Uronic acids decarboxylation

In all of these reactions, the formation of enols and ene-diols seems to play a part, unlike the 4-epimerisations, aminations and uronic acid decarboxylations, where the ketonic character of the 4-oxo intermediate is considered paramount. 

Electron donating a-substituents favour the non-Kolbe reaction but the radical intermediates in these anodic processes can be trapped during co-electrolysis with an alkanoic acid. Anodic decarboxylation of sugar uronic acids leads to formation of the radical which is very rapidly oxidised to a carbonium ion, stabilised by the adjacent ether group. However, in the presence of a tenfold excess of an alkanoic acid, the radical intermediate is trapped as the unsymmetrical coupling product [101]. Highly functionalised nucleotide derivatives such as 20 will couple successfully in the mixed Kolbe reaction [102], Other examples include the co-electrolysis of 3-oxa-alkanoic acids with an alkanoic acid [103] and the formation of 3-alkylindoles from indole-3-propanoic acid [104], Anodic oxidation of indole-3-propanoic acid alone gives no Kolbe dimer [105],... 

The fact that 27 is produced by dehydration both of uronic acids and of pentoses has led to the suggestion112 that pentoses may be intermediates in decarboxylation reactions of uronic acids, and that treatment of such glycuronans as pectin with strong acids results in the production of pentosans.113 Little evidence supports this theory, be-... 

The formation of such chromones as 3,8-dihydroxy-2-methyl-chromone by treating uronic acids or pentoses with dilute acid was reported by Aso,119 and studied by Popoff and Theander,120 who obtained a number of these compounds in 3.5% yield, as well as some catechols. Although nothing is yet known about the mechanism of formation of these compounds, the fact that the chromones contain 10 carbon atoms and are produced both from pentoses and uronic acids suggests that they may be derived from 2-furaldehyde or re-ductic acid, or produced from a decarboxylated intermediate. 

Investigations of the mechanism of decarboxylation of hexuronic acids have largely involved kinetic and tracer studies. When either D-xylo-5-hexulosonic acid or D-glucuronic acid is converted into 27 in acidified, tritiated water, the resulting 27 contains 18% and 15%, respectively, of the activity of the solvent as carbon-bound tritium.21 Further degradation studies showed that the isotope is situated on the furan ring at either position 3 or 4, or both these atoms correspond to C-3 or C-4 of the starting uronic acid. 

All of the possibilities mentioned are credible perhaps each participates to some extent in the reaction. No studies similar to those with aldoses have been made of substituted uronic acids, and no intermediates have been isolated. Aso,96 who first suggested 72 as a precursor for 2-furaldehyde and reductic acid,92 has prepared 72, but its significance in the decarboxylation reactions has not been fully examined.123... 

The formation of reductic acid and 2-furaldehyde from uronic acids is believed to occur through the same intermediate (72a) that is generated on decarboxylation of 3,4-dideoxy-D-g/t/cero-hex-3-enos-uluronic acid (71) (see Section III,3 p. 191). However, little is... 

One of the most important reactions of hexuronic acids and glycuronans is the decarboxylation caused by treating with strong acids (usually 12% hydrochloric acid). The rapid and stoichiometric loss of 1 mol of carbon dioxide per mole has been developed as an analytical method by many workers.227 Methods based on the formation of colored phenolic compounds in strongly acidic media are widely used to assay total uronic acid.228,229 The method of Blumenkrantz and Asboe-Hausen228 has been adapted for microtiter plates.230... 

D. M. W. Anderson and S. Garbutt, Studies on uronic acid materials. Part VII. The kinetics and mechanism of the decarboxylation of uronic acids, J. Chem. Soc., (1963) 3204-3210. 

M. S. Feather and J. F. Harris, Relationships between some uronic acids and their decarboxylation products, J. Org. Chem., 31 (1966) 4018-4021. 

The use of the hypervalent iodine reagent [bis(trifluoroacetoxy)iodo]benzene has been reported to be effective in the synthesis of C-nucleoside-like compounds. Radical decarboxylation of a suitably protected uronic acid, initiated photochem-ically, followed by addition of a heterocyclic base provided the C-nucleoside in high yield.154 The mode of action involves initial radical formation of 122 (Scheme 33), followed by introduction of the base and radical coupling.155 The anomeric selectivity was high in some examples, and low in others—lepidine gave the highest proportion of the ( anomer. Isolated yields were poor to moderate. 

