Acylation and Alkylation of Hydroxyl Groups in 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. 

The choice of conditions for alkylation (or any protection method) is dictated by the existing functionality within the carbohydrate. Where there are alternatives within a category, the specific choice is usually made on the basis of previous experience or if the method offers a desired level of selectivity for protection of a particular site. For example, the alkylation conditions within category (a) above [with the exception of (vi)] and that under (c) lead to more or less equivalent levels of selectivity (summarised in section 2.2, Hydroxyl group reactivity) and will not be discussed further. However alkylations and acylations proceeding via 0-stannyl intermediates (b) and via copper chelates [(a)(vi)] follow set patterns that may differ from these general trends. These alternatives are discussed within sections 23.1, Benzyl ethers and 2.3.2, Allyl ethers. 

Other reactions of carbohydrates include those of alcohols, carboxylic acids, and their derivatives. Alkylation of carbohydrate hydroxyl groups leads to ethers. Acylation of their hydroxyl groups produces esters. Alkylation and acylation reactions are sometimes used to protect carbohydrate hydroxyl groups from reaction while a transformation occurs elsewhere. Hydrolysis reactions are involved in converting ester and lactone derivatives of carbohydrates back to their polyhydroxy form. Enolization of aldehydes and ketones leads to epimerization and interconversion of aldoses and ketoses. Addition reactions of aldehydes and ketones are useful, too, such as the addition of ammonia derivatives in osazone formation, and of cyanide in the Kiliani-Fischer synthesis. Hydrolysis of nitriles from the Kiliani-Fischer synthesis leads to carboxylic acids. 

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