Pyrroles, Pyridines

The volatile compounds formed by heating food include numerous pyrrole and pyridine derivatives. Of special interest are the N-heterocycUc compounds with the foUowing structural feature  

This characteristic feature appears to be required for a roasted odor. In fact, aU the pyrroUnes and pyridines listed in Table 5.23 as weU as 2-acetyl-thiazole, 2-acetylthiazoline (cf. Table 5.22) and acetylpyrazine (cf. Table 5.23) contain this structural element and have a roasted or cracker-hke odor. However, the thresholds of these compounds vary greatly. The lowest values were found for 2-acetyl-and 2-propionyl-l-pyrrohne. The length of the alkanoyl group also influences the aroma activity because in the transition from 2-propionyl- to 2-hutanoyl-l-pynoline, the roasted note suddenly disappears and the odor threshold increases hy several powers of ten. 2-Acetyl-l-pyrrohne (Apy) is responsible for the typical aroma of the cmst of white bread and it 

Strecker degradation of both proline (cf. Formula 5.14) and ornithine (cf. Formula 5.15). 

In the baking of white bread, ornithine comes from yeast where it is found in a concentration about four times that of free proline. 

In addition, triose phosphates occurring in yeast have been identified as precursors. They yield on heating, e. g., 2-oxopropanal from di- 

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Besides being useful precursors to pyrroles pyridine-2-ones -4-ones, -4-thiones. and -4-imines 4-alkylidene-dihydropyridines thiophenes 1,2,4-triazoles thiapyrane-2-thiones, isoquinoline-3-ones isoben-zothiophenes and 4-mercaptoimidazolium hydroxide inner salts, mesoionic thiazoles are potentially useful in the construction of molecules with herbicidic (39). central nerve stimulating, and antiinflammatory properties (40,41). Application in dye synthesis has likewise been reported (42). 

The nomenclature used to describe the fused benzene-pyrrole-pyridine system of the compounds under discussion has been repeatedly modified, and the compounds have been numbered in an astonishing variety of ways since Perkin and Robinson introduced the name carboline for the ring system, which was encountered for the first time in the harmala alkaloids. In the earliest version of carboline nomenclature, the parent compound of the series, whose trivial name was norharman, was referred to as 4-carboline and numbered as in 1. Harmine (2) then became ll-methoxy-3-methyl-4-carboline. 

Pyrrole, pyridine, pyrazole, and imidazole derivatives as insecticides and acari-cides 99UK773. 

Pyrrole Pyridine 4- Hydroxy pyridine Indole Quinoline... 

Noteworthy is the fact that one can utilize the appropriate bromopyrrole (e.g., 48) in conjunction with the desired Grignard reagent in a one-step operation to afford the corresponding substituted pyrroles (e.g., 49) [43]. The mixed pyrrole-pyridine heterocycle 50 was made in this fashion [45]. 

Volatiles produced by further interactions pyrroles, pyridines, pyr-azines, imidazoles, oxazoles, thiazoles, thiophenes, dithiolanes, trithi-olanes, dithianes, trithianes, furanthiols... 

Since proline already contains a pyrrolidine ring it provides a potential source of nitrogen heterocyclics in the MaiUard reaction, and a number of proline-con-taining model systems have been examined. Tressl et al. [32] identified more than 120 proline-specific compounds in the reaction of proline or hydroxypro-line with various sugars. These include pyrrolines, pyrroles, pyridines, indolines, pyrrolizines and azepines, but relatively few of the compounds have been identified among food volatiles. 

The SHMO orbitals of pyrrole, pyridine, and pyridinium are shown in Figure 11.6. The HOMO of pyrrole is the same as that of butadiene. Thus pyrrole is more reactive than benzene toward electrophilic attack. Attack, leading to substitution, occurs mainly at the... 

Dans le tableau 2, nous n avons porte que les valeurs moyennes resultant de plusieurs mesures obtenues par les deux methodes. La constante K du complexe pyrrole-pyridine est comparee ailleurs [Bj avec celle indiquee par Linnell [10]. 

The SHMO orbitals of pyrrole, pyridine, and pyridinium are shown in Figure 11.6. The HOMO of pyrrole is the same as that of butadiene. Thus pyrrole is more reactive than benzene toward electrophilic attack. Attack, leading to substitution, occurs mainly at the 2- and 5-positions where the electron density of the HOMO is concentrated. In the case of pyridine (Figure 11.6b), the HOMO is not the n orbital, but the nonbonded MO, wn, which would be situated at approximately a - 0.5 //. Thus, it is not pyridine but pyridinium (Figure 11.6c) which undergoes electrophilic attack and substitution. The reactivity is much less than that of benzene, although this could not be deduced directly from the SHMO calculation. Neither does the calculation suggest the reason that electrophilic substitution occurs mainly at the 3- and 5-positions, since the n HOMO is... 

Aassoc) roR Pyrrole-Pyridine Complexes and pK or the Associated Pyridine"... 

Viscosity,160,160a surface tension,161 and cryoscopic162 measurements have, also been applied to the study of the pyrrole-pyridine complex. 

A rather unique synthetic example based on the well-known pyrrole-pyridine carbene insertion reaction involves the following ring expansion.16... 

The imidazole part of the IQ-compounds suggests creatinine as a common precursor. The remaining parts of the IQ-compounds could arise from Maillard reaction products, e.g., 2-methylpyri-dine or 2,5-dimethylpyrazine. These two compounds could be formed through Strecker degradation. In Maillard reactions, this is induced by a-dicarbonyl compounds derived from carbohydrates, which are thereby converted to pyrroles, pyridines, pyrazines, etc. (8). 

Following the application of 3-cresol and anisole, induction of P. putida TVA8 was achieved and there are some studies (e.g., Boyd and Sheldrake, 1998), suggesting several substrates and mono- and poly-cyclic aromatic rings may be cleaved by toluene dioxygenase. Other cyclic compounds (e.g., pyrrole, pyridine, and furan) also induce P. putida TVA8, but the specific mechanism and enzymes involved are unknown. Further studies on this aspect are required to comprehensively address this question. 

A good example of the effect of pH98 is that observed on a xylose-lysine model system (1 M each, refluxed 1 h with diethyl ether in a Likens and Nickerson apparatus, initial pH 4.9, either kept at pH 5 with NaOH additions or left, when the final pH is 2.6) 54 and 28 volatiles were identified, respectively, 2-furaldehyde dominating with 52.2 and 99.9% (w/w). Total yield and number of nitrogen-containing compounds were greater at higher pH values of the former system, and monocyclic pyrroles, pyridines, and 2,3-dihydro-lH-pyrrolizines were identified only in that system. 

Nitrogen compounds (one through eight cond. ring pyrroles, pyridines) Hydroxy-Nitrogen compounds... 

All quantitative calculations are carried out by computer programs (2,6,16). Calibration data are at present available for 95 heteroaromatic compound types, including one-to-seven or -eight ring thiophenes, furans, mono- and dihydroxyaromatics, pyrroles, pyridines, and nitrogen/oxygen compounds. Semiquantitative data are available on an essentially unlimited number of additional heteroaromatics. 

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