Galacturonic lactones

The pyranose structure of D-galacturonic acid, which lactonizes with great difficulty, has been confirmed by Levene and Kreider66 from a study of the methylated ester of the uronoside. Methyl a-D-galacturonoside (XXXIII) was methylated with silver oxide and methyl iodide and there was obtained methyl 2,3,4-trimethyl-a-D-galacturonoside methyl ester (XXXVI) which on oxidation was converted to d,l(2,3,4-trimethylmucic) acid (XXXVII) and h-arabo-trimethoxyglutaric acid (XXXVIII). 

By analogy with the biosynthesis of L-ascorbic acid and with other oxidation-reduction enzyme systems, it seems likely that D-galactose is oxidized to D-galacturonic acid (XXIII) on reduction at Cl, this yields L-galactonic acid (XXIV), the 1,5-lactone (XXV) of which, on reduction at Cl, would give L-galactose (XXII). 

The major pathway probably proceeds via D-galacturonic acid and L-galactono-1,4-lactone, as these are converted to L-ascorbic acid by mitochondria prepared from peas and mung beans, but neither L-gulono-1,4-... 

In model experiments,205 it was found that concentration of an aqueous solution of the aldonic acids in the presence of hydrochloric acid gives exclusively the 1,4-lactones, which, on subsequent trimethyl-silylation, give only one peak on the chromatogram. This method was successfully employed for the separation of D-galacturonic, D-glucuronic, and D-mannuronic acids. Of the 1,4-lactones examined, only the trimethylsilyl ether of D-mannono-1,4-lactone was obtained in crystalline form. However, all of the derivatives showed characteristic differences in their infrared spectra in the range of 1500 to 600 cm"1. When this method was applied to the determination of uronic acids in a variety of polysaccharides,205 it was impossible to find any hydrolytic conditions under which the uronic acids were quantitatively released and then reduced, a problem experienced by other workers.20 The method was, however, successful in affording a qualitative, microscale procedure for the identification of hexuronic acids (which otherwise are difficult to detect). 

L-fhreo-Pentos-2-ulose (9) was prepared by several procedures (see Scheme 3). In procedure A, D-galacturonic acid (14) was reduced with sodium amalgam to L-galactonic acid (15), which lactonized to L-ga-lactono-1,4-lactone (16) by removal of water. Treatment of 16 with ammonia in methanol afforded L-galactonamide (17) which, on oxidation with sodium hypochlorite, gave L-lyxose (18). Reaction of 18 with phenylhydrazine yielded the corresponding osazone (19) which, on hydrolysis, afforded 9. 

Whereas mannuronic and galacturonic acids exist in both a- and /3-d-pyranoid forms, glucuronic acid is only found in the /3-d form. Glucuronic and mannuronic acids form crystallizable lactones, but that of galacturonic acid has not yet been crystallized. On isolation, D-glucuronic acid appears as needle-like crystals of /3-D-glucopyranuronic acid (I), which is... 

Acetylation of the formazan of n-galacturonic acid leads to a lactone acetate which has three acetyl groups and is an appropriate starting material for the preparation of the corresponding tetrazolimn compound. 

Nowadays, no saturated or unsaturated mannuronic acid derivatives are commercially available in a monomeric form, thereby limiting their uses in chemical synthesis. In contrast to o-glucuronic, o-galacturonic, or L-iduronic acids, only the D-mannurone lactone form (D-mannofuranurono-6,3-lactone) was available for... 

The acidity constants of o-galacturonic and o-glucuronic acids in water have been determined by conductance and e.m.f. measurements/ The structures of the isomeric products obtained on trimethylsilylation of naturally occurring hexuronic acids and their sodium salts and lactones have been established using n.m.r. spectroscopy (for the anomeric configuration) and mass spectrometry (for the ring-size)/ ° The equilibria of some of the hexuronic acids in DMSO involved a larger proportion of furanoid forms than those in water or pyridine. 

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