The reduction of acetylated or benzoylated glycosyl halides or selcnides with low concentrations of lribulylstannane leads to 2-dcoxy sugars. An important step in this radicalchain reaction is the cis-selective migration or an ester group. The broad applicability of this method is demonstrated by thc synthesis of mono-and dideoxy sugars, or PI-and Pdeoxy sugars, of dcoxypyranoses and deoxyiuranoses, of dcoxypentosyl-and -hexosyl carbohydrates.
E.s.r. spectra of C-2, -3, and -4 carbohydrate free radicals, generated regiospecifically by halogen abstraction from acylated deoxyhexopyranoses are recorded in benzene solution. Analysis of the hyperfine splittings reveals the retention of the 4C, chair conformation of the parent compounds in the radicals.In Part 2 ' the structures of various pyranosyl radicals with the radical centre at the anomeric (C-1) carbon atom have been examined by e m . spectroscopy. In non-aqueous media the pyranos-1-yl radicals exist as planar n-type radicals and their equilibrium conformations strongly depend on the type of carbohydrate. Thus, diastereoselectivities in radical C-C coupling reactions of carbohydrates have to be discussed in terms of carbohydrate conformations2 rather than in terms of 0configuration at the radical centre as had been suggested earlier. 3 Recently, it has been shown that diastereoselective C-C coupling reactions could also be performed at the C-2 position of the carbohydrate backbone, giving 2-deoxy sugars in reasonably good yield^.^ Similarly, radical-induced C-C bond formation is also possible at C-3 and -4 of the pyranosyl rings5The e.s.r. spectroscopic investigation of C-2, -3, and -4 deoxypyranosanyl radicals, which is presented here, should provide better insight into the dependence of diastereoselectivity on radical conformation. Structurally related radicals have been investigated in the reaction of unprotected sugar derivatives with O H radicals in aqueous s ~l u t i o n . ~. ~
Results and DiscussionThe deox ypyranosanyl radicals (Rl)-(R8) were regiospecifically generated in benzene solution from the corresponding bromoor iodo-compounds (1)-( 8) by reaction with trimethylstannyl radicals, generated photolytically from hexamethylditin (see Table ). The carbohydrates (lb), (2b), (3), and (6)-( 8) are new compounds that were synthesized by standard procedures. The e.s.r. spectra, recorded in the temperature range from -8 to
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