Aldehydes factors influencing acidity

In Ugi four-component reactions (for mechanism, see Section 1.4.4.1.) all four components may potentially serve as the stereodifferentiating tool65. However, neither the isocyanide component nor the carboxylic acid have pronounced effects on the overall stereodiscrimination60 66. As a consequence, the factors influencing the stereochemical course of Ugi reactions arc similar to those in Strecker syntheses. The use of chiral aldehydes is commonly found in substrate-controlled syntheses whereas the asymmetric synthesis of new enantiomerically pure compounds via Ugi s method is restricted to the application of optically active amines as the chiral auxiliary group. 

One recent study showed that extraction of volatile substances from the wood dropped off sharply after 12 months of aging, though there was high variability between wines, highlighting the influence of such major oenological factors as the tartaric acid content and the potassium content and also the SO2 concentration in the wines, since SO2 can combine with certain furanic derivatives and phenolic aldehydes (Anci n et al. 2004 Ortega-Heras et al. 2007). 

The discrimination between different aldehyde carbonyls with a Lewis acid is rather difficult owing to their high reactivity and inherent coordination aptitude to these acids. The complexation with aldehydes is consistent with anti to the substituent of an aldehydic carbon [3], hence steric factors have less influence on the discrimination events (Fig. 2-8). 

Flavor is an outstanding characteristic of dry-cured ham. The odor-active compounds of this product have been recently identified, and these data have shown the great importance of both lipid and amino acid derived compounds. To obtain samples with odor characteristics similar to those of dry-cured ham, several temperatures were applied to meat samples, and a mild temperature was selected. Compounds previously identified as dry-cured ham odorants were researched by SPME-GC-MS, and the effect of several factors on them was checked. The increase of sodium chloride content caused a general increase in aldehydes, except for 3-methylbutanal. The addition of sodium nitrite caused a general decrease, specially in straight-chain aldehydes. The addition of cysteine and proline on the odorants identified was less important. Reaction time influenced the odorants, with a marked effect on pentanal and hexanal. 

One of the most important characteristics of the aldol reaction is its easy reversibility under many conditions. Since this factor has such a generally profound effect, we briefly introduce the topic here. For an aldol addition reaction that is carried out under the influence of a catalytic amount of acid or base in protic medium A//° and AG° can be estimated from thermochemical data. Guthrie has estimated A// and AG for the aldol addition depicted in equation (3) to be -9.8 kcal mol and -2.4 kcal mol , respectively (1 cal = 4.18 J). ° The thermochemical values used in this estimation, and the derived values of A// and AG% refer to species at equilibrium with the covalently hydrated aldehydes. It is not expected that values for the free aldehydes would be greatly different. The equilibrium constant for equation (3) is 4(X) M- ... 

In summary, several factors determine the stereochemical outcome of aldol addition reactions. The diastereochemical preference of the syn or anti isomer is determined by the configuration of the enolate and the orientation of the aldehyde within the TS. Chirality in either reactant introduces another stereochemical influence. The use of chiral auxiliaries can promote high facial selectivity in the approach of the aldehyde and thus permit the preparation of enantiomerically enriched products. The same outcome can be achieved using chiral Lewis acids as reaction catalysts. 

In conclusion, the conversion of AA by LAB generates a variety of aroma compounds, including branched-chain acids (cheesy) and aldehydes (malty), aromatic alcohols and aldehydes (floral), and volatile sulfur compounds (garlic, cabbage, boiled potato) (Table 19.1). The expression of LAB enzymes in situ is influenced by a number of factors detailed in Section 19.5. 

Mukaiyama aldol reactions, whereby trimethylsilyl enol ethers react with aldehydes in aqueous solution to form -ketoalcohols, have been promoted by new chiral lanthanide-containing complexes and a chiral Fe(II)-bipyridine complex with 0 outstanding diastereo- and enantio-selectivities. Factors controlling the diastereoselec-tivity of Lewis-acid-catalysed Mukaiyama reactions have been studied using DFT to reveal the transition-state influences of substituents on the enol carbon, the a-carbon of the silyl ether, and the aldehyde. The relative steric effects of the Lewis acid and 0 trimethyl silyl groups and the influence of E/Z isomerism on the aldol transition state were explored. Catalytic asymmetric Mukaiyama aldol reaction of difluoroenoxysilanes with /-unsaturated a-ketoesters has been reported for the first time and studied extensively. ... 

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