Intermediates common

Aldolases cataly2e the asymmetric condensation of intermediates common in sugar metaboHsm, such as phosphoenolpymvic acid, with suitable aldehyde acceptors. Numerous aldolases derived from plants or animals (Class I aldolases) or from bacteria (Class II) have been examined for appHcations (81). Efforts to extend the appHcations of these en2ymes to the synthesis of unusual sugars have been described (2,81). 

Process Primaries Intermediates (common usage) Colorants (common usage)... 

Aprotic polar solvents have to be used for several reasons. They are often good solvents for both monomers (including phenolates) and amorphous polymers. In addition, they can also stabilize the Meisenheimer intermediates. Common aprotic polar solvents, such as DMSO, /V,/V-dimcthyl acetamide (DMAc), DMF, N-methyl pyrrolidone (NMP), and cyclohexylpyrrolidone (CHP) can be used. Under some circumstances, very high reaction temperature and boiling point solvents such as sulfolane and diphenyl sulfone (DPS) have to be used due to the poor reactivity of the monomers or poor solubility of the resulting, possibly semicrystalline polymers, as in the PEEK systems. 

Fig. 8.5. Mechanism postulated for competitive, specific base catalyzed hydrolysis and acyl migration of catechol monoesters, as seen with 4-pivaloyl-L-dopa (8.81) [114a]. Deprotonation (Reactions a and b) accelerates intramolecular nucleophilic attack (Reactions c and d) to form a tetrahedral transition state. The latter is postulated to be the intermediate common to hydrolysis (Reaction e) and acyl migration. 

The metabolism of NNK is also under investigation. When NNK was incubated with rat liver microsomes under the usual conditions and the mixtures extracted and analyzed by GLC-MS, both myosmine and the keto alcohol, 4-hydroxy-l-(3-pyridyl)-l-butanone were indent fied as indicated in Figure 14. These results provide evidence for hydroxylation of the N-methyl group of NNK, a metabolic step which produces intermediates common to NNK and NNN. 

The formation of acyclic enediols is, apparently, the initial reaction that leads to dehydration products. Sugar enediols are transitory compounds that have never been isolated. However, because, when treated with either acid or base, an aldose gives rise to its 2-epimer, as well as to its 2-keto isomer, a persuasive argument is provided for the 1,2-enediol as the intermediate common to each of the products. The evidence in favor of these intermediates is based primarily on isotope-exchange experiments, on reactions that involve isomeriza-tions of O-methyl sugars, and on kinetic measurements.10... 

The disproportionation activity in the supported species is parallel to the increased activity of ethylene polymerization on supported catalysts. Many of the steps in the reaction may be identical for example, the initial coordination of olefin to the metal center will be common to both systems. Indeed, some of these catalysts are also ethylene polymerization catalysts (see Table IV) although their activities are much less than the corresponding zirconium derivatives. A possible intermediate common to both disproportionation and polymerization could be the hydrocarbyl-olefin species (Structure I). Olefin disproportionation would result if the metal favored /3-hydrogen elimination to give the diolefin intermediate (Structure II) which is thought to be necessary for olefin disproportionation. Thus, the similarity between the mechanism and activation of olefin disproportionation and polymerization is suggested. 

Several studies have tackled the structure of the diketopiperazine 1 in the solid state by spectroscopic and computational methods [38, 41, 42]. De Vries et al. studied the conformation of the diketopiperazine 1 by NMR in a mixture of benzene and mandelonitrile, thus mimicking reaction conditions [43]. North et al. observed that the diketopiperazine 1 catalyzes the air oxidation of benzaldehyde to benzoic acid in the presence of light [44]. In the latter study oxidation catalysis was interpreted to arise via a His-aldehyde aminol intermediate, common to both hydrocyanation and oxidation catalysis. It seems that the preferred conformation of 1 in the solid state resembles that of 1 in homogeneous solution, i.e. the phenyl substituent of Phe is folded over the diketopiperazine ring (H, Scheme 6.4). Several transition state models have been proposed. To date, it seems that the proposal by Hua et al. [45], modified by North [2a] (J, Scheme 6.4) best combines all the experimentally determined features. In this model, catalysis is effected by a diketopiperazine dimer and depends on the proton-relay properties of histidine (imidazole). R -OH represents the alcohol functionality of either a product cyanohydrin molecule or other hydroxylic components/additives. The close proximity of both R1-OH and the substrate aldehyde R2-CHO accounts for the stereochemical induction exerted by RfOH, and thus effects the asymmetric autocatalysis mentioned earlier. 

