Elimination of ammonia

An important general method of preparing indoles, known as the Fischer Indole synthesis, consists in heating the phenylhydrazone of an aldehyde, ketone or keto-acld in the presence of a catalyst such as zinc chloride, hydrochloric acid or glacial acetic acid. Thus acrtophenone phenylhydrazone (I) gives 2-phenyllndole (I V). The synthesis involves an intramolecular condensation with the elimination of ammonia. The following is a plausible mechanism of the reaction ... 

A/-(4-Hydroxyphenyl)glycine can be prepared from 4-aminophenol and chloracetic acid (199,200) or by alkaline hydrolysis of the corresponding nitrile with subsequent elimination of ammonia (201). 

The reaetion of 4-dialkylamino(alkoxy)but-3-en-2-one with benzonitrile N-oxide (35°C, EtaO, 8 h) follows a seheme of 1,3-dipolar eyeloaddition with elimination of ammonia, the eorresponding amine, or aleohol to form 3-phenyl-4-aeetylisoxazole 324 in 60-67% yield (83ZOR2281). 

Heating of an aryl hydrazone 1 in the presence of a catalyst leads to elimination of ammonia and formation of an indole 2. This reaction is known as the Fischer indole synthesisy and is somewhat related to the Benzidine rearrangement. 

That electrocyclic reaction is related to the Claisen rearrangement of phenyl vinyl ether. In a final step a cyclization takes place with subsequent elimination of ammonia to yield the indole 2 ... 

The pharmacological versatility of this general substitution strategy is further illustrated by diazonium coupling of 14 with 2-nitrobenzenediazonium chloride to produce biarylal-dehyde 18. Formation of the oxime with hydroxylamine is followed by dehydration to the nitrile. Reaction with anhydrous methanolic hydrogen chloride leads to imino ether and addition-elimination of ammonia leads to the antidepressant amid-ine, nitrafudam (20). ... 

When arylhydrazones of aldehydes or ketones are treated with a catalyst, elimination of ammonia takes place and an indole is formed, in the Fischer indole synthesis,Zinc chloride is the catalyst most frequently employed, but dozens of others, including other metal halides, proton and Lewis acids, and certain transition metals have also been used. Microwave irradiation has been used to facilitate this reaction. Aniline derivatives react with a-diazoketones, in the presence of a... 

The balance between those molecules which are protonated and those which are not has emerged as an inportant function in determining the course of any reaction occurring within the Interlamellar region. One carefully studied reaction is that of the elimination of ammonia from amines (21), a reaction proceeding by two sinple steps... 

The presence of PCHE in the HDN product (Table 1) indicates that at least part of the HDN reaction of PCHA proceeds through elimination of ammonia rather than by direct hydrogenolysis. Nevertheless, the direct product of the elimination reaction (allylcyclohexylamine) was not observed. This must be due to its strong adsorption and fast hydrogenation to PCHA. 

It has been tentatively suggested that one mechanism underlies the Willgerodt reaction and the Kindler modification of it. A labile intermediate is first formed which has a carbon—carbon bond in the side chain. The scheme is indicated below It postulates a series of steps involving the addition of ammonia or amine (R = H or alkyl), elimination of water, re-addition and elimination of ammonia or amine until the unsaturation appears at the end of the chain then an irreversible oxidation between sulphur and the nitrogen compound may occur to produce a thioamide. 

Alkyl and aryl nitriles 151 react with thiosemicarbazide 138 under acidic conditions to give 1,3,4-thiadiazoles (Scheme 14 and Table 8) <1995BML1995, 1996IJB273, 1997IJB394>. The acidic conditions promote the elimination of ammonia from the intermediate iminothioacylhydrazine 152. 

Melamine and its salts are widely used in formulations of fire retardant additives, particularly of the intumescent type (4-71. The role played by melamine structures in these additives is however not yet understood. The thermal behaviour is of paramount importance in studies of the fire retardance mechanism. It is known that melamine undergoes progressive condensation on heating with elimination of ammonia and formation of polymeric products named "melam", "melem", "melon" (8.91. The following schematic reaction is reported in the literature (10-121 ... 

Owing to overlapping of different processes in this range of temperature, quantitative considerations based on data obtained in programmed temperature TG cannot be made. However, it can roughly be estimated that up to 410 C, that is to the end of the process characterised by the narrow DTG peak 3 in Figure 12, about one mole of ammonia 1s evolved per mole of melamine. This would support the formation of melam by Reaction 13, followed by elimination of ammonia (reaction 14). 

If the hydroformylation of olefins is conducted in the presence of aromatic hydrazines and Bronsted or Lewis acids indoles can be obtained directly in one pot [91-93,95]. Hydroformylation of the olefin gives an intermediate aldehyde, which is trapped immediately by the present aromatic hydrazine as an aromatic hydrazones similar to the formation of imines under hydroformylation conditions. Under acid mediation these aromatic hydrazones undergo a Fischer indolization, consisting of a [3,3]-sigmatropic rearrangement followed by a cyclization and elimination of ammonia (Scheme 38). 

