Aliphatic amides hydrolysis

Aliphatic amides may be hydrolysed by boiling with 10 per cent, sodium hydroxide solution to the corresponding acid (as the sodium salt) the alkahne solution should be acidified with dilute sulphuric acid any water-soluble acid may then be distilled from the solution. Alternatively, hydrolysis may be eflfected with 10-20 per cent, sulphuric acid. The resulting ahphatic acid (usually a liquid) may be characterised as detailed in Section 111,85. 

While ester, carbonate, carbamate and anilide hydrolyses have been catalysed effectively by antibodies, the difficult tasks of hydrolysis of an aliphatic amide or a urea remain largely unsolved. Much of this problem hinges on the fact that breakdown of a TF is the rate-determining step, as established by much... 

The first systematic study of the metabolic hydrolysis of primary aliphatic amides was carried out by Bray et al. in 1950 [1]. The substrates were incubated in rabbit liver preparations for 5 h at 37°. In Fig. 4.2, the effect of chain length on the degree of hydrolysis of amides containing 1 to 18 C-atoms (4.1) is shown. The extent of hydrolysis was very small for the first three homo-... 

A series of branched aliphatic amides were prepared to evaluate the role of amide hydrolysis on the pharmacokinetics and anticonvulsant activity of valpromide analogues. Table 4.1 summarizes the structures investigated, the fraction of amide hydrolyzed (/m), and the stability in blood. These results were obtained after intravenous administration to dogs [6], The structures are classified here in order of decreasing fm. 

N-Methyl substitution does not seem to influence dramatically the hydrolysis of the amide bond. Indeed, a similar relationship between hydrolysis and chain length has been described for unsubstituted aliphatic amides... 

Like the simple aliphatic secondary amides discussed above, structurally more-complex compounds may also be expected to undergo hydrolysis. However, very few such results are available, implying either that xenobio-tics are relatively stable, or that they have been insufficiently studied. It seems that the former reason is the more likely, since the amide bond, in general, is chemically stable and is metabolized over only a narrow range of structures (see, e.g., the /V-alkyl-substituted amides discussed above). Some of the few reported examples of structurally complex xenobiotics that undergo amide hydrolysis are discussed below. 

The last group of aliphatic amides to be discussed are the tertiary amides, which, by definition, carry two alkyl substituents on the N-atom. Investigations of their chemical stability have disclosed a surprising difference between tertiary and secondary amides, since the rate of acid-catalyzed hydrolysis of N,N-dimethyl amides is higher than that of A-methyl amides. If steric fac-... 

F. R. Murphy, V. Krupta, G. S. Marks, Drug-Induced Porphyrin Biosynthesis. XIII. Role of Lipophilicity in Determining Porphyrin-Inducing Activity of Aliphatic Amides after Blockade of Their Hydrolysis by Bis-(p-nitrophenyl)phosphate , Biochem. Pharmacol. 1975, 24, 883-889. 

A Streptomyces enzyme that catalyzes hydrolysis of capsaicin is described by Koreishi et The substrate is an A -vanillyl aliphatic amide, and the authors found that their enzyme also accepted A lauroyl amino acids as substrates. The enzyme was used successfully to catalyze the reaction in the opposite direction, driving the equilibrium toward synthesis by running it in buffer containing 78% glycerol. Yields of 5-40% were obtained for a wide range of natural L-amino acids. In the case of L-lysine the enzyme catalyzed acylation at both amino groups, with a clear preference for the e-NH2. 

Nitrilase [EC 3.5.5.1], also known as nitrile aminohy-drolase and nitrile hydratase, catalyzes the hydrolysis of a nitrile to produce a carboxylate and ammonia. The enzyme acts on a wide range of aromatic nitriles. Nitrile hydratase [EC 4.2.1.84], also known as nitrilase, catalyzes the hydrolysis of a nitrile to produce an aliphatic amide. The enzyme acts on short-chain aliphatic nitriles, converting them into the corresponding acid amides. However, this particular enzyme does not further hydrolyze these amide products nor does the enzyme act on aromatic nitriles. 

