Protected carbohydrate group

In Fig. 3 one finds a variety of functional groups that may be subdivided as follows haloalkyl groups [14-16], saturated esters (acetate [17], and benzoate [18,19]) unsaturated esters (methacrylate [19,20], acrylate [21], cinnamate [20], and others [21]) malonate and related esters [22-24] ethers and oligo(oxyethylenes) [25-28] imides [29,30] alkylsilyloxy groups [31] perfluoroalkyls [32-34] and protected carbohydrate groups [35]. Vinyl ethers with mesogenic (liquid-crystal forming) substituents will be treated separately in Section II. B.2. 

Partially protected carbohydrates can be selectively oxidized at the primary hydroxy group to uronic acids at the nickel hydroxide electrode. At the same electrode, in polyhydroxy steroids, a preferential oxidation of the sterically better accessible hydroxyl groups is achieved [142]. By way of the mediator, TEMPO, carbohydrates that are only protected at the anomeric hydroxyl group are selectively oxidized at the primary hydroxyl group (Fig. 27) [143-145]. 

The effects observed for the ether-protected carbohydrates are likely a result of their lower degree of positive charge destabilization than the corresponding ester groups, leading to side reactions such as ring contraction and ehmination. ... 

The reactions of protected carbohydrates with and in anhydrous hydrogen fluoride have been well investigated and arc covered by some excellent reviews29,33 34"277 279 288 289 (see also Flouben-Weyl, Vol. E14a/3, pp 627-646). The anomeric (glycosidic) center is as a rule the most reactive position of a carbohydrate moiety. Practically all functional groups located in... 

Prom the viewpoint of a synthesis chemist, carbohydrates would appear to be severely overfunctionalized. Thus, in a hexopyranose, one has to contend with five hydroxyl groups distributed over six carbon atoms. Furthermore, four of the hydroxyl groups are chiral. Obviously, to cany out synthetic manipulations on such molecules one has to learn to protect hydroxyl groups (or amino groups in the case of aminodeoxy sugars) to leave free only those destined for reactions. Therefore, a rich repertoire of protecting gronp manipulations for this purpose has evolved [1,2]. Table 1 shows some of the more common ones in current use. 

The electrochemical deprotection of carbonyl compounds proved to be a useful method especially in cases where alternative chemical reactions are unsuccessful, a-Keto- and a-hydroxythioacetals, when oxidized in MeCN-HoO (9 1 v/v) on a Pt anode, are transformed into the corresponding a-diones and a-ketols [142]. Diethyl dithioacetals of sugars were anodically oxidized in MeCN-H20 (5% H2O) on Pt electrode, and the substrates were successfully deprotected producing the correspondent carbohydrates in 65-85% yield [143]. It is noteworthy that protected hydroxy groups as esters or cyclic acetals were not affected. Selective deprotection to carbonyl compounds electrooxidizing mixtures of thioacetals, like a ketone and an aldehyde thioacetal, the former being preferentially deprotected, was described [144]. 

In all the above methods for oxidizing carbohydrates a stoichiometric oxidant is added to the reaction mixture. This can be avoided by using an electrochemical oxidation. A nickel hydroxide electrode has been applied for oxidizing isopropylidene-protected carbohydrates in aqueous base [26]. While secondary hydroxy groups fail to react under these conditions, the hemiacetal at the anomeric center is oxidized to the lactone in good yield [26]. 

A commonly used, protected carbohydrate containing a secondary hydroxy group is diiso-propylideneglucofuranose 23. Oxidation to the corresponding ketone 24 illustrates some of the most widely applied methods for oxidation of secondary alcohols (O Table 4). Again, the reactions can be divided into three main categories oxidations mediated by activated DMSO, oxidations with chromium(VI) oxides, and oxidations catalyzed by mthenium oxides. For oxidations with activated DMSO the Swern procedure is the most widely used [27]. 

Other partially protected carbohydrates also undergo very regioselective oxidation. Noteworthy is the oxidation of isopropylideneglucofuranose 29 to 5-ketofuranose 30 (O Scheme 10) [109]. For oxidation of the axial hydroxy group in cis-1,2 diols, the dibutylstannylene acetal method is often employed. Oxidation of methyl fucoside 31 with this procedure gives ketone 32 in good yield (O Scheme 10) [111]. 

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