Transesterification acylation/deacylation

Study of the action of ammonia upon carbohydrate acyl esters originated some eighty years ago and, since then, many aspects of this complex reaction have been investigated. The processes that take place in this reaction include migrations, degradations, transesterifications, and deacylations, and their simultaneous occurrence makes the interpretation of the whole scheme very difficult. The present article provides a general description of the facts and a discussion of the different variables that play a role in the yields of products formed and in the mechanisms involved. 

Enantioselective lipase-catalyzed transesterification involving deacylation of esters of racemic primary or secondary alcohols with primary alcohols, most frequently -butanol, serving as an acyl acceptor, is fairly common. Recent examples include esters of amino alcohols, isoserine, chlorohydrins, and various to-syloxybutanoate esters (eq 8). ... 

In summary, it may be stated that the reaction of acyl esters of aldoses and aldobioses (see Section III, p. 92) with ammonia consists of a set of competitive pathways, including intramolecular O —> N migrations of acyl groups, deacylations, and transesterifications, with formation of aldose amides and variable proportions of the free sugar as the principal products. Significant proportions of basic or insoluble polymeric substances were not observed with aldose acetates or benzoates, although occurrence of extensive browning indicates the probable formation of soluble melanoidins. 

Because a substantial excess of the halide is used in the Koenigs-Knorr synthesis of cardenolides, it is usually difficult, if not impossible, to secure the acylated intermediate in crystalline form. Hence, Reichstein nd coworkers18 introduced a procedure whereby the whole of the products arising from the coupling reaction are saponified, employing a solution of potassium hydrogen carbonate in aqueous methanol. This modified transesterification procedure renders all extraneous carbohydrate materials water-soluble the methanol is removed by evaporation, leaving an aqueous solution from which the (deacylated) cardenolide may be obtained by... 

ChemoselecHve transesterification. A survey of enzymatic regioselective acylation (and deacylation) of carbohydrates has been given. ... 

Protection of Alcohols.—The acylation of alcohols by polymers (48) of 3-acyl-2-oxazolone has been reported using fluoride ion as the catalyst, selective acylation of primary alcohols in the presence of secondary alcohols has been observed. Transesterification, mediated by titanium alkoxides, has been investigated and shown to be a selective, mild, and inexpensive technique for example, for the deacylation of alcohols. ... 

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