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 delta 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 delta 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 delta C-H bond over a secondary one. This intramolecular oxidation method can be extended to the oxidation of the tertiary gamma' C-H bond of ketones 9 and 10. For ketone 11 with two delta C-H bonds and one gamma' C-H bond linked respectively by a sp(3) hydrocarbon tether and a sp(2) ester tether, the oxidation took place exclusively at the delta 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.
Wong
Man-Kin
MK
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong.
Chung
Nga-Wai
NW
He
Lan
L
Wang
Xue-Chao
XC
Yan
Zheng
Z
Tang
Yeung-Chiu
YC
Yang
Dan
D