Aromatic compounds, formation from carbohydrates

Olsson K., Pernemalm P.A., PopofT T. and Theander O. (1977) Formation of aromatic compounds from carbohydrates. V. Reaction of D-glucose and methylamine in slightly acidic, aqueous solution. Acta Chem. Stand. B31, 469-74. 

The three aromatic amino acids that are biosynthesized in the shikimic acid pathway have much in common. The many stereochemical events occurring between the condensation of compounds 288a and 289 derived from carbohydrates to the formation of prephenic acid 296 have been extensively reviewed including a recent review by ourselves (82), and so we have summarized the stereochemistry of the biosynthesis in Scheme 79. Prephenic acid 296 leads to phenylalanine 297 and tyrosine 298. The mem-substituted amino acids 299 are derived from chorismate 295, as is tryptophan 302, as shown. 

Nelson, D.A. Hallen, R.T. Theander, O. "Formation of aromatic compounds from carbohydrates X. Reaction of xylose, glucose and gluconic acid in acidic solution at 300 C." In This Volume. 

For several years our respective groups have investigated the formation of aromatic compounds from carbohydrates in aqueous solution at various pH-values under reflux or hydrothermolytic conditions. For instance, previous papers(1-6) in this series concerned the degradation of hexoses, pentoses, erythrose, dihydroxyacetone, and hexuronic acids to phenolic and enolic components. Of particular interest were the isolation and identification of catechols, an acetophenone, and chromones from pentoses and hexuronic acids at pH 4.5 (1,2). The formation of these compounds, as well as reductic acid(7),was found to be more pronounced than that of 2-furaldehyde(2) under acidic conditions. 

A consideration of the biosynthesis of p-aminobenzoic acid (PABA) involves primarily the mechanism of formation of the ring structure of aromatic compounds. Studies with mutants of E. colt indicated that shikimic acid (Fig. 3) was the precursor of the aromatic ring as occurring in PABA and also in tyrosine, tryptophan, phenylalanine, and p-hydroxy-benzoic acid (12). Much is now known about the manner in which kimic acid is formed from intermediates of carbohydrate metabolism (IS, ISa). Certain E. colt mutants also accumulate shikimic acid-5 -phosphate (ISb). Recent work of Weiss and Srinivasan (13c) demonstrated that PABA could be formed from shikimic acid-5 -phosphate and L-glutamine in an enzyme system derived from baker s yeast. Free shikimic acid was utilized very poorly for PABA synthesis but was more active when incubated in the presence of ATP. Glutamine was a specific amino donor and could not be replaced by glutamic acid or asparagine. When uniformly C -labeled shikimic acid was used as substrate for PABA synthesis in the enzyme system, the PABA formed had the same molar specific activity as the initial shikimic acid 5 -phosphate. PABA synthesis was also dependent on small amounts of yeast or liver concentrates but the nature of the cofactor and the mechanism of the over-all reaction are not known. 

Schiff, for example, had included carbohydrates in his studies on the formation of Schiff bases from aldehydes and amino compounds. He heated D-glucose with aniline or withp-toluidine in substantia [11], and described that these components, under loss of water, formed yellow glass-like condensation products. These supported his suggestions on the formation of Schiff bases from aldehydes and suitable aromatic amines [12]. 

The compound 3,5-dihydroxy-2-methyl-5,6-dihydropyran-4-one (V in Formula 4.67) is also formed from the pyranoid hemiacetals of l-deoxy-2,3-hexodiulose (Formula 4.75). In comparison, maltol is preferentially formed from disaccharides like maltose or lactose (Formula 4.76) and not from dihydroxypyranone by water elimination. The formation of maltol from monosaccharides is negligible. A comparison of the decomposition of 1-deoxyosones from the corresponding cyclic pyranone structure clearly shows (cf. Formula 4.75 and 4.76) that the glyco-sidically bound carbohydrate in the disaccharide directs the course of water elimination in another direction (Formula 4.76). It is the stabilization of the intermediates to quasi-aromatic maltol which makes possible the cleavage of the glycosidic bond with the formation of maltol. Parallel to the formation of maltol, isomaltol derivatives which still contain the second carbohydrate molecule are also formed from disaccharides (Formula 4.77). Indeed, the formation of free isomaltol is possible by the hydrolysis of the... 

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