Synthesis of 4,4-dimethyl-3,4-dihydro-3,3,5-trisubstituted-2H-pyrazoles and N-benzoyl derivatives: Method for “Hydrolysis” of unreactive amides and carbamates

Addition of organolithium reagents to 4,4-dimethyl-3,5-disubstituted-4H-pyrazoles produced a series of 4,4-dimethyl-3,4-dihydro-3,3,5-trisubstituted-2H-pyrazoles, 2–6 , in good yield. The reaction was stereoselective: addition of organolithium compounds occurred only to carbon-3 of 4,4-dimethyl-3-alkyl-5-aryl-4H-pyrazoles. The 3,4-dihydro-2H-pyrazoles were found to be of high sensitivity to oxygen. For long term storage and ease of handling, N-benzoyl derivatives were synthesized. Removal of the protecting group could not be accomplished by use of many standard sets of conditions. Deprotection was accomplished in high yield by reaction of the N-benzoyl-4,4-dimethyl-3,4-dihydro-3,3,5-trisubstituted-2H-pyrazoles with anhydrous potassium t-butoxide in toluene [heated under reflux (ultra-pure argon)] in the presence of a phase transfer catalyst (18-Crown-6). Cleavage of a N-carbamate derivative was also achieved by this phase transfer approach. This methodology should be applicable to the hydrolysis of unreactive amides and carbamates in general.     

N1-substituted 3,5-dimethoxy-4-halogeno-1H-pyrazoles: Synthesis and NMR study

The synthesis of 1-methyl- and 1-benzyl-3,5-dimethoxy-4-halogeno-1H-pyrazoles is described. Moreover, detailed nmr spectroscopic studies with the title compounds are presented. The highly shielded pyrazole C-4 atom in the corresponding 4-iodo congeners was found to have a ¹³C-chemical shift of only ?27 ppm.     

Reaction of 3-benzylidene-2,4-pentanedione and 3-methoxymethylene-2,4-pentanedione with aroylhydrazines

Aroylhydrazines 2 reacted with 3-benzylidene-2,4-pentanedione ( 1 ) to give 1-aroyl-3,5-dimethyl-1H-pyrazoles 5 and benzaldehyde aroylhydrazones 6 . From the reaction of 3-methoxymethylene-2,4-pentanedione ( 7 ) with aroylhydrazines 2 the unknown N-aroyl-4-acetyl-1H-pyrazoles 9 were exclusively isolated in good yields.     

Synthesis and reactions of pyrazole-4-carbaldehydes

1-, 3-, and 5-Alkylpyrazoles, as well as linearly bridged bis-pyrazoles, were converted into the corresponding 4-formyl derivatives by Vilsmeier-Haak reaction both under standard conditions and under microwave activation in DMF over a period of 10 min. 1,1′-(Hexane-1,6-diyl)bis(3,5-dimethyl-1H-pyrazole) and 1,1′-(benzene-1,4-diyldimethylene)bis(3,5-dimethyl-1H-pyrazole) gave rise to 4-formyl derivatives at both pyrazole rings. 5-Chloro-1,3-dialkyl-1H-pyrazoles failed to undergo formylation according to Vilsmeier-Haak or under microwave activation. 1,1′-Bridged bis-3,5-dimethyl-1H-pyrazoles reacted with 2-sulfanylethanol on heating in the presence of chloro(trimethyl)silane to give the corresponding bridged bis-4-(1,4,6-oxadithiocan-5-yl)-1H-pyrazoles.     

Synthesis of monomeric and oligomeric 1,1′-methylenebis-(1H-pyrazoles) contaning ethynyl fragments

1,1′-Methylenebis(1H-pyrazole) and 1,1′-methylenebis(3,5-dimethyl-1H-pyrazole) reacted with iodine in the presence of iodic acid to give the corresponding 4,4′-diiodo derivatives. Polycondensation of the latter with p-diethynylbenzene led to the formation of oligomeric compounds. 1,1′-Methylenebis(4-iodo-1H-pyrazoles) were converted into 4,4′-diethynyl derivatives by the Sonogashira and reverse Favorskii reactions, and their oxidative polycondensation in the presence of copper(I) chloride in pyridine also gave oligomeric products with a molecular weight exceeding 9000.     

