Arylhydrazones, oxidation

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. 

Other non-traditional preparations of 1,2,3-triazoles have been reported. The rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-l,2,4-oxadiazole into (2-aryl-5-phenyl-27/-l,2,3-triazol-4-yl)ureas was investigated mechanistically in terms of substituents on different pathways <06JOC5616>. A general and efficient method for the preparation of 2,4-diary 1-1,2,3-triazoles 140 from a-hydroxyacetophenones 139 and arylhydrazines is reported <06SC2461>. 5-Alkylamino-] //-], 2,3-triazoles were obtained by base-mediated cleavage of cycloadducts of azides to cyclic ketene acetals <06S1943>. Oxidation of N-... 

The thermally induced rearrangements in the furoxan series have also been found. In particular, the transformation of 3-R-substituted 4-(3-ethoxycarbonylthioureido)-l,2,5-oxadiazole 2-oxides into derivatives of 5-amino-3-(a-nitroalkyl)-l,2,4-thiadiazole and into (5-amino-l,2,4-thiadiazol-3-yl)nitroformaldehyde arylhydrazones has been reported (Equation 8) <2003MC188>. 

Oxidation of a-diketone bis arylhydrazones using a variety of reagents results in the formation of 2-aryl-1,2,3-triazoles. This reaction, which is... 

Examples have been described of cyclization of bis(azostilbene) derivatives by reaction with hydrogen sulfide and hydrochloric acid, by irradiation, and by heat. 2,4,5-Triphenyltriazole is formed in each case. Nonoxidative cyclization of bis arylhydrazones is also reported. A reaction related to these is the oxidation of benzaldehyde... 

Oxidation of arylhydrazones 143 with HTIB in the presence of diisopro-pylethylamine produces pyrazoles 145 in one pot. The intermediate a-tosyloxy compounds 144, which are generated in situ by the oxidation of 143, undergo intramolecular participation of amino group in displacement of the tosyloxy group, thereby yielding cyclized products 145 (91SC1583). 

Arylhydrazones of /3-aryl-a,/3-unsaturated ketones are converted into pyrazoles by 3 mol equivalents of T CIO4 (Scheme 2). The ring closure occurs via radical cation 45 and is not simply oxidation of a preclosed dihydropyrazole, as evidenced by dimerization (- 46) of such a possible intermediate upon treatment with the radical cation (Scheme 3) (88JOC1973). 

Acyl substituents at the 3- and/or 4-positions result in decreased hydrolytic stability compared with the alkyl and aryl derivatives described above. Despite this constraint most of the usual reactions of the carbonyl group are possible. Aldehydes <9ILA1211> and ketones are oxidized to the carboxylic acid, borohydride reduction affords the expected alcohols, and epoxides are formed on reaction with diazomethane. Oximes and arylhydrazones are formed with hydroxylamine and arylhydrazines, and the products may subsequently undergo monocyclic rearrangement involving the oxadiazole to give the corresponding isomeric furazans and 1,2,3-triazoles (Section 4.05.5.1.4). 

Mild oxidation of o-nitrobenzaldehyde arylhydrazones with either bromine and sodium acetate or lead tetraacetate results in the overall loss of two hydrogen atoms and production of a class of iV-aryl heterocycles, tiie structure of which was the subject of uncertainty and some controversy for more than fifty years. These oxidation products were first prepared and investigated by Chattawayj " who described them as isodiazomethanes and formulated them as the triaziridine derivates 72. Structural assignment was based entirely on evidence from degradation studies in particular, Chattaway showed that reduction of... 

Several examples of the mesoionic 3-aryl-l,2,3,4-oxatriazole-5-oxide ring system 123 have been prepared by cyclisation of the arylhydrazones 122 these in turn are prepared from aryldiazonium salts and bromonitromethane <99KGS413>. 

Nitrobenzaldehyde arylhydrazones with a halogen give compounds (453), which on treatment with base form 2-aryl-4-oxido-l,2,3-benzotriazinium betaine 1-oxides (454 R = Ar) (74JOC2710). 

