The cycloaddition reaction of diazomethane with diethyl 1,3-butadienylphosphonates containing electronegative substituents on the α-carbon atom gave diethyl (4-alkenyl-Δ1-pyrazolin-3-yl)phosphonates in high yields. Thermolysis and the base-catalyzedreaction of the pyrazolinylphosphonates produced functionalized 2,4-pentadienylphosphonates and pyrazoles, respectively, in good yields.