Synthesis of Tetracyclic Flavonoids via Palladium-Catalyzed Intramolecular Oxidative Cyclization

Palladium-catalyzed oxidative cyclization of terminal-olefin-tethered phenols was achieved in good yields (61-90% yields). The reaction was optimized and White catalyst {[1,2-bis-(phenylsulfinyl)ethane]palladium(II)acetate} provided the best yield in the presence of benzoquinone as an oxidant. The reaction tolerated both electron-donating and electron-withdrawing substituents on the substrates, and the structure of the tetracyclic flavonoids was confirmed using single-crystal X-ray analysis. The study represents the first example of successful allylic C-H activation/C-O bond formation using terminal-olefin-tethered phenols.