The stereochemistry of thermolysis of 4-alkylidene-1-pyrazolines

The products of thermolysis of 11 4-alkylidene-1-pyrazolines were isolated and identified. It is observed that these products cannot be rationalized in terms of a planar singlet trimethylenemethane type of intermediate, nor can they be reconciled in terms of the orthogonal trimethylenemethane singlets. Evidence is presented that for 4-ethylidene-1-pyrazolines the carbon anti to the methyl has the greatest propensity to become a cyclopropane carbon, and that which is syn is most likely to become the exocyclic methylene.