Acylation rate

Figure 13 indicates burst kinetics. As discussed before, such biphasic curves indicate the reaction to occur through two steps involving an acylated intermediate. The initial slopes for the presteady state can be taken as the measure of acylation rates, and the slopes of the later straight line for steady-state can be taken as the measure of deacylation rates. 

We initially tested Candida antarctica lipase using imidazolium salt as solvent because CAL was found to be the best enzyme to resolve our model substrate 5-phenyl-l-penten-3-ol (la) the acylation rate was strongly dependent on the anionic part of the solvents. The best results were recorded when [bmim][BF4] was employed as the solvent, and the reaction rate was nearly equal to that of the reference reaction in diisopropyl ether. The second choice of solvent was [bmim][PFg]. On the contrary, a significant drop in the reaction rate was obtained when the reaction was carried out in TFA salt or OTf salt. From these results, we concluded that BF4 salt and PFg salt were suitable solvents for the present lipase-catalyzed reaction. Acylation of la was accomplished by these four enzymes Candida antarctica lipase, lipase QL from Alcaligenes, Lipase PS from Burkholderia cepacia and Candida rugosa lipase. In contrast, no reaction took place when PPL or PLE was used as catalyst in this solvent system. These results were established in March 2000 but we encountered a serious problem in that the results were significantly dependent on the lot of the ILs that we prepared ourselves. The problem was very serious because sometimes the reaction did not proceed at all. So we attempted to purify the ILs and established a very successful procedure (Fig. 3) the salt was first washed with a mixed solvent of hexane and ethyl acetate (2 1 or 4 1), treated with activated charcoal and passed into activated alumina neutral type I as an acetone solution. It was evaporated and dried under reduced... 

The inclusion of DMAP to the extent of 5-20 mol % in acylations by acid anhydrides and acyl chlorides increases acylation rates by up to four orders of magnitude and permits successful acylation of tertiary and other hindered alcohols. The reagent combination of an acid anhydride with MgBr2 and a hindered tertiary amine, e.g., ( -Pr)2NC2H5 or 1,2,2,6,6,-pentamethylpiperidine, gives an even more reactive acylation system, which is useful for hindered and sensitive alcohols.105... 

N-Carbobenzoxy-L-alanine-/>-nitrophenyl ester is a specific substrate for elastase in which the rate-limiting step is deacylation, that is, hydrolysis of the acyl-enzyme intermediate. In 70% methanol over a reasonable temperature range the energy of activation of the turnover reaction, that is, deacylation, is 15.4 kcal mol. In the pH 6-7 region in this cryoprotective solvent, the turnover reacdon can be made negligibly slow at temperatures of -50 C or below. Under such conditions/i-nitro-phenol is released concurrent to acyl enzyme formation in a 1 1 stoichiometry with active enzyme in the presence of excess substrate. In other words, even at low temperatures, the acylation rate is much faster than deacylation and the acyl enzyme will accumulate on the enzyme. The rate of acyl-enzyme formation can be monitored by following the rate of p-nitrophenol release, and thus the concentration of trapped acyl enzyme may be determined. This calculadon has been carried out and... 

The inclusion of DMAP to the extent of 5-20 mol% in acylations by acid anhydrides and acyl chlorides increases acylation rates by up to four orders of magnitude and permits successful acylation of tertiary and other hindered alcohols. 

To generate high-molecular-weight perfluorinated polyimides, it is first necessary to determine how fluorine affects the reactivity of the monomers. In particular, the effect of substituting hydrogen with fluorine on diamine reactivity is important because kinetic studies of the acylation of conventional monomers have revealed that acylation rate constants can differ by a factor of 100 among different dianhydrides, and by a factor of 10 among different diamines. ... 

When the ester is mixed with the enzyme, there is a rapid exponential phase followed by a linear increase in the absorbance due to the nitrophenol. The rate constant for acylation and the dissociation constant of the enzyme-substrate complex may be calculated from the concentration dependence of the rate constant for the exponential phases (Chapter 4, equation 4.46). (The rate constant of the linear portion gives the deacylation rate, but this is a steady state measurement.) Unfortunately, nitrophenyl esters are often so reactive that the acylation rate is too fast for stopped-flow measurement. 

A similar deacylation mechanism is proposed in a micellar catalysis. Lauryl-imidazolylhydroxamic acid in a cationic micelle gives a high acylation rate, deacylation rate and turnover number, which are estimated to be 159 M-1 - sec-1, 4x10 3 sec-1 and 3.2x10 3 sec 1, respectively (88). 

