Protection of the carbohydrate hydroxyl groups

Interestingly, glycopeptides containing the 6-deoxysugar L-fucose, which undergoes acid-catalysed hydrolysis only S-6 times faster than the 

---

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. 

Benzyl and triphenylmethyl ethers of carbohydrates are preferred over methyl ethers when selective removal of protecting groups is important. The relatively high nucleophilic activity of the 5-hydroxyl group in glycosides and 1,2-O-alkylidene derivatives of 4 permits its benzylation and triphenylmethylation under mild conditions. Thus, treatment of 33 (Ref. 34) and 36 (Ref. 57) with benzyl bromide and... 

Besides acetyl, benzoyl, and benzyl protecting groups for the carbohydrate hydroxyl groups, silyl, isopropylidene, and p-methoxybcnzyl groups have also been employed in solid-phase glycopeptide synthesis. The synthesis of a glycopeptide... 

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 chemical transformations of dialkyl dithioacetals have been reviewed in detail [47] and offer routes to a variety of useful carbohydrate derivatives. Dialkyl dithioacetal derivatives of sugars continue to play an important role in modem synthetic carbohydrate chemistry through reactions of die dithioacetal function and manipulation of the sugar hydroxyl groups. Dithioacetals also provide a convenient method for temporary protection of sugar carbonyl groups in the synthesis of noncarbohydrate natural products. 

In carbohydrate chemistry, condensatiMi with carbonyl compounds such as acetone, acetaldehyde, formaldehyde and benzaldehyde is of great importance for the protection of pairs of hydroxyl groups. Trityla-tion may also be used in these instances for the protection of individual primary hydroxyl groups before carrying out the condensation with carbonyl compounds. After the condensation, the individual hydroxyl groups may be liberated by suitable detritylation methods. As yet, these possibilities have been explored very little. Wolfrom and coworkers described the condensation of 1,6-ditrityldulcitol with benzaldehyde to l,6-ditrityl-2,3,4,5-dibenzylidenedulcitol. Acetals other than those obtained by direct condensation are thus available. 

---