Regioselective Synthesis of 14-Membered Biaryl Ethers : Total Synthesis of RA-VII and Deoxybouvardin

Abstract

In order to obtain a key compound (22a") for synthesis of RA-VII (1) and deoxybouvardin (2), construction of the 14-membered ring system was performed by means of thallium trinitrate-mediated oxidation of the tetrahalogeno amides 5-7. The dibromo dichloro amide 6 or the bromo trichloro amide 7 gave a natural type of 14-membered ring dienone (23a or 23c), whereas the tetrabromo amide 5 gave an unnatural type of product, 19a. The formation of the latter product 19a could be understood on the basis of energy calculations on plausible intermediates 26a-c and 27a-c in the transition state in the oxidaive coupling reaction. Compound 23a was further converted to 22a" through conventional procedures (aromatization; methylation; catalytic hydrogenation). This intermediate was readily converted to 1 and 2. Thus, total synthesis of RA-VII (1) and deoxybouvardin (2) was achieved for the first time.