Common Protecting Groups in Carbohydrate Synthesis

Hereafter the anomeric center is protected. When the ceramide building block has to be connected to the oligosaccharide, normally selectively deprotection of the C-la position of the lactose is necessary. The thexyldimethylsilyl (TDS) group is a less common trialkylsilyl protecting group in carbohydrate synthesis. Advantageously it survives some rather harsh conditions. The liquid TDSC1 42 with imidazole 43 or DMAP as basic activator in a dipolar aprotic solvent like DMF forms the C-l protected compound 8 as a mixture of anomers.21... 

The radical approach for C-C bond formation is a popular method within organic chemistry. The use of radical chemistry in carbohydrate synthesis has certain advantages. Firstly the reaction conditions are very mild and tolerant of a range of functional and protecting groups. Anomeric radicals are also stable towards elimination and epimerisation. Most significantly, the chemistry required to incorporate an appropriate substituent at C-1, employed in the initial homolytic cleavage step, is common within carbohydrate chemistry. The use of such radical techniques can be subdivided into two classes, intermolecular and intramolecular reactions. 

The base-catalyzed reaction of nitroalkanes and sugar-based aldehydes (the Henry reaction) is one of the most common procedures for the lengthening of the carbon skeleton of a carbohydrate.16 The mild reaction conditions required for the formation of C C bonds by this method are usually compatible with most of the protective groups and masked functionalities involved in multistep synthesis from sugars.17... 

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. 

Glycopeptides are more difficult to synthesize than the conventional peptides, because common protection-deprotection reactions used in the peptide synthesis can cause serious problems to the protective groups used for carbohydrates. Many of these problems have been solved by recent technical innovations [21]. The combination of 9-fluorenylmethyloxycarbonyl (Fmoc) group for N-protection and pentafluo-rophenyl (PFP) group as the activating group for the carboxylic acid allows GlcNAc-Asn... 

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