One-pot synthesis of 5-fluoroalkyl-5-hydroxy-4-hydroxy-imino-1-isonicotinoyl-4,5-dihydro-1<Emphasis Type="Italic">H</Emphasis>-pyrazoles and their tuberculostatic activity

5-Fluoroalkyl-5-hydroxy-4-hydroxyimino-1-isonicotinoyl-4,5-dihydro-1H-pyrazoles were synthesized via a one-pot procedure from polyfluoroalkyl-containing 1,3-diketones, sodium nitrite in acetic acid, and isoniazid. Acetylacetone gave rise to 3-hydroxyiminopentane-2,4-dione monoisonicotinoylhydrazone which underwent intramolecular cyclization to 5-hydroxy-4-hydroxyimino-1-isonicotinoyl-3,5-dimethyl-4,5-dihydro-1H-pyrazole on heating in ethanol. The synthesized compounds exhibited moderate tuberculostatic activity.     

Identification of some isomeric 3,5-unsymmetrically substituted 1-aroyl-pyrazole derivatives by NMR spectroscopy

The ¹H and ¹³C nmr chemical shifts are used for the structural assignment of isomeric 1-aroyl-4,5-dihydro-5-hydroxy-4,4-dimethyl-1H-pyrazoles 1 unsymmetrically substituted with phenyl or methyl in the 3,5-positions of the pyrazole ring. The ¹H nmr spectra of 1-aroyl- or 1-acetyl-4-methyl-1H-pyrazoles 2 are useful in structure elucidation of unsymmetrically 3- or 5-methyl substituted derivatives.     

Thermolysis of hexasubstituted-4,5-dihydro-3H-pyrazoles: Synthesis of 1-alkoxy- and 1-acetoxy-1,2,2,3,3-pentasubstituted-cyclopropanes

A series of 5-alkoxy- and 5-acetoxy-4,4-dimethyl-3,3,5-trisubstituted-4,5-dihydro-3H-pyrazoles 2a-f (hexasubstituted pyrazolines) was synthesized by oxidation of the corresponding 3,4-dihydro-2H-pyrazoles with lead tetraacetate in the appropriate solvents. The 5-acetoxy compounds were produced when the oxidations were carried out in methylene chloride. Oxidation with lead tetraacetate in dry alcoholic solvents resulted in the formation of the 5-alkoxy derivatives as the major products. Thermolysis of the hexasubstituted-4,5-dihydro-3H-pyrazoles 2a-f in cyclohexane or as the melt at high temperature yielded the 1-alkoxy- and 1-acetoxy-1,2,2,3,3-pentasubstituted cyclopropanes 3a-f in good yields. Trace amounts of alkene products were formed in several reactions. No products attributable to cycloreversion pathways were detected. The product distributions were consistent with extrusion of nitrogen gas from 2a-f to yield the singlet 1,3-diradical, closure of which resulted in cyclopropane formation with partial retention of stereochemistry.     

External electrolyte-free electrochemical one-pot cascade synthesis of 4-thiocyanato-1H-pyrazoles

A practical synthetic method for 4-thiocyanato-1H-pyrazoles through the electrochemical cascade reaction of hydrazines, 1,3-diones and NH₄SCN under metal-, chemical oxidant-and external electrolyte-free conditions was established. Importantly, both a gram-scale synthesis of 4-thiocyanato-1H-pyrazoles and five one-pot sequential transformations starting from hydrazine were successfully accomplished.     

