Ester intermediates, and

The overall reaction catalyzed by epoxide hydrolases is the addition of a H20 molecule to an epoxide. Alkene oxides, thus, yield diols (Fig. 10.5), whereas arene oxides yield dihydrodiols (cf. Fig. 10.8). In earlier studies, it had been postulated that epoxide hydrolases act by enhancing the nucleo-philicity of a H20 molecule and directing it to attack an epoxide, as pictured in Fig. 10.5, a [59] [60], Further evidence such as the lack of incorporation of 180 from H2180 into the substrate, the isolation of an ester intermediate, and the effects of group-selective reagents and carefully designed inhibitors led to a more-elaborate model [59][61 - 67]. As pictured in Fig. 10.5,b, nucleophilic attack of the substrate is mediated by a carboxylate group in the catalytic site to form an ester intermediate. In a second step, an activated H20... 

If the cation introduced by ion exchange is capable of multiple valence, the clay may serve as a catalyst for oxidation or reduction reactions. For example, montmorillonite treated with iron(III) nitrate is so reactive that it has to be stored under an inert atmosphere the clay catalyzes reactions of the nitrate ion, such as oxidation of secondary alcohols to ketones (via nitrite ester intermediates) and organic hydrazides to azides, and the nitration of phenols. 

Alternatively, according to Kwart and George,28 the available experimental data are coherent with a hydrogen transfer by way of a cyclic five-membered transition state. A mechanism as in Equation below would be consistent both with a manganese ester intermediate and with the five-membered transition state suggested by Kwart and George. 

Diolefin crystals (I) were prepared through half-ester intermediates and, for the (II) - (V) crystals, by two-step aldol condensations. Further details of the preparation of these monomers have been described in recent papers (13). 

A second application of QM/MM methods to the quantification of steric effects is provided by the lMOMM(Becke3LYP MM3) study on the origin of enantioselectivity in the dihydroxylation of styrene by the (DHQD)2PYDZ-0s04 complex (21). The size and complexity of the catalyst, which can be seen in Figure 8, preclude the possibility of accurate pure QM calculations on the problem, because the selectivity is decided precisely by the bulky substituents in the NR3 cinchona group. The selectivity is defined by the initial approach of the substrate to the catalyst to form an osmate ester intermediate, and consequently a number of possible paths were analyzed. In particular, there are 12 such paths, defined by the three possible regions (A, B, and C) of approach of the substrate... 

Figure 11 Three possible mechanisms for the dehalogenation of frans-3-chloroacrylate. (a) Addition of water to the double bond, followed by enzyme-catalyzed or chemical decomposition of a short-lived halohydrin intermediate to afford malonate semialdehyde, (b) Conjugate addition reaction where the chlorine atom is displaced by a water-derived hydroxyl group, followed by tautomerization of the enol intermediate, (c) Conjugate addition reaction where the chlorine atom is displaced by an active site carboxylate group, followed by hydrolysis of the covalent ester intermediate, and tautomerization of the enol intermediate.

W. H. Daly and T. S. Evenson. Grafting of vinyl polymers to carboxylated poly(arylene ether sulfone) utilizing barton ester intermediates and nitrox-ide mediation. Polymer, 41(13) 5063-5071, June 2000. 

Daly WH, Evenson TS, lacono ST, Jones RW (2001) Recent developments in cellulose grafting chemistry utilizing barton ester intermediates and nitroxide mediation. Macromol... 

Dinitrochlorobenzene Dinitrofluorobenzene Diphencyprone Squaric acid dibutyl ester Intermediate and precursor products 2,6-Dichloropurine... 

Protection and Deprotection.—N-Protected a-amino-acids are readily esterified by methanol or ethanol in 60—80% yield after reaction with an enamine (e.g. from isobutyraldehyde and piperidine) and t-butyl isocyanate. Such amino-acids can also be esterified efficiently with alkyl halides under phase-transfer conditions with no racemization. Direct esterification of a-amino-acids with ethyl toluene-p-sulphonate in boiling ethanol gives a-amino-acid ethyl esters in 80—90% yield as the sulphonate salts. The protection of acid functions by formation of the 2-chloro-(or bromo-)ethyl esters has been discussed. These derivatives survive exposure to both moderately acidic and basic conditions and are removable by conversion into the iodoethyl analogues followed by zinc reduction. Alternatively, they may be converted into hydrophilic ammonium or phosphonium salts which exhibit enhanced acid stability but which are cleaved by very dilute base. Yet another method for the removal of such groups using supernucleophilic Co phthalocyanin anions has been reviewed. Further routes to 2,2,2-trichloroethyl esters have been described, one of which employs an activated ester intermediate and is suited to acid-labile substrates. 

In the Claisen condensation mechanism, the sodium methoxide removes an acidic hydrogen from the carbon a to the carbonyl carbon, resulting in the formation of an enolate ester (Wiedermann et al., 1961). This reaction produces a carbanion, a powerful nucleophile. This nucleophile will attack carbonyl groups, forming a P-keto ester intermediate and a glycerylate. The glycerylate is now free to attack other carbonyl carbons and exchange esters intra- and intermolecularly. Once this carbanion is created, the same considerations as for the usual carbonyl carbon chemistry apply. 

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