Esters, carbohydrate, reaction with

The alcohol groups of carbohydrates undergo chemical reactions typical of hydroxyl functions They are converted to esters by reaction with acyl chlorides and carboxylic acid anhydrides... 

ESTERIFICATION Like all free OH groups, those of carbohydrates can be converted to esters by reactions with acids. Esterification often dramatically changes a sugar s chemical and physical properties. Phosphate and sulfate esters of carbohydrate molecules are among the most common ones found in nature. 

For SrnI C-alkylation of the conjugate anions of 2-nitropropionate esters, on reaction with p-nitrobenzyl chloride, only moderate asymmetric induction is induced when the ester is derived from a chiral carbohydrate the 8-phenyl menthol moiety is more effective. 

Cellobiose was prepared first by Skraup and Konig by the saponification of the octaacetate with alcoholic potassium hydroxide, and the method was improved by Pringsheim and Merkatz.3 Aqueous barium hydroxide also has been employed for the purpose, and methyl alcoholic ammonia has been used extensively for the hydrolysis of carbohydrate acetates. The method of catalytic hydrolysis with a small quantity of sodium methylate was introduced by Zemplen,i who considered the action to be due to the addition of the reagent to the ester-carbonyl groups of the sugar acetate and the decomposition of the addition compound by reaction with alcohol. The present procedure, reported by Zemplen, Gerecs, and Hadacsy, is a considerable improvement over the original method (see Note 2). 

The importance of reactions with complex, metal hydrides in carbohydrate chemistry is well documented by a vast number of publications that deal mainly with reduction of carbonyl groups, N- and O-acyl functions, lactones, azides, and epoxides, as well as with reactions of sulfonic esters. With rare exceptions, lithium aluminum hydride and lithium, sodium, or potassium borohydride are the... 

Carbohydrate lactones have been used as the carbonyl reagent in the Reformatsky reaction. Thus, 2,3 5,6-di-O-cyclohexylidene-D-mannono-1,4-lactone [44, obtained by oxidation of the mannofuranose derivative (49)] reacted with ethyl bromoacetate and zinc to give the protected 2-deoxy-3-octulosonic acid ethyl ester (45a) in 69% yield (50). Ketonic hydrolysis with potassium hydroxide in aqueous methanol, followed by acidification and heating, afforded the 1-deoxyheptulose derivative 45b. Similarly, starting from compound 44, the 1-C-substituted allyl and propar-gyl lactols were prepared on reaction with allyl or propaigyl bromides in the presence of zinc (51). 

Until the introduction of triflate or imidazylate esters in carbohydrates, SN2-displacements of carbohydrate sulfonates with charged nucleophiles in certain positions of hexopyranose orfuranose derivatives were not possible, or gave low yields of substituted products owing to the predominance of elimination or rearrangement reactions [6,22,32,33]. 

Reformatsky reaction.1 This form of zinc is highly effective for Reformatsky reactions with both a-bromo- and a-chloro esters and with both cyclic and acyclic ketones. Reactions proceed in THF at -78 to 0° in 10-30 minutes. It even effects diastereospecific Reformatsky reactions with chiral carbohydrate ketones. 

The value of the sulphonate esters in carbohydrate chemistry lies in their ability to undergo cleavage by reaction with nucleophiles in one of two possible ways, (i) cleavage of the S—O bond with regeneration of the hydroxyl group as a result of SN2 attack at the sulphur atom, and (ii) cleavage of the C—O bond as a result of SN2 attack at the carbon atom. 

---