A thermally induced rearrangement of arylhydrazones of furoxan-3-carbonyl compounds into 2-aryl-5-[(hydroximino)arylmethyl]-2//-l,2,3-triazole 1-oxides has been observed for the first time.156 2-(2,2-Dicyano-l-hydroxyethenyl)-l-methylpyrroles (192) are readily rearranged to their 3-isomers (193) in nearly quantitative yield when heated to 75-142 °C. The inter- or intra-molecular auto-protonation of a pyrrole ring... 

This chapter covers the literature from mid-1995 to mid-2007. Recent advances include the preparation of the first A2-l,2,3,4-oxatriazolines 7 by intramolecular [3+2] cycloaddition (Section 6.08.9), synthesis of mesoionic derivatives 4 using bromonitroformaldehyde iV-arylhydrazones as the starting materials (Section 6.08.9), and detailed studies of the biologically active mesoionic oxatriazoles in the context of their nitric oxide donating capabilities (Section... 

Other approaches to related bicyclic systems include the reaction of 1,3-dicarbonyl compounds with l-amino-2(l//)-pyridin-2-amines or -pyrimidin-2-amines (Scheme 14) (94JHC1157] and the oxidation of 2- or 4-pyrimidinyl ketone arylhydrazones to yield triazolo[l,5-a - or -ll,5-c]pyrimidinium salts (Scheme 15). In this reaction 2,4,4,6-... 

Pyrazolo[5,l-c][l,2,4]triazoles (272) have been prepared by lead tetraacetate oxidation of arylhydrazones (270) via azine intermediates (271) (79TL1567). 

Dioximes or bis(arylhydrazones) of /3-diketones, react with chlorides or oxides of sulfur, selenium or tellurium, to produce various polyheterapentalenes, as indicated in Scheme 5. 

The mechanism of osazone formation has been studied by the isotopic-tracer technique, by using an N -labeled arylhydrazone, treating it with an unlabeled arylhydrazone, and making an isotopic assay of the reaction products. The results were characteristic of Weygand s mechanism involving the oxidation of the hydrazone to a l-imino-lV -2-keto derivative (40) and subsequent osazone formation with the elimination of ammonia-iV . 

As in the case of imines, during the oxidation of arylhydrazones nucleophilic attack occurs at the a-carbon [Eq. (16)]. If the nucleophile is water, and R is an aryl substituent, cleavage under formation of the parent ketone, nitrogen, and ArH is observed [118]. 

L-f/ireo-glycerol-l-yl)-pyrazoline-5-one (49) when its solution in alkali was acidified with acetic acid (50). The reaction was further extended to other bis(arylhydrazones) (51), The structure of the phenyl analogue (49) was established by oxidation to the known 3-carboxy-l-phenyl-4-phenylazopyrazolin-5-one (50). Later, on the basis of NMR data (39), the structure of this group of compounds was formulated as the hydra-zones (51). Acylation of 51 aflForded the tri-O-acylated derivatives (51), while periodate oxidation of 51 gave 3-formyl-l-aryl-4,5-pyrazoledione-... 

As a specific kind of intramolecular amination, one can consider the oxidative cyclization of arylhydrazones of benzazolyl-2-ketones by the action of N-bromosuccinimide (67AG272) [Eq. (22)]. This reaction occurs most easily for hydrazones of benzimidazoles, and this process is more difficult in the case of benzothiazoles weakly basic hydrazones of benzo-xazolyl-2-ketones do not take part in this conversion. 

This type of compound was prepared from aldehydes 216 by conversion into the bis(arylhydrazone) 225, which, upon reaction with copper sulfate, afforded the corresponding triazoles 227 (Scheme 48). When 216 was allowed to react with amines or hydrazines having substituents that differ from that in 216, the corresponding mixed bis(hydrazones) 228-230 were prepared. Attempted oxidation of bis(benzoylhydrazone) 231 with iodine and mercuric oxide to give 232 was investigated (80MI7). 

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