ACETYLATION OF AROMATIC AMINES

ACETYLATION OF AROMATIC AMINES Acetyl derivatives of aromatic amines may be prepared either with acetic anhydride or acetic acid or with a mixture of both reagents. Primary amines react readily upon warming with acetic anhydride to yield, in the first instance, the mono-acetyl derivative, for example  

If the heating be prolonged and excess of acetic anhydride is employed, variable amounts of the diacetyl derivative are formed  

The production of diacetyl derivatives is facilitated by the presence of substituents in the ortho position thus an excellent yield of diacetyl-o-toluidine results when o-toluidine is heated with an excess of the reagent  

In general, however, the diacetyl derivatives are unstable in the presence of water, undergoing hydrolysis to the mono-acetyl compound, so that when they (or a mixture of mono- and di-acetyl derivatives) are crystallised from an aqueous solvent, e.g., dilute alcohol, only ihe mono-acetyl derivative is obtained. A further disadvantage of the use of acetic anhydride in the absence of a solvent is that all the impmrities in the amine are generally present in the reaction product. HeavUy substituted amines, e.g., 2 4 6-tribromoaniline, react extremely slowly with acetic anhydride, but in the presence of a few drops of concentrated sulphuric acid as catalyst acetylation occurs rapidly, for example  

The disadvantages attending the use of acetic anhydride alone are absent when the acetylation is conducted in aqueous solution according to the following procedure. The amine is dissolved in water containing one equivalent of hydrochloric acid, slightly more than one equivalent of acetic anhydride is added to the solution, followed by enough sodium acetate to neutralise the hydrochloric acid, and the mixture is shaken. The free amine which is Bberated is at once acetylated. It must be pointed out that the hydrolysis of acetic anhydride at room temperature is extremely slow and that the free amine reacts much more readily with the anhydride than does the water this forms the experimental basis for the above excellent method of acetylation. 

Acetylation with acetic anhydride is comparatively expensive because of the cost of the reagent. The use of the inexpensive glacial acetic acid depends upon the displacement of the reversible equilibrium  

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J. LeWalter, U. Korallus, Blood Protein Conjugates and Acetylation of Aromatic Amines New Findings on Biological Monitoring , Int. Arch. Occup. Envir. Elealth, 56,179-196 (1985). 

The most important kind of acetylation reaction is the acetylation of aromatic amines. This converts the ionizable amine group to a nonionizable group, to which the acetyl group is attached. 

As a consequence, some acetylated products are not as soluble in water as the parent compounds. In some cases, acetylation of aromatic amines makes them less active as toxicants, particularly in binding with DNA, whereas in other cases, they are made more active. In the latter case, activity can be due to a cytochrome P-450 catalyzed attachment of an -OH group to the acetylated nitrogen, leading to a positively charged electrophilic species capable of binding with DNA. 

One common polymorphism in the United States is for N-acetyltransferase, an enzyme involved in phase II reactions. N-acetyltransferase catalyzes the acetylation of aromatic amines and hydrazines, and other classes of xenobiotics. People characterized as slow acetylators have relatively low N-acetytransf-erase activity. Consequently, slow acetylators are more sensitive to the toxic effects of certain types of drugs, including sulfa drugs. In addition, a study of workers exposed to benzidine in the dye industry suggested a link between the slow acetylator phenotype and the development of bladder cancer. 

Acetylation of amines, phenols, and thiols. This reagent readily transfers two acetyl groups to amines, phenols, and thiols under mild conditions (25°) with conversion to 1,4-diacetylglycouril. Ahphatic primary amines react most readily in fact, primary amino groups can be selectively acetylated in the presence of a secondary amino group. Acetylation of aromatic amines is slower, but can be catalyzed by an acid (e. g., acetic acid). Phenols are also acetylated, but aminophenols can be selectively N-acetylated. Thiophenols and alkyl thiols are acetylated readily, but alcohols react only slowly, even at elevated temperatures. ... 

Experiments on the role of the adrenal hormones in fat metabolism have suggested that the cortical hormones exert a direct effect on fatty acid oxidation preceding the formation of acetyl-CoA (Lipsett and Moore, 1952). Neither the acetylation of aromatic amines (Dumm and Ralli, 1951) nor the formation of ketone bodies from pyruvate in a malonate blocked system (Lipsett and Moore, 1952) were influenced by adrenalectomy. 

At the present time the data suggest multiple points of action of the adrenocortical hormones. Of the reactions which Villee et al. (1952) suggested might be influenced by the adrenocortical hormones, the condensation of pyruvate with oxaloacetate is known to require coenzyme A, as was described earlier. However, both Lipsett and Moore s (1952) experiments showing that the production of ketone bodies from pyruvate was not influenced by adrenalectomy and the observation that the acetylation of aromatic amines was not influenced by adrenalectomy (Dumm and Ralli, 1951) appear to exclude the reactions leading to the production of acetyl-CoA from pyruvate as probable points of action of the adrenocortical hormones. Therefore, of the reactions leading to the production of citrate from pyruvate and oxaloacetate, the most likely to be influenced by the adrenocortical hormones would appear to be the final condensation of acetyl-CoA with oxaloacetate. If further work should establish a direct influence of the adrenocortical hormones on the condensation of acetyl-CoA with oxaloacetate, an additional basis for the interrelations between pantothenic acid and the functions of the cortical hormones would be established. 

Differences in the metabolic route between species are common, e.g., the rat mainly hydroxylates amphetamine leading to conjugated products whereas the rabbit and guinea pig (and man) mainly deamin-ate amphetamine. Acetylation of aromatic amines occurs in humans but not dogs while glucuronic conjugation is very poor in the cat. A good example of a compound that has the same routes of metabolism in different species but different rates is caffeine, as shown in Table 3. Species differences can be... 

Amine Acetylation. The first acetylation reaction to be studied intensively was the acetylation of aromatic amines, of which sulfanilamide was used as a representative (II). This system was used for the first assays of CoA. The amine-acetylating enzyme has been partially purified. 

Acetylation of aromatic amine drugs has been studied extensively and found to occur in most animal species (Williams, 1959). Early studies of drug acetylation in human volunteers and in animals were carried out mainly with sulfonamides. They showed that large difierences exist be-... 

Coenzyme A was discovered by Lipmann in 1945 as necessary for the acetylation of sulfanilamide by pigeon liver extracts. Shortly thereafter this same coenzyme was identified as the activator of choline acetylation earlier observed by Nachmansohn and Berman, as well as Feldberg and Mann. Subsequently, work from Lipmann s laboratory, as well as from other laboratories, extended the role of CoA to a large variety of transacetylation reactions, i.e., acetylation of aromatic amines, synthesis of acetylcholine, of citrate, of acetoacetate, of fatty acids, of sterols, and of phospholipids. ... 

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