Pyrrolidine nitrosation

Evidence exists that the relative solubility of amines and inhibitors in heterogeneous oil-water systems could be decisive in formation of nitrosamines and blocking these reactions, Nitrosopyrrolidine formation in bacon predominates in the adipose tissue despite the fact that its precursor, proline, predominates in the lean tissue (5,6,7). Mottram and Patterson (8) partly attribute this phenomenon to the fact that the adipose tissue furnishes a medium in which nitrosation is favored, Massey, et al, (9) found that the presence of decane in a model heterogeneous system caused a 20-fold increase in rate of nitrosamine formation from lipophilic dihexylamine, but had no effect on nitrosation of hydrophilic pyrrolidine. Ascorbic acid in the presence of decane enhanced the synthesis of nitrosamines from lipophilic amines, but had no effect on nitrosation of pyrrolidine. The oil-soluble inhibitor ascorbyl palmitate had little influence on the formation of nitrosamines in the presence or absence of decane. 

How proline is converted to NPYR has not yet been fully elucidated and could conceivably occur by either of two pathways (29, ). One pathway involves the initial N-nitrosation of proline, followed by decarboxylation, while in the other, proline is first decarboxylated to pyrrolidine followed by N-nitrosation to NPYR. Since the conversion of N-nitrosoproline (NPRO) to NPYR occurs at a much lower temperature than the transformation of proline to pyrrolidine, the pathway involving intermediacy of NPRO is thus the more likely route ( ). It has been reported that preformed NPRO in raw bacon is not the primary precursor of NPYR in cooked bacon (29,33-5), as shown by the fact that ascorbyl paImitate, when added to bacon, inhibits the formation of NPYR (33). However, this by no means rules out the intermediacy of NPRO which could be formed at the higher temperatures attained during the frying process (29,36). 

Since nicotine is the major precursor to NNN in tobacco and tobacco smoke, the reaction of nicotine with sodium nitrite was studied to provide information on formation of other tobacco specific nitrosamines, especially NNK and NNA, which could arise by oxidative cleavage of the l -2 bonds or l -5 bond of nicotine followed by nitrosation (26). The reaction was investigated under a variety of conditions as summarized in Table I. All three nitrosamines were formed when the reaction was done under relatively mild conditions (17 hrs, 20 ). The yields are typical of the formation of nitrosamines from tertiary amines (27). At 90 , with a five fold excess of nitrite, only NNN and NNK were detected. Under these conditions, both NNK and NNA gave secondary products. NNK was nitrosated a to the carbonyl to yield 4-(N-methyl-N-nitrosamino)-2-oximino-l-(3-pyridyl)-1-butanone while NNA underwent cyclization followed by oxidation, decarboxylation and dehydration to give l-methyl-5-(3-pyridyl)pyrazole, as shown in Figure 4. Extensive fragmentation and oxidation of the pyrrolidine ring was also observed under these conditions. The products of the reaction of nicotine and nitrite at 90 are summarized in Table II. 

The nitrosation of 2,5-diphenylpyrrolidine in an ice-cold solution of ethanol and hydrochloric acid with sodium nitrite produced the expected iV-nitroso-2,5-diphenylpyrrolidine. The product could be separated into trans and cis isomers by fractional crystallization using acetone-water mixtures. The ratio of trans to cis isomer was found to be 2.5 1. When the nitrosation was carried out in an acetic acid solution, the ratio of trans to cis isomer remained the same although the yield of the identified products was somewhat lower (71 vs 56 %) [29]. The ratio of the isomers of the starting pyrrolidine was not reported. 

For both the in vitro studies with morpholine and pyrrolidine and the in vivo study with proline, it was observed that partially decomposed [Fe2(SMe)2(NO)4] was a much more potent nitrosation agent than the pure complex (110). The nature of the decomposition was not reported, and it is not clear what the significance of this report may be, although nitrite was probably present in the decomposed material. 

Mirvish (15) showed that weak bases are more easily N-nitrosated than strong bases under mildly acidic conditions. Since the secondary amine nitrogen on the 1,3-thiazolidine ring is much less basic (pKa 6.3) than in the pyrrolidine ring (pKfl 11.3), the formation of N-nitroso-l,3-thiazoline should be favorable and hence would be predicted to occur in products such as bacon where N nitrosopyrrolidine is of major concern. 

Several ascorbic acid derivatives were examined by Pensabene et al. (570) for their ability to inhibit nitrosation of pyrrolidine in a model system developed to simulate the lipid-aqueous-protein composition of bacon. While sodium ascorbate was quite effective in the aqueous phase, a combination of an ascorbyl ester with sodium ascorbate gave a better effect in the lipid phase (Table XX). The use of ascorbates and tocopherol as inhibitors of nitrosamine formation and oxidation in foods of the aqueous and lipid type has been reviewed by Newmark and Mergens (326). These compounds in combination could be markedly useful in preventing food contamination with nitrosamines and/or nitros-amides in cured meats such as bacon. 

Table XX. Ascorbyl Monoesters as Inhibitors of the Nitrosation of Pyrrolidine in a Model System (570)...

Lovenberg, 1974). Besides, when putrescine and cadaverine are cooked they may be converted into pyrrolidine and piperidine, respectively (Yamamoto et al., 1982). These secondary amines, as well as spermidine and spermine, may undergo nitrosation forming the extremely carcinogenic compound nitrosamine. The aromatic amines P-phenylethylamine, tyramine, isopentylamine, and 3-(2-aminoethyl) indole (tryptamine), are responsible for dietary disturbances, including migraines and hypertension (Stratton et al., 1991 Anderson et al., 1993). In wine, these compounds are present as odorless salts, but at the pH in the mouth they may have repulsive smells. 

Many other secondary amines have been identified in tobacco smoke but for most of them no corresponding NNA has been identified in smoke. These include a series of (V-substituted anilines, all amenable to A-nitrosation XXXV. Others include the alkyl derivatives of pyrrolidine XXXVI, piperazine XXXVII, andpiperidine XXXVIII. The amines pyrrolidine (XXXVI), piperazine (XXXVII, and 1,2,3,6-tetrahydropyridine (XXXIX) have been identified in cigarette MSS but not piperazine (XXXVII) (see structures in Table XV-7). For each piperazine, mono- and... 

Pyrrolidines and TV-substituted pyrrolidines undergo reactions typical of secondary or tertiary alkylamines. They can be alkylated, quatemized, acylated and nitrosated. The basicity and nu-cleophilicity of pyrrolidines are, however, greater than those of diethylamine (pyrrolidine pX = 11.27, diethylamine pKa = 10.49). Because of these properties, pyrrolidine is very suitable for the conversion of carbonyl compounds into enamines ... 

The metabolic pathway has as the initial step the N-nitrosation of the azacyclopentane (pyrrolidine) nitrogen. Oxidation and ring opening (compare Real Life 21-2) then take place, giving a mixture of two N-nitrosodialkanamines (N-nitrosamines), each of which is a known powerful carcinogen. 

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