Mechanisms Strecker degradation

Scheme 18.—Mechanism for Acylpyrrole formation from 3-Deoxyhexos-2-ulose by the Strecker Degradation.

Several mechanisms have been proposed for the formation of pyrazines in food flavours [18, 23, 25], but the major route is from a-aminoketones, which are products of the condensation of a dicarbonyl with an amino compound via Strecker degradation (Scheme 12.3). Self-condensation of the aminoketones, or condensation with other aminoketones, affords a dihydropyrazine that is oxidised to the pyrazine. 

Isatin has been used in the Strecker degradation of a-amino acids to aldehydes,434-437 and in the formation of benzaldehydes from benzyl-amines.431,435,438-440 These conversions have been the subject of a review, and mechanisms have been proposed.441 This formation of aldehydes from primary amines may, in part, explain some of the... 

Probably the most important reactant in the formation of volatile meat flavor compounds is hydrogen sulfide. It can be formed by several pathways during meat cookery, but one mechanism is Strecker degradation of cysteine in the presence of a diketone as established by Kobayashi and Fujimaki (29). The cysteine condenses with the diketone and the product in turn decarboxylates to amino carbonyl compounds that can be degraded to hydrogen sulfide, ammonia and acetaldehyde. These become very reactive volatiles for the formation of many flavor compounds in meat and other foods. 

Several mechanisms have been reported for pyrazine formation by Maillard reactions (21,52,53). The carbon skeletons of pyrazines come from a-dicarbonyl (Strecker) compounds which can react with ammonia to produce ot-amino ketones as described by Flament, et al. (54) which condense by dehydration and oxidize to pyrazines (Figure 6), or the dicarbonyl compounds can initiate Strecker degradation of amino acids to form ot-amino ketones which are hydrolyzed to carbonyl amines, condensed and are oxidized to substituted... 

Hodge7 has advanced several possible routes for the conversion of the enol form of the 1-amino-l-deoxy-2-ketose into melanoidin, and the evidence to support these mechanisms is considerable. Thus, the enol may be converted into the Schiff base of a furaldehyde, or to a reductone by loss of water. It may also be broken down into smaller fragments (for example, hydroxy-2-propanone or pyruvaldehyde), which react further with amino compounds. The enol may also react with an a-amino acid and be converted to an aldehyde by a Strecker degradation. The compounds thus formed from... 

The identification of 49 pyrazines in heated beef and other meats has been extensively revieved (32, 43). Several mechanisms have been proposed for pyrazine formation by the Maillard reaction. Dlcarbonyl compounds can initiate Strecker degradation of amino acids to yield ot-amino ketones, vhich in turn can undergo condensations and oxidizations to form substituted pyrazines (13). 

Scheme 5.13 Mechanism proposed for the formation of2-acetylpyrido 3,4-d imidaz,ole via Strecker degradation of histidine with 2-oxopropanaP44...

Figure A shows a proposed mechanism for the formation of 2,A,5-trlmethyloxazole and A,5-dlmethyloxazole from the Strecker degradation of cysteine with 2,3-butanedlone (Al).

Pyrazines have been prepared by heating 1,2-dicarbonyl compounds with a-amino acids. Thus Rizzi (308) observed that under the conditions of the Strecker degradation, equimolar amounts of DL-valine (44) and butane-2,3-dione in refluxing bis(2-methoxyethyl) ether, diglyme, gave isobutyraldehyde, tetra-methylpyrazine (9%), and a mixture of cis- and trans-2-isopropyl-4,5-dimethyl-3-oxazoline (4%). He proposed a reductive amination mechanism in which butane-23-dione was converted to 2-aminobutan-3-one which underwent self-condensation to the pyrazine. Tetramethylpyrazine was also prepared when the same reactants were heated in dimethylformamide at 123° for 5 hours (and other pyrazines prepared similarly) (308a). 

The mechanisms of generating the compounds shown in Table 12.1.2 were already discussed in Section 3.5. The main mechanisms are the decarboxylation by CO2 elimination or the water elimination with the formation of a dipeptide and further of the diketopiperazines. By Strecker degradation, amino acids may also be converted to aldehydes. 

