Arylhydrazone

Retrosynthesis a in Scheme 7,1 corresponds to the Fischer indole synthesis which is the most widely used of all indole syntheses. The Fischer cyclization converts arylhydrazones of aldehydes or ketones into indoles by a process which involves orf/io-substitution via a sigmatropic rearrangement. The rearrangement generates an imine of an o-aminobenzyl ketone which cyclizes and aromatizes by loss of ammonia. 

The issue of regioselectivity arises with arylhydrazones of unsymmetrical ketones which can form two different enehydrazine intermediates. Under the conditions used most commonly for Fischer cyclizations, e g. ethanolic HCI, the major product is usually the one arising from the more highly substituted enehydrazine. Thus methyl ketones usually give 2-methy indoles and cycliz-ation occurs in a branched chain in preference to a straight chain. This regioselectivity is attributed to the greater stability of the more substituted enhydrazine and its dominance of the reaction path. 

Another issue of regioselectivity arises with meta-substituted arylhydrazones from which either 4- or 6-substitutcd indoles can be formed. Robinson has tabulated extensive data on this point[9]. A study comparing regioselectivity of cyclization as catalysed by HCl/EtOH and ZnClj was carried out for several m-substituted arylhydrazones of diethyl ketone[10]. The results given in Table 7.1 show some dependence on catalyst but mixtures are obtained under all conditions studied. 

Folyphosphoric acid trimethylsilyl ester (PPSE)[1] can be used in sulfolane, CH,Cl2 or nitromethane. It is similar to polyphosphoric acid but the overall conditions arc milder and the work-up more convenient. PPSE has been used in the cydization of ris-arylhydrazones of cyclohexane-l,2-diones to give indolo[2,3-a]carbazole analogues[2],... 

The Japp-Klingeraann coupling of aryidiazonium ions with enolates and other nucleophilic alkenes provides an alternative route to arylhydrazones. The reaction has most frequently been applied to P-ketoesters, in which deacylation follow S coupling and the indolization affords an indole-2-carboxylate ester. 

The oxygen analogue of the Fischer cyclization requires the formation of 0-vinyl derivatives of iV-arylhydroxylarnines. These are readily converted to indoles but are less readily accessible than the arylhydrazones used for the Fischer cyclization. 

Thiazole carboxylic acid hydrazides were prepared in a similar way (444, 445). Thus by refluxing thioacetamide or thiobenzamide with y-bromoaceto acetic ester arylhydrazones (83) for several hours in alcohol the 4-carboxythiazole derivatives (84) listed in Table II-ll were obtained (Scheme 36) (656). This reaction is presumed to proceed via dehydration of the intermediate, thiazoline-S-oxide. 

Arylhydrazones of ethyl ester and amide of 4-phenyIthia2oIyIglyoxalic acid R., N ]i i NNHR, r c COjEt ... 

The reaction of a ,)3-alkynic nitriles (317) with hydroxylamine gave the 5-aminoisoxazoles (318) regiospecifically, whereas in the presence of sodium hydroxide the 3-aminoisoxazoles (319) were obtained exclusively (66CPB1277). Similarly, the course of the cyclization of arylhydrazones (320) was influenced by a change in the base employed (75JOC2604). 

Jacobson-Hugershofi synthesis benzothiazoles, 5, 135 Japp-KJingemann reaction JV-arylhydrazones from, 4, 337 1,2,4-triazole synthesis by, 5, 768 Jasmine lactone synthesis, 3, 847 Jasminine occurrence, 2, 626 y-Jasmolactone synthesis, 4, 674 cis-Jasmone synthesis, 1, 422, 427 Jatropham synthesis, 1, 426 Josephson junctions, 1, 359 Juliprospine... 

The Fischer indole synthesis can be regarded as the cyclization of an arylhydrazone 1 of an aldehyde or ketone by treatment with acid catalyst or effected thermally to form the indole nucleus 2. ... 

The first indolization of an arylhydrazone was reported in 1983 by Fischer and Jourdan" by treatment of pyruvic acid 1-methylphenylhydrazone 3 with alcoholic hydrogen chloride. However, it was not until the following year that Fischer and Hess identified the product from this reaction as 1-methyl indole-2-carboxylic acid 4. 

A large number of Brpnsted and Lewis acid catalysts have been employed in the Fischer indole synthesis. Only a few have been found to be sufficiently useful for general use. It is worth noting that some Fischer indolizations are unsuccessful simply due to the sensitivity of the reaction intermediates or products under acidic conditions. In many such cases the thermal indolization process may be of use if the reaction intermediates or products are thermally stable (vide infra). If the products (intermediates) are labile to either thermal or acidic conditions, the use of pyridine chloride in pyridine or biphasic conditions are employed. The general mechanism for the acid catalyzed reaction is believed to be facilitated by the equilibrium between the aryl-hydrazone 13 (R = FF or Lewis acid) and the ene-hydrazine tautomer 14, presumably stabilizing the latter intermediate 14 by either protonation or complex formation (i.e. Lewis acid) at the more basic nitrogen atom (i.e. the 2-nitrogen atom in the arylhydrazone) is important. 

