Aldol Strecker reaction

The Lewis acidity of lanthanide complexes has been known for a long time. It was exploited extensively in their use as NMR shift reagents, mainly Eu(fod)3. They show strong affinity toward carbonyl oxygens and, therefore, have been widely used as catalysts for cycloaddition of dienes with aldehydes [25]. Moreover, the ability of catalytic amounts of lanthanide compoimds to activate coordinating nitriles as well as imines has also been recognized [26]. In recent years lanthanide (III) complexes have demonstrated clear effectiveness in catalyzing not only hetero-Diels-Alder reactions, but also Michael, aldol, Strecker and Friedel-Crafts acylation reactions [27]. 

In 2003, Rawal reported the use of TADDOLs 177 as chiral H-bonding catalysts to facilitate highly enantioselec-tive hetero-Diels-Alder reactions between dienes 181 and different aldehydes 86 (Scheme 6.29A) [82], and also BINOL-based catalysts 178 were found to facilitate this reaction with excellent selectivities [83]. TADDOLs were also successfully used as organocatalysts for other asymmetric transformations like Mukaiyama aldol reactions, nitroso aldol reactions, or Strecker reactions to mention a few examples only [84]. In addition, also BINOL derivatives have been employed as efficient chiral H-bonding activators as exemplified in the Morita-Baylis-Hilhnan reaction of enone 184 with different carbaldehydes 86 [85]. The use of chiral squaramides for asymmetric reactions dates back to 2005 when Xie et al. first used camphor-derived squaric amino alcohols as ligands in borane reductions [86]. The first truly organocatalytic application was described by Rawal et al. in 2008 who found that minute amounts of the bifunctional cinchona alkaloid-based squaramide 180 are... 

In addition to the studies mentioned above, chiral alcohols have been used as H-bonding catalysts in a vinylogous aldol reaction of Chan s diene with aldehydes [73], in an enantioselective Strecker reaction [74], and in the enantioselective addition of aza-enamines to imines [75]. Taddol has also found use as a memory of chirality enhancer in the stereoselective synthesis of (i-lactams from amino acid derivatives [76, 77]. 

Thus, IQ may arise from creatinine, 2-methylpyridine and formaldehyde or a related Schiff base, formed from glycine through Strecker degradation. The initial step may be a Mannich reaction or an aldol condensation. By analogy MelQ may arise from creatinine, alanine and 2-methylpyridine, and MelQx from creatinine, glycine and 2,5-dimethylpyrazine according to the scheme in Figure 1. 

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. 

Heterocyclic aroma compounds found in meat primarily arise from interactions between mono- and dicarbonyl compounds, H2S and ammonia. The carbonyl compounds are derived from the Maillard reaction, including Strecker degradation of amino acids, oxidation of lipids and aldolization reactions. H2S is produced by thermal degradation of sulfur amino acids and ammonia by amino acid pyrolysis. 

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... 

In this chapter, remarkable advances in the research devoted to peptide catalysed alcohol esterifications, 1,4-conjugate additions, aldol reactions, Strecker synthesis, asymmetric cyanohydrin synthesis and alkene epoxida-tion are discussed. 

Since the introduction of the first peptide organocatalyst in the 1980s, a considerable number of new peptide frameworks have been developed that are able to effectively catalyse several important transformations including alcohol esterifications, 1,4-conjugate additions, aldol reactions, Strecker synthesis, asymmetric cyanohydrin synthesis and alkene epoxidation are discussed. A few successful examples of solid-supported peptides and reactions in ball milling under solvent-free conditions have been demonstrated. These methods combine the advantages of being economically and environmentally friendly processes. 

By retro-aldol cleavages of the 1-deoxyosone short-chain dicarbonyls like diacetyl, methylglyoxal or hydroxydiacetyl are formed which - like the deoxyosones - may participate in the Strecker degradation of amino acids, thus creating volatile flavor components 17,18). On the other hand, the 1-deoxyosone undeigoes several cyclization reactions yielding e.g. acetylfiiran, maltol, isomaltol, furaneol and 5-hydroxy-5,6-dihydromaltol (DHM) (75). 

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