Partially Alkylated Carbohydrates

In carbohydrate trityl ethers and the partially acylated carbohydrate derivatives obtained from them, the free hydroxyl groups may be etheri-fied with other alcohols. This method has been used repeatedly, particularly for the preparation of partially methylated compounds. With the alkali resistant trityl ethers, any alkaline alkylation method may be used, for example, dimethyl sulfate and alkali, or methyl iodide and silver oxide. In choosing an alkylation method for partially acylated compounds, one must be selected which does not cause acyl migration or saponification. 

In this manner 6-methyl-D-glucopyranose (XXIX) and some of its derivatives were synthesized from n-glucose derivatives containing a free hydroxyl group only in the 6-position. The 6-methyl ethers of D-mannose and of methyl a-n-mannoside were obtained similarly. Haworth and coworkers synthesized 2,3,4-trimethyl-n-glucose from 6-trityl-D-glucose and used the ether for the elucidation of structural problems. 

The 6-trityl ether of methyl 3-tosyl-D-glucopyranoside was employed by Goebel and coworkers for the unequivocal synthesis of methyl 2,4-dimethyl- 9-D-glucopyranoside. This synthesis was an important link in elucidating the structure of the capsular polysaccharide of Type III pneumococcus. 

For the structural clarification of euxanthic acid, Robertson and Waters prepared methyl 2,3,4-trimethyl-a-D-glucopyranoside from the 6-trityl ether. 

Many partially alkylated carbohydrates may be obtained in an analogous manner by using trityl ethers and the partially acylated carbohydrates obtainable from them. 

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The phase-transfer technique of partial alkylation introduced by Garegg in the carbohydrate-field (see Sect. 2.2) can be used also for the preparation of partially allylated derivatives. It was found that the reaction of benzyl 4-0-benzyl-a-L-rhamnopyrano-... 

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. 

Of the numerous alkyl protecting groups known [8, 9], only those widely used in synthetic chemistry of carbohydrates will be discussed. Methyl ethers — with hundreds of selective methylations far exceeding the scope of the review — were excluded for their unimportant role as synthetic intermediates (great majority of partial methylations are carried out to complete the list of reference compounds for the methylation analysis of polysaccharides, or simply to prepare ethers widely occurring in biologically important molecules). 

Sashiwa, H. and Shigemasa, Y. 1999. Chemical modification of chitin and chitosan. 2 Preparation and water soluble property of N-acylated or N-alkylated partially deacetylated chitins. Carbohydr. Polym. 39 127-138. 

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