It has been proposed that protonation or complex formation at the 2-nitrogen atom of 14 would enhance the polarization of the r,6 -7i system and facilitate the rearrangement leading to new C-C bond formation. The equilibrium between the arylhydrazone and its ene-hydrazine tautomer is continuously promoted to the right by the irreversible rearomatization in stage II of the process. The indolization of arylhydrazones on heating in the presence of (or absence of) solvent under non-catalytic conditions can be rationalized by the formation of the transient intermediate 14 (R = H). Under these thermal conditions, the equilibrium is continuously pushed to the right in favor of indole formation. Some commonly used catalysts in this process are summarized in Table 3.4.1. 

The keto arylhydrazone 22 indolized to give only the 3-H substituted indole 23 upon treatment under the cyclization conditions. Indolization had occurred toward the more substituted carbon atom. ... 

The arylhydrazone 24 generally gives the 3-alkyl-2-methylindole 25 as major product. However, the indolization of ethyl methyl ketone has been reported to provide both 2,3-dimethyl indole and 2-ethyl indole. " ... 

The arylhydrazone 29 of a 2-substituted cyclohexanone gave a mixture of indolenine 30 and tetrahydrocarbazole 30. It was reported that the relative amounts of 30 and 31 produced depended upon the catalyst employed. For example, glacial acetic acid as catalyst provided largely 30, whereas aqueous sulfuric acid gave 31 as the major product. 

Cyclization of an o-substituted arylhydrazone provides one isomer in many cases. However, the cyclization is more sluggish than the m- or p-substituted analogs. Sometimes the cyclization gives low yields of the desired indole products along with side products. Cyclization of the 2-substituted arylhydrazone can occur either to the unsubstituted side to provide the normal indole product (50), or to the substituted side, where other reactions take place (53 and 56). 

Aniline 77 was converted into its diazonium salt with nitrous acid and this was followed by reduction with stannous chloride to afford the corresponding arylhydrazine 78. Condensation of 78 with 3-cyanopropanal dimethylacetal 79 gave the arylhydrazone 80. Treatment of 80 with PPE resulted in cyclization to indole 81. The nitrile group was then reduced to the primary amine by catalytic hydrogenation. Reaction of the amine with excess formalin and sodium borohydride resulted in Imitrex (82). 

Surprisingly, arylhydrazones 88a-d, upon treatment with 1-benzenesulfonyl-hydrazide 89 in refluxing ethanol, afforded very good yields of pyrazol-3-ones 90a-d (85JIC54) (Scheme 25). 5-Aminopyrazol-3-(Mies have also been synthesized from /3-cyanoesters (Section II,A,2) and from /3-iminoesters (Section II,A,3). 

It is sometimes impossible to isolate the resultant jS-ketonitrile derivatives as such as they are unstable and readily polymerize, e.g. cyano-acetone. They are therefore identified as their corresponding arylhydrazones. The simpler alkyl-w-cyanomethyl ketones are, however, much more stable and their production through the cleavage of 5-alkylisoxazoles has been suggested as a preparative synthetic method. ... 

Another route to suitable arylhydrazones is offered by the Japp-Klingemann reaction. 

Arylhydrazones from reaction of /3-dicarbonyl compounds with arenediazonium salts... 

The coupling of arenediazonium compounds 1 to 1,3-dicarbonyl substrates 2 (Z = COR) is known as the Japp-Klingemann reaction As suitable substrates, /3-ketoacids (Z = COOH) and /3-ketoesters (Z = COOR) can be employed. As reaction product an arylhydrazone 4 is obtained. 

In addition to /3-diketones, /3-ketoacids and /3-ketoesters, cyanoacetic ester and related compounds are suitable starting materials. The arylhydrazones 4 thus obtained are of great importance as starting materials for the Fischer indole synthesis, as well as for the preparation of other iV-heterocycles. ... 

Barbituric acid can be considered as a cyclized malonic acid diamide (malonyl-urea). It is therefore a cyclic diketone that may be classified, in the sense of the compounds discussed in Section 12.6, as a coupling component with a methylene group activated by two carbonyl groups in the a- and a -positions. The reaction with arenediazonium salts was studied by Nesynov and Besprozvannaya (1971). These authors obtained coupling products (in good yield) that they considered to be arylhydrazones. Coupling with 4-(phenylazo)benzenediazonium chloride was studied by Chandra and Thosh (1991). The lH NMR spectra of these compounds are consistent with the arylhydrazone structure 12.68. 

As far as we are aware, the azo coupling of an ethyne derivative was only investigated over half a century ago Ainley and (Sir Robert) Robinson (1937) investigated the reaction of phenylethynes (phenylacetylenes) with diazonium ions (Scheme 12-59). Unsubstituted phenylethyne did not give identifiable products with the 4-nitrobenzenediazonium ion, but with the more nucleophilic 4-methoxyphenyl-ethyne an azo compound (12.119) was formed. On reaction with water it gives an arylhydrazone of an a-ketoaldehyde (12.120). 

Compounds containing carbon-nitrogen double bonds can be hydrolyzed to the corresponding aldehydes or ketones. For imines (W = R or H) the hydrolysis is easy and can be carried out with water. When W = H, the imine is seldom stable enough for isolation, and hydrolysis usually occurs in situ, without isolation. The hydrolysis of Schiff bases (W = Ar) is more difficult and requires acid or basic catalysis. Oximes (W = OH), arylhydrazones (W = NHAr), and, most easily, semicarbazones (W = NHCONH2) can also be hydrolyzed. Often a reactive aldehyde (e.g., formaldehyde) is added to combine with the liberated amine. 

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