Strecker aldehydes, reaction

Strecker reactions provide one of the most efficient methods for the synthesis of a-amino nitriles, which are useful intermediates in the synthesis of amino acids and nitrogen-containing heterocycles. Although classical Strecker reactions have some limitations, use of trimethylsilyl cyanide (TMSCN) as a cyano anion source provides promising and safer routes to these compounds.133-351 Consequently, we focused our attention on tributyltin cyanide (Bu3SnCN), because Bu3SnCN is stable in water and is also a potential cyano anion source. Indeed, the Strecker-type reactions of aldehydes, amines, and Bu3SnCN proceeded smoothly in water (Eq. 9).1361 It should be noted that no surfactants are required in this reaction. Furthermore, Complete recovery of the toxic tin compounds is also possible in the form of bis(tributyltin) oxide after the reaction is over. Since conversion of bis(tributyltin) oxide to tributyltin cyanide is known in the literature, this procedure provides a solution to the problem associated with toxicity of tin compounds. 

The aldehydes 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, methional, and phenylacetaldehyde are so-called Strecker aldehydes, formed as a result of a reaction between dicarbonyl products of the Amadori pathway and amino acids, having one less carbon atom than the amino acid (i). 

Proline and hydroxyproline differ from the other amino acids in that they contain a secondary amino group in a pyrrolidine ring therefore, they do not produce aminoketones and Strecker aldehydes in the reaction with dicarbonyls, ffowever, nitrogen heterocyclics are produced, including 1-pyrroline, pyrrolidine, 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine (Scheme 12.5) [19]. 

Aliphatic carbonyl compounds, such as diacetyl, which has a butter-like odour, also may contribute to the aromas derived from the MaiUard reaction, and many of the Strecker aldehydes also have characteristic aromas (Table 12.1). 

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. 

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). 

It becomes clear that analytical methods based on the evaluation of the end products of deteriorative reactions will not be satisfactory. Therefore in our own experiments amino acid analysis of Amadori compounds and gas chromatography of volatile Strecker aldehydes were applied to detect the onset of the Maillard reaction well before detrimental sensory changes occurred. 

The Strecker amino acid synthesis, which involves treatment of aldehydes with ammonia and hydrogen cyanide (or equivalents) followed by hydrolysis of the intermediate a-amino nitriles to provide a-amino acids (Scheme 1), was first reported in 1850 [1], This method has been applied on an industrial scale toward the synthesis of racemic a-amino acids, but more recently interest in nonproteinogenic a-amino acids in a variety of scientific disciplines has prompted intense activity in the asymmetric syntheses of a-amino acids [2]. The catalytic asymmetric Strecker-type reaction offers one of the most direct and viable methods for the asymmetric synthesis of a-amino acid derivatives. It is the purpose of this Highlight to disclose recent developments in this emerging field of importance. 

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. 

The question of the fate of the "Strecker" aldehydes requires an answer. By converting the amino acid phenylalanine to yield aroma compounds, phenyl acetaldehyde is liberated. Because of its phenyl ring it is a good detector compound. We were able to establish some of its reaction products. For example, we have identified, among others, phenylethylpyrazine, phenylfuran, phenylethylpyrrole and phenylpyri-dine. We assume that aldol condensations are responsible for the formation of these compounds. Figure 5 illustrates our assumption.We have identified several compounds the structures of which make probable an aldol condensation (3-(2 -furyl)-2-phenyl-2-propenal, phenyl hydroxyketones) likely. This assumption is supported by the identification of pyrazines with up to 5 carbon atom side chains in other experiments. 

Tressl et al7r J1 designated the linear polymers as Type I and the branched ones as Type II. In most melanoidins, they would represent domains (or substructures), unsubstituted pyrroles and Strecker aldehydes, for example, being integrated into the melanoidin backbone, giving a complex macromolecular structure overall. Tressl et aV1 consider the oligomerisation/polycondensation reactions described as the only experimentally established pathways by which simple Maillard products generated from hexoses and pentoses are easily and irreversibly converted into macromolecules. 

For the thiophen mentioned above, all the carbon atoms stemmed from cysteine, carbohydrate apparently not having a role. On the other hand, theoretically, thiophens can be derived from furans simply by reaction with H2S. Belitz and Grosch251 postulate derivation from 2-mercaptoethanal (Strecker aldehyde from cysteine) and acrolein or butenal (aldol condensation product from acetaldehyde). Vemin and Parkanyi216... 

Aldehydes, ketones, and acetals react with allyltrimethylsilane in the presence of a catalytic amount of BiX3 (X = C1, Br, OTf) to give homoallyl alcohols or homoallyl alkyl ethers (Equation (52)).91-93 The BiX3-catalyzed allylation of aldehydes and sequential intramolecular etherification of the resulting homoallylic silyl ethers are involved in the stereoselective synthesis of polysubstituted tetrahydropyrans (Equation (53)).94,95 Similarly, these Lewis acids catalyze the cyanation of aldehydes and ketones with cyanotrimethylsilane. When a chiral bismuth(m) catalyst is used in the cyanation, cyanohydrines are obtained in up to 72% ee (Equation (54)). a-Aminonitriles are prepared directly from aldehydes, amines, and cyanotrimethysilane by the BiCl3-catalyzed Strecker-type reaction. 

In another thorough flavor study Wu et al.(55) determined both the volatile and nonvolatile flavor compounds found in mushroom blanching water. They used HPLC to determine such non volatile flavor components as sugars, amino acids and nucleotides. The free amino acids were analyzed to determine if they might be involved in any thermal reactions which might produce Amadori compounds or Strecker aldehydes which in turn would produce aroma components. 

