Strecker degradation of a-amino acids

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... 

The flavors produced by the Maillard reaction also vary widely. In some cases, the flavor is reminiscent of caramelization. The Strecker degradation of a-amino acids is a reaction that also significantly contributes to the formation of flavor compounds. The... 

Schonberg A. and Moubacher R. (1952) The Strecker degradation of a-amino acids. Chem. Rev. 50, 261-77. Schonberg A., Moubacher R. and Mostafa A. (1948) Degradation of a-amino acids to aldehydes and ketones by interaction with carbonyl compounds. J. Chem. Soc. 176-82. 

Schonberg, A., and Moubacher, R. 1952. The Strecker degradation of a-amino acids. 

Two branches of the above reaction pathways provide active reagents for the degradation of a-amino acids to aldehydes and ketones of one less carbon atom (Strecker degradation), which is another arm of the Maillard reaction. Strecker aldehydes from these reactions are important flavor compounds (18). 

Rohan had suggested that the operative reaction in the development of chocolate aroma might be a Strecker degradation of the amino acid fraction. Bailey et al. (8) demonstrated quantitatively that three aldehydes, which could be related to leucine, valine, and alanine, were prominent in the volatiles from a typical sample of roasted, ground cacao beans. 

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. 

Aroma of Strecker Degradation Products of a-Amino Acids with Isatin (Diketodihvdroindole) ... 

Strecker aldehyde are generated by rearrangement, decarboxylation and hydrolysis. Thus the Strecker degradation is the oxidative de-amination and de-carboxylation of an a-amino acid in the presence of a dicarbonyl compound. An aldehyde with one fewer carbon atoms than the original amino acid is produced. The other class of product is an a-aminoketone. These are important as they are intermediates in the formation of heterocyclic compounds such as pyrazines, oxazoles and thiazoles, which are important in flavours. 

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... 

Figure 2.10. Strecker degradation of amino acids and a-dicarbonyls to form heterocyclic compounds. For glycine, R = H. Reprinted with permission from Wong, J. W., and Shib-amoto, T. (1996). Geotoxicity of the Maillard reaction products. In The Maillard Reaction. Consequences for the Chemical and Life Sciences, Ikan, R., ed., John Wiley Sons, Chichester, UK, 129-159.

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... 

Also acetic acid may arise from a reaction of this type. Most important compounds of this pathway are pyruvic aldehyde, diacetyl, hydro-oxyacetone and hydroxydiacetyl which can easily react with amino acids. The Strecker degradation is a reaction where the amino acid is de-carboxylated and loses its amino group. Reaction products are the Strecker aldehyde and - as an intermediate - an aminoketone which forms a pyrazine by dimerization. This pathway is considered to be most important for the origin of pyrazines in thermal aromas. However, only limited knowledge is available about the fate of the Strecker aldehydes. As we will demonstrate they are very reactive. 

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