N-2 extrusion paths of the smallest monocyclic azoalkane, diazetine, its tetramethyl-substituted derivative, and their three-membered cyclic isomers were investigated by ab initio calculations. Systematic considerations of the orbital correlation diagram predicted that the routes conserving the C-s point group are symmetry allowed. CASSCF(2,2)/6-31G*, CASSCF(4,4)/6-31G*, and CASSCF(2,2)/6-311+G(2d,p) geometry optimizations were carried out and showed that the decomposition paths of the isomers followed the Woodward-Hoffmann rule. However, the paths of diazetines did not follow it. The dual orbital-mixing rule proposed here correctly explained the Jahn-Teller effect. Reaction enthalpy and activation energy were satisfactorily reproduced by the BCCD(T)/6-31G*//CASSCF(2,2)/6-31G* energies.