Oxidative Coupling of Phenols

Nature makes extensive use of oxidative reactions to generate bonds between carbons, particularly in the coupling of phenols, which is a striking feature in many biosynthetic pathways. The coupled phenols may exhibit the phenomenon called axial “chirality” or handedness. The Kozlowski group uses the atom economical oxidant O2 and metal-based catalysts which, developed in-house, mimic the active sites of the enzymes to bring about these transformations. Vanadium catalysts have been extensively applied to the coupling of phenols and carbazoles with great efficiency and results. Here, this method is applied to the synthesis of bismurrayaquinone-A, an antitumor compound that occurs naturally in the roots of the curryleaf tree, Murraya koenigii. In this report, the routes for synthesis of the coupled phenol and carbazoles are highlighted. In the scaleup synthesis, 515 mg of the coupled carbazole was synthesized with an overall of 68% yield and 91% ee. A key oxidative coupling intermediate in the synthesis of bismurrayaquinone-A was afforded in 51% ee.