Carbohydrates triphenylmethylation

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. 

In carbohydrate chemistry, the preparation of ethers that are stable in the presence of acids, bases, and aqueous alkali is an important analytical and synthetic tool. The methods used for the etherification of hydroxyl groups51 generally employ reactions of unprotected sugars and glycosides with methyl, allyl, benzyl, triphenylmethyl, and alkylsilyl halides in the presence of a variety of aqueous and nonaqueous bases. 

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... 

Methods now available for the partial protection of carbohydrate hydroxyl groups by alkylation are critically evaluated. Of the numerous alkyl protecting groups known, only those widely used in synthetic chemistry of saccharides are discussed. Benzyl, allyl, triphenylmethyl and other arylalkyl ethers, trimethylsilyl, rer/-butyidimethylsilyi, and rm-butyldiphenylsilyl ethers are the main topics of the article. 

Williams and Blann developed a route to cyclobutanols starting from carbohydrate-derived precursors [59]. The ketyl intermediates formed on treatment of precursors 35 with SmU in THF/HMPA underwent 4-exo-ketyl-olefin cyclisations to afford cyclobutanols 36 as isomer pairs (major isomer shown). Good yields were obtained for R = t-butyldimethylsilyl and trityl (triphenylmethyl), but for R = pivaloyl the reaction was diverted into an elimination pathway. 

The triphenylmethyl ethers of the carbohydrates constitute a group of derivatives of particular interest because of the special properties inherent in the trityl group. Before entering into a discussion of these carbohydrate derivatives, a few words regarding the preparation, properties and special behavior of trityl ethers would appear to be in order. 

For an alternate route to this compound see Chapleur, Y, Germain, F, Aubry, A, Bayeul, D, Elimination reactions of 2-C-methyl-2-deoxy sugars application to the synthesis of methyl 2,3, 4-trideoxy-2,4-di-C-methyl-6-0-triphenylmethyl-D-/yxo-/iexo-pyranojiiie and its X-ray crystal structure, 7. Carbohydr. Chem., 3, 443-459, 1984. 

As with many carbohydrates, the primary alcohol group at C-6 undergoes ready triphenylmethylation (tritylation) with chlorotriphe-nylmethane and pyridine (Figure 4.8). 

Other Alkyl and Aryl Ethers. - The preparation of selectively 0-alkylated saccharides, including benzyl, allyl, triphenylmethyl, and arylalkyl ethers has been reviewed. Analogues of lipids A and X incorporating 3-O-alkyl (rather than acyl) linkages have been described, and some carbohydrate monoalcohol derivatives have been alkylated with haloacetic acid ester derivatives. ° 3-C -Carboxymethyl D-glucose and D-fhictose compounds have been coupled via the carboxylic acid group to L-lysine in a search for acrosin inhibitors."... 

The triphenylmethyl (trityl) group is removed under even milder conditions, and is an important hydroxyl-protecting group, especially in carbohydrate chemistry. This group is introduced by reaction of the alcohol with triphenylmethyl chloride via an SnI substitution. Hot aqueous acetic acid suffices to remove the trityl group. The ease of removal can be increased by addition of electron-releasing substituents. The p-methoxy derivatives have been employed for this purpose. ... 

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