smith, C.T. and WolflF, S.P. (1990). Autooxidative glycosylation and possible involvement of peroxides and free radicals in LDL modification by glucose. Diabetes 39, 1420-1424. 

I. Addition of C-Radicals to Nitrones Recently (525), the addition of alkyl radicals to chiral nitrones as a new method of asymmetrical synthesis of a-amino acids has been described. Addition of ethyl radicals to glycosyl nitrone (286) using Et3B as a source of ethyl radicals appears to proceed with a high stereo-control rate. 

Anomeric halides follow the typical reactivity order F < Cl < Br < I for nucleophilic substitutions. They have been used in stereoselective O-glycosylation, nucleophilic displacement, and carbanion as well as in radical reactions. 

Thioglycosides can also be activated by a one-electron transfer reaction from sulfur to the activating reagent tris-(4-bromophenyl)ammoniumyl hexachloroanti-monate (TBPA+) [102,103]. The use of this promoter was inspired by an earlier report where activation was achieved under electrochemical conditions to give an intermediate S-glycosyl radical cation intermediate [104], and the reactivity and mechanism have also been explored [105,106]. 

Scheme 4.102 Cation and radical-cation pathways for selenoglycoside glycosylation.

Selenoglycosides are attractive donor species owing to the wide range of reagents that can promote cation- and radical-cation-based processes for their activation and subsequent O-glycosylation (Scheme 4.102) [223,512],... 

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). 

Skrydstrup, Beau and co-workers122 have adapted Stork s method to the SmI2-reduction of glycosyl pyridyl sulfones bearing a silicon-tethered unsaturated group at HO-C(2). An example is shown with the synthesis of methyl a-C-zso-maltoside 172 from alkyne 170 via the 5-exo-dig radical cyclization of 171 (Scheme 56).144... 

The stereochemistry of the reaction of glycosyl radicals is strongly influenced by the anomeric effect. Glucopyranosides and manno-pyranosides afford stereo selectively the a-C-glycosyl compounds whereas in furanosidic structures the stereochemistry is not always predictable. 

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