PAL is a tetramer of 226-340 kDa, and is composed of either identical or very similar subunits [20, 21]. PAL catalyzes the trani-elimination of ammonia and the... 

The second reaction took place during drying of the membrane at 110°C. The residual polyethylenlmine under the polyamide surface skin was crosslinked by elimination of ammonia between adjacent amine groups (see Reaction II). 

Porphyrin rings are formed in nature by a process that is remarkably similar to that shown above. Though the sequence contains some rather unusual features, the coupling process also involves nucleophilic attack on to an alkylidene pyrrolium cation. This may be generated from the precursor porphobilinogen by elimination of ammonia. 

From a simplified scheme of reduction of the amide function it can be seen that the first stage is formation of an intermediate with oxygen and nitrogen atoms linked to an sp carbon. Such compounds tend to regenerate the original sp system by elimination of ammonia or an amine. Thus an aldehyde is formed and may be isolated, or reduced to an alcohol. Alternatively the product is an amine resulting from direct hydrogenolysis of the sp intermediate. 

A specialized amino acid residue that serves as an essesn-tial electrophilic center in several enzymatic reactions, including those catalyzed by L-phenylalanine ammonia lyase (Reaction L-phenylalanine tranx-cinnamate + NH3) and L-histidine ammonia lyase (Reaction L-histi-dine urocanate + NH3). The former facilitates the elimination of ammonia and the pro-S hydrogen of phe-nylanine, and the initial step is nucleophilic attack of... 

In 1892, Tauber reported the synthesis of carbazole (1) by the cyclization of 2,2 -diaminobiphenyl (509). This reaction relies on high temperatures and acidic conditions involving cyclization with the elimination of ammonia (491,492). This method works well with chloro- and nitro-substituted 2,2 -diaminobiphenyls. However, the general application of this methodology to the synthesis of carbazoles is limited by the limited availability of 2,2 -diaminobiphenyls (493) (Scheme 5.5). 

The cyclohexanone phenylhydrazone (512), obtained by reacting cyclohexanone (510) with phenylhydrazine (511), on indolization, furnished tetrahydrocarbazole 513 which, on dehydrogenation, afforded carbazole 514. The success of the reaction is dependent on the reagent used for indolization and the dehydrogenating agent. The mechanism for the formation of the tetrahydrocarbazole involves a tautomeric equilibrium and the formation of a new C-C bond via a [3,3]-sigmatropic rearrangement followed by elimination of ammonia (495,496,498) (Scheme 5.7). 

Although 2 protonation of pyrrole is favoured, there is sufficient of the 3-protonated form to initiate the addition of hydroxyl-amine. Once this occurs, ring opening may follow, as shown below. Subsequent addition of a second equivalent of hydroxylamine and elimination of ammonia (as ammonium chloride) then gives the dioxime. 

PAL is one of the best-characterized enzymes of plant secondary metabolism. It converts l-phenylalanine into tran -cinnamate ( -cinnamate) by the tra 5-elimination of ammonia and the pro-StS proton (see Ref. 4 for a full reaction discussion). The enzyme, which requires no cofactor, is a tetramer of 310-340 kDa. A cDNA for PAL was first isolated from Petrose-linum crispum (parsley), and others have subsequently been isolated from numerous species. Often PAL is produced from a multigene family and is present in a variety of isoenzyme forms. 

As with other enaminones, aqueous KOH effects addition of OH- to C-4 and subsequent elimination of ammonia involving conversion to 3-acyl tetramic acids (14) (Section III). The reaction pathway, according to this mechanism, depends on the steric requirements of the substituents at C-5. The reaction proceeds rapidly and to completion in the case of R2 = R3 = Me and R2—R3 = —(CH2)4—. Slightly more sterically hindered substituents like R2 = R3 = Et considerably hinder the reaction (87TH2 89MI1). (See Fig. 6.)... 

Chloro-triammino-platinous Chloride, [Pt(NH3)3Cl]Cl, may be obtained by the addition of ammonia to dichloro-diammino-platinum, [Pt(NH3)2Cl2], or by the elimination of ammonia from tetrammino-platinous chloride by the action of hydrochloric acid thus ... 

Both methods, however, give very poor yields, as the addition or elimination of ammonia cannot be regulated, and therefore cannot be stopped at the required stage. 

This series of compounds may be produced by the elimination of ammonia from the previous series by means of hydrochloric acid. It is probable that the acid, [Pt(NH3)R.3]H, is formed, but this has not been isolated, and only the potassium, ammonium, and silver salts are known. 

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