- Direct, mild, and selective synthesis of unprotected dialdo-glycosides
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A direct and highly convenient organocatalytic method for the preparation of 1,5-dialdo-pyranosides and 1,4-dialdo-furanosides is presented. The method relies on the chemoselective properties of TEMPO in combination with trichloroisocyanuric acid under very mild, basic conditions. Unprotected glycosides are prepared in a single step in high yields and are efficiently purified with the use of solid-phase imine capture. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.
- Angelin, Marcus,Hermansson, Magnus,Dong, Hai,Ramstrom, Olof
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- Process for oxidizing primary alcohols
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Primary hydroxyl groups in a substrate having both primary and secondary hydroxyl groups can be selectively oxidized to carbaldehyde and/or carboxyl groups by contacting the substrate with a cyclic nitroxyl compound in the presence of a peroxosulfate as a co-oxidant and by carrying out the reaction at a temperature below 30° C. and at a pH below 9. The process is halogen-free and metal-free and is especially suitable for oxidizing polysaccharides.
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- Process of oxidizing primary alcohols
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In a new process for oxidizing a primary and/or secondary alcohol, an oxidizing agent is used in the presence of a di-tertiary-alkyl nitroxyl, in an aqueous reaction medium at a pH of below 7. The di-tertiary-alkyl nitroxyl is especially 4-hydroxy-TEMPO, and the process is particularly advantageous for oxidizing carbohydrates such as starch.
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- Relative reactivities of glucopyranosides towards permanganate and chromic acid in perchloric acid medium
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The oxidative behaviour and relative reactivity of methyl-α-D-glucopyranoside and methyl-β-D-glucopyranoside towards permanganate and acid chromate have been studied in perchloric acid medium. The reactions are first order with respect to [glucopyranoside] and [oxidant], The reaction rates increase with the increase in [H+]. Activation parameters of the reactions have been evaluated. The mechanism of the reactions has been discussed.
- Tribedi, Partha Sarathi
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p. 287 - 289
(2007/10/03)
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- Bromide-free TEMPO-mediated oxidation of primary alcohol groups in starch and methyl α-D-glucopyranoside
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TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation of potato starch and methyl α-D-glucopyranoside (MGP) was performed in the absence of sodium bromide (NaBr) as co-catalyst, solely using sodium hypochlorite (NaOCl) as the primary oxidant. The low reaction rate associated with a bromide-free process was increased by performing the oxidation at increased temperatures. The reaction proceeded stoichiometrically and with high selectivity and with only minor depolymerisation, provided that temperature and pH were kept ≤20°C and a corresponding oxidation catalysed by NaBr at 2°C. Consequently, this is a simple approach to raise the TEMPO/NaOCl reaction rate under bromide-free conditions while still maintaining good product properties. At higher oxidation temperatures (≥25°C) and under more alkaline conditions (pH ≥ 9.0) degradation of the starch skeleton occurred. Simultaneously, side-reactions of the nitrosonium ion lowered the yield of the oxidation. Despite the absence of the NaBr catalyst, the reaction rate-controlling step was found to be the oxidation of the primary hydroxyl groups with the nitrosonium ion. The reaction was first- order in MGP and in TEMPO. (C) 2000 Elsevier Science Ltd.
- Bragd, Petter L.,Besemer, Arie C.,Van Bekkum, Herman
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p. 355 - 363
(2007/10/03)
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- Oxidation of methyl and n-octyl α-D-glucopyranoside over graphite-supported platinum catalysts: Effect of the alkyl substituent on activity and selectivity
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The oxidation of methyl and n-octyl α-D-glucopyranoside to methyl and n-octyl α-D-glucopyranosiduronate with molecular oxygen over a graphite-supported platinum catalyst was investigated. An increase of the length of the n-alkyl substituent from methyl to n-octyl resulted in a ten-fold decrease of the catalyst activity and an increase of the selectivity at pH 8.0 and 323 K. The selectivity decreased with increasing pH. The lower activity for a longer n-alkyl substituent is attributed to steric effects upon adsorption on the platinum surface and not to internal diffusion limitations. A tentative reaction scheme is presented, which describes the formation of side products through oxidation of secondary hydroxyl groups, ring cleavage and hydrolysis. Major side products are mono- and di-carboxylates with 2, 4, and 6 carbon atoms and mono-carboxylates, resulting from the oxidation of the alkyl substituent. C-C-Bond cleavage mainly occurs between C-2 and C-3 or C-4 and C-5, the former being less important for a longer alkyl substituent. The higher selectivity for a longer alkyl substituent is attributed to its protecting ability against hydrolysis and the exposition of neighboring hydroxyl groups to the platinum surface.
- Vleeming, Johannes H.,Kuster, Ben F.M.,Marin, Guy B.
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p. 175 - 183
(2007/10/03)
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