Concise Synthesis of Cyclic Ethers via the Palladium-Catalyzed Coupling of Ketene Acetal Triflates and Organozinc Reagents. Application to the Iterative Synthesis of Polycyclic Ethers

The reaction of the ketene acetal triflates 9a−e and a zinc homoenolate 10 in the presence of a catalytic amount of Pd(PPh₃)₄ gave the enol ethers 11a−e in good yields. The products were converted to the corresponding cyclic ethers 14a and 14b by hydroboration and lactonization. The present methodology allowed us to synthesize the DE and GH ring segment of gambierol in a concise manner. Iterative syntheses of the polycyclic ethers 26 and 32 are also described.