Derived 4H-pyrazole as a reactive intermediate in the transformation of cyclic malonic hydrazide into 1,5-dichloro-1,4-cyclohexadiene through a Diels–Alder reaction with substituted cyclopropenes. AM1 semiempirical and B3LYP density functional theory studies of the cycloaddition reaction of one pyrazole tautomer with cyclopropene