Abstract
Oxidative cyclization of 5,e- and «,f-unsaturated enol silyl ethers 4a and 4b with cupric triflate and cuprous oxide or ceric ammonium nitrate and sodium bicarbonate in acetonitrile provides the tricyclic ketones 5a and 5b stereoselectively. These cyclizations proceed by oxidation of 4 to the cation radical 24 followed by cyclization of 24 to cation radical 27. This cation radical undergoes a second cyclization to give cation radical 30, which loses the silyl group, undergoes a second oxidation, and loses a proton to give 5. The stereochemistry of the cycloadduct is controlled by the stereochemistry of the enol ether. The Z-enol methyl ether (Z)-65 leads mainly to 5a while the £-enol methyl ether (£)-65 leads mainly to 6a. The oxidative cyclizations of 7,13, 21, and 43 are also described. Oxidation of a-allyl silyl enol ethers 34a and 34b leads mainly to oxidation without cyclization to give the a,/3-unsaturated ketones 36a and 36b. Oxidative cyclizations of alkynyl silyl enol ethers 56 and 60 lead to 15 and 64, respectively. Oxidation of siloxycyclopropane 74 with Cu(BF4)2 generates cation radical 75, which cyclizes to 76, which is oxidized to give 21% of cyclopentane 77. This suggests that cation radicals are intermediates in the oxidative dimerization of siloxycyclopropanes.