Strecker acid, formation

Amino acid formation in the Urey-Miller experiment and almost certainly in the prebiotic environment is via the Stecker synthesis shown in Figure 8.3. This reaction mechanism shows that the amino acids were not formed in the discharge itself but by reactions in the condensed water reservoir. Both HCN and HCO are formed from the bond-breaking reactions of N2 and H2O in a plasma, which then react with NH3 in solution. The C=0 group in formaldehyde or other aldehydes is replaced by to form NH and this undergoes a reaction with HCN to form the cyano amino compound that hydrates to the acid. The Strecker synthesis does not provide stereo-control over the carbon centre and must result in racemic mixtures of amino acids. There is no room for homochirality in this pathway. 

A strong argument in favor of the involvement of Strecker synthesis is that aldehydes and HCN are among the initial species formed in electric discharge experiments leading to a-amino acid formation. Mechanistic and thermodynamic studies (20) of systems derived from aqueous mixtures containing... 

In the presence of oxygen, intermediate 10 can undergo oxidation and give rise to the formation of the aminoketo compound 11 and subsequently an acid, the so-called Strecker acid [36], The formation of Strecker acid, however, is not possible if dicarbonyls such as 4 are present in form of their stable cyclic hemiacetals, as shown by Hofmann and co-workers [36]. The same group revealed also that Strecker aldehydes can be formed via an oxidative degradation of Amadori compounds, as shown for the Amadori compound of phenylalanine and glucose [37],... 

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... 

Furthermore, the results revealed that, in addition to MBT level, 3-(methylthio)propanal and phenylacetaldehyde levels were significantly increased in the illuminated sample (Table 1). The formation of such Strecker aldehydes induced by light has also been observed earlier in model systems containing catechin, riboflavin, and the respective precursor amino acids [16] and has been reported for light-treated skim milk [17]. However, the mechanism of Strecker aldehyde formation induced by light is still unclear. 

The carboxyl terminated ACPA, 4,4 -azobis-(4-cya-nopentanoic acid), turned out to be a suitable reagent in condensation reactions. This compound can be prepared by Strecker s synthesis from levulinic acid following the method of Haines and Waters [12]. Regarding the formation of polymeric azo initiators, Matsakuwa et al. [13] reported on the condensation of ACPA with various diols and diamines in the presence of a condensation agent, I-methyl-2-chlorpyridinium iodide, and a cata-... 

There are expressions of uncertainty concerning the mechanism of the first step of the Strecker amino acid synthesis13-17. The reaction can proceed via the formation of an imine and subsequent nucleophilic attack of cyanide (path ). Alternatively, it has been speculated that the reaction of the aldehyde with hydrogen cyanide furnishes a cyanohydrin (path ), which then is subjected to a nucleophilic displacement of the hydroxy group by the amino function. 

Like the Strecker synthesis, the Ugi reaction also involves a nucleophilic addition to an imine as the crucial step in which the stereogenic center of an a-amino acid derivative is formed4. The Ugi reaction, also denoted as a four-component condensation (A), is related to the older Passerini reaction5 (B) in an analogous fashion as the Strecker synthesis is to cyanohydrin formation. In both the Ugi and the Passerini reaction, an isocyanide takes the role of cyanide. 

As has been outlined for the Strecker synthesis, the Ugi reaction also proceeds via initial formation of a Schiff base from an aldehyde and an amine. The imine intermediate is attacked by the isocyanidc, a process which is supported by protonation of the imine by the carboxylic acid component. The resulting a-amino nitrilium intermediate is immediately trapped by the carboxylate to give an 6>-acyl imidiate. All steps up to this stage are reversible. Only the final oxygen to nitrogen acyl shift is irreversible and delivers the A-acyl-a-amino amide as the thermodynamically favored product which contains two amide groups. 

As early as 1996, Nelson and Miller (1996) reported initial successful experiments. They synthesized AEG and small amounts of ethylenediamine diacetic acid from mixtures containing ethylenediamine, formaldehyde and HCN thus the formation of AEG (Fig. 6.12) is possible via the Strecker synthesis (see Sect. 4.1). 

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. 

Serine has been prepared by the Strecker method from glycol-aldehyde 1 and from ethoxyacetaldehyde,2 3 by the condensation of ethyl formate with ethyl hippurate followed by reduction and hydrolysis,4 5 from the reaction product of chloromethyl ether with ethyl sodium phthalimidomalonate,6 and by amination of a-bromo-/3-methoxypropionic acid with subsequent demethyla-tion.7... 

The ease of the Strecker synthesis from aldehydes makes a-aminonitriles an attractive and important route to a-amino acids. Fortunately, the microbial world offers a number of enzymes for carrying out the necessary conversions, some of them highly stereoselective. Nitrilases catalyze a direct conversion of nitrile into carboxylic acid (Equation (11)), whereas nitrile hydratases catalyze formation of the amide, which can then be hydrolyzed to the carboxylic acid in a second step (Equation (12)). In a recent survey, with a view to bioremediation and synthesis, Brady et al have surveyed the ability of a wide range of bacteria and yeasts to grow on diverse nitriles and amides as sole nitrogen source. This provides a rich source of information on enzymes for future application. 

The formation of aldehydes (flavorings) in roasting of cocoa beans, for example, is caused by Strecker degradation of 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... 

Hydrogen cyanide and methanal are especially reasonable starting materials for the prebiotic synthesis of amino acids, purine and pyrimidine bases, ribose and other sugars. Formation of glycine, for example, could have occurred by a Strecker synthesis (Section 25-6), whereby ammonia adds to methanal in the... 

The first such process was realized over one and a half centuries ago with the discovery of the Strecker reaction [10] which has a cyanide ion as the nucleophile, leading to the formation of a-amino nitriles 10 (Scheme 7.2). These highly versatile synthetic intermediates can be hydrolyzed to a-amino acids or can be converted to other molecules [11, 12]. 

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

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 5. Formation of secondary reaction products involved in production of l-(methylthio)ethanethiol by Strecker degradation of amino acids. (Reproduced from Ref. 49. Copyright 1972, American Chemical Society.)...

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