Investigation of Mn(III)-Based Oxidative Free Radical Cyclization Reactions toward the Synthesis of Triptolide:  The Effects of Lanthanide Triflates and Substituents on Stereoselectivity

Mn(III)-mediated oxidative free radical cyclization reactions are useful for construction of polycyclic compounds. In the total synthesis of triptolide ( 3 ) and its related compounds, construction of tricyclic skeletons 4 and 5 was achieved by the Mn(OAc) 3 -mediated oxidative radical cyclization of a series of acyclic precursors 6 − 9 . The addition of a catalytic amount of lanthanide triflates was found to significantly improve the rate and stereoselectivity of those radical cyclization reactions. The effects of the benzylic oxygen substituents and α -substituents (chloro and methyl) on the radical cyclization reactions were also investigated. Transition-state models were proposed to explain the observed stereoselectivity.