Investigation on the regioselectivities of intramolecular oxidation of unactivated C-H bonds by dioxiranes generated in Situ

We found that dioxiranes generated in situ from ketones 1-6 and Oxone underwent intramolecular oxidation of unactivated C-H bonds at δ sites of ketones to yield tetrahydropyrans. From the trans/cis ratio of oxidation products 1a and 2a as well as the retention of the configuration at the δ site of ketone 5, we proposed that the oxidation reaction proceeds through a concerted pathway under a spiro transition state. The intramolecular oxidation of ketone 6 showed the preference for a tertiary δ C-H bond over a secondary one. This intramolecular oxidation method can be extended to the oxidation of the tertiary γ′ C-H bond of ketones 9 and 10. For ketone 11 with two δ C-H bonds and one γ′ C-H bond linked respectively by a sp3 hydrocarbon tether and a sp2 ester tether, the oxidation took place exclusively at the δ C-H bonds. Finally, by introducing proper tethers, regioselective hydroxylation of steroid ketones 12-14 have been achieved at the C-17, C-16, C-3, and C-5 positions.