Addition decarboxylative

Mercuric carboxylates, which decarboxylate by a chain mechanism when initiated by peroxides, also decarboxylate under UV irradiation (123,128,129,131-140,142,144-146,153-155). In addition, decarboxylation was observed for mercuric benzoate and mercuric a-naphthoate (123). Side reactions [Eqs. (24), (25), (109)] observed in peroxide initiated reactions also occurred on UV irradiation, and mercurous salt formation [Eq.(24)] was more extensive under the latter conditions. Decarboxylation giving methylmercuric acetate occurred on irradiation of mercuric acetate in aqueous solution and is considered to be of environmental significance (156,157). Stepwise decarboxylation giving (CF3)2Hg occurred on irradiation of solid mercuric trifluoroacetate at -196° C (158), but, at 20° C, trifluoromethyl radicals diffused from the solid and dimerized (158). No other diorganomercurial has been formed by radical decarboxylation, and the reaction is not preparatively competitive with the thermal decarboxylation synthesis of (CF3)2Hg (26,27) (Section III,A). 

Phenylthioacetylene has been prepared by elimination of thiophenol and dehydrobromination of cis-1,2-bis(phenylthio)ethylene5 and cis-1-bromo-2-phenylthioethylene,2 7 respectively. The latter was obtained by addition of thiophenol to propiolic acid in ethanol and subsequent one-pot bromine addition, decarboxylative dehalogenation, and careful distillation to remove the trans isomer.2.7 on the other hand, cis-1,2-bis(phenylthio)ethylene was prepared by double addition of thiophenol to cis-1,2-dichloroethylene.5a d Although these procedures can provide useful amounts of phenylthioacetylene, they were found to be somewhat less satisfactory in our hands as far as operation and/or overall yields are concerned. Furthermore, we have encountered problems with regard to the reproducibility of one-pot dehydrobrominations of phenylthio-1,2-dibromoethane.6 However, the stepwise execution of the double dehydrobromination, as described in the modified procedure reported here, provides preparatively useful quantities of phenylthioacetylene in a practical manner. 

Photodecomposition products included acidic compounds and five methylated derivatives (Plimmer, 1970). When picloram in an aqueous solution (25 °C) was exposed by a high intensity monochromatic UV lamp, dechlorination occurred yielding 4-amino-3,5-dichloro-6-hydroxy-picolinic acid which underwent decarboxylation to give 4-amino-3,5-dichloropyridin-2-ol. In addition, decarboxylation of picloram yielded 2,3,5-trichloro-4-pyridylamine which may undergo dechlorination yielding 4-amino-3,5-dichloro-6-hydroxypicolinic acid (Burkhard and Guth, 1979). 

The peanut chalcone synthase and parsley stilbene synthases have been cloned, expressed in E. colt, and purified to homogeneity [135,137]. The enzymes appear to be mechanistically similar each catalyzes the formation of a tetraketide from three molecules of malonyl CoA that are decarboxylated and condensed with a starter unit derived from p-coumaroyl CoA or a similar CoA thioester (Fig. 6). No reductions or dehydrations occur during either chalcone or stilbene synthesis, and some products spontaneously cyclize following their release from the enzyme. A major feature that distinguishes chalcone and stilbene synthases is that the latter perform an additional decarboxylation to remove a carbon atom that is present in chalcone products [132,138]. The presence of this additional carboxyl group results in a different cyclization pattern for chalcone products. The precise mechanisms by which chalcone and stilbene synthases determine the fate of this carbon atom are not known. 

Rg.6. Reactions catalyzed by chalcone and stilbene synthases. Each enzyme condenses three malonyl CoA extender units onto p-coumaroyl-CoA. Stilbene synthases catalyze an additional decarboxylation, resulting in a different pattern of cyclization for chalcone versus stilbene products... 

In addition decarboxylation of the (2 + 4)-adduct occurs to give a diene that is trapped as (175) by a second addition of maleimide. A further report by the same group states that the cycloaddition of maleimides with 2-pyrone carboxylates in the solid-state yields endo adducts. This is in contrast to the sensitized cycloaddition that leads exclusively to the exo products. 

Fatty acids have predominantly even numbers of carbon atoms because they are effectively formed from acetyl (C2) units, which are derived from glucose in the presence of various enzymes, coenzymes and carrier proteins. An overall scheme for saturated fatty acid biosynthesis is presented in Fig. 2.13, in which it can be seen that the first step is the formation of acetyl coenzyme A (often abbreviated to acetyl-CoA). One molecule ofacetyl-CoA undergoes addition of CO, to form malonyl-CoA, while the acetyl group on another molecule is transferred to an enzyme (fatty acid synthase). The malonyl unit (C3) is added to the enzyme-bound acetyl unit, which produces a butyryl group following loss of C02, dehydration and reduction. Six further steps of combined malonyl addition, decarboxylation, dehydration and reduction occur to yield palmitate (C16). Higher acids are built from palmitate in a similar... 

Esters 106 (R = Me, Et or Pr = Et, Pr, r-Bu or PhCHi) of aliphatic carboxylic acids react with lithium acetylides 107 (R = H, C5 Hi i or Ph) in the presence of boron trifluoride etherate in THE to give acetylenic ketones 108 (equation 18). Palladium-[tetrakis(triphenylphosphine)]-copper(I) iodide catalyses the oxidative addition-decarboxylation of propargyl methyl carbonates, e.g. 109, with terminal alkynes to yield 1,2-dien-4-ynes (allenylacetylenes) 110. The regiochemistry of the palladium-catalyzed addition of phenylacetylene to the allenic ester 111 depends on the nature of the catalyst used palladium(III) acetate-triphenylphosphine yields a 81 19 mixture of adducts 112 and 113, while in the presence of tetrakis(carbomethoxy)palladacyclopentadiene-tris(2,4,6-trimethoxyphenyl)phosphine the ratio is reversed to 9 91 k... 

The primary degradation reaction was dissociation of allophanate into phenyl isocyanate and alkyl carbanUate, followed by dissociation of the alkyl carbanUate into phenyl isocyanate and alcohol. Decarboxylation of the ethyl carbanUate fragment also took place slowly. A small amount of diphenyl carbodimide was observed at the pyrolysis temperature of 450°C. In addition, decarboxylation of the isopropyl carbanUate fragment took place at 550°C. A small amount of diphenyl carbodimide was observed from 350°C to 550°C. 

To convert coproporphyrinogen III to protoporphyrin IX, the ultimate precursor to heme and chlorophyll, additional decarboxylations and an oxidation are necessary (Scheme 14.30). The former results in the replacement of the propionate side chains of rings A and B with vinyl substituents, and the latter fully aromatizes the macrocyle. 

Methyl-2-(trifluoromethyl)chroman-4-ones 194a,b were obtained in good yields by reaction of chromene-4-imines 141 with malonic acid, which acts as methylating agent via addition-decarboxylation-hydrolysis sequence [98] (Scheme 62). 

The Effect of pH on Reaction Rate, Hydrothermal experiments in gold bag liners, as well as stainless steel and titanium vessels, have shown that the rate of decarboxylation of the acidic form of acetic acid is faster than the rate for the anion form (Kharaka et al. 1983 Palmer and Drummond 1986 Bell 1991 Bell et al. 1993). In addition, decarboxylation was found to be first order with respect to either acetic acid or acetate (Bamford and Dewar... 

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