Vilsmeier-Haak formylation of 3,5-dimethylpyrazoles

Formylation of N-alkyl-3,5-dimethyl-1H-pyrazoles according to Vilsmeier-Haak led to the formation of the corresponding 4-formyl derivatives. 3,5-Dimethyl-1H-pyrazole having no substituent on the nitrogen atom failed to undergo formylation at the 4 position under analogous conditions. 3,5-Dimethyl-1H-pyrazole-4-carbaldehyde was synthesized by alkaline hydrolysis of methyl β-(4-formyl-3,5-dimethyl-1H-pyrazol-1-yl)propionate and subsequent heating of the acid thus formed.     

One-Pot Synthesis of 4-Thiocyanato-1H-pyrazoles through Electrochemical Multicomponent Thiocyanation under Metal- and Oxidant-Free Conditions

An electrochemical oxidative three-component tandem reaction of arylhydrazines, 1,3-diketones and ammonium thiocyanate has been established to construct 4-thiocyanato-1H-pyrazoles under metal-, oxidant-free and mild conditions in an undivided cell. Various 4-thiocyanato-1H-pyrazole derivatives could be obtained with broad substrate scope, and excellent functional group tolerance from available raw materials in moderate to excellent yields. This thiocyanation protocol with practicability and scalability features economical and eco-friendly.     

Reaction of ethyl 4-[(<Emphasis Type="Italic">E</Emphasis>)-1-chloro-3-oxoprop-1-en-1-yl]-3,5-dimethyl-1<Emphasis Type="Italic">H</Emphasis>-pyrrole-2-carboxylate with hydrazines

Ethyl 4-[(E)-1-chloro-3-oxoprop-1-en-1-yl]-3,5-dimethyl-1H-pyrrole-2-carboxylate reacted with substituted hydrazines in different solvents to give mixtures of regioisomeric 3- and 5-substituted pyrazoles. Conditions were found for selective formation of 1-aryl(alkyl)-5-(5-ethoxycarbonyl-2,4-dimethyl-1H-pyrrol-3-yl)-1H-pyrazoles.     

1,3-Dipolar cycloaddition reactions of nitrilimines with 1-aroyl-4,5-dihydro-3,4,4-trimethyl-5-methylene-1H-pyrazoles. Synthesis of 6-aroyl-8,9,9-trimethyl-1,2,6,7-tetrazaspiro[4.4]nona-2,7-dienes

From the 1,3-dipolar cycloaddition reactions of 5-methylene-1H-pyrazoles 3 with N-arylnitrilimines the novel spiro-cycloadducts 4 were isolated, in addition to the corresponding 5-(2-aroylhydrazono-1,1-dimethylpropyl)-1H-pyrazoles 5. These pyrazoles 5 were the only products from the reactions of 3 with N-methylnitrilimine 2d. The chemical behaviour of the spiro-cycloadducts 4 was also examined.     

A study on m-chloroperbenzoic acid and lead tetraacetate oxidation of some 1-aroyl-4,5-dihydro-4,4-dimethyl-5-methylene-1H-pyrazoles

1-Aroyl-4,5-dihydro-5-methylene-1H-pyrazoles 1 are converted upon oxidation with m-chloroperbenzoic acid to the pyrazolones 2 . The same aroyl enamides 1 are also oxidized with LTA to form the acetoxy derivatives 7 and 8 . The reaction mechanisms are discussed.     

Diels-alder reaction of (E)-4-(2-nitroethenyl)-1H-imidazoles and methyl (E)-3-(1-imidazol-4-yl)propenoates

Diels-Alder reaction of methyl (E)-3-(1H-imidazol-4-yl)propenoates 2, 3a-c and (E)4-(2-nitroethenyl)-1H-imidazoles 3d,e with 2,3-dimethyl-1,3-butadiene, cyclopentadiene, and cyclohexa-1,3-diene gave the corresponding cycloadducts 6–9 .     

Novel 4,5-bis(trifluoromethyl)-1H-pyrazoles through a concise sequential iodination-trifluoromethylation reaction

An efficient protocol for the assembly of a new series of 1,3-disubstituted 4,5-bis(trifluoromethyl)-1H-pyrazoles – through a concise sequential iodination-trifluoromethylation reaction at the C-4 position of an initial series of 1-aryl-3-(alkyl/aryl)-5-trifluoromethyl-1H-pyrazoles (electronically deactivated heterocycles), using NIS and MFSDA, at good conversion yields and in short reaction times – is reported.     