Various Isopentyl-substituted pyrazines, such as 2-lsopentyl-3-methylpyrazine, 2-isopentyl-5-methylpyrazine, 2-isopentyl-6— methylpyrazlne, 2-i8opentyl-5,6-disiethylpyrazlne, 2-isopentyl-3,5-dimethylpyrazlne and 2-isopentyl-3,6-dimethylpyrazlne were identified from the thermal reaction of glucose and leucine (12). The formation mechanisms for these compounds may also involve the reaction of 3,6-dihydropyrazine with isovaleraldehyde, the Strecker aldehyde of leucine. Kltamura and Shibamoto (13) described 2-lsopen-tyl-5,6-dimethylpyrazlne as having a caramel-like, coffee and sweet aroma. Although isopentyl-substituted pyrazines have not yet been reported in cocoa, they could, if present, be very Important contributors to that characteristic aroma. 

Several examples of the Strecker-type reaction were tested. For all the compounds investigated, including aromatic, aliphatic, heterocyclic, and aj3-unsaturated aldehydes, the reactions proceeded smoothly to afford the corresponding a-amino nitriles in high yield. The adducts, a-(A-benzhydryl)amino nitriles, were readily converted to a-amino acids [78], and Strecker-type reactions using other amines such as aniline and benzylamine also proceeded smoothly to afford the corresponding adducts in high yields. 

Aromatic and aliphatic aldehydes in the presence of dialkylamines and an equivalent of acid such as hydrochloric, perchloric or p-toluenesulfonic acid give iminium salts, which add cyanide ion to form a-(dialkylamino)nitriles. An alternative preparation involves the reaction of the aldehyde with dialkylamines in the presence of acetone-cyanohydrin, a-(A, -dialkylamino)isobutyronitiiles, diethyl phosphorocyanidate or TMS-CN. Another route to a-aminonitrile starts with an aldehyde, the salt of an amine and KCN in organic solvents under solid-liquid two-phase conditions by combined use of alumina and ultrasound. Chiral a-aminonitriles were prepared by Strecker-type reactions, cyano-silylation of Schiffs bases, amination of a-siloxynitriles or from an A -cyanomethyl-l,3-oxazolidine synthon. Reaction of tertiary amines with CIO2 in the presence of 5.1 mol equiv. of aqueous NaCN as an external nucleophile affords a-aminonitrile. °... 

Strecker [34] discovered that the reaction between amino acids (glycine, alanine, leucine) and the tricarbonyl compound alloxan yields CO2 and aldehydes. The Strecker degradation of amino acids occurs also with dicarbonyl compounds [35] including those that are formed in the course of the Maillard reaction, in particular deoxyglycosones and some of the smaller sugar fragments like 7 and diacetyl. Fig. 3.22 shows the reaction pathway that involves the formation of an imine 8, followed by decarboxylation and liberation of the resulting aminoketo compound and the Strecker aldehyde from the intermediate 10. Odour-active Strecker aldehydes which... 

Hofmann, T, Mlinch, P, Schieberle, P. Quantitative model studies on the formation of aroma-active aldehydes and acids by Strecker-type reactions. J. Agric. Food Chem. 2000, 48, 434-440. 

Proteases degrade the proteins to free amino acids and oligopeptides, thus imparting sweet, sour and mainly a bitter taste to the product. Further reactions lead to the corresponding Strecker aldehydes 2-methylbutanal and 3-methylbutanal with a pungent, malty, cocoa note. 

Cyanosilylation of imines (Strecker-type reaction) is efficiently promoted by conventional Lewis acids such as ZnX2, AlCl , and TiCLj [604]. Kobayashi et al. recently disclosed that Yb(OTf)3 has high catalytic activity in this cyanosilylation (Scheme 10.237) [622]. In the competitive reaction of aldehydes and the corresponding imines with TMSCN, Yb(OTf)3 activates imines to give only a-aminoni-... 

The premise is to utilise a liquid film to provide a reaction environment which can be dynamically controlled in terms of heat and mass flux (influx/effiux) and to complement this with the on-line monitoring technique of Atmospheric Pressure chemical Ionisation (APcI)-Ion Trap Mass Spectrometry (ITMS). This technique allows the flux of protonated molecular ions (Mlf to be directly monitored (mass spectral dimension 1) and to fragment these species under tailored conditions within the ion trap (Collision Induced Dissociation (CID),mass spectral dimension 2), to produce fragment ions representative of the parent ion. This capability is central to allowing species with a common molecular weight to be quantified, for example butan-2,3-dione (MW=86 MH =87, glucose degradation product) and 3-methylbutanal (MW=86 MH =87, Strecker aldehyde from leucine). 

The results listed in Table I demonstrate that the reaction of DWGH with glucose produces much greater amounts of volatile confounds than is produced by the interaction of WGH with glucose. About 17 times the amount of Strecker aldehydes, 8 times the amount of other carbonyls, 2.5 times the amount of hydrocarbons, 3.7 times the amount of furans, 18 times the amount of pyrazines, 4 times the amount of other N-containing confounds, and 3 times the amount of S-containing compounds were produced in the DWGH-G system than in the WGH-G system. 

This acid is formed by oxidation of the corresponding Strecker aldehyde (C.12). For Ho et al. (1993), it is generated by a Maillard reaction. 

---