Amides, acid/base hydrolysis

Acid/base stress testing is performed to force the degradation of a drug substance to its primary degradation products by exposure to acidic and basic conditions over time. Functional groups likely to introduce acid/base hydrolysis are amides (lactams), esters (lactones), carbamates, imides, imines, alcohols (epimerization for chiral centers), and aryl amines. 

The characterisation of a primary aromatic amide is based upon its own m.p. and the identification of the acid (see Section IV,175) produced on hydrolysis. A crystalline derivative may be prepared directly with xanthhydrol (for experimental details, see Section 111,110, 1). 

The acid-base reactions that occur after the amide bond is broken make the overall hydrolysis irreversible m both cases The amine product is protonated m acid the car boxylic acid is deprotonated m base... 

Nitriles are classified as carboxylic acid derivafives because fhey are convened fo car boxylic acids on hydrolysis The condifions required are similar fo fhose for fhe hydrol ysis of amides namely healing m aqueous acid or base for several hours Like fhe hydrolysis of amides nilrile hydrolysis is irreversible m fhe presence of acids or bases Acid hydrolysis yields ammonium ion and a carboxylic acid... 

Nitriles are classified as carboxylic acid derivatives because they are converted to carboxylic acids on hydrolysis. The conditions required are similar- to those for the hydrolysis of amides, namely, heating in aqueous acid or base for several hours. Like the hydrolysis of amides, nitrile hydrolysis is ineversible in the presence of acids or bases. Acid hydrolysis yields fflnmonium ion and a carboxylic acid. 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

Further evidence for this mechanism is that a small but detectable amount of 0 exchange (see p. 425) has been found in the acid-catalyzed hydrolysis of benz-amide. (The exchange has also been detected for the base-catalyzed... 

In an approach toward the tetraponerine family of alkaloids, Piehiers and co-workers utilized base hydrolysis of the ester 90 followed by acid-catalyzed decarboxylation to obtain the corresponding amide 91 in good yield (Equation 19) <2000CJC1030>. 

Brown, R.S. (2000). Studies in amide hydrolysis the acid, base and water reactions. In The Amide Linkage. Structural Significance in Chemistry, Biochemistry and Materials Science, Greenberg, A., Breneman, C.M. and Liebman, J.F. (eds), p. 85. John Wiley Sons, Inc., New York... 

Enzyme-catalyzed transfer of KDO from CMP-KDO (127) into a lipid-A acceptor has been studied by Heath and coworkers,1 using a cell-free system from Escherichia coli O 111 J-5. LPS from the organism did not function as an acceptor, and only weak acceptor-activity was displayed147-148 by lipid A preparations obtained by mild, acid-catalyzed hydrolysis of LPS. Base-catalyzed, hydrolytic removal of the ester-linked, but not the amide-linked, fatty acid residues from lipid A resulted in an acceptor of maximal activity1 (see Scheme 38 compare Scheme 39). 

Fig. 3.1. General scheme for chemical hydrolysis of carboxylic acid esters and amides. Pathway a Proton (general acid) catalyzed hydrolysis. Pathway b HO (general base) catalyzed...

Table 6w4 Effect of Cobalt(lll) Coordination on Rate Constants for Base Hydrolysis of Amino Acid Amides...

The reaction progresses because the amide anion, once a small amount is released, abstracts a proton from the carboxylic acid product. Again, we have an analogy with the last step in the base hydrolysis of esters, and the ionization becomes an essentially... 

Proteins are fundamentally polymers of a-amino acids linked by amide linkages (see Section 13.1). It is a pity that biochemists refer to these amide linkages as peptide bonds remember, a peptide is a small protein (less than about 40 amino acid residues), whereas a peptide bond is an amide. Therefore, peptides and proteins may be hydrolysed to their constituent amino acids by either acid or base hydrolysis. The amide bond is quite resistant to hydrolytic conditions (see above), an important feature for natural proteins. 

The sequence is completed by base hydrolysis of the amide and removal of the protecting group. This is much the same as an ester hydrolysis, and needs to include as last stage the ionisation of acetic acid since RNH is a poor leaving group, it is this ionisation that allows the reaction to proceed. 

Acid hydrolysis of an amide yields a carboxylic acid and an ammonium ion. The mechanism for acid hydrolysis is shown in Figure 12-36. Base hydrolysis of an amide, on the other hand, yields ammonia and a carboxylate ion. You can see this mechanism in Figure 12-37. To identify similarities, compare these mechanisms to the mechanisms for the hydrolysis of esters (refer to Figures 12-34 and 12-35). 

The cleavage mechanism of the caspases is shown schematically in Fig. 15.5. They use a typical protease mechanism with a catalytic diad for cleavage of the peptide bond. The nucleophilic thiol of an essential Cys residue forms a covalent thioacyl bond to the substrate during the catalysis. The imidazole ring of an essential histidine is also involved in catalysis and this facilitates hydrolysis of the amide bond in the sense of an acid/base catalysis. 

Esters are less reactive than acid chlorides and acid anhydrides. They are converted to carboxylic acid hy acid or base hydrolysis, and to another ester by acid or base alcoholysis (transesterification). The 1°, 2° or 3° amides are obtained from esters by treatment with ammonia or 1° or 2° amines, respectively. 

Amides are the least reactive of the carboxylic acid derivatives, and undergo acid or base hydrolysis to produce the parent carboxylic acids, and reduction to appropriate amines (see Section 4.3.10). They can also be dehydrated to nitriles, most commonly with boiling acetic anhydride, (AcO)20, sulphonyl chloride (SOCI2) or phosphoms oxychloride (POCI3) (see Section 4.3.18). Amines (with one less carbon) are prepared from amides by the treatment of halides (Br2 or CI2) in aqueous NaOH or KOH. This reaction is known as Hofmann rearrangement (see Section 4.3.10). 

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