- Cyclocondensation of arylhydrazines with 1,3-bis(het)arylmonothio-1,3- diketones and 1,3-Bis(het)aryl-3-(methylthio)-2-propenones: Synthesis of 1-Aryl-3,5-bis(het)arylpyrazoles with complementary regioselectivity
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Two efficient highly regioselective routes for the synthesis of unsymmetrically substituted 1-aryl-3,5-bis(het)arylpyrazoles with complementary regioselectivity starting from active methylene ketones have been reported. In the first protocol, the newly synthesized 1,3-bis(het)aryl-monothio-1,3-diketone precursors (prepared by condensation of active methylene ketones with het(aryl) dithioesters in the presence of sodium hydride) were reacted with arylhydrazines in refluxing ethanol under neutral conditions, furnishing 1-aryl-3,5-bis(het)arylpyrazoles 7, in which the het(aryl) moiety attached to the thiocarbonyl group of monothio-1,3-diketones is installed at the 3-position. In the second method, the corresponding 3-(methylthio)-1,3-bis(het)aryl-2- propenones (prepared in situ by base-induced alkylation of 1,3- monothiodiketones) were condensed with arylhydrazines in the presence of potassium tert-butoxide in refluxing tert-butyl alcohol, yielding 1-aryl-3,5-bis(het)arylpyrazoles 9 with complementary regioselectivity (method A). The efficiency of this protocol was further improved by developing a one-pot, three-component procedure for the synthesis of pyrazoles 9, directly from active methylene ketones, by reacting in situ generated 3-(methylthio)-1,3-bis(het)aryl-2-propenones with arylhydrazines in the presence of sodium hydride (instead of potassium tert-butoxide as base). The structures and regiochemistry of newly synthesized pyrazoles were confirmed from their spectral and analytical data along with X-ray crystallographic data of three pairs of regioisomers.
- Kumar, S. Vijay,Yadav, Santosh K.,Raghava,Saraiah,Ila,Rangappa,Hazra, Arpan
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p. 4960 - 4973
(2013/06/27)
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- β-Thioketones. Part 6. Electronic Absorption Spectra of Aromatic β-Thioketones. A Study of Enol-Enethiol Tautomerism
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Aromatic β-thioketones exist in solution as mixture of rapidly interconverting Z-enol and Z-enethiol tautomers.The electronic absorption spectra exhibit in general four absorption bands in the u.v.-visible region at ca. 265 (ArC=O ?,?*; ArC=C ?,?*), 330 (ArC=S ?,?*; O=CC=CS ?,?*; C=O n,?*), 415 (OC=CC=CS ?,Π*), and 520 nm (C=S n,Π*), respectively.The β-thioxoketones are converted by sodium hydroxide into the corresponding anions.CNDO/B Calculations predict that the negative charge in the β-thioxoketonates is delocalized over the OCCCS system, suggesting simultaneously sickles or W shaped conformations.Two characteristic absorption bands found for the β-thioxoketonates at ca. 275 and 400 nm are assigned to ?,?* transitions involving the Ar-CCC-Ar' and SCCCO chromophores, respectively.The enol-enethiol tautomeric equilibrium has been studied by means of low temperature spectroscopy.At room temperature equilibrium constants (K293) of 3-5 have been found corresponding to a 4:1 enol-enethiol concentration ratio.The reaction entropy (ΔSr) has been found to be negative for the enethiol->enol conversion,reflecting the intramolecular O-H...S hydrogen bonds to be considerably stronger than the corresponding O...H-S hydrogen bond.Variation in ΔSr and K293 as functions of substitution in the aryl group next to oxygen are discussed.
- Carlsen, Lars,Duus, Fritz
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p. 1768 - 1773
(2007/10/02)
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