Aldose transformations

Aldoses, for example mannose, fucose, arabinose, and others, have been detected at the subpicomole level by their reduction to aldites, which are then transformed into pemaphthoates by means of naphthoylimidazole. These pemaphthoates show strong fluorescence at 374 nm, and can be easily separated by HPLC.[46]... 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

P9. Postemak, T., and Rosselet, J. P., Action de la phosphoglucomutase du muscle sur des acides aldose-l-phosphoriques. Transformation de l acide galactose-l-phos-phorique. Helv. Chim. Acta 37, 246-250 (1954). 

Aldoses generally undergo benzilic acid-type rearrangements to produce saccharinic acids, as well as reverse aldol (retro-aldol) reactions with j3-elimination, to afford a-dicarbonyl compounds. The products of these reactions are in considerable evidence at elevated temperatures. The conversions of ketoses and alduronic acids, however, are also of definite interest and will be emphasized as well. Furthermore, aldoses undergo anomerization and aldose-ketose isomerization (the Lobry de Bruyn-Alberda van Ekenstein transformation ) in aqueous base. However, both of these isomerizations are more appropriately studied at room temperature, and will be considered only in the context of other mechanisms. 

Hydrolysis of the diacetamides is effected by acids. Dilute hydrochloric, sulfuric and nitric acid have been used. An aldose with one carbon atom less than the original nitrile is then liberated from the combination. Only exceptionally can this aldose be isolated in pure condition without transforming it into an insoluble derivative in these instances, the method has been employed for preparative work. In most cases the sugar has been characterized as an osazone. The aldose, without great purification, has been employed successfully for reduction - or oxidation experiments. ... 

Using the corresponding (6R) isomer (from D-valine and DL-alanine), alkylation with 2-chloro-3-methyl quinoxaline followed by further transformation has yielded an aldose reductase inhibitor in optically pure form (90T7745). 

Transketolase catalyzes the reversible transfer of a hydroxyacetyl fragment from a ketose to an aldehyde. Because the ketose products formed by transketolase reactions are not acceptors for a consecutive transformation by the same enzyme, we have investigated the option to include a xylose (glucose) isomerase (Xyll E.C. 5.3.1.5), which has similar stereochemical specificity, for ketose to aldose equilibration (Scheme 2.2.5.13). Starting from racemic lactaldehyde 32a, the transketolase forms 5-deoxy-D-xylulose 35a, which indeed was accepted by the Xyll in situ for diastereospecific conversion into 5-deoxy-D-xylose 36a. The latter again proved to be a substrate of transketolase which completed a tandem operation to furnish 7-deoxy-sedoheptulose 37a as the sole bisadduct in 24% overall yield and in enantio- and diastereomerically pure quality [35, 36]. All four stereocenters of the resulting product are completely controlled by the enzymes during this one-pot operation. The procedure profits from the limited tolerance of the isomerase... 

For a long time, this reaction was conducted exclusively with acety-lated nitriles of aldonic acids, and the products obtained were known in general as "aldose-amides. Fischer2 used this reaction to transform tetra-O-acetyl-L-rhamnononitrile into l,l-bis(acetamido)-l,5-dideoxy-L-arabinitol, whose subsequent hydrolysis and oxidation allowed him to determine the configuration of dextro-tartaric acid (L-threaric acid). 

K. Tsdano, H. Maeda, M. Hoshino, Y. Iimura, and T. Suami, A novel transformation of four aldoses to some optically pure pseudobexopyranoses and a pseudopentofonnose, carbocyclic analogs of hexopytanoses and perrtofuranoses, J. Org. Chan. 52 1946 (1987). 

Natural D-mannose is the aldehyde of natural D-mannitol, and is transformed by the action of bromine water into D-mannonic acid, which was isolated as its phenylhydrazide. The acid was regenerated from the phenyl-hydrazide and isolated as its crystalline lactone. Kiliani hod obtained the enantiomorph of this lactone on applying the cyanohydrin reaction to natural L-arabinose. A mixture of both lactones formed a racemate. Then, by taking recourse to his newly discovered reduction of the lactones to the aldoses, a reaction which Fischer designated the most significant in the... 

In 1914, Fischer discovered another transformation of the sugars on treating the acetohalogenoses with zinc dust. Glucose thereby forms glucal (24). The reaction is general for the aldoses. One molecule of glucal... 

Rates of Enolization Reactions. For a better understanding of the transformation and oxidation reactions of reducing sugars, methods have been developed to measure the primary rates of enolization (18). One of these methods depends on the rate at which tritium ions are released from aldoses-2- to the solvent. This is measured by separation of the water-, sublimation, and radiochemical assay of the water as the reaction proceeds. The rate constant is calculated from the first-order equation ... 

Several compounds of this type, formally derived from aldoses by oxidation of the terminal CH2OH group to -CHO, have been prepared. Dialdoses arise as intermediates in structural studies, but they are also valuable starting materials for synthetic conversions, in particular for natural-product synthesis. A branched-chain dialdose, streptose (167), occurs as a component of the antibiotic streptomycin. The structure of streptose was elucidated after extensive investigations of its derivatives and transformation products.371 The 2,5-dimethoxytetrahydrofuran fulvanol (168), an analogue of apiose, has been isolated from the plant Hemerocallis fulva 12... 

Simple radicals derived from aldoses have been considered exclusively in the preceding sections, from the point of view of structure and transformation. Application of radical reduction to tertiary anomeric radicals, produced from ketoses, higher sugars and other substituted analogues, and having generally alkoxy and... 

It has been postulated (37) that lactulose is formed from lactose by the Lobry de Bruyn and Alberda van Ekenstein transformation, whereby glucose is isomerized to fructose via an enol intermediate. In turn, two mechanisms have been proposed for the degradation of this intermediate (38)- One involves the addition of a proton to the enediol resulting in epimeric aldoses and the original ketose, while the other involves 8-elimination to yield galactose and saccharinic acids. The authors experimental data would tend to better support the second pathway. 

In weakly alkaline solutions aldoses and ketoses undergo rearrangements. An example is the Lobry de Bruyn -Alberda van Ekenstein transformation of... 

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