Carbohydrates ester formation

Because carbohydrates are so frequently used as substrates in kinetic studies of enzymes and metabolic pathways, we refer the reader to the following topics in Ro-byt s excellent account of chemical reactions used to modify carbohydrates formation of carbohydrate esters, pp. 77-81 sulfonic acid esters, pp. 81-83 ethers [methyl, p. 83 trityl, pp. 83-84 benzyl, pp. 84-85 trialkyl silyl, p. 85] acetals and ketals, pp. 85-92 modifications at C-1 [reduction of aldehydes and ketones, pp. 92-93 reduction of thioacetals, p. 93 oxidation, pp. 93-94 chain elongation, pp. 94-98 chain length reduction, pp. 98-99 substitution at the reducing carbon atom, pp. 99-103 formation of gycosides, pp. 103-105 formation of glycosidic linkages between monosaccharide residues, 105-108] modifications at C-2, pp. 108-113 modifications at C-3, pp. 113-120 modifications at C-4, pp. 121-124 modifications at C-5, pp. 125-128 modifications at C-6 in hexopy-ranoses, pp. 128-134. 

The chemistry of enolates has provided excellent routes to highly complex structures, in particular in the total synthesis of natural products. Because of the highly oxygenated structures of carbohydrates, enolate formation could easily result in p-elimination of a suitably located oxygenated group (ethers, esters, and such) to provide enone. For these reasons, the chemistry of carbohydrate enolates has been poorly documented. 

Although the orthocarboxylic acids, R—C(OH)3, are unstable, many of their derivatives are known, stable compounds. The known carbohydrate orthoesters are all derived from the structure that results when two of the three hydroxyl groups of an orthoacid are involved in ester formation with two hydroxyl groups of the sugar molecule. General formula A represents those types which are of particular interest in the field of carbohydrates. The two adjacent carbon atoms of the five-membered ring constitute carbon atoms number 1 and 2 of an aldose and either 1 and 2, or 2 and 3 of a ketose. In certain instances it has been... 

Hydrolysis is one of the most important reactions of fats and oils, just as it was for carbohydrates. The treatment of fats or oils with water and an acid catalyst causes them to hydrolyze to form glycerol and fatty acids. This reaction is simply the reverse of ester formation ... 

The chemistry of carbohydrates has sometimes been referred to as a marriage between the chemistry of alcohols and that of aldehydes and ketones. In the preceding sections, we examined oxidations and reductions of carbohydrates, cyanohydrin formation, Fischer glycosidation, and processes that involve enolization. All of these reactions involve carbonyl groups in carbohydrates. In this section, we will consider the reactions of the hydroxyl groups of carbohydrates, starting with their conversion to esters. 

Under mildly alkaline conditions and in the presence of excess cyclo-heptaamylose the rate of degradation of penicillin is increased 2(U90-fold compared with the rate of alkaline hydrolysis (Tutt and Schwartz, 1971). Michaelis-Menten kinetics are observed which are indicative of complex formation. The apparent binding constant of 6-substituted penicillins varies little with the length of the alkyl side chain although it is increased about 10-fold for diphenylmethyl penicillin. The reaction is catalytic and hydrolysis proceeds by the formation of a penicilloyl- 3-cyclodextrin covalent intermediate, i.e. ester formation, by nucleophilic attack of a carbohydrate hydroxyl on the P-lactam. 

Polysaccharides are capable of ester formation with either organic or inorganic acids. Reaction may, in general, be effected by any of the well-known esterification procedures after their adaptation and modification to fit the possible special requirements of carbohydrate macromolecules. The esters produced are derivatives of high polymers and as such portray typical macromolecular properties in addition to natural ester behavior. Esterification with organic acids usually alters the properties from hydrophilic to hydrophobic, and, hence, alters the solubility so that the ester is no longer soluble in water but is soluble in organic liquids. 

These reactions are catalyzed by kinases, some of viiich have already been discussed in this chapter. The reaction may be virtually irreversible as in phosphate ester formation. Here the phosphate acceptor may be a hydroi l group of a carbohydrate (glucose, ycerol, fructose, nucleotides, etc.), (Moline, or pantetheine. The terminal phosphate may also be transferred to an acceptor without loss of high chemical potential, such as to nucleoside mono- or diphosphate or to a nitro n atom. These reactions are freely reversible. Nucleotide diphosphate may also donate its terminal phosphate as in the nucleotide monophosphate kinase reaction. 

The application of lipases to the preparation of sugar esters has been reviewed, and a short section with 18 refs, on the formation and hydrolysis of carbohydrate esters is included in a review on the use of esterolytic and lipolytic enzymes in organic synthesis. ... 

NMR spectroscopy has been used to determine the thermodynamics of borate ester formation by three readily grafted carbohydrates. Boron specia-tion in borate-poly(amido amine) dendrimer has been performed using B NMR spectroscopy. The role of inositol in the synthesis of FAU- and LTA-type zeolites has been investigated using H, C, Na, and Al NMR spectroscopy. Ulvan-boron complex formation has been studied using B and NMR spectroscopy. The complexation of borate ions and humic acid fractions has been analysed with B and H NMR spectroscopy. ... 

As a catalyst for ester and amide formation from acyl chlorides or anhydrides, 4-(di-methylamino)pyridine has been recommended (DMAP G. Hdfle, 1978). In the presence of this agent highly hindered hydroxyl groups, e.g. of steroids and carbohydrates, are acylated under mild conditions, which is difficult to achieve with other catalysts. 

Metabolic Functions. The formation of phosphate esters is the essential initial process in carbohydrate metaboHsm (see Carbohydrates). The glycolytic, ie, anaerobic or Embden-Meyerhof pathway comprises a series of nine such esters. The phosphogluconate pathway, starting with glucose, comprises a succession of 12 phosphate esters. 

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