Synthesis and chemistry of structurally unique hexasubstituted pyrazolines

A review focused on our contributions to the synthesis and chemistry of hexasubstituted pyrazolines is presented. The development of a synthetic route to penta-substituted 2H-pyrazoles 1 provides the key starting materials that are used in the synthesis of several unique series of highly substituted pyrazolines 2-6. Thermolysis of pyrazolines 2-4 allows the facile synthesis of hexasubstituted cyclopropanes. Autoxidation of pentasubstituted 2H-pyrazoles 1 in acetone produces a series of hydroperoxy substituted pyrazolines 5 which are effective oxygen-atom transfer reagents. The thermal decomposition of 5 produces beta-keto radicals, the transformation of which in the presence of oxygen provides a good route for the synthesis of 3-hydroxy-1,2-dioxolanes. The reaction of tosyl chloride with a pentasubstituted 2H-pyrazole yields a chloro-substituted pyrazoline 6 rather than the expected N-tosyl product. Thermolysis of 6 yields products, the structure of which is consistent with the formation of an unstable intermediate chloro-substituted cyclopropane.