The acylimidazoles (Table 6.1, entry 10) and the A-acylpyridinium salts (entry 12) occupy additional leading positions with respect to their acylation rates. In the acylimidazoles the free electron pair of the acylated N atom is essentially unavailable for stabilization of the C=0 double bond by resonance because it is part of the 17-electron sextet, which makes the imidazole ring an aromatic compound. For a similar reason there is no resonance stabilization of the C=0 double bond in A-acylpyridinium salts in the corresponding resonance form, the aromatic sextet of the pyridine would be destroyed in exchange for a much less stable quinoid structure. 

The acylation rate (ka) did not differ mudi between die terpolymer (PHA-MIm-AAm) and a copolymer which contains only die hydroxamate functional group (PHA-AAm, 7). In contrast, die hydrolysis of the acelyl hydroxamate intermediate was greatly accelerated by the assistance of the intrapolymeric imidazole group (60—80 fold increase in kj). 

PNPA may be hydrolyzed via acylation (k,) and deaqdation (k ) of the PHA unit (more efficient route), or directly by the action of the Vim unit (kyim)- Th acylation rate of die hydroxamate unit is again not much affected by die introduction of the Vim unit, while the deacjiation rate is remarkably increased. The imidazole group can accelerate deacjdation by general-basic and nucleophilic catalyses. 

However, the acylation rate with PNPA is 10 to 20 times greater than that of ITIA-Vlm-AAm 23, again indicating the unusual reactivity of the zwitterionic nucleophile. 

Fig. 5—3 summarizes pH-rate profOes of acylation of various cata c polymers. It is clear that hydroxamate polymers ve acylation rates that are much faster than those of imidazole pdiymeis (Vm-AAm, Mhn-AAm), over the whole pH range. Fig. 

It is apparent that file catalytic efficiency of synthetic catatysts is never much inferior to that of a-chymotrypsin. In the synthetic systems, the highest acylation rate (160—200 M sec" at pH 8) was observed for oxime-bound polyvinylpyri-dine ( yOx) (section S—2) and micellar, bifunctimial catalyst (LImHA-CTAB,... 

Ref. 14S), and the greatest dauylation rate at pH 8 (0.009—0.013 sec" ) was found with bifunctional polymer catalysts (Elm -HA, PVP -HA and HlA-VIm-AAm) (Section S—3). The acylation rate can be further increased by catalysis at higher pH s and by using substrates with longer alkyl chains. 

The DCC/HOSu proceduref l has been extensively apphed to fragment condensations in solution. In racennization-prone coupling steps this procedure was generally found to be superior to the DCC/HOBt method (see Section 3.5.1.2.2) even if HOBt (14) is more efficient as an additive in terms of acylation rates. 

A number of attempts have been made to use reactions of this type for synthetic purposes. The ligand (49) can be used as an acylating reagent (Scheme 13). Acylation rates in Me2SO are greatly increased by the addition of transition metal ions such as Zn , Co" and Ni". As a result it is possible to prepare acylating reagents whose reactivity can be switched on by addition of a transition metal ion. 

Figure 28 Investigation of interchain and intrachain acylation in dimeric VibF. (a) Vibrobactin. (b) VibF heterodimer for interchain and intrachain kinetic assay and ESI-FTMS analysis, (c) Obtained best-fit kinetic parameters for VibF intrachain and interchain acylation, Pinterchain - probability of interchain flux, /rinterohain - interchain acylation rate, /rintra0hain - intrachain acylation rate.

The steady-state level of occupancy of the active site, which is dependent on the ratio of deacylation rate to the acylation rate (fcdeacyi/ acyi) [14], was greater than 0.999. The low deacylation rate was reflected in a very low net hydrolysis rate (Fig. 1), with 1 M- s... 

The lipase-catalysed enantioselective acylation of allylic alcohols in an ionic liquid solvent was demonstrated by Itoh et al. [16] (Fig. 7.7). They found that the acylation rate was strongly dependent on the counter anion of the imidazolium salt, while the lipase-catalysed acylation proceeded with high enantioseleclivity in all ionic liquid tested. Good results were obtained when the reaction was carried out in [bmimT [PFg ] or [bmun" ][BF ]. Other examples of kinetic resolution of allylic alcohols catalysed by lipases in ionic liquids were also reported by these authors [71, 72]. The transesterification of 5-phenyl-l-penten-3-ol under reduced pressure at 27 hPa and 40°C was carried out using methyl phenylthioacetate as acyl donor in [bmim+] [PF ] and [bdmim ][BF ], for obtaining the corresponding acylated compound in optically pure form [71], The acetylation of methyl mandelate catalysed by immobilised P5L in [bdmim ][BF ] is another example reported by these authors about the successful application of ionic liquids as reaction media in racemic resolutions... 

In order to investigate the influence of the amount of catalyst on the acylation rate, the proportion of zeohte HY (Si02/Al203 = 29) was varied between... 

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