Total Synthesis of (−)-Balanol

Abstract

The efficient total synthesis of (−)-balanol, a potent inhibitor of the protein kinase C, is described. (−)-Balanol consists of a chiral hexahydroazepine-containing fragment and a benzophenone fragment, both of which were prepared via novel synthetic routes. The hexahydroazepine fragment was prepared in racemic form through either Bu3SnH- or SmI2-promoted radical cyclization of oxime ethers 2ab intramolecularly connected with the formyl group. SmI2-promoted