Multistep oxidations catalyzation

Subsequent multistep oxidations catalyzed by one P450 have been found not only within steroid metabolism but also in the degradation ofxenobiotics by mammalian and nonmammalian P450s. 

Scheme 5.8 Demethylation of lanosterol via multistep oxidations catalyzed by CYP51.

Although some mechanistic studies have been conducted, the mechanisms of multistep oxidations catalyzed by one P450 on a single substrate remain unclear in most cases. 

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

Fig. 4.51 Multistep CYP24Al-catalyzed oxidation of The figure emphasizes the probable role of the 23-alcohol 25-hydroxyvitamin D3, including a C-C bond cleavage as a side product rather than as an obligatory intermediate step, to the metabolite 24,25,26,27-tetranor-23-OH-D3. in the side-chain cleavage sequence...

Scheme 5.4 Multistep oxidation of cholesterol to pregnenolone catalyzed by CYPllAl.

Scheme 5.5 Possible multistep oxidation of deoxycorticosterone to aldosterone catalyzed by CYP11B2. Bold arrows indicate the experimentally confirmed biosynthesis pathway toward...

Scheme S.IO Multistep pyrene oxidations catalyzed by CYP1A2.

Scheme 5.11 Multistep oxidations of ethyl carbamate (a) and ethanol (b) catalyzed by ...

Scheme 5.12 Multistep oxidations of ent-kaurene toward gibberellin 12 (GAjj) catalyzed by CYP701A3 and CYP88A enzymes.

Adipinic acid is produced industrially by oxidation of cyclohexane in a multistep oxidation process with cyclohexanol/cyclohexanone as important intermediates. The process operates at 125-165 °C and 8-15bar and uses 50-65% nitric acid as the oxidant. As coupling product of the oxidation dinitrogen oxide (N2O) is formed that has to be removed from the process flue gas due to its ozone layer depletion effect and its very high global warming potential (298 times that of CO2). The reaction is catalyzed by vanadium and copper salts and reaches product yields of 96%. 

As discussed for Figure 10.3B, a large baseline signal is encountered in HPLC-PAD for the oxide-catalyzed detections of amino acids and sulfur compounds (Mode n). Furthermore, the large baseline current is frequently observed to drift to large anodic values, especially for new or freshly polished electrodes. This drift is the consequence of a slow growth in the true electrode surface area as a result of surface reconstruction caused by the oxide on-off cycles in the applied multistep waveforms. As listed in Table 10.2, Mode II detections performed with PAD are subject to a number of disadvantages because of the formation of surface oxide, which is required and concomitant with the detection of amine- and sulfur-based compounds. 

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

Passage of 1.0 mol of electrons (one faraday, 96,485 A s) will produce 1.0 mol of oxidation or reduction—in this case, 1.0 mol of Cl- converted to 0.5 mol of Cl2, and 1.0 mol of water reduced to 1.0 mol of OH- plus 0.5 mol of H2. Thermodynamically, the electrical potential required to do this is given by the difference in standard electrode potentials (Chapter 15 and Appendix D) for the anode and cathode processes, but there is also an additional voltage or overpotential that originates in kinetic barriers within these multistep gas-evolving electrode processes. The overpotential can be minimized by catalyzing the electrode reactions in the case of chlorine evolution, this can be done by coating the anode with ruthenium dioxide. 

Fatty acid oxidation is a multistep process requiring orchestration of reactions in the cytoplasm and mitochondria (Fig. 9-1). Free fatty acids enter the cell and are activated to their coenzyme A (CoA) thioesters in the reaction catalyzed by fatty acyl-CoA synthetase ... 

In this section, we seek to identify materials that are the reasonable first structures to arise from biomass deconstruction, and to describe how chemically catalyzed processes are being developed for their production. For that reason, commercially practiced processes that use catalysis, such as the reduction of glucose to sorbitol, are mentioned only briefly or not at all. Chemical catalysis will certainly play an additional role in the further conversion of these initial building blocks into secondary intermediates or final marketplace products (e.g., oxidative conversion of levulinic acid into succinic acid), but such multistep possibilities are outside the scope of this discussion. 

The reaction of 3-bromo-4-nitroquinoline 1-oxide (220) with 1-morpholinocyclohex-ene 14 gave furoquinoline 228 after treatment with base (equation 48)118. It was demonstrated that the reaction proceeds via a multistep ionic process involving the initial formation of 225 via 221-224 followed by a base-catalyzed enolization to 226 and cyclization to 227 and 228. Apparently, the strong electron-withdrawing effect of the nitro group is essential for the reaction to occur, because similar compounds without the nitro group do not undergo this conversion. 

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