Reactions decarboxylative

A large variety of organic oxidations, reductions, and rearrangements show photocatalysis at interfaces, usually of a semiconductor. The subject has been reviewed [326,327] some specific examples are the photo-Kolbe reaction (decarboxylation of acetic acid) using Pt supported on anatase [328], the pho-... 

The first step of this reaction, decarboxylation of pyruvate and transfer of the acetyl group to lipoic acid, depends on accumulation of negative charge on the carbonyl carbon of pyruvate. This is facilitated by the quaternary nitrogen on the thiazolium group of thiamine pyrophosphate. As shown in (c), this cationic... 

As can be expected, the high-temperature processing runs the risk of enhancing side and consecutive reactions. Decarboxylation of the main product was found and increases with temperature (see Fig. 7). This is illustrated at the example of the synthesis of 2,4,6-trihydroxy benzoic acid from phloroglucinol, as this molecule is even more sensitive to thermal destruction due to the enhanced electron richness of the aromatic core by presence of a third hydroxyl group (Hessel et al. 2007). 

In contrast to glutamate, y-aminobutyric acid (y-aminobutyrate, GABA) is an inhibitory compound. Metabolically derived from glutamate, GABA production illustrates a reaction, decarboxylation, common to the production of serotonin and the catecholamines (Figure 4.9). 

Increasingly sophisticated catalytic domains have been synthesized and used as adducts to the framework polymers. These synthetic macromolecules show substantially enhanced catalytic effects on hydrolytic reactions, decarboxylation, Schiffbase hydrolysis, aromatic nucleophilic substitution, and oxidation [63-69]. Several of these synthetic polymers are effective peptidases and nucleases. 

The discovery, in these waters, of high concentrations (up to 1 X 10" mg L ) of mono- and dicarboxylic acid anions, phenols, and other reactive organic species has led to numerous field and laboratory studies to determine their distribution and importance in inorganic and organic interactions. The observed concentrations are minimum values, because the organics are degraded by bacteria and by thermal decarboxylation reactions. Decarboxylation rates estimated from... 

Lawrence, N. J., Muhammad, E. Ramberg-Backlund type reactions of phosphonium salts. Tetrahedron Lett. 1994, 35, 5903-5906. Wladislaw, B., Marzorati, L., Russo, V. P. T., Zim, M. H., Di Vitta, C. Novel reaction decarboxylative Ramberg-Backlund rearrangement in some a-isopropylsulfonyl carboxylic esters. Tetrahedron Lett. 1995, 36, 8367-8370. 

Crosby, J., Stone, R. and Lienhard, G.E. (1970). Mechanisms of thiamine-catalyzed reactions. Decarboxylation of 2-(l-carboxy-l-hydroxyethyl)-3,4-dimethylthiazolium chloride. J. Am. Chem. Soc. 92, 2891-2900... 

Hammick reaction. Decarboxylation of a-pi-colinic or related acids in the presence of carbonyl compounds accompanied by the formation of a new carbon-carbon bond. 

Hunsdiecker reaction. Decarboxylative bromination is achieved photochemically by this reagent combination. 

In each of these transformations, one of the bonds to the cr-carbon of the amino acid substrate is broken in the first step of the reaction. Decarboxylation breaks the bond joining the carboxyl group to the cr-carbon transamination, racemization, and o , 8-elimination break the bond joining the hydrogen to the cr-carbon and —Cp bond cleavage breaks the bond joining the R group to the a-carbon. 

In 1966 Dunathan (18) proposed that, in PLP-mediated reactions, the bond to be broken in the substrate-cofactor compound should be perpendicular to the plane of the extended conjugated system so that there would be maximum a-n overlap between the breaking bond and the ring-imine n system. Thus 3a, 3b, and 3c represent the conformations of the imine 3 best suited to achieve transamination reactions, decarboxylation reactions, and retroaldol reactions, respectively. The enzyme will be responsible for the orientation of the amino acid-PLP complex and thus dictate the nature of the resultant reaction. An example of an enzyme catalyzing two distinct reactions was found for serine hydroxymethyltransferase (EC 2.1.2.1), which normally catalyzes the retroaldol process outlined in 3c when L-serine or L-threonine are substrates. When D-alanine was used as substrate, however, a slow transamination was observed (19). Comparison of the conformations of the amino acid-PLP complexes, 3c and 17, respectively, for the retroaldol and transamination reactions shows that both the proton removed from the... 

A useful method of a-substitution in carboxylic acids is via the a-bromo derivative, synthesized using the Hell-Volhard-Zelinsky reaction. Decarboxylation... 

FIGURE 19.4 The mechanism of the pyruvate dehydrogenase reaction. Decarboxylation of pyruvate occurs with formation of hydroxyethyl-TPP (Step 1). Transfer of the two-carbon unit to lipoic acid in Step 2 is followed by formation of acetyl-CoA in Step 3. Lipoic acid is reoxidized in Step 4 of the reaction. 

As a reaction mechanism for the strong base-induced cycioaddition reaction of homophthalic anhydrides and dienophiles, two routes are possible (Scheme 4). One is route a via the Diels-Alder reaction, decarboxylation and de-HX. The other is route b via Michael-type addition, intramolecular cycli-zation, decarboxylation and de-HX. Several studies showed that route a was the more preferable. After cycioaddition of the homophthalic anhydrides with dienophiles, the reaction is always accompanied by decarboxylation and aromatization. The stereochemistry of the cycioaddition reaction of the homophthalic anhydride is not considered. 

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