Radical glucosyl

The synthesis of the C-disaccharide 1 of kojibiose from the glucosyl radical 2 and the alkene 3 is a good example for the C-C bond formation by a radical approach (Scheme 2) [2-4],... 

Because of the high reactivity of the OH radical, the selectivity with respect to the site of attack is greatly lowered. From D-glucose, for example, six different primary D-glucosyl radicals are formed in about equal yields.10... 

A. Alberti, M. A. Della Bona, D. Macciantelli, F. Pelizzoni, G. Sello, G. Torri, and E. Vismara, Reactivity of glucosyl radical in the presence of phenols, Tetrahedron, 52 (1996) 10241-10248. 

Intermolecular free radical reactions. Giese notes the diastereoselectivity of reactions of acrylonitrile with cyclic 5- and 6-membered ring radicals can be controlled by adjacent substituents. Thus an axial 3-substituent can favor axial attack, whereas an equatorial 3-substituent favors equatorial attack in the case of 6-membered cyclic radicals. Glucosyl radicals, regardless of the precursor, yield a-substi-tuted products (88 12). 

Two equatorial /i-substituents are present in cyclic carbohydrate radicals, in which the pyranose ring forms a heterocyclic cyclohexyl radical37. The glucosyl radicals A, B, and C below all show preferred equatorial attack in addition reactions to alkenes the actual selectivity depends on the radical and alkene substituents. In reactions of cyclohexyl radicals with only one equatorial /1-substituent, addition of 1,2-disubstituted alkenes yields only the equatorial addition product. 

Radical reactions can also be used in the construction of bridged carbohydrate derivatives. The 3-(2-propenyl)-l-glucosyl radical 9 cyclizes to the bridged tetrahydropyran derivative 10 in 53 % yield, consisting of a 90 10 mixture of C-l epimers. Due to geometrical constraints, the corresponding 6-(2-propenyl) radical only yields the noncyclized product68. 

In a reaction of a C-5 glucosyl radical axial attack has also been observed48. 

As illustrated by the preparation of a-D-glucosyl-(7 )-alanine 75, the method is simple and efficient (Scheme 13). Alkene 72a is treated with iodide 73, sodium cya-noborohydride, and tributyltin chloride in r-butanol to give exclusively the dia-stereoisomer 74 (88%) arising from addition of the glucosyl radical on its a-face followed hydrogen-atom transfer to the intermediate oxazolidinonyl radical anti to the bulky t-butyl group [47]. 

Glycosyl radicals appear to adopt a conformation such that the half-occupied orbital at the radical centre is largely coplanar with the -C-0 bond (o orbital). The glucosyl radical thus... 

As for the reactions, another convenient photoreductive process catalyzed by ruthenium trw-bipyridyl has been exploited for the generation of glucosyl radicals from the corresponding halides (amines are added as reducing agents for the generation of the active Ru species). The mild generation of radicals under this conditions allows the addition to alkenes (see Scheme 27). ... 

The synthetic process for sucrose can be regarded as a condensation reaction in which the elements of water of hydrolysis are replaced by those of phosphoric acid. The process can also be considered as a transglucosida-tion reaction in which the D-glucosyl radical from a-D-glucose 1-phosphate is transferred to a D-fructofuranose radical, serving as an acceptor. 

The glycoside-enzyme complex in the second and third stages of the mechanism may react with an alcohol instead of water (10), The free sugar is not formed, but rather the glucosyl radical is transferred and a new glycoside is formed according to the equation ... 

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