Radicals in Carbohydrate Chemistry

METHODS OF INVESTIGATION OF RADICALS IN CARBOHYDRATE CHEMISTRY AND BIOCHEMISTRY... 

Although the generation of nitroalkyl radicals by oxidative transition-metal-mediated reactions is known for many years, their application in carbohydrate chemistry was not investigated until recently.41... 

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The wide versatility of the tin hydride method in carbohydrate chemistry exists because anomeric radicals can be generated from many functional groups at the anomeric... 

Of all the radical reactions, the exo-l,5-cycIization of a hex-5-enyl radical to cyclopen-tylmethyl radical and its subsequent trapping by various reagents have attracted the most attention from synthetic chemists (Scheme 1) [4-7]. Starting materials that are most often used for the tin method (initiation of the chain by trialkyl tin radical) are halides, sulfides, selenides, or thionocarbonates. The generation and cyclization of the radical proceeds under exceptionally mild neutral conditions, and these conditions are compatible with a wide variety of common functional groups. A prototypical example of an application in carbohydrate chemistry is shown in Scheme 2 [8]. Readily available 2,3-di-O-isopropyl-ideneribonolactone 1 was converted into the bromoacrylate 2 in three steps. Radical... 

This use of free radical reactions offers an alternative to ionic reactions which are well known in carbohydrate chemistry. The neutral conditions and mild work-up of these phototransformations are often of great interest in the synthesis of multifunctional molecules. 

Another important application of mixed 0,Se-acetals is the generation from radicals at the anomeric center in carbohydrate chemistry.244-248... 

The most representative examples of intramolecular free radical cyclization in carbohydrate chemistry are the syntheses of C-branched nucleosides derivatives. The key step in C-branched nucleoside preparation is the regio- and stereo-controlled formation of a new C-C bond at the branching point of the ribofuranose ring [58]. Among published reports, a temporary silicon connection is becoming a growing interest in the syntheses of C-branched nucleosides by intramolecular radical cyclization. 

Isopropylidene-protected ct.v-diols, which are frequently utilized in carbohydrate chemistry, are found to have a large influence on the stereoselectivity in addition reactions when they are located adjacent to the radical center35,75. The selectivity seems to be somewhat lower when the radical is located at the anomeric center compared to other ring positions. 

Very recently, cerium(IV)-mediated C-C bond formations were applied for the first time in carbohydrate chemistry [9]. Again, radicals are generated from malo-nates la at low temperatures, and even sensitive glycals 16 are stable under such conditions. The regioselectivity of the additions is controlled by favorable orbital interactions and the adduct radicals are oxidized to cations, which are trapped by the solvent or CAN to afford the carbohydrate C-analogs 17 in good yields (Scheme... 

Scheme 5. CAN-mediated radical reactions in carbohydrate chemistry...

The y -0-mannopyranoside linkage is traditionally one of the most challenging anomeric linkages to form in carbohydrate chemistry. One popular solution to this problem has been to generate an equatorial radical and allow it to isomerize to its more stable axial position prior to quenching. 

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