Base-mediated reaction of the Bestmann-Ohira reagent with nitroalkenes for the regioselective synthesis of phosphonylpyrazoles

1,3-Dipolar cycloaddition of the anion of diethyl 1-diazomethylphosphonate, generated in situ from diethyl 1-diazo-2-oxopropylphosphonate (Bestmann-Ohira reagent), with conjugated nitroalkenes provides regioisomerically pure phosphonylpyrazoles in moderate to good yield. These pyrazoles are formed in one pot via spontaneous elimination of the nitro group. However, nitropyrazoles could be synthesized by the same strategy using alpha-bromonitroalkenes. The methodology works for the synthesis of phosphonylpyrazoles fused to other carbo- and heterocycles as well. [reaction: see text]