Since phenyl a-D-galactopyranoside did react, although slowly, it is apparent that this substance was converted to some intermediate common to it and the jS-D-galactopyranoside, or else it was degraded by a mechanism unique to itself. The conversion of phenyl /3-D-mannopyranoside to l,6-anhydro-/3-D-mannopyranose9 was effected with relative ease, whereas phenyl a-D-mannopyranoside reacted at about the same rate but the intermediate product, which was apparently not stabilized by 1,6-anhydride formation, underwent extensive destruction in the alkali.8... 

Most SN reactions of carboxylic acids and their derivatives follow one of three mechanisms (Figures 6.2, 6.4, and 6.5). A key intermediate common to all of them is the species in which the nucleophile is linked with the former carboxyl carbon for the first time. In this intermediate, the reacting carbon atom is tetrasubstituted and thus tetrahedrally coordinated. This species is therefore referred to as the tetrahedral intermediate (abbreviated as Tet. Intermed. in the following equations). Depending on the nature of the reaction partners, the tetrahedral intermediate can be negatively charged, neutral, or even positively charged. 

Metallacycles have been claimed to play pivotal roles in many transition metal-mediated multi-component coupling reactions [1]. For example, [2 -i- 2 -i- 2] alkyne cyclo-trimerization leading to benzenes - the Reppe reaction - has been considered to proceed via metallacyclopentadiene and elusive metallacycloheptatriene intermediates ("common mechanism ), while metallacyclopentenes have been proposed as intermediates for the [2 -i- 2 -i- 1] cyclo-coupling reactions of an alkyne, an alkene, and CO leading to a cyclopentenone (the Pauson-Khand reaction). A metallacyclic compound - which is defined here as a carbocyclic system with one atom replaced by a transition metal element - can be generally formed by oxidative cyclization of two unsaturated molecules with a low-valent transition metal fragment [2-4]. Alter-... 

Problem 23.11 The roactkm of ii-biitylMiiiiie with sodium nitrite and hydrochloric acid yields nitrogen and the following mixture ii-butyl alcohol, 25% sec-butyl alcohol, 13% 1-butene and 2-butene, 37% ir-butyl chloride, 5% jec butyl diloride, 3%. (a) What is the most likely intermediate common to all of th products (b) Outline reactions that account for the various products. 

An alternative mechanism for Scheme 5.6.18 involves a palladium-catalyzed formal [2+2+2] cycloaddition 55 56b, followed by oxidative addition of Pd(0) to give 56a as an intermediate common to both cascades. Apparently, both mechanisms may operate, since mixtures of 56b and 57 are obtained under certain conditions. Significantly, under the conditions of Scheme 5.6.18, 56b is not converted to 57. 

Second, in the reversal to olefin one should expect that a three-center intermediate common to both octalins might not remember the original location of the double bond, i.e., reversal should give isomerization to a common product ... 

The large inverse deuterium isotope effects, that is the ratio of the formation rates for CsDfiO and C3HgO, nj/rir of 3-5, much greater than the possible range of the true deuterium inverse isotope effects3, have been rationalized qualitatively within the framework of the reaction scheme (equation 1), by assuming that C—H versus C—D bond rupture determines the further total oxidation of the intermediate, common to both epoxide formation and full oxidation to C02. 

Peroxides are a common source of radical intermediates. Commonly used initiators include benzoyl peroxide, f-butyl peroxybenzoate, di-f-butyl peroxide, and r-butyl hydroperoxide. Reaction generally occurs at relatively low temperature (80° -100°C). The oxygen-oxygen bond in peroxides is weak ( 30kcal/mol) and activation energies for radical formation are low. Dialkyl peroxides decompose thermally to give two alkoxy radicals. ... 

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