Copper-Catalyzed Synthesis of Substituted 4-Acylpyrazole Derivatives through a Cascade Transformation from N-Propargylic Sulfonylhydrazones and Diaryliodonium Salts

Abstract

A concise strategy for the synthesis of substituted 4-acylpyrazole derivatives from N-propargylic sulfonylhydrazones and diaryliodonium salts has been developed. The pyrazole derivatives are formed through a five-step cascade sequence that includes intramolecular cyclization, hydroxylation, elimination, copper-catalyzed aerobic oxidation, and intramolecular rearrangement.

Key words

propargylic sulfonylhydrazones - diaryliodonium salts - cascade transformation - acylpyrazoles

• References and Notes

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• 14 Pyrazoles 3a–m; General Procedure A 10 mL round-bottomed flask was charged with the appropriate N-propargylic sulfonylhydrazone 1 (0.5 mmol), diaryliodonium salt 2 (0.75 mmol), and Cu(OTf)₂ (18.1 mg, 0.05 mmol). Undried DBE (5 mL) was added, and the mixture was stirred at 80 °C in air until the reaction was complete (TLC). The solvent was removed under vacuum, and the crude residue was purified by column chromatography (silica gel, eluent: petroleum ether/EtOAc = 30/1). (1,3-Diphenyl-1H-pyrazol-4-yl)(phenyl)methanone (3a) White solid; yield: 92 mg (57%); mp 117–119 °C. ¹H NMR (500 MHz, CDCl₃): δ = 7.34–7.37 (m, 2 H), 7.43 (t, J = 7.7 Hz, 2 H), 7.50–7.56 (m, 4 H), 7.74–7.77 (m, 2 H), 7.80–7.88 (m, 5 H), 8.31 (s, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 119.6, 121.2, 127.5, 128.1, 128.3, 128.6, 128.9, 129.4, 129.6, 132.1, 132.2, 132.6, 138.9, 139.2, 154.0, 190.0. HRMS (ESI): m/z [M + Na] calcd for C₂₂H₁₆N₂NaO: 347.1155; found: 347.1160. [3-(4-Bromophenyl)-1-(4-tolyl)-1H-pyrazol-4-yl](phenyl)-methanone (3b) White solid; yield: 102 mg (49%); mp 139–141 °C. ¹H NMR (400 MHz, CDCl₃): δ = 2.43 (s, 3 H), 7.30 (d, J = 8.0 Hz, 2 H), 7.44–7.52 (m, 4 H), 7.58 (t, J = 7.3 Hz, 1 H), 7.64–7.72 (m, 4 H), 7.86 (d, J = 7.7 Hz, 2 H), 8.23 (s, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 21.0, 119.5, 120.8, 122.8, 128.4, 129.4, 130.1, 130.5, 131.1, 131.2, 132.5, 132.6, 136.8, 137.7, 138.9, 152.6, 189.7. HRMS (ESI): m/z [M + Na] calcd for C₂₃H₁₇BrN₂NaO: 439.0416 and 441.0396; found: 439.0419 and 441.0399.