Synthesis and chemistry of structurally unique hexasubstituted pyrazolines

Abstract

Abstract A review focused on our contributions to the synthesis and chemistry of hexasubstituted pyrazolines is presented. The development of a synthetic route to pentasubstituted 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 β-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.