Mechanism 3-oxidation pathway

Michael reactions and, 895 Beta-keto ester, 851 alkylation of, 859-860 cyclic, 892-893 decarboxylation of, 857, 860 Michael reactions and. 895 pKd of, 852 synthesis of, 892-893 Beta-lactam antibiotics, 824-825 Beta oxidation pathway, 1133-1137 mechanism of, 1133-1136 Beta-pleated sheet (protein), 1038 molecular model of, 1039 secondary protein structure and, 1038-1039 Betaine, 720 Bextra. structure of, 544 BHA, synthesis of, 629 BHT, synthesis of. 629 Bicycloalkane. 129 Bijvoet. J. M., 299 Bimolecular, 363... 

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

Most in vitro studies of xanthines have centered around the enzyme xanthine oxidase. Bergmann and co-workers 40-4)) have examined the main oxidative pathways in the xanthine oxidase catalyzed oxidation of purines. The mechanism proposed by these workers 41 > is that the enzyme binds a specific tautomeric form of the substrate, regardless of whether or not that form represents the major structure present in solution. It is then proposed that the purine, e.g., xanthine, undergoes hydration at the N7=C8 double bond either prior to or simultaneously with dehydrogenation of the same position. Accordingly, the process would involve either pathway a or b. Fig. 15. Route a would give a lactim form of the oxidized purine, while b would give the cor-... 

Its chemical structure does not allow elimination of HNO, thus supporting the oxidative pathway of activation to NO, a mechanism still possible in this blocked SIN-1A derivative. The final product of the NO-release was found to be l-amino-2-cyanomorpholine (110) [106]. Its formation can be rationalized assuming that, after the oxidative NO-release, deprotonation occurs at the a-position of the morpholine, followed by migration of the cyano group and hydrolytic cleavage of the hydrazone moiety. 

Cyclic. S -Mannich bases are rarely encountered in medicinal chemistry. The (R)-thiazolidine-4-carboxylic acids (11.113, Fig. 11.15), which are used as derivatives and chemical delivery systems for L-cysteine (11.114), provide an excellent example of S-Mannich bases. These compounds underwent activation by two distinct mechanisms, directly by nonenzymatic hydrolysis to cysteine and the original aldehyde (Fig. 11.15, Pathway a), and oxidatively (Pathway b) [138]. The latter route involved first oxidation by mitochondrial enzymes to the (f )-4,5-dihydrothiazole-4-carboxylic acid (11.115), followed by (presumably nonenzymatic) hydrolysis to /V-acylcysleine, and, finally, cytosolic hydrolysis to cysteine (11.114). 

Rats exposed to a fteptone-containing atmosphere excreted a variety of metabolites resulting from oxidative pathways [176]. The major metabolites were isomeric mono-alcohols and ketones, but small amounts of 2-ethyl-5-methyl-2,3-dihydrofuran (11.171, R = Et, R = Me, Fig. 11.22,a) and 5-ethyl-2-methyl-2,3-dihydrofuran (11.171, R = Me, R = Et) were also detected. These metabolites are believed to arise from 6-hydroxyheptan-3-one (11.170, R = Et, R = Me) and 5-hydroxyheptan-2-one (11.170, R = Me, R = Et). The postulated mechanism of formation of 2,3-dihydrofurans involves their equilibrium with the corresponding linear y-hydroxy ketones, as shown in Fig. 11.22,a. Such a reaction has been documented for linear y-hydroxy aldehydes [177],... 

Several products were also detected in base-degraded D-fructose solution acetoin (3-hydroxy-2-butanone 62), l-hydroxy-2-butanone, and 4-hydroxy-2-butanone. Three benzoquinones were found in the product mixture after sucrose had been heated at 110° in 5% NaOH these were 2-methylbenzoquinone, 2,3,5-trimethylbenzoquinone, and 2,5-dimethyl-benzoquinone (2,5-dimethyl-2,5-cyclohexadiene-l,4-dione 61). Compound 62 is of considerable interest, as 62 and butanedione (biacetyl 60) are involved in the formation of 61 and 2,5-dimethyl-l,4-benzenediol (63) by a reduction-oxidation pathway. This mechanism, shown in Scheme 10, will be discussed in a following section, as it has been proposed from results obtained from cellulose. 

