- Catalytic consequences of borate complexation and pH on the epimerization of l-arabinose to l-ribose in water catalyzed by Sn-Beta zeolite with borate salts
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Sn-Beta zeolite with sodium tetraborate cooperatively catalyzes the epimerization of aldoses via an intramolecular 1,2 carbon-shift mechanism. l-Arabinose is one of the seven common sugars and its epimer, l-ribose, is a valuable rare sugar with applications in antiviral and anticancer agents. Here, a full factorial experimental design is performed to demonstrate the catalytic consequences of varying key reaction parameters such as pH, borate to sugar ratio, and reaction time. Reactivity data revealed that isomerization is favored under acidic pH conditions (pH 7.8). Using a 5 wt% arabinose feed and 100:1 sugar-metal ratios at 343 K, conversions ranging from 20% to 30% were obtained with selectivities of 75%, 84%, and 91% for boron-sugar ratios of 0.2:1, 0.5:1, and 1:1, respectively. The predominance of epimerization over isomerization products with substoichiometric borate suggests that one borate can influence the reactivity of several sugar molecules and may influence the Sn active site directly. Reaction data obtained under differential conditions revealed that the epimerization reaction follows first order kinetics over a wide temperature range with an apparent activation energy of 98 kJ/mol and pre-exponential factor of 1.9 × 1014 L mol Sn-1 s-1.
- Gunther, William R.,Duong, Quynh,Román-Leshkov, Yuriy
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- Aldol Reaction between Small Sugars. Preparation of DL-threo-2-Pentulose and DL-lyxo-3-Hexulose and their Isolation as O-Isopropylidene Derivatives
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The improved diastereoselectivity obtained with strongly basic anion-exchange resin as catalyst in aldol condensation between two-, three- and four-carbon "sugars" has been utilised in the preparation of DL-threo-2-pentulose and DL-lyxo-3-hexulose, which were isolated as their O-isopropylidene derivatives.A possible reason for the observe preference of formation of the lyxo-diastereomer in condensation between glycolaldehyde and glycero-tetrulose is suggested.
- Morgenlie, Svein
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p. 745 - 748
(2007/10/02)
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- Kinetics, Catalysis, and Mechanism of the Secondary Reaction in the Final Phase of the Formose Reaction
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In the final phase of the formose reaction sugars are formed by the reaction of glycolaldehyde, glyceraldehyde and dihydroxyacetone.The application of high-pressure liquid chromatography allows for the first time to investigate intermediate and final products quantitatively.The results of kinetical investigations allow to suggest a reaction mechanism for the secondary reaction in the final phase of the formose reaction.This mechanism is compared with that of the starting phase and other known mechanisms.From the results metal ion-catalyzed aldol reactions have to be assumed.
- Harsch, Guenther,Bauer, Hermann,Voelter, Wolfgang
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p. 623 - 635
(2007/10/02)
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