Phloroglucinol Tautomerism

Constitution. Tautomerism.—The constitution of each of the poly-phenols which we have considered has not been taken up because it has been sufficiently established by the syntheses and reactions as given. In the case of phloroglucinol, however, we have another case of tautomerism. Its constitution as tri-hydroxy benzene is established by the syntheses given for it and the fact that it yields tri-acyl derivatives. Toward other reagents, however, it acts otherwise than as a hydroxyl compound. Hydroxylamine, H2N—OH, which is the characteristic reagent for aldehydes and ketones (p. 124), yields a... 

Also phloroglucinol the 1-3-5-tri-hydroxy benzene may have the tautomeric constitution of a tri-ketone (p. 621) in which case it is a tri-oxy derivative of hexa-hydro benenze. 

The most popular method involves 2-thiobarbituric acid (TBA) two molecules of 2-thiobarbituric acid are condensed with malonaldehyde. The emergent chromogen — the two tautomeric structures of the red TBA-malonaldehyde adduct — is determined at 532 nm, and also often at 450 nm, to determine aUcenals and aUcanals, respectively. The qualitative Kreis test was based on a similar principle it involved detection of the epihydrine aldehyde — a tautomeric malondialdehyde — in a color reaction with resorcine or phloroglucinol. The popularity of the TBA test stems from a correlation between the results and sensory evaluations. Paradoxically, this is related to the most important drawback of the TBA technique — its lack of specificity. In addition to the reaction with malonaldehyde, TBA forms compounds of identical color with other aldehydes and ketones, products of aldehyde interaction with nitrogen compounds, and also with saccharides, ascorbic acid, creatine, creatinine, trimethylamine oxide, trimethylamine, proteins, and amino acids. For this reason, the TBA test may even be treated as a proteolysis indicator (Kolakowska and Deutry, 1983). Recently, TBA-reactive substances (TEARS) were introduced, primarily to stress that the reaction involves hydroperoxides in addition to aldehydes. Due to the nonspecificity of the TEARS test, its results reflect the rancidity of food better than other conventional methods, especially off-flavor, which is caused by volatiles from lipids as well as being affected by products of lipids interaction with nitrogenous compounds. 

Most of the hop resins contain -di- and p-tri-carbonyl functions which are enolized. It is seldom possible to isolate the individual tautomers but estimates of the relative proportions can be obtained by proton magnetic resonance (PMR) spectroscopy and other physical methods. Irrespective of the major component a tautomeric mixture can react in any form if, for example, a minor tautomer is consumed in a reaction it will be regenerated from the major tautomers, according to the equilibrium, until the reaction is complete. Thus phloroglucinol, the parent of the hop resin, exists almost entirely in the trienol form (17) but can react as cyclohexone-1,3,5 trione (18) to form a trioxime (19). 

Succinosuccinic ester was discovered by Fehling and investigated by Remsen. It was synthesised and its conversion into p-dihydroxyterephthalic acid observed by F. Herrmann. Baeyer synthesised terephthalic acid from succinosuccinic acid, and with W. A. Noyes synthesised succinosuccinic acid by reducing dihydroxyterephthalic acid. Baeyer found that phloroglucinol forms an oxime, and hence can function as triketohexamethylene as well as trihydroxybenzene, a recognition of tautomerism. He introduced the de-colorisation of alkaline permanganate solution as a test for unsaturation. 

Keto-enol tautomerism has been studied by computation for a range of polyphenols such as resorcinol (1,3-dihydroxybenzene) and phloroglucinol (1,3,5-trihydroxy-benzene). In addition to the obvious favouring of enols associated with aromaticity, less obvious through-bond effects are also highlighted. 

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