Glycosyl free radicals

Glycosyl free radicals (242), generated by thermolysis (91T6559) or photolysis of 2,5-anhydroaldonic acids (241, R = H) in the presence of hyperva-lent iodine compounds (HVICs) [91TL6559 92TL7575 93JCS(P1)2417],... 

Recently, C-nucleoside synthesis by glycosyl free radical coupling with protonated nitrogen heterocycles started to gain impetus. Thus, photoirradiation of the l-(2,5-anhydro-D-allonoyloxy)pyridine-2-thione derivative 798 gave the D-ribofuranosyl free radical 799 that couples with substituted pyridines to give a mixture of the two anomers of 2-pyridyl C-nucleosides... 

Plumb GW, De Pascual-Teresa S, Santos-Buelga C, Cheynier V, Williamson G (1998) Antioxidant properties of catechins and proanthocyanidins effect of polymerisation, galloylation and glycosylation. Free Radic Res 29 351-358... 

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. 

Diets rich in millet have been associated with endemic goiter in parts of West Africa where millet is a staple. The damage has been attributed to vitexin, a C-glycosyl flavone, that in rats has antithyroid activity and that in vitro inhibits thyroid peroxidase and the free radical iodination step in thyroid hormone biosynthesis. Isoflavones have produced similar antithyroid effects in rats, but clinical studies in adults have not. " However, this remains a possible concern in infants fed soya-based milk-replacers, especially if iodine supply is compromised. 

The next two chapters give the reader an excellent perspective and laboratory-proven methods for the synthesis of C-glycosyl compounds in general, and glycosylarenes in particular, utilizing free-radical reactions and Lewis acid-catalyzed oxonium ion chemistry, respectively. 

C-Glycosyl Compounds from Free Radical Reactions... 

Another possible precursor to conduct free radical reactions is the glycosyl-cobait(III) dimethylglyoximato complex 33 [22,23], These organometallic compounds can readily be prepared by the displacement of the halide atom in 17 with the highly nucleophilic cobalt(I) anion 32. The latter can be generated from the dimeric Co(II) complex 31 under reducing conditions. 

The theme of C-glycosyl compounds is covered by two chapters on free radical- and cwis acid-mediated transformations that address the stereocontrolled synthesis of carbon abstitnents at the anomeric position. 

AP sites may be spontaneous or induced, and can block the replication. Instability of the glycosylic site constitutes the principal cause of AP sites formation. Cellular DNA can undergo puric base losses, estimated at between 250 and 500 per hour. AP sites can be regenerated during reparation processes by base excision and free radicals. 

Jain SK and Lim G (2001) Pyridoxine and pyridoxamine inhibits superoxide radicals and prevents lipid peroxidation, protein glycosylation, and (Na+ + K+)-ATPase activity reduction in high glucose-treated human erythrocytes. Free Radicals in Biology and Medicine 30,232-7. 

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. 

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