DABCO-promoted synthesis of pyrazoles from tosylhydrazones and nitroalkenes

Abstract

An efficient synthesis of pyrazoles from tosylhydrazones and nitroalkenes was developed. In comparison with the previously reported 1,3-dipolar cycloaddition reaction of diazo compounds with electron-deficient alkenes or alkynes, this methodology proceeded with a sequential Baylis-Hillman/intramolecular cyclization mechanism and a variety of reversed regioselectivity products were prepared in good yields.

Acid‐Mediated Three Component Assembly of 4‐Fluoropyrazoles from α‐Fluoronitroalkenes, Hydrazines, and Aldehydes

“…From the mechanistic point of view, concerted and stepwise mechanisms of [3+2]‐cycloaddition are possible for this reaction , , . While stepwise mechanisms involving Michael addition/cyclization are known to be more favored for annulations of nitroalkenes in basic conditions involving strong nucleophilic reaction partners, mechanism of acid‐mediated annulations is a subject of discussions , . Comparatively to α‐unsubstituted nitroalkenes and even α‐alkylated nitroalkenes halonitroalkenes are considered to be weaker Michael acceptors.…”

Fast and Efficient Continuous Flow Method for the Synthesis of Ynones and Pyrazoles

“…3,5‐Diphenyl‐1 H ‐pyrazole (5a): 1 H‐NMR (400 MHz, CDCl 3 ): δ = 7.74–7.72 (m, 4H), 7.43–7.32 (m, 6H), 6.84 (s, 1H). 13 C NMR (100 MHz, CDCl 3 ): δ = 148.8, 131.3, 128.9, 128.3, 125.7, 100.2.…”

“…3,5-Diphenyl-1H-pyrazole (5a): [38] [41] [43] 1 H-NMR (400 MHz, CDCl 3 ): δ = 11.95-10.86 (bs, 1H), 7.61 (dd, J = 7.4, 1.8 Hz, 2H), 7.29-7.21 (m, 4H), 6.62 (s, 1H), 6.51 (d, J = 3.2 Hz, 1H), 6.31 (dd, J = 3.2, 2.0 Hz, 1H). 13 [ [45] 5-Benzyl-1,3-diphenyl-1H-pyrazole (5i): [46] [44] [47] 1 H-NMR (400 MHz, CDCl 3 ): δ = 9.86 (s, 1H), 7.72 (d, J = 7.2 Hz, 2H), 7.38-7.26 (m, 3H), 6.34 (s, 1H), 2.60 (t, J = 7.8 Hz, 2H), 1.66-1.59 (m, 2H), 1.32-1.25 (m, 8H), 0.87 (t, J = 6.8 Hz, 3H).…”

Transition metal-free one-pot synthesis of nitrogen-containing heterocycles

“…Later on, in the same year, Tan published another process for the synthesis of substituted pyrazoles 135a, 135b from tosylhydrazones 133 and nitroalkenes 134 using a base and DABCO. The pathway for this pyrazole formation was belived to happen via a Baylis-Hillman mechanism followed by an intermolecular cyclization (Scheme 40) [89].…”