Glycoside exchange

Scrutiny of Scheme 10 suggests that it should be possible to capture the intermediate IV with an alcohol. Such a reaction would lead to VIII, and the overall process I —> VIII would be an acetal exchange-or in the case of sugars, a glycoside exchange. This has proved to be possible. 

The potential of this procedure for the syntheses of oligosaccharides is obvious. However, various questions must be answered (some dealing with mechanistic details, stereocontrol of the glycoside exchange, etc.) before the merits of the reaction can be appreciated. 

Dowex 2-X8 1.2 0.75 Strongly basic (but less basic than Dowex 1 type) anion exchanger with S-DVB matrix for deionization of carbohydrates and separation of sugars, sugar alcohols, and glycosides. 

Inhibition of the Na+/K+-ATPase leads to a loss of potassium and an increase of sodium within the cell. Secondary intracellular calcium is increased via the Na VCa -exchanger. This results in a positive inotropic effect in the myocardium, with an increase of peak force and a decrease in time to peak tension. Besides this, cardiac glycosides increase vagal activity by effects on the central vagal nuclei, the nodose ganglion and increase in sensitivity of the sinus node to acetylcholine. 

A recently developed application of the Ramberg-Backlund reaction is the synthesis of C-glycosides. The required thioethers can be prepared easily by exchange with a thiol. The application of the Ramberg-Backlund conditions then leads to an exocyclic vinyl ether that can be reduced to the C-nucleoside.95 Entries 3 and 4 in Scheme 10.6 are examples. The vinyl ether group can also be transformed in other ways. In the synthesis of partial structures of the antibiotic altromycin, the vinyl ether product was subjected to diastereoselective hydroboration. 

VII. Foemation of Sucrose and Other Disaccharides through Exchange of Glycosidic Linkages... 

Sucrose phosphorylase can then be regarded as a rather versatile transglucosidase, capable of exchanging glycosidic and ester bonds and of donating D-glucose to a variety of substrates such as ketoses, an aldose, inorganic phosphate and arsenate. 

It is of interest that similar enzymes, capable of exchanging glycosidic linkages, appear to be involved in the synthesis of some polysaccharides. Thus the polysaccharide dextran is formed from sucrose by enzyme preparations from Leuconostoc46 while the polysaccharide levan is produced from sucrose or raffinose by enzymes of other bacteria.49 The reactions may be written as follows ... 

Reformatsky product (46). Treatment of 46 with methanol and an acid ion-exchange resin afforded the methyl glycoside 47a, the product of partial hydrolysis (47b), and the 3,8-anhydro sugar 48. Synthetic transformations were performed on these products (52). 

When passage of a sugar solution at 90°C through a weak-base anion exchange resin was interrupted, an explosion occurred. This was attributed to an exothermic Maillard reaction (interaction of an amino acid with a glycosidic OH group) under the poor heat transfer conditions in a particulate bed without fluid flow. 

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