Ex Situ Generation of 18 O 2 and 17 O 2 from Endoperoxides for *O-Labeling and Mechanistic Studies of Oxidations by Dioxygen.

Near-stoichiometric amounts of 18 O 2 and 17 O 2 were generated ex situ from endoperoxides in a two-chamber glassware to oxidize various substrates. This strategy gave [*O 2 ]endoperoxides, [*O 1 ]quinones, [*O 1 ]phenols, and [*O x ]artemisin in moderate to good yields and high isotopic enrichments (up to 84%) at affordable costs. Moreover, mass spectrometry and 17 O NMR of the [*O]products provided valuable information about the chemical mechanisms involved.