Hydroxy-4,5-dimethyl-2 formation mechanisms

A maltol-ammonia browning reaction produced thirteen pyrazines, two pyrroles, two oxazoles, and one pyridine (12). The major products of this system were 2-ethyl-3-hydroxy-6-methylpyridine and 2-ethyl-3,6-dimethylpyrazine. It is difficult to construct possible formation mechanisms for these compounds from maltol and ammonia. All the carbon atoms must come from maltol. It is possible, then, that maltol degrades into smaller carbon units and that these fragments recombine to form larger carbon units, producing these compounds. Recently, the formation of thiophenones and thiophenes from the reaction of 2,5-dimethyl-4-hydroxy-3(2H)-furanone and cysteine or cystine was reported (13. 14). All these reaction mixtures were reported to possess a cooked meat-like flavor. 

Several products were also detected in base-degraded D-fructose solution acetoin (3-hydroxy-2-butanone 62), l-hydroxy-2-butanone, and 4-hydroxy-2-butanone. Three benzoquinones were found in the product mixture after sucrose had been heated at 110° in 5% NaOH these were 2-methylbenzoquinone, 2,3,5-trimethylbenzoquinone, and 2,5-dimethyl-benzoquinone (2,5-dimethyl-2,5-cyclohexadiene-l,4-dione 61). Compound 62 is of considerable interest, as 62 and butanedione (biacetyl 60) are involved in the formation of 61 and 2,5-dimethyl-l,4-benzenediol (63) by a reduction-oxidation pathway. This mechanism, shown in Scheme 10, will be discussed in a following section, as it has been proposed from results obtained from cellulose. 

Although the hydroxy group is a relatively poor leaving group, its base-catalyzed nucleophilic substitution by the mechanism shown in Scheme 69 accounts not only for the hydrogenolysis of the 3-hydroxymethylindoles, but also for their SN reactions with ethoxide ions, cyanide ions and with piperidine. Nucleophilic substitution on 2-hydroxymethyl-pyrroles is generally precluded by the faster formation of the bis(2-pyrrolyl)methanes, but the synthesis of 2-cyano-2-(2,5-dimethyl-3-pyrroIyl) propanes from 2,5-dimethylpyrrole, propanone and potassium cyanide probably results from an SN reaction of the cyanide ion upon the initially formed 3-pyrrolylcarbinol (81USP4248784). The formation of (294)... 

As in the hydration of glycals, competing elimination reactions interfere. 5-Methoxylevulaldehyde dimethyl acetal (13) and 2-(hydroxymethyl)-5-(methoxymethyl)furan (14), respectively, have been reported as having been found amongst the products of reaction of methanolic hydrogen chloride with n-xylal and n-glucal, respectively, and evidence for the production of 4,5-dihydro-4-hydroxy-2,5-bis(methoxymethyl)furan (15) in the latter reaction has been obtained. The mechanism of formation of these substances has not yet been discussed. 

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