Synthesis of 1-substituted 3-nitroquinolin-4(1H)-ones

A versatile synthetic method for preparing 1-substituted 3-nitroquinolin-4(1H)-ones from corresponding 2-fluoro-α-nitroacetophenones is demonstrated by the synthesis of 6,7-difluoro derivatives 7a-c . The method involves sequential treatment of the starting nitroacetophenone with triethyl orthoformate and the appropriate amine, followed by a nucleophilic cyclization reaction under mild conditions. The C-7 fluorine atom of 7 can be displaced by cyclic amines. Substituted 6-fluoro-7-(4-methyl-1-piperazinyl)-3-nitroquinolin-4(1H)-ones 8a-c were prepared in this way.     

H4SiW12O40-catalyzed cyclization of epoxides/aldehydes and sulfonyl hydrazides: An efficient synthesis of 3,4-disubstituted 1H-pyrazoles

A simple and efficient method for the synthesis of pyrazoles through a silicotungstic acid(H₄Si W₁₂O₄₀)-catalyzed cyclization of epoxides/aldehydes and sulfonyl hydrazides has been developed. Various epoxides/aldehydes were smoothly reacted with sulfonyl hydrazides to furnish regioselectivity 3,4-disubstituted 1H-pyrazoles. The application of such an earth-abundant, readily accessible, and nontoxic catalyst provides a green approach for the construction of 3,4-di...     

The synthesis of pyrazolo[1,2-a]pyrazoles. Regio- and stereo-selective 1,3-dipolar cycloadditions of (1Z)-rel-(4R,5R)-1-arylmethylene-4-benzoylamino-5-phenyl-3-pyrazolidinon-1 -azomethinimines

Reaction of rel-(4R,5R)-4-benzoylamino-5-phenyl-3-pyrazolidinone (4) with aromatic aldehydes 5a-f gave the corresponding (1Z)-rel-(4R,5R)-1-arylmethylene-4-benzoylamino-5-phenyl-3-pyrazolidinon-1-azomethinimines 6a-f . 1,3-Dipolar cycloadditions of azomethinimines 6a-f to various dipolarophiles, which were found to proceed regio- and stereo-selectively, afforded the corresponding pyrazolo[1,2-a]-pyrazoles 8a-f, 10 , and 13–16 . Reaction of azomethinimine 6a with hydrogen cyanide gave rel-(5R,6R)-6-benzoylamino-5,6-dihydro-3,5-diphenyl-1-oxo-1H,7H-pyrazolo[1,2-a][1,2,3]triazole (18) as a representative of a new ring system.     

Synthesis of 6-aryl-1,3-dimethyl-6,7-dihydro-6-azalumazin-7-(6H)ones and their conversion into 2-aryl-1,2,4-triazine-3,5-(2H,4H)odiones. A new synthesis of 1-aryl-6-azauracils

Treatment of 6-amino-5-arylazo-1,3-dimethyluracils with urea or N,N′-carbonyldiimidazole gave the respective 6-aryl-1,3-dimethyl-6,7-dihydro-6-azalumazin-7-(6H)ones, which were hydrolyzed with alkali to afford 2-aryl-2,3,4,5-tetrahydro-3,5-dioxo-1,2,4-triazine-6-carboxylic acids (1-aryl-6-azauracil-5-carboxylic acids). Thermal decomposition of these carboxylic acids gave the corresponding 2-aryl-1,2,4-triazine-3,5-(2H,4H)diones (1-aryl-6-azauracils). Methylation of the latter with methyl iodide gave the corresponding 2-aryl-4-methyl-1,2,4-triazine-3,5-(2H,4H)diones (1-aryl-3-methyl-6-azauracils).