Direct dehydration pathway

Dihydroxyacid dehydratase of the branched-chain amino acid biosynthetic pathway catalyzes the dehydration and tautomerization of 2,3-dihydroxy-3-methyl-(butyrate and pentanoate) to 2-keto-3-methyl(butyrate and pentanoate). The enzyme isolated from spinach recently has been shown to have not a [4Fe-4S] cluster, but rather a spectroscopically unusual [2Fe-2S] cluster in its active site (68,69). The EPR spectrum of the reduced enzyme is similar to that seen for Rieske Fe-S proteins (71) with a g-average of 1.91. Upon addition of substrate the g-average of the EPR spectrum shifts to 1.96 (opposite the effect of substrate on aconitase), and then reverts back to a g-average of 1.90 when only the product is present The dramatic changes in the EPR spectra upon addition of substrate suggest, in analogy to aconitase, that the Fe-S cluster may be directly involved in catalysis. 

Furoxans are not formed by direct oxidation of furazans, but they can readily be prepared by ring closure or cycloaddition pathways. The most synthetically useful routes are the oxidative cyclization of 1,2-dioximes, the dehydration of a-nitroketoximes and, for symmetrically substituted furoxans, dimerization of nitrile oxides. For asymmetrically substituted analogues care must be taken in selecting the route in order to avoid formation of mixtures of 2- and 5-oxide isomers. 

Theoretical investigations indicated641 642 that two main pathways are possible for the first, dehydration step. One proceeds via a methoxy surface intermediate (10) formed as a result of the adsorption of methanol on acidic site of the zeolite (HO-Z) to give dimethyl ether [Eqs (3.63) and (3.64)]. The other, direct route is a reaction between two methanol molecules adsorbed simultaneously on the same active site [Eqs (3.65) and (3.66)] ... 

The reaction of metal N-alkylcarbamates M(C)2CN 11 R) (M = Na, Mn(II), Co(II) R = Ph, Pr, Cy) with R C(0)C1 (R = Me, Ph) takes place, at ambient temperature, in a more complex way with the formation of isocyanates (RNCO), carboxylic anhydrides (R C(0)0C(0)CR ), amides (RNHC(O)R ) and C02. Amide formation and the evolution of C02 can be due to (i) the decomposition of mixed anhydride RNHC(0)0C(0)R obtained by addition of the acyl chloride to the oxygen atom of the carbamate group or (ii) the direct reaction of acyl chloride at the carbamic nitrogen atom of M(02CNHR) . The mixed anhydride RNHC(O) 0C(0)R might also decompose via another route so as to afford isocyanate and carboxylic acid. However, a different pathway (Scheme 6.6) has been also envisaged for the formation of RNCO and R C(0)0C(0)CR, which excludes any intermediacy of the mixed anhydride [61a], Two acetic acid molecules, bound to the same metal or to different metal centers, would then be dehydrated and acetic... 

Levulinic Acid. Dehydration of glucose or other monomeric and polymeric C6 sugars leads to the direct formation of levulinic acid (LA) as a potential primary building block for the biorefinery, and several reviews have described its potential commercial utility 477,478 The preparation of levulinic acid is not difficult, although the mechanism of its formation from carbohydrates is complex, and offers several alternative decomposition pathways (equation 3).479... 

OH, O and Nj radicals were reported to react with chloro-and hydroxy-derivatives of aniline at diffusion-controlled rates with k> dm mol s and the rates are nearly identical for all isomers of chloroaniline. The OH radical reaction involves both addition and direct H abstraction and the extent of the two reactions is determined by the position of the substituent. The initially formed OH adduct then undergoes dehydration to give the anilino radical. The attack of the OH radical at the carbon bonded to Cl is a minor pathway (not more than 15%) as was confirmed by the detection of Cl ions. 

Evidently, however, another species arises in a side, self-poisoning, reaction and extensively covers the surface, inhibiting the progress of the above main reaction in the sequence of steps shown (89-91) In situ IR spectroscopy shows that this species is principally chemisorbed CO, bridged or linearly bonded to surface metal atoms. Its behavior is similar to that observed with CO directly chemisorbed at a Pt electrode from the gas phase. However, the mechanism of its catalytic formation from HCOOH is unclear. It is well known that CO can be formed from HCOOH by dehydration, but such conditions do not obtain at a Pt electrode in excess liquid water. Hence a catalytic pathway for adsorbed CO formation has to be considered. The species C=0 or C—OH are not to be regarded as the kinetically involved intermediates in the main reaction sequence (Section IV). Because the poisoning species seems to be formed in the presence of coadsorbed, H steps such as... 

Although the reactions of peroxisomal /3-oxidation are similar to those in mitochondria, there are some notable differences. First, the initial reaction in the peroxisomal pathway is catalyzed by a different enzyme. This reaction is a dehydration catalyzed by an acyl-CoA oxidase. The reduced coenzyme FADH2 then donates its electrons directly to 02 instead of UQ (coenzyme Q). 

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