As a general mechanism, the degradation of PVA starts outside the cells via enzymatic attack on the polymer. The resulting products are a mixture of acetoxy hydroxy and hydroxy fatty acids. Upon intracellular enzymatic deacetylation, hydroxy fatty acids are generated that can be further metabolised via the classical p-oxidation pathway and TCA cycle. 

Of the aromatic hydrocarbons, the oxidative pathways of benzene have been studied most exhaustively. Fuji et al. proposed a global mechanism in the early 1970s, in which the C—H bond of benzene is broken to form the phenyl (CgHs ) radical that... 

The impact of oxygen on wine aroma is likely to involve several oxidation mechanisms. One pathway involves pol) henol quinones, particularly in the case of the removal of unwanted sulfur-containing off-odors (RSH Mestres et ah, 2000), as illustrated in Fig. 4.4A. 

A large number of studies have been published about the mechanism of oxidation of benzene to maleic anhydride, and the structure and role of catalysts and promoters.999,1006 1007 At present hydroquinone formed through a peroxidic adduct is well established to be the intermediate in selective oxidation1008 [Eq. (9.179)], whereas p-benzoquinone is the intermediate in the nonselective oxidation pathway [Eq. (9.180)] ... 

High-Temperature Oxidation The high-temperature oxidation of methane has been studied extensively, and the mechanism is better established than that of the low temperature oxidation. At high temperatures the methylperoxy radical (CH302) is no longer stable, and the low-temperature oxidation pathway initiated by reaction (R15) is not active. Furthermore the chain-branching reaction... 

While in lab experiments or on-site the pH can be controlled, in an in situ application it will always decrease due to the formation of organic acids. This will effect shifts in the oxidation mechanism toward the direct oxidation pathway and in the chemical equilibrium of the soil. Furthermore, both ozone applications will result in changes in the soil chemical constituents, i. e. the cation exchange layer and the humic fraction. The consequences of these changes are still mostly unknown. A special lag-phase and a selection of bacteria in regrowth might be caused by the ozonation. 

One of the important mechanistic uses of isotopic substitution is that it can selectively affect different steps in a stepwise mechanism and so help resolve mechanisms and pathways. For example, in reactions where there is hydride transfer coupled to C—C bond formation, the measurement of both 13C/12C and D/H kinetic isotope effects can distinguish whether the steps are concerted or stepwise. Consider, for example, the oxidative decarboxylation of malate catalyzed by the malic enzyme, which could occur either via an intermediate mechanism (2.81) or in a concerted mechanism (2.82).59... 

The molecular mechanism of the selective oxidation pathway is believed to be the one shown in Scheme 2 (Section I). Adsorbed butene forms adsorbed 7r-allyl by H abstraction in much the same way as xc-allyl is formed from propene in propene oxidation (28-31). A second H abstraction results in adsorbed butadiene. Indeed, IR spectroscopy has identified adsorbed 71-complexes of butene and 7t-allyl on MgFe204 (32,33). On heating, the 7r-complex band at 1505 cm 1 disappears between 100-200°C, and the 7t-allyl band at 1480 cm-1 disappears between 200-300°C. The formation of butadiene shows a deuterium isotope effect. The ratio of the rate constants for normal and deuterated butenes, kH/kD, is 3.9 at 300°C and 2.6 at 400°C for MgFe204 (75), 2.4 at 435°C for CoFe204, and 1.8 at 435°C for CuFe204 (25). The large isotope effects indicate that the breaking of C—H (C—D) bonds is involved in the slow reaction step. 

On the other hand, above 20mol% SbF5, a small but increasing amount of unionized SbF5 can be observed, which may rationalize the change in the mechanism of alkane activation from the protolytic to the oxidative pathway, when the concentration of SbF5 increases over 20mol% (see Section 5.1.1). 

The Oxidative Pathway. For a long time, one of the difficulties in understanding the mechanism of the superacid-catalyzed transformations of alkanes was that no... 

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