N-Acetylimidazole

This catalysis follows Scheme II that catalysis occurs means that N-acetylimidazole is more susceptible to hydrolysis than is the ester. (This does not explain why the N-acetylimidazole is formed, however.)... 

This mechanistic interpretation is supported by the isolation of N-acetylimidazole as a major product and the observation of only first order catalysis by N-methylimidazole irrespective of the pH... 

In ref. [74] it is emphasized that N-acetylimidazole reacts with all tyrosyl residues in copolymers and denatured proteins but only with free tyrosyl residues in native proteins. In ref. [75] it is noted that Af-acetylimidazole reacts extensively with both lysine and tyrosine chains. 

A Lewis acid is also necessary for the acetylation of tetracarbonylferrate using N-acetylimidazole. In the absence of a Lewis acid, a Claisen-type condensation product was formed, which has been synthesized independently from 2 moles of A-acetylimidazole with sodium terf-butanolate in tert-butyl alcohol (55% yield) or with imidazole sodium in THF (95% yield) ... 

Dimethylamino)propanol (201 R = H) is known to be acetylated by N-acetylimidazole (206) by a mechanism that involves intramolecular general base... 

Diamines of varying structure show rate enhancements of 20-200 fold, compared to monofunctional aliphatic amines, in nucleophilic reactions with N-acetylimidazole (Page and Jencks, 1972). These were attributed to intramolecular general base catalysis of proton removal from the attacking nitrogen, viz.. 

Jencks and Carriuolio275 have shown that the pH-rate profile for the hydrolysis of N-acetylimidazole (25°C, solvent HaO) is given by... 

Oxidation of two out of 13 tryptophan residues in a cellulase from Penicillium notatum resulted in a complete loss of enzymic activity (59). There was an interaction between cellobiose and tryptophan residues in the enzyme. Participation of histidine residues is also suspected in the catalytic mechanism since diazonium-l-H-tetrazole inactivated the enzyme. A xylanase from Trametes hirsuta was inactivated by N-bromosuc-cinimide and partially inactivated by N-acetylimidazole (60), indicating the possible involvement of tryptophan and tyrosine residues in the active site. As with many chemical modification experiments, it is not possible to state definitively that certain residues are involved in the active site since inactivation might be caused by conformational changes in the enzyme molecule produced by the change in properties of residues distant from the active site. However, from a summary of the available evidence it appears that, for many / -(l- 4) glycoside hydrolases, acidic and aromatic amino acid residues are involved in the catalytic site, probably at the active and binding sites, respectively. 

Product inhibition occurs if the catalyst binds to the product with a similar tightness to the substrate. In a similar geometric approach of reactants, however, catalysis was successful (Mackay, 1994). Besides the above-mentioned Diels-Alder reaction, the substrate-free porphyrin trimer also catalyzes the acetylation of 4-hydroxymethylpyridine with N-acetylimidazole. Although both reactants are bound to the porphyrin trimer, their ground states are bound less tightly than the... 

Numerous aryl-pyranosides have been shown to bind more strongly to Con A than their alkyl analogs(32,39,40). Also, acetylation studies with N-acetylimidazole in the presence and absence of carbohydrate have implicated the involvement of tyrosine residues(41). It is quite clear from the three-dimensional structure that Tyr 12 and 100, which are found in this region and exposed to... 

Figure 4. Modifications of phenolic groups (a) N-acetylimidazole (b) iodination (c) TNM (d) diazotization.

TMS derivatives of biogenic amines are used in combination with acyl derivatives for electron-capture detection. Horning et al. [97] presented retention data of TMS-N-acetyl and TMS-N-HFB derivatives of a number of these substances on SE-30, OV-1 and OV-17. The derivatives were prepared by the following procedure. A 1-mg amount of the amine or amino hydrochloride was dissolved in 0.1 ml of acetonitrile and 0.2 ml of TMS-imida-zole was added. After heating for 3 h at 60°C, 5 mg of N-acetylimidazole (or 0.1 ml of HFB-imidazole) were added and the solution was heated at 80°C for 3 h (30 min at 60°C). The solution was used directly for the GC analysis. 

The variation in reactivity of N-acetylimidazoles (and other azohdes) in nucleophilic reactions involving the carbonyl group is paralleled by the marked shift in the carbonyl bands (toward higher frequencies for the more reactive compounds).212 This shift, i.e., increase in the C-0 force constant, can also be attributed to increased electron attraction by the heterocyclic rings.213... 

Acetylhydroxy-L-proline, 1014 Acetyl hypobromite, 12-13 N-Acetylimidazole, 13 S-Acetylmercaptosuccinic anhydride, 13 Acetyl methyl ursolate, 823... 

Acetylation Acetic anhydride. N-Acetoxyphthalimide. 2- and 3-Acetoxypyridine. Acetyl chloride. N-Acetylimidazole. Boron trifluoride. Catalysts (see Acetic anhydride). Ketene. Magnesium. Methyl oxocarbonium hexafluoroantimonate. Perchloric acid. Phenyl acetate. Pyridine. Sodium acetate. Tetraelhylammonium acetate. p-Toluenesulfonic acid. Tri-n-hexylethyl ammonium hydroxide. 2,4,6-Triisopropylbenzenesulfonyl chloride. Trityl-sodium. Zinc chloride. 

Figure 8-9. The catalytic properties of imidazole. See text and Bruice and Schmir, 1957. b N-acetylimidazole has been isolated in the B reaction sequence.

Price and Radda (338) found that N-acetylimidazole could acetylate up to six tyrosine residues without loss of activity or alteration of Km for substrate however, reaction of about one tyrosine per subunit results in desensitization toward GTP, but the response to ADP is not abolished even by extensive 0-acetylation. Essentially the same results are observed upon nitration with tetranitromethane (TNM). Acetylation does not grossly alter the molecular weight, as measured by sedimentation velocity, or the conformation, as determined by ORD. The GTP site is not protected by NADH alone, but is partially protected (25-50%) by GTP and is at least 75% protected by inclusion of both GTP and NADH in the reaction mixture. Piszkiewicz et al. (339) confirmed these findings by modification with TNM. The reaction is biphasic with initial rapid formation of one residue of 3-nitrotyrosine per subunit. The primary site of reaction is tyrosine-406 in the linear sequence (340). Later (338) the same effect was obtained with chicken GDH with both enzymes there is no influence on activation by ADP. Further, the pH optima of the enzymes are not influenced by the degree of nitration or the inhibition by GTP or activation by ADP (338). 

The acetylation of L-rhamnal and L-fucal has been studied. When acetyl chloride, N-acetylimidazole, benzoyl chloride, or -benzoyl-imidazole were used, the esterification of L-rhamnal occurred primarily at the allylic hydroxy group. With acetic anhydride-pyridine it was mainly the homoallylic hydroxy group which ester-ified. Selective acylations of L-fucal could not be achieved since acyl migration from 0-3 to 0-4 occurred. The ratios of the products were studied under various conditions of solvent, temperature, and time. 

At first sight the reactivity of acylated imidazoles is therefore unexpected but their acylating ability is comparable with that of acid anhydrides or chlorides. This can be seen in a comparison of the susceptibility to hydrolysis of N-acetyIpyrrole and N-acetylimidazole (6). 

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