Carbohydrate coupling

Silver(I)-loaded zeolites and silica-alumina can be used in carbohydrate coupling. The Ag(I) activates the 1-Br-substituent in this nucleophilic substitution. Because of the size of the reactants only the outer surface of the zeolites is active. 

Carbohydrate-coupling or glycosylation, is a major synthesis method in carbohydrate preparation. Silver silicates and Ag(I)-exchanged zeolite A - so-called insoluble Ag(I) - have been advocated as promoting agents, applied in more than stoichiometric amount (Fig. 9). All hydroxyl groups except the attacking one are suitably protected. 

The basic form of ketosis occurs in starvation and involves depletion of available carbohydrate coupled with mobihzation of free fatty acids. This general pattern of metabohsm is exaggerated to produce the pathologic states found in diabetes meUitus, twin lamb disease, and ketosis in lactating catde. Nonpathologic forms of ketosis are found under conditions of high-fat... 

LAB are non-respiring microorganisms, principally generating ATP by fermentation of carbohydrates coupled to substrate-level phosphorylation. The two major pathways for the metabolism of hexoses are homofermentative or glycolysis (Embden-Meyerhof pathway), in which lactic acid is virtually the only end-product, and heterofermentative (phosphoketolase pathway), in which other end-products such as acetic acid, C02, and ethanol are produced in addition to lactic acid (Axelsson et al., 1989 Kandler, 1983 Zourari et al., 1992). 

N. M. Spijker, C. A. A. van Boeckel, Double stereodifferentiation in carbohydrate coupling reactions The mismatched interaction of donor and acceptor as an unprecedented factor governing a/fi ratio of glycoside formation, Angew. Chem. Int. Ed., 30 (1991) 180-183. 

The most important by-product of the analytic and synthetic work accomplished so far is knowledge about the stereochemistry and reactivity of natural compounds. There is an enormous potential for the chemists of the twenty-first century lying in the mastery and application of this knowledge in order to produce organized and finally functional materials. Typical contemporary examples include surface monolayers on metals and colloids made of fatty acid and steroid derivatives, the regio- and stereoselective assembly reactions between steroids and carbohydrates, coupled redox chains of metalloporphyrins and vitamins, noncovalent fibers made of amino acids, nucleotides, and saccharides, and the functionalization of proteins by incorporation of reactive molecules. The field of supramolecular or noncovalent natural compound chemistry has been scientifically fruitful for several decades and is presently exploited for the development of useful molecular devices and machines as well as for medical applications. 

Cid MB, Alonso I, Alfonso F, Bonilla IB, Lopes-Prados J, Martin-Lomas M (2006) Simultaneous regio- and enantio-differentiation in carbohydrate coupling. Eur I Org Chem 2006 3947-3959... 

The concept of torsional deactivation was expanded further by Ley and coworkers in their exploration of 1,2-diacetal systems. The acetal and ketal groups, when fused to carbohydrate coupling donors, impart rigidity into the structure, making more difficult the conformational changes required for glycosylation, hence the reactivity of the sugar is diminished. 

The most important CO2 activation process is photosynthesis, in which solar photons drive a reaction that would otherwise be uphill thermodynamically the reduction of CO2 to carbohydrates coupled to water oxidation to O2. Many metalloenzymes are involved in these processes, the one that fixes CO2 is ribulose diphosphate carboxylase, in which an enolate anion of the sugar nucleophilically attacks the CO2 carbon. Cu(II), Mn(II), and are all present in the active enzyme, and one of these probably plays a role in polarizing the CO2, perhaps via an tj -OCO complex. 

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