Selective Protection of Carbohydrates

Except for a very limited number of cases, in order to address the regioselectivity issue and to achieve an efficient glycosyl synthesis, the first problem to face is a suitable derivatization 

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Bu3Sn)20, toluene, reflux BnBr, A/-methylimidazole, 95% yield. " Equatorial alcohols are benzylated in preference to axial alcohols in diol-containing substrates. The application of the stannylene method for the selective protection of carbohydrates has been reviewed. ... 

Some of the original work in the carbohydrate area in particular reveals extensive protection of carbonyl and hydroxyl groups. For example, a cyclic diacetonide of glucose was selectively cleaved to the monoacetonide. A summary describes the selective protection of primary and secondary hydroxyl groups in a synthesis of gentiobiose, carried out in the 1870s, as triphenylmethyl ethers. 

The selective protection of hydroxyl groups is obviously most frequent in carbohydrate synthesis and, in fact, photosensitive protecting groups have been used to this effect in oligosaccharide synthesis, nucleotide synthesis, and saccharide modification. Here, as well as in other Sections, special attention will be devoted to 2-nitrobenzyl derivatives, whose re-... 

Thermodynamic control is important in other ways in carbohydrate chemistry, because control over ring size allows selective protection of the hydroxyl groups of sugars. Compare these two reactions. Both of them give acetals from the same starting material, mannitol. 

Selective protection of primary alcohols. Selective protection of primary hydroxyl groups of carbohydrates is possible by reaction of this chlorosilane and polyvinylpyridine in CH2CI, or THF in the presence of HMPT. Amberlite A-26 in the F form is recommended for desilylation. 

As modified oligonucleotides have become a major field of investigation for chemists, methods for the suitable protection/deprotection applied to the synthesis of nucleoside monomers have become equally important. Selective protection of a multifunctional compound is a challenging problem in organic synthesis. In nucleoside chemistry, selective manipulation of the hydroxyl groups on the carbohydrate moiety over amino groups of the bases is synthetically challenging and... 

Imidazole carboxylic esters of secondary or tertiary alcohols, such as 1053 and 1054,, form carbonates exclusively with primary alcohols (Scheme 253). Thus, 1054 is a useful reagent for the BOC-protection of primary hydroxyl groups < 19990L933>. In some cases, 1054 offers better selectivity than pivaloyl chloride in the protection of carbohydrates <1998S1787>. Similarly, l-(methyldithiocarbonyl)imidazole 1055 and its methyl triflate salt convert alcohols to ri -methyldithiocarbonates <1997SL1279> and amines to 6 -methyldithiocarbamates or thioureas <2000T629>. 

Selective Benzylation Selective benzylation of carbohydrate hydroxyl functions by direct one-step protection is difficult to achieve. Therefore, several techniques for the selective protection have been developed over the years and the most common are discussed below. 

Selective protection of fran -diequatorial 1,2-diols in carbohydrate systems has been reported via formation of di-spiroacetals. ... 

This group was developed for C-2 protection of carbohydrates. It selectively directs glycosylation to give primarily the (3-glycoside. This group has the advantage over the pivalate, whichhas a similar directing effect in that it is easily cleaved with catalytic DBU in MeOH. ... 

Carbohydrate synthesis usually involves selective protection of some groups, reaction of the others, then deprotection. The enzymatic aldol reaction can produce some sugars without the need for any protecting groups.251 An example is the preparation of 6-deoxy-L-sorbose (9.39).252... 

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