Oxidative chemistry of 2-nitro and 4-nitroestradiol: Dichotomous behavior of radical intermediates and novel potential routes for oxyfunctionalization and B-ring fission of steroidal scaffolds

Oxidation of 4-nitro-17β-estradiol (1) with the peroxidase/H2O2 system gave the symmetric C2-linked dimer (3) through phenoxy radical coupling. Similar oxidation of 2-nitro-17β-estradiol (2), in which the nitro group is coplanar with the aromatic ring, yielded 9α- and 9β-hydroxy-2-nitro-17β-estradiol (4a,b), (17β)-2-nitroestra-1(10),2,4,9(11)-tetraene-3,17-diol (5), and (12α,17β)-2-nitroestra-1(10),2,4,9(11)-tetraene-3,12,17-triol (6). With higher concentrations of H2O2, the novel secoestra-1(10),2,4-trien-9-one derivative 7 was obtained from 2. Theoretical calculations suggested that the peculiar behavior of 2 may be due to the generation of a relatively stable radical intermediate at C9, which would then be converted to the reactive quinone methide 8. The chemistry described in this paper appears to be an intriguing example of control of the site of substitution over evolution of phenoxy radicals, and opens new vistas toward selective oxyfunctionalization of the estrane skeleton.