Xylose reaction with bases

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... 

Both linear and cyclic forms in the pentose series have been examined. In their reactions with a dialkyl hydrogenphosphonate, both 2,4 3,5-di-0-ethylidene-L-xylose (in its reaction with diethyl hydrogenphosphonate in the presence of KF) and 2,3 4,5-di-0-isopropylidene-D-arabinose, single products were obtained to which the 15 configuration was assigned, based on NMR studies on the [(pentaacetyloxy)pentyl]phosphonic... 

Oxazolidinone aiudharies based on o-xylose [66] and D-mannitol [67] residues were converted into corresponding carboxylic acids 79 and 80 which after activation were used for stereoselective Staudinger reaction with diaryl, or aryl-styryl imines. hi the first case, an excellent diastereoselectivity was obtained to afford 81 with high preponderance [66]. hi the second case. 

Application of the Grenacher reaction (condensation of aldehydes with rho-danine) to di-(9-isopropylidene derivatives of a/de/zydo-L-arabinose, -D-arabinose, and -L-xylose followed by base-catalysed hydrolysis gave sugar derivatives of the type RCH=CH(SH)C02H where R = glycosyl. Photoirradiation of phenyl 2,3,5-tri-(9-acetyl-l-seleno-a, 3-ribofuranose results in migration of the sugar residue to the ortho-position with concomitant formation of the diselenide dimer. ... 

Initiated by a paper from Carrell and co-workers [52] reporting the structure of D-xylose isomerase from Streptomyces rubiginosus refined to 1.9 A resolution, in which the authors claimed experimental support for the enediol mechanism, controversy on the true mechanism of the enzymatic catalysis arose. This group had designed the aldose analogue 32, a suicide inhibitor of the isomerase, and had determined three crystal structures that of the native enzyme, of the enzyme with bound substrate/product, and the enzyme after reaction with the mechanism-based inhibitor (Scheme 10). 

Recently, development activities for alkyl polypentosides have been described. The products are based on hemicellulose, which, after hydrolysis, leads to a mixture of glycosides (mainly pentoses such as xylose and arabinose). These pentoses are transformed into the desired alkyl polypentosides via the reaction with fatty alcohols under standard reaction conditions. The products are reported to have similar properties to alkyl polyglycosides. These developments have been carried out by Wheatoleo, a joint venture between Oleon and A.R.D./Soliance [57]. 

The chemistry can be illustrated with the case of adenine isodideoxynucleoside. The key precursor for the coupling reaction was compound 14 (Scheme 7), which can be synthesized in excellent yields (17%) from D-xylose. Condensation of 14 with adenine stereospecifically and regiospecifically was carried out by reaction with this base in the presence of potassium carbonate and 18-crown-6 in DMF. Deprotection of the resulting product with sodium methoxide in methanol gave the target isodideoxynucleoside 15 in 55% yield (for the last two steps). The structure of 15 was confirmed by its UV spectrum (260 nm, e 14,000), H and C NMR data (single compound and absence of astereoisomer), and optical rotation ([a]p = -26.6°). TTiis magnitude of levorotation is the... 

Catechol and related phenolics 13,16,19, 31, and 32 were also isolated after alkaline treatment of D-glucose and sucrose. Several other substituted acetophenones were isolated. The mechanism of formation of phenolic compounds from monosaccharides under alkaline conditions has yet to be thoroughly investigated. The similarity in the types of aromatic products from D-glucose and D-xylose indicates the formation of the same C2, C3, or C4 fragments, with subsequent recombination and cycliza-tion. Base-catalyzed aldol reactions are, no doubt, predominant pathways in the initial formation of these aromatic products. 

It is precisely to the production of meatlike flavors that the great majority of patents based on the Maillard reaction have been directed. Mos of them indicate cysteine or cystine as the essential sulfur-containing compound. Other patents claim alternative sources for sulfur, e.g., derivatives of mercaptoacetaldehyde (36), mercaptoalkylamines (37), S-acetylmercaptosuccinic acid (38), 2-thienvltetrasulfide (39), "a sulfide" (40), and hydrogen sulfide (heated with aqueous xylose without any amino acid) (41). 

In 27 runs at various conditions, the operators of this pilot plant found to their distress that the furfural yields were only in the order of 30 percent, in harsh contrast to their expectations of 85 percent, derived from calculations based on the known kinetics in water. The designers had made a fundamental mistake They had measured the rate of pentose disappearance in the sulfite liquor, but not the rate of furfural formation, and as the rate of pentose disappearance was far greater than what had to be expected when the liquor s hydrogen ion concentration was used in the known kinetics of xylose disappearance in acid water, they had concluded that the lignosulfonate of the liquor had a special catalytic effect on the pentose-to-furfural conversion. They had overlooked that the fast disappearance of the pentose in the liquor was not due to a mysterious catalysis but caused by loss reactions of the pentose with lignosulfonate and other ingredients of the liquor. 

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