Dermatan sulfate may be distinguished from chondroitin 4- and 6-sulfates in that it is not degraded by testicular hyaluronidase and, furthermore, the desulfated mucopolysaccharide is unattacked by testicular and bacterial hyaluronidases (M17). Further diflFerentiation of dermatan sulfate from hyaluronic acid and the foregoing chondroitin sulfates is readily made on the basis of color reactions given by the different uronic acid components. Dermatan sulfate shows equimolar ratios of uronic acid ihexosamine sulfate when the uronic acid content is determined by the orcinol (K7) or decarboxylation (T4) methods, whereas significantly lower values are obtained by the carbazole method (D8). 

The Leffivre-Tollens decarboxylation method for the determination of uronic acids has been applied to soils. This method gives good results with plant material adequately prepared. With soils, however, unbelievably high values for uronic acid, up to 40 % of the soil organic matter, are obtained. The decarboxylation method has been shown to be unsuited for the... 

Decarboxylation with hydroiodic acid Tl6) was the basis for a procedure used in determining uronic acid levels in dietary fiber fractions (17). The carbon dioxide from decarboxylation was purified, trapped in a cell containing standard sodium hydroxide, and conductivity changes were measured using an Ingold electrode. 

Sugar nucleotides have also been shown to undergo oxidation at the primary alcohol group and decarboxylation of the uronic acid moiety. ... 

There is no standardised procedure for the hydrolysis of dissolved polymeric uronic acids. Mopper (1977) su ests much milder procedures than are used for sugars since uronic acids are much more labile towards decarboxylation or transformation reactions. The hydrolysis procedure suggested by Burney and Sieburth (1977) may therefore prove to be adequate. The use of cation-exchange resin in the form may also be effective. 

Only 2-pyridyl reverse C-nucleosides are known. Coupling saccharide free radicals 831 and 834 with protonated pyridine derivatives gave the 2-pyridyl reverse C-nucleosides 832 and 835, respectively. Free radical 831 was obtained by decarboxylative photolysis of the uronic acid derivative 830 in the presence of hypervalent iodine compounds (92TL7575 (Scheme 232), whereas free radical 834 was obtained by thermal homolysis of the carbon-iodine bond in the 6-iodo-6-deoxy-o-galactopyranose derivative 833 in the presence of benzoyl peroxide (93JOC959) (Scheme 233). 

Analysis of dietary fiber. Total dietary fiber including both water-soluble and water-insoluble components was analysed with an enzymic method as described by Asp et al. (18). The dietary fiber was characterized by gas-liquid chromatographic assay of monosaccharides after acid-hydrolysis and gravimetric determination of acid insoluble lignin. Uronic acids were assayed with a decarboxylation method. These analyses were performed as described by Theander and Aman (19). 

The reaction of carboxylic acids with the PhI(OAc)2-iodine system may result in a decarboxylation ieading to the intermediate formation of a carbon-centered radical, which can be further oxidized to a carbocation and trapped by a nucleophile. This process has been utilized in several syntheses [97, 615,616, 617]. In a typical example, the oxidative decarboxylation of uronic acid derivatives 568 in acetonitrile under mild conditions affords acetates 569 in good yields (Scheme 3.225) [615]. A similar oxidative decarboxylation has been be used for the synthesis of 2-substituted pyrrolidines 571 from the cyclic amino acid derivatives 570 [616,617]. 

Potentially the cyclitol pathway could be more elaborate, with various epimerisation or substitutions occurring at the cyclitol level and not by way of sugar nucleotides. As Loewus has pointed out, any blockade at C-1 of myoinositol would eventually prevent decarboxylation of the uronic acid and pentose formation, while allowing UDPGalA to form. However, there is little evidence directly for or against such mechanisms. Nevertheless, the cyclitol pathway is extremely important as a major route and part of a potential regulatory system in plants, as the evidence of Rubery and Northcote (1970) shows (see Chapter 5). 

Most secondary monosaccharides are formed from primary sugars by the following types of reactions (for biosynthesis of some secondary monosaccharides by decarboxylation of uronic acids, see D 1.2). 

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