The hydroxide-catalysed hydrolysis of aliphatic amides is generally first-order in hydroxide ion. Bruylants and Kezdy30 showed that, for a series of alkyl amides RCONH2 ranging from R = CH3 to R = C13C, the rates of alkaline hydrolysis obey the Taft relationship... 

The types of enzymes that bring about hydrolysis are hydrolase enzymes. Like most enzymes involved in the metabolism of xenobiotic compounds, hydrolase enzymes occur prominently in the liver. They also occur in tissue lining the intestines, nervous tissue, blood plasma, the kidney, and muscle tissue. Enzymes that enable the hydrolysis of esters are called esterases, and those that hydrolyze amides are amidases. Aromatic esters are hydrolyzed by the action of aryl esterases and alkyl esters by aliphatic esterases. Hydrolysis products of xenobiotic compounds may be either more or less toxic than the parent compounds. 

Some phase 1 metabolic transformations aromatic hydroxylation, aliphatic hydroxylation, A-hydroxylation, A-oxidation, deamination, A-, S- and (9-dealkylation, alcohol oxidation, azo and nitro reduction, ester and amide hydrolysis. 

Table 2.5 summarizes the hydrolysis kinetic data for a number of aliphatic amides. 

It is apparent that only those aliphatic amides which contain substituents that withdraw electron density from the carbonyl group (e.g., halides), making it more susceptible to nucleophilic attack, will have appreciable hydrolysis rates at pH 7 at 25°C. Alkyl substituents on nitrogen will increase hydrolysis half-lives due to steric effects (compare ti/2 for acetamide to A-methyl- and A-ethylacetamide). 

PAAm undergo the general reactions of the aliphatic amide group. The most important reactions are hydrolysis, Hofmann degradation and Mannich reaction. At very extreme pH values hydrolysis occurs. At low pH values (lower than 2.5) inter- and intramolecular imidization occurs [370], which leads to partially insoluble products (Figure 4). 

O vs. N Protonation.—Controversy over the site of protonation of amides has been raised anew by Liler. Using u.v. spectroscopy, she has suggested that benzamide is 50 50 O N protonated in 80 % sulphuric acid. However, a considerable weight of evidence has been produced to indicate that protonation or Lewis acid complexation of amides consistently occurs by co-ordination with oxygen. Studies have included the acidity-dependent changes in the tt-tt and n-it u.v. absorption bands of aliphatic amides kinetic evidence based on rate data for acid-catalysed amide hydrolysis in both dilute and concentrated acid an n.m.r. study of proton exchange rates, where for A -methylacetamide the molar ratio of 0 N protonated species exceeds 10 a n.m.r. study of adducts of boron trifluoride and antimony pentachloride with N-labelled ureas, where the hybridization-dependent —H coupling constants were inconsistent with N-co-ordination ... 

Isocyanates derived from the higher aliphatic amides react more rapidly with the haloamide salts than with water and alkali, so that, when these amides are subjected to the Hofmann reaction in aqueous mediiun, only small amounts of the expected amines are formed. Although amines arise from the hydrolysis of the alkyl acyl ureas, they are largely oxidized to nitriles by the excess of hypobromite present. 

Frankenberger and Tabatabai have described an assay for soil amida.ses, which catalyse the hydrolysis of aliphatic amides to carboxylic acids and ammonia. Their assay requires toluene-treated soils to be incubated at 37°C with 0.05M substrate (formamide, acetamide, propionamide) for either 2 or 24h. The suspensions were buffered at pH 8.5 the choice of buffer influenced the amounts of NH4 released, which formed the basis for estimating activities. The pH optimum for hydrolysis of all substrates was about pH 8.5 the optimum temperature was about 65°C. Reaction rates in the presence of toluene were linear both with time and with the amounts of soil when substrate concentrations were 0.05M. At this concentration, and above, the reactions exhibited zero-order kinetics for the recommended assay periods. 

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