Epimerization of sugars

Epimerization of sugar, mechanisms 778 Epimers, definition of 163 Epinephrine (adrenaline) 542,553, 553s Episomes. See plasmid Epithelial cells 29 Epitheliocytes 25 Epoxides, alkyation by 254 Epoxide hydrolases 591 EPR (electron paramagnetic resonance) spectroscopy 398, 399 of glutamate mutase 873 in study of phosphotransferases 639 EPSP (enolpyruvoylshikimate-3-phosphate) 687s... 

Thymidine D-galacturonic acid sugar beet epimerization of sugar beet ... 

Closely related to this reaction is the base-catalyzed epimerization of aldonic acids and their lactones (see Lundt and Madsen, this voL). This reaction is, of course, even older than the LdB-AvE process, and was first used by Emil Fischer [29]. Potassium hydroxide and tertiary amines (pyridine, quinoline) have been used as bases. The reaction is much slower than the epimerization of sugars and requires prolonged heating, but the aldonic acids are much more stable to the action of bases than the sugars, and no side reactions occur. The reaction proceeds even if the hydroxyl group at C-2 is methylated [30]. The kinetics and the mechanism of the interconversion of the aldopentonic acids in potassium hydroxyde solution have been studied this reaction also occurs via the enediol and the removal of H-2 is the rate-determining step [31]. It is of industrial importance, as indicated by numerous patents [32]. 

Extensive studies have indicated that only pyruvate is acceptable as the NeuA donor substrate, with the exception of fluoropyruvate [49], but that the enzyme displays a fairly broad tolerance for stereochemically related aldehyde substrates as acceptor alternatives, such as a number of sugars and their derivatives larger or equal to pentoses [36,48,50,51]. Permissible variations include replacement of the natural D-manno configured substrate (4) with derivatives containing modifications such as epimerization, substitution, or deletion at positions C-2, -4, or -6 [16,27]. Epimeriza-tion at C-2, however, is restricted to small polar substituents owing to strongly... 

Intramolecular nitroaldol reactions are a useful choice for the conversion of sugars into polyhydroxylated nitro cyclopentanes, nitro cyclohexanes and their derivatives.46 Baer et al. in the course of their studies on the cyclization of 6-deoxy-6-nitrohexoses under kinetic and thermodynamic control,47 established the reaction pathway involved in the formation of nitroinositols mediated by intramolecular Henry reactions. Firstly, a nitronate is formed and then, under thermodynamic control conditions, an epimerization occurs before cyclization. But, under kinetic controlled conditions, the cyclization occurs first.48... 

Note that harsher conditions may lead to further changes, e.g. epimerization at C-3 in fmctose, plus isomerization, or even reverse aldol reactions (see Section 10.3). In general, basic conditions must be employed with care if isomerizations are to be avoided. To preserve stereochemistry, it is usual to ensure that free carbonyl groups are converted to acetals or ketals (glycosides, see Section 12.4) before basic reagents are used. Isomerization of sugars via enediol intermediates features prominently in the glycolytic pathway of intermediary metabolism (see Box 10.1). 

Epimerization. In weakly alkaline solutions, glucose is in equilibrium with the ketohexose D-fructose and the aldohexose D-mannose, via an enediol intermediate (not shown). The only difference between glucose and mannose is the configuration at C-2. Pairs of sugars of this type are referred to as epi-mers, and their interconversion is called epimerization. 

This enzyme [EC 5.1.3.3], also known as mutarotase, catalyzes the epimerization of the hemiacetal carbon atom of aldoses (thus, anomerization). Hence, a-D-glu-cose is reversibly converted to /3-D-glucose. Other sugars can act as substrates (e.g., L-arabinose, D-xylose, D-galac-tose, maltose, and lactose). 

Removal of the sugar portion allows epimerization of the 3P-OH group, with a decrease in activity and an increase in toxicity due to changes in polarity. 

The reduction of ketopentose-, as well as ketohexose-, nucleosides with metal hydrides has been used to obtain biologically important nucleosides and rare sugar nucleosides, by epimerization of a chiral center. Moreover, reduction of unsaturated ketonucleosides with borohydride provides new and direct routes to unsaturated and deoxy-nucleosides. 

L-lduronic acid synthesis Synthesis of L-iduronic acid residues occurs after D-glucuronic acid has been incorporated into the carbohydrate chain. Uronosyl 5-epimerase causes epimerization of the D- to the L-sugar. 

Hydride-Transfer Reactions. The hydride-transfer mechanism for rearrangement of sugars (20) yields a ketose anion (5) by direct transfer of the C-2 hydrogen atom with its electrons to C-l. Reversal of the process leads to epimerization at C-2 (Scheme II). 

Pseudo-cc-DL-allopyranose (61) has been prepared from 54 by epimerization of the C-3 configuration as follows. O-Isopropylidenation of 54 with 2,2-dimethoxypropane gave l,2 4,6-di-0-isopropylidene-pseudo-a-DL-glucopyranose (56). On oxidation with ruthenium tetroxide and sodium metaperiodate, 56 gave the 3-oxo derivative (57), which was converted into l,2 4,6-di-0-isopropylidene-pseudo-a-DL-allopyranose (58) exclusively by catalytic hydrogenation under the presence of Raney nickel. Conven-. tional acetylation of 58 furnished the 3-O-acetyl derivative (59). Hydrolysis of 59 with aqueous acetic acid, followed by acetylation afforded pseudo-a-DL-allopyranose pentaacetate (60), which gave the free pseudo-sugar 61 on usual alkaline hydrolysis [22] (Scheme 13). 

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