Pathways competing

Scheme 36). Interestingly, the higher order cuprate 206 underwent conjugate addition with only moderate selectivity. This is likely due to the intervention of an electron transfer pathway. Competing electron transfer reactions involving a-alkoxymetal reagents of this type have also been reported by Cohen [81]. 

There are three groups of eicosanoids that are synthesized from C20 eicosanoic acids derived from the essential fatty acids linoleate and a-linolenate, or directly from dietary arachidonate and eicosapentaenoate (Figure 23-5). Arachidonate, usually derived from the 2 position of phospholipids in the plasma membrane by the action of phospholipase Aj (Figure 24-6)—but also from the diet—is the substrate for the synthesis of the PG2, 1X2 series (prostanoids) by the cyclooxygenase pathway, or the LT4 and LX4 series by the lipoxygenase pathway, with the two pathways competing for the arachidonate substrate (Figure 23-5). 

Poly(3HB) synthesis in various subcellular compartments could be used to study how plants adjust their metabolism and gene expression to accommodate the production of a new sink, and how carbon flux through one pathway can affect carbon flux through another. For example, one could study how modifying the flux of carbon to starch or lipid biosynthesis in the plastid affects the flux of carbon to acetyl-CoA and poly(3HB). Alternatively, one could study how plants adjust the activity of genes and proteins involved in isoprenoid and flavonoid biosynthesis to the creation of the poly(3HB) biosynthetic pathway in the cytoplasm, since these three pathways compete for the same building block, i. e., acetyl-CoA. 

A mechanistic scheme, (Scheme 4) similar to that previously reported (72) for nucleophilic substitution, in which pre-association, free-ion and ion-pairs pathways compete, can therefore also be proposed for bromination. Based on this scheme, the stereochemical behaviour depends on both solvent and substituents. In trifluoroethanol, the reaction occurs independently of the substituents on the double bond, via free ions ... 

Most bacterial proteins that use the TAT pathway bind cofactors, which may interact with the translocation machinery. However, other types of proteins also seem to use this pathway. So, Sec-dependent and TAT-dependent pathways compete with each other. Increasing the hydropho-bicity of a TAT-directing signal can convert it into a Sec-directing signal (Cristobal et al., 1999). 

In longer chain hydrocarbon radicals, isomerization reactions become more important these pathways compete with bimolecular oxidation reactions and can impact ignition rates at low temperatures. For instance, -propylperoxy radical can undergo several unimolecular dissociations/rearrangements ... 

Notice the particular features of this kind of ohgonucleotide the hexameric sequence is said to be self-complementary, since two identical molecules can form a duplex via Watson and Crick bases. It may also be noted from Figure 7.5 that two parallel pathways compete for the formation of the template T, namely the template-dependent, autocatalytic pathway, and the template-independent, non-autocatalytic one. This competition is the reason why the initial rate of the autocatalytic synthesis was found to be proportional to the square root of the template concentration -something that von Kiedrowski and colleagues called the square-root law of autocatalysis. As Burmeister (1998) put it ... 

The kinetics of radical reactions can be studied by direct methods (discussed in Chapter 18 of this volume) or by indirect methods. Indirect kinetic studies require no special instrumentation, and are popular for obtaining relative or absolute rate constants for an intermediate that might be formed in a specific conversion. The radical of interest is generated in a reaction where two pathways compete, the reaction of interest and the basis reaction, for which a rate constant is known. Relative rate constants are then determined from the product mixture by spectroscopy or chromatography and used with the basis rate constant to calculate the absolute rate constant for the reaction of interest. The method is most easily applied... 

Boron cations, Me2B+ and (MeO)2B+, are highly reactive towards carbonyl compounds, but are not easily studied in condensed phase, hi the gas phase, two pathways compete 183 (i) C=0 cleavage, with OH abstraction leaving behind a... 

A number of disilanes have been investigated for their suitability as potential silene precursors. The formation of silenes by the 1,3-silyl migration pathway competes with dehydrosilylation, which also gives silenes, and with homolytic Si—Si bond cleavage to give silyl radicals. 

Activation energies for unimolecular 1,3-hydrogen shifts connecting ketones and enols are prohibitive, so that thermodynamically unstable enols can survive indefinitely in the gas phase or in dry, aprotic solvents. Ketones are weak carbon acids and oxygen bases enols are oxygen acids and carbon bases. In aqueous solution, keto-enol tautomerization proceeds by proton transfer involving solvent water. In the absence of buffers, three reaction pathways compete, as shown in Scheme 2. 

The competing pathways in the reactions of intermediate l-alkylidene-2-oxyallyl cations (47) with furan have been investigated.54 Cycloaddition pathways compete with the electrophilic substitution pathway which initially forms the cation (48) from which a number of products may form. 

When migratory aptitudes of a-substituents of alkylidene carbenes are relatively poor, the MCI pathway competes with the MCR reaction. Reaction of the alkynyliodane with benzenesulfinate anion in water leads to a mixture of the MCI and the MCR products, because of a moderate migratory aptitude of aryl-sulfonylgroups [Eq. (114)] [170]. 

Theoretical studies have been reported for the neutral29 and alkaline30,31 hydrolysis of formamide. A theoretical study of the acid hydrolysis of iV-formylaziridine concluded that both N- and O-protonated pathways compete.32 In an historical overview of tetrahedral intermediates in the reactions of carboxylic acid derivatives with nucleophiles, several citations of amide reactions are included.33... 

Figure 28.4. Mechanism of quinone drug redox cycling by liver microsomal NADPH cytochrome P450 reductase. A one-electron pathway competes with a two-electron process (catalyzed by DT-diaphorase) and is coupled with the monoxygenation and glucuronidation and sulfation systems.

Acyclic ( -pentadienyl)Fp complexes also behave as Diels-Alder dienes in some cases, although two other reaction pathways compete. With TCNE and maleic anhydride, (17 R = H, Me) undergoes cycloaddition in moderate yields. Highly electron-deficient alkynes, on the other hand, undergo Fe bond insertion reactions reminiscent of SO2 reactions with ( -allyl)Fp complexes, to give (30). Finally, when the pentadiene is disubstituted at C-5, the Diels-Alder route is effectively blocked, and [3 + 2] cycloadditions result with TCNE at the 2,3-double bond. ... 

The initial disclosure of a [6 + 4] cycloaddition between tropone (1) and a 1,3-dipole involved diphe-nylnitrilimine as depicted in Scheme In this instance the [6 + 4] pathway competed rather poorly with various alternative [4 + 2] pathways and only a small quantity of the adduct in which the dipole added across the 2- and 7-positions of (1) was recovered. The three [4 + 2] adducts (45)-(d7) that were isolated from the reaction mixture presumably arose from a base-catdyzed hydrogen shift that occurred subsequent to the initial cycloaddition. Efforts to account for the divergent behavior of dienes and 1,3-dipolar species in their reactions with tropone have included invoking a dipole repulsion between the large positive charge located on the central atom of the 1,3-dipole and the partial positive charge on tropone, which must come into close proximity in the transition state of a concerted [6 + 4] cycloaddition. A ca-... 

For the parent (20 l,3,S-hexatriene (1), the main pathways competing with the desired electrocycliza-tion to (2) are the isomerization of the central double bond to give the ( )-triene (50) as well as die formation of other cyclized products (51) and (52) and the vinylallene (53). The sensitized irradiation of isomeric trienes results primarily in rapid (Z)-(E) isomerization about the central double bond and dimerization. By contrast to singlet state photochemistry, no electrocyclic or sigmatropic rearrangements occur under sensitized conditions. ... 

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