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Oxidation of Δ 4 ‐ and Δ 5 ‐Steroids with Hydrogen Peroxide Catalyzed by Porphyrin Complexes of Mn III and Fe III
Author(s) -
Rebelo Susana L. H.,
Simões Mário M. Q.,
Neves M. Graça P. M. S.,
Silva Artur M. S.,
Cavaleiro José A. S.,
Peixoto Andreia F.,
Pereira Mariette M.,
Silva Manuela R.,
Paixão José A.,
Beja Ana M.
Publication year - 2004
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200400461
Subject(s) - chemistry , porphyrin , chemoselectivity , stereoselectivity , epoxide , hydrogen peroxide , catalysis , substrate (aquarium) , metal , stereochemistry , medicinal chemistry , organic chemistry , oceanography , geology
In this paper we describe a new environmentally friendly method to promote the stereoselective epoxidation of Δ 4 ‐ and Δ 5 ‐steroids. Metalloporphyrins efficiently catalyze the epoxidation reactions of 17β‐acetoxy‐4‐androstene ( 1 ), 4‐cholestene ( 2 ) and 3β‐acetoxy‐5‐cholestene ( 3 ) in the presence of H 2 O 2 as oxygen donor. Modeling the molecular structure of the porphyrin as well as the central metal allows the control of the preferential formation of α‐ or β‐epoxides. Porphyrins with bulky, electron‐withdrawing groups in the ortho positions of the meso phenyls and with Mn III as the central metal ion, such as [Mn(TDCPP)Cl], gave preferentially the β‐epoxide of Δ 4 ‐ and Δ 5 ‐steroids. [Fe(TPFPP)Cl] catalyzes preferentially the α‐epoxidation of Δ 4 ‐steroids and also increases the stereoselectivity for the α‐epoxide in Δ 5 ‐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 1 H and 13 C NMR resonances as well as the stereochemistry of these products. A mechanistic proposal involving oxo species for the β‐approach and peroxy species for the α‐approach is proposed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)