2-Acetylpyrrole, formation

Scheme 19.—Mechanism for Formation of 2-Acetylpyrrole and 2-Methyl-3-pyridinol.

Predictably, nitrosation of 2-acetylpyrrole and pyrrole-2-carboxylic esters with alkyl nitrites or nitrous acid preferentially yields the relatively stable 4-nitroso derivatives, whilst 2,4-dialkyl- or -diaryl-pyrroles are nitrosated at the 5-position. Further reaction of the dialkyl and diaryl nitrosopyrroles with an excess of alkyl nitrite in the absence of a base can result in the formation of the nitropyrroles, whereas the reaction with nitrous acid converts the nitrosopyrroles into diazopyrroles (B-77MI30502). 

Some pyrrole derivatives have pleasant flavor. For example, pyrrole-2-carboxaldehyde gives a sweet and corn-like odor and 2-acetylpyrrole has caramel-like flavor. However, some pyrroles have been found to contribute to off-flavor of food products (24). Pyrroles have not received as much attention as flavor components as other heterocyclic Maillard reaction products such as pyrazines and thiazoles even though the number of derivatives identified is almost the same as that of pyrazines (Figure 1). Proposed formation mechanisms of pyrroles in the Maillard reaction systems are similar to those of thiophenes (Figures 2). 

Table 2 shows the relative quantities of component groups in the volatiles after heating mixtures of glucose or fructose with serine at 120°, 150° and 180° C. It shows optimal formation of furans and pyranones at 120° C, whereas furanones possess a maximum at 150° C. Compounds of the other groups are formed preferentially at 150° C, while the formation of pyrazines proceeds better the higher a reaction temperature was chosen. Also pyrroles need higher temperatures for their formation. So we could demonstrate that only acetylpyrrole and 5-methylpyrrole-2-aldehyde has been formed at 120° C whereas many additional pyrroles appear at 150° C or 180°C respectively (12). 

In Maillard reactions with lactose, the 4-hydroxy group is substituted and hence /1-dicarbonyl formation is blocked, thus preventing the production of 2-acetylpyrroles.30... 

Potassium salts of pyrrole, 2-acetylpyrrole, 1,2,3,4-tetrahydrocarbazole, 9-carbazole, indole, imidazole, and 1,2,4-triazole react with [Tr-CjHjFe ( 0)2 ] to form ct-N derivatives, e.g., [7r-C5H5Fe(CO)2(CT-N-pyrrolyl)] (III) (400, 421), shown to be intermediates in the formation of the n-complexes e.g., (IV) (400). Anions [M(CO)5L] (M = Cr, Mo, W) were similarly prepared from the hexacarbonyls and alkali metal derivatives of succinimide, phthalimidine, and saccharin (49). 

A type Ib cyclization involving ketene-iV,S-acetals led to the formation of highly functionalized pyrroles including 4-acetylpyrrole-2-carboxylates <05SC693> and 3,4-diarylpyrroles <05TL475>. The latter are useful building blocks for the preparation of the lamellarin alkaloids. Another approach to the lamellarin framework involved a 1,5-electrocyclization of azomethine ylides <05TL7531>. 

A -heterocyclic compounds other than pyrazines such as pyrrolines, pyrrrolidines, piperidines and pyrroles are also very important flavor compounds. The formation of pyrrolines and pyrrolidines are reported to be generated from the reaction of proline with glucose (Shigematsu et aL, 1975 Tressl et aL, 1985a). The pyrrolidines possess smoky and roasty aromas while 2-acetyl-1-pyrroline was reported by Tressl et al. (1985b) to have a cracker-like odor. The pyrrole rings from proline and hydroxyproline are present in many of their reaction products. N-acetylpyrrole exhibits a cookie-like and mushroom-like odor (Tressl et ai, 1986). 

The key intermediates in the formation of furan and pyrrole derivatives from 3-deoxyglycos-2-uloses via P-dicarbonyl compounds are 3-deoxy-2,4-diuloses and l-amino-l,3-dideoxy-2,4-diuloses. Cyclisation, dehydration and keto-enol tautomerisation of diuloses yield 2-acetylpyrrole derivatives as the main products (Figure 4.96). 

Low temperatures (70 C) favor the formation of early products (1 and 7) and slow down their degradation to more stable compounds (Figure 11). On the other hand higher temperatures lead to more rapid production of early Maillard reaction products. At 130 C for exanple, p-pyranone, cyclopentenone and 3-furanone are barely detectable, whereas late products such as glucosyl isomaltol, acetylpyrrol and pyrraline become predominant. 

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