The basic reactions contributing to thermal flavour generation are the Maillard reaction, Strecker degradation, lipid oxidation and thiamin degradation. Interaction of products formed by these different mechanisms additionally leads to further flavour components. 

Using labeled precursors, Blank et al. (1996) explained the formation of homofuraneol in reactions of xylose with alanine (preferentially to glycine). The proposed mechanism suggested the incorporation of the Strecker degradation product, acetaldehyde. This mode of formation is preferred to sugar fragmentation. 

According to Carlin et al. (1986), the exact mechanism of oxazole formation is not known, despite the previous schemes proposed by Vitzthum and Werkhoff (1974a,b) and by Ohloff and Flament (1978). Formation pathways were proposed by Baltes and Bochmann (1987d) and Mottram (1991). For Vitzthum and Werkhoff (1974b), one pathway could be the decarboxylation of serine or threonine into ethanolamine or methylethanolamine condensation with an aldehydic compound into an oxazolidine, then oxidation into an oxazole unsubstituted or methylated on position 5 and bearing an alkyl or an acyl radical on position 2. Another pathway could be the condensation of amino acids with a-dicarbonyl compounds, followed by a Strecker degradation, formation of an a-amino ketone which, after acylation... 

Its formation during roasting could result from a trimolecular reaction between ethylglyoxal, ammonia and acetaldehyde resulting from the Strecker degradation of alanine. The mechanism of chain elongation reactions of glyoxal has been elucidated by Yaylayan and Keyhani (1998). 

The latter authors also found it when serine and/or threonine reacted with sucrose. It was previously identified in other model reactions by treating furfural with hydrogen sulfide and ammonia (Shibamoto, 1977) or by heating rhamnose with ammonia (Shibamoto and Bernhart, 1978). Ho and Hartman (1982) proposed a plausible mechanism for the formation 2,4,5-trimethyloxazole from the reaction of dl-alanine or L-cysteine and 2,3-butanedione. Ho el al. (1982) also studied the formation of oxazoles and oxazolines in the Strecker degradation of dl-methionine and L-cysteine with 2,3-butanedione. In their proposed mechanisms, the authors suggested the formation of intermediate 3-oxazolines. 

Other possible mechanisms for the formation of pyrazines during smoking are possible. For instance, leaf carbohydrates could be degraded either through pyrolysis or Maillard reactions to form a-dicarbonyl compounds, which could, in turn, react with amino acids to undergo a Strecker degradation forming... 

Hodge et al. [49] originally proposed a mechanism for the formation of pyrroles, which is also supported by Tressl et al. [50]. This basically is the participation of proline and hydroxy proline in the Strecker degradation to yield pyrroles. If the food system does not contain either proline or hydroxy proline, then a sugar of at least five or more carbons is required for pyrrole formation [51]. 

The most important reactions which provide volatile carbonyl compounds were presented in sections 3.7.2.1.9 (lipid peroxidation), 4.2.4.3.3 (caramelization) and 4.2.4.4.T (amino acid decomposition by the Strecker degradation mechanism). 

Fig. 5.17. Cysteine decomposition by a Strecker degradation mechanism formation of H2S (I) or 2-mer-captoethanal (II)...

Monoaminomonocarboxylic a-amino acids with a primary amino group produce sensory active aldehydes called Strecker aldehydes. Strecker degradation of P-amino acids yields alkan-2-ones known as methylketones (see Section 8.2.4.1.2). By analogy, alkane-3-ones (ethylketones) are formed from y-amino acids. The general reaction is schematically indicated in Figure 2.43. The reaction mechanism, however, varies considerably depending on the type of oxidant and amino acid. 2-Imino acids and 2-oxoacids can in some cases apparently form as intermediates, analogous to enzymatically catalysed transamination and oxidative deamination of amino acids (see Section 2.5.1.3.2). Some Strecker aldehydes readily decompose, such as methional, or yield cyclic products, such as 5-aminopentanal, which dehydrates to 2,3,4,5-tetrahydropyridine. 

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