Carbohydrate radicals 1 Conformational analysis / ESR spectroscopy / Anomeric effect Acetylated 1-cyano-and 1-chloro-pyranos-1-y1 radicals and 5-acetoxycarbonyl-, 5-methoxycarbonyl-, and 5-unsubstituted pyranosan-5-yl radicals were generated from the corresponding bromides by bromine abstraction with trimethyltin radicals. The conformation of these radicals, as deduced from the ESR hyperfine splittings, is explained by the combined action of a quasi-anomeric and a homo-anomeric stabilization effect.
A captodative substitution pattern of the radical center does not influence the conformations.
Free radical reactions have become a standard methodology in organic synthesis in recent years'). Because these reactions may be carried out under nonpolar conditions, they constitute a valuable supplement to other synthetic methods. Experience has shown that free radical reactions may also be applied successfully in the area of carbohydrate chemistry, sometimes leading to products of high stcreoselectivity 3! These observations were the incentive to study the conformation of the intermediate carbohydrate radicals by ESR s p e c t r o s ~o p y ~~~) .
The synthetically useful generation of carbohydrate radicals by halogen abstraction from organohalogen compounds also proved viable for the observation of their ESR spectra. Whereas free-radical chain reactions are used in synthesis, for spectroscopic studies it is more appropriate to generate the carbohydrate radicals by nonchain processes.
As in our earlier work'^^-^), in this study we used the photochemical cleavage of hexamethylditin to generate trimethyltin radicals which act as halogen abstractors in inert solvents, viz. benzene, fluorobenzene, or tetrahydrofuran. Though the carbohydrate radicals often gave ESR spectra of low signal-to-noise ratio, evaluation of the spectral parameters was always possible.
F 6
We report here on the conformation of C-1-cyano and -chloro-substituted pyranos-I -yl (1 R -6R), C-5-methoxycarbonyl-substituted (7R-SR), C-5-unsubstituted, and C-5-acetoxymethyl-substituted (9R -10R) pyrnnosan-5-yl