Oxidation of Δ4- and Δ5-Steroids with Hydrogen Peroxide Catalyzed by Porphyrin Complexes of MnIIIand FeIII

Abstract

In this paper we describe a new environmentally friendly method to promote the stereoselective epoxidation of Delta4- and Delta5-steroids. Metalloporphyrins efficiently catalyze the epoxidation reactions of 17beta-acetoxy-4-androstene (1), 4-cholestene (2) and 3beta-acetoxy-5-cholestene (3) in the presence of H2O2 as oxygen donor. Modeling the molecular structure of the porphyrin as well as the central metal allows the control of the preferential formation of alpha- or beta-epoxides. Porphyrins with bulky, electron-withdrawing groups in the ortho positions of the meso phenyls and with MnIII as the central metal ion, such as [Mn(TDCPP)Cl], gave preferentially the beta-epoxide of Delta4- and Delta5-steroids. [Fe(TPFPP)Cl] catalyzes preferentially the alpha-epoxidation of Delta4-steroids and also increases the stereoselectivity for the alpha-epoxide in Delta5-steroids, similar to the results obtained with m-CPBA (m-chloroperbenzoic acid) as oxidant. The substrate structure strongly influences the chemoselectivity of the reactions. The X-ray structures of two main products were determined, and two-dimensional NMR techniques allowed the full assignment of 1H and 13C NMR resonances as well as the stereochemistry of these products. A mechanistic proposal involving oxo species for the beta-approach and peroxy species for the alpha-approach is proposed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)