Dehydrative decarboxylation

Pyruvic acid is the simplest homologue of the a-keto acid, whose established procedures for synthesis are the dehydrative decarboxylation of tartaric acid and the hydrolysis of acetyl cyanide. On the other hand, vapor-phase contact oxidation of alkyl lactates to corresponding alkyl pyruvates using V2C - and MoOa-baseds mixed oxide catalysts has also been known [1-4]. Recently we found that pyruvic acid is obtained directly from a vapor-phase oxidative-dehydrogenation of lactic acid over iron phosphate catalysts with a P/Fe atomic ratio of 1.2 at a temperature around 230°C [5]. 

Several processes often occur in lipids, including oxidation, hydration, dehydration, decarboxylation, esterification, aromatization, hydrolysis, hydrogenation and polymerization. In fact, the chemistry of these materials can be affected, for example, by heat (anthropogenic transformations), humidity, pH, and microbial attacks. 

Decarboxylation of L-ascorbic acid in acid solutions has been proposed to involve one of two possible mechanisms. One pathway required dehydration, decarboxylation, and formation of 2-furaldehyde. The second pathway involved a rearrangement to the 3-keto form followed by a... 

Dehydrative decarboxylation. The convc sion of p-hydroxy carboxylic acids to alkenes by reaction with a DMF dialkyl acetal iirvolves an r/n/Z-elimination, and thus is complementary to the known. yv/i-elimination of these hydroxy acids via a P-lactone (5, 22 9, 504). These reactions were used to obtain both the (E)- and the (Z)-l-alkoxy-l,3-... 

Natural product organsim AGP (Gys Ser) HMGS (Dehydration) (Decarboxylation)... 

Catalytic vapour cracking makes deoxygenation possible through simultaneous dehydration-decarboxylation over acidic zeolite catalysts. At 450 C and the atmospheric pressure oxygen is rejected as H O, CO, and CO producing mostly aromatics (46). The low H/C ratio in the bio-oils imposes a relatively low limit on the hydrocarbon yield and, in addition, the technical feasibility is not yet completely proven. The catalyst deactivation still raises many concerns for both routes. The processing costs are high and the products are not competitive with fossil fuels (47). 

Recently, several syntheses of pyridazines from monosaccharides have been described. Calcium 2,5-diketo-D-gluconate reacts with monosubstituted hydrazines to give the corresponding pyridazinium derivatives of the zwit-terionic type 95. ° The reaction proceeds by hydrazone formation, dehydration, decarboxylation, and cyclization. In a similar manner, other... 

The thermal decomposition of a number of organic acids has been studied by DTA by Wendlandt and Hoiberg (60,6l). Since ihe acids were decomposed in an argon atmosphere, only endothermic peaks were observed in the DTA curves. These peaks were caused by such reactions as dehydration, decarboxylation. sublimation, decomposition, and phase transitions from the solid to the liquid state. The maximum peak temperatures for the phase transitions were 10 to 30 higher than the reported melting-point temperatures. The DTA curves for some of the acids are given in Figure 7.33. 

Dehydrative decarboxylation of fi-hydroxy carboxylic acids. Olefins are formed when 3-hydroxy carboxylic acids are refluxed with excess DMF dimethyl acetal in CHCI3 for 5-10 hr. Yields generally are fair to good (50-90%). 

The metabolic intermediate (e.g., pyrnvate) nndergoes complete oxidation to CO2, throngh the pathway referred to as tricarboxylic acid cycle (TCA cycle). The following reactions are involved in TCA cycle. The hrst step involves conversion of pyrnvate to acetyl-CoA throngh decarboxylation and prodnction of NADH. The acetyl-CoA (2 carbon) combines with the fonr-carbon componnd oxalacetate, leading to the formation of citric acid (6 carbon). The TCA cycle is also referred to as citric acid cycle. A series of reactions inclnding dehydration, decarboxylation, and oxidation are involved in the conversion of citric acid to carbon dioxide. The electrons released are transferred to enzymes containing the coenzyme NAD+. 

Benzofurans can be prepared from a-halo-coumarin compoimds. Base-catalyzed ring-contraction followed by dehydrative decarboxylation gives rise to the desired benzofuran. 

Scheme 7.17 One-pot procedure for the synthesis of cyclohexenones by Michael/ aldol/dehydration/decarboxylation sequence.

Under subcritical conditions, biological chemicals undei o a range of reactions, such as depolymerization, dehydration, decarboxylation, etc., resulting in fragments of light molecules. Simultaneously, unstable and reactive firagments repolymerize into oil compounds. 

Heating of 2-deoxy-D-ara6/ o-hexonic acid resulted in appreciable lactoniza-tion on melting. Subsequent decomposition of the lactone involved dehydration, decarboxylation, and scission to yield water, carbon dioxide, carbon monoxide, 5-(2-hydroxyethylidene)-2(5/f)-furanone, acrylaldehyde, and acetic acid. 3-Deoxy-D-rj6( -hexono-l,4-lactone - which cannot undergo j3-elimination -is thermally more stable and gives products resulting from scission and dehydration. 

Multzer s group has applied the dehydrative decarboxylation of (3-hydroxy-acids to the synthesis of buta-l,3-dienes. Thus, the addition of acetate anion to the a -unsaturated ketone (204) followed by reaction with dimethylformamide dimethyl acetal affords the ( )-dienes (205) stereospecifically, in high yields... 

A-Iodosuccinimide reacts with a-hydroxy-carboxylic acids in an oxidative decarboxylation sequence to give carbonyl compounds. A dehydrative decarboxylation of 2,3-disubstituted 3-hydroxycarboxylic acids, using dimethyl-formamide acetals, has been reported. Photosensitized oxygenation of diethyl... 

A technique that now enables the synthesis of a variety of unstable 1,3-diaryI-1,3-dienes under mild conditions is the dehydrative decarboxylation of /8-hydroxycarboxylic acids (61) with dimethylformamide dimethylacetal. The E configuration of the starting a,/8-unsaturated ketone (60) is unaffected during the reaction sequence, as illustrated in Scheme 27. Catalytic amounts of... 

DMF dineopentyl acetal has been used as a reagent for dehydrative decarboxylation in order to avoid the possibility of competing 0-alkylation of the carboxyl group. This conversion of P-hydroxy carboxylic acids to alkenes by reaction with a DMF acetal involves an anti elimination, and it is thus complementary to the syn elimination of these hydroxy acids via the p-lactone. These reactions have been used to obtain both )- and (Z)-1 -alkoxy-1,3-butadienes (eq 6). For additional examples of alkenes obtained from p-hydroxy acids with DMF acetals, see Scheeren et al. ... 

In the case of stoichiometric reactions where there is only partial mass loss (e.g. dehydration, decarboxylation), Gcan be calculated using the equation... 

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