Shiv P. Singh

FORMATION AND DEHYDRATION OF A SERIES OF 5-HYDROXY-5-TRIFLUOROMETHYL-4,5-DIHYDROPYRAZOLES

Abstract Reaction of five (3-oxo-4,4,4-trifluorobutanoyl)heterocycles with hydrazine hydrate under mild conditions gave the corresponding 3-heterocyclyl-5-hydroxy-5-trifluoromethyl-4,5-dihydropyrazoles. Thermal elimination of water from the 3-(thien-2-yl), 3-(pyridin-2-yl) and 3-(pyridin-4-yl) compounds readily gave the corresponding pyrazoles but acid catalysis was required to form 3-(benzothiazol-2-yl)-5-trifluoromethylpyrazole and 3-(1-methylbenzimidazol-2-yl)-5-trifluoromethylpyrazole. More forcing conditions were required for the analogous dehydration/aromatisations giving 3,5-bis(trifluoromethyl)-1-(4-nitrophenyl)pyrazole and 3,5-bis(trifluoromethyl)-1-pentafluorophenylpyrazole.

Hypervalent Iodine Oxidation of 1, 3, 5-Trisubstituted Pyrazolines: A Facile Synthesis of 1,3,5-Trisubstituted Pyrazoles

Abstract 1, 3, 5-Trisubstituted pyrazolines (1a–i) undergo facile oxidation to the corresponding pyrazoles (2a–i) in the presence of iodobenzene diacetate.

Approaches towards the synthesis of 5-aminopyrazoles

Summary The biological and medicinal properties of 5-aminopyrazoles have prompted enormous research aimed at developing synthetic routes to these heterocyles. This review focuses on the biological properties associated with this system. Various synthetic methods developed up to 2010 for these compounds are described, particularly those that involve the reactions of β-ketonitriles, malononitrile, alkylidenemalononitriles and their derivatives with hydrazines, as well as some novel miscellaneous methods.

Reaction of hydrazinoquinolines with trifluoromethyl-β-diketones: structural and mechanistic studies

Abstract Reaction of 2-hydrazino-4-methylquinoline with a series of trifluoromethyl-β-diketones gives 3-substituted-5-hydroxy-1-(4-methylquinolin-2-yl)-5-trifluoromethyl-4,5-dihydropyrazoles and, in some cases, 5-substituted-1-(4-methylquinolin-2-yl)-3-trifluoromethylpyrazoles, depending on the substitution of the diketone. Dehydration of the hydroxydihydropyrazoles can be effected with sulphuric acid in acetic acid to give the regioisomeric 3-substituted-1-(4-methylquinolin-2-yl)-5-trifluoromethylpyrazoles. In contrast, the reaction of two 4-hydrazinoquinolines with 1,1,1-trifluoropentane-2,4-dione afforded a different isolable intermediate, the corresponding hydrazone formed at the 4-carbonyl. Dehydration gave the 1-(substituted-quinolin-4-yl)-3-methyl-5-trifluoromethylpyrazoles. The regioisomeric identity of the pyrazoles was established using 19 F NMR.

REACTION OF HYDRAZINES AND HYDROXYLAMINE WITH TRIFLUOROMETHYL-β-DIKETONES: SYNTHESIS OF TRIFLUOROMETHYLPYRAZOLE AND ISOXAZOLE DERIVATIVES

The reaction of trifluoromethyl-(3-diketones with hydrazines and hydroxylamine finds interesting application in the synthesis of trifluoromethylpyrazole and isoxazole derivatives. The review covers the literature of the reactions mentioned up to 2007. Mechanistic aspects leading to the formation of these heterocycles and their NMR spectral characteristics are also described.

Hypervalent Iodine Oxidation of 2-Aryl-1,2,3,4-tetrahydro-4-quinolones: An Easy Access to 2-Aryl-4-quinolones

Abstract Oxidation of 2-aryl-1,2,3,4-tetrahydro-4-quinolones (1a-e) using iodobenzene diacetate in methanolic potassium hydroxide leads to dehydrogenation of 1 thereby providing an easy access to 2-aryl-4-quinolones (2a-e).

Hypervalent iodine oxidation of o-aminochalcones: A novel synthesis of 3-(β-styryl)-2,1-benzisoxazoles

Hypervalent iodine oxidation of o-aminochalcones using C6H5I(OAc)2KOHMeOH leads to a novel and useful route for the synthesis of 3-(β-styryl)-2,1-benzisoxazoles. A plausible mechanism for this novel rearrangement is proposed.

Synthesis of Heterocyclic Compounds from the Reactions of Dehydroacetic Acid (DHA) and Its Derivatives

3-Acetyl-4-hydroxy-6-methyl -2-oxo-2H-pyran (Dehydroacetic acid, DHA) and its simple derivatives such as 3-cinnamoyl-4-hydroxy-6-methyl-2-oxo-2H-pyrans (Chalcone analogues of DHA) find interesting applications in the synthesis of various heterocyclic compounds. The review highlighting this aspect covers literature up to 2003.

Hypervalent Iodine Oxidation of Hydrazones of Some Nitrogen Heterocyclic Ketones and Aldehydes: An Efficient Synthesis of Fused 1,2,3-Triazoloheterocycles

Abstract Fused 1,2,3-triazoloheterocycles have been prepared by iodobenzene diacetate mediated oxidation of hydrazones of nitrogen heterocyclic ketones and aldehydes.

Structural Revision in Pyrazole Chemistry

Reaction of several heteroarylhydrazines with β-diketones has been incorrectly reported to generate triazepines or diazepines. It has now been firmly established that these reactions lead to the formation of pyrazoles. Further, many workers have reported the formation of pyrazoles in the reaction of heteroarylhydrazines with trifluoromethyl 1,3-diketones, whereas the actual products were found to be 5-hydroxy-5-trifluoromethyl-4,5-dihydro-1H-pyrazoles. It was also established that with a trifluoromethyl β-diketone and hydrazines, the location of the CF 3 group at position 3 or 5 of pyrazoles depends on the nature of the hydrazine. Erroneous reports concerning the structure of the products obtained by the reaction of dehydroacetic acid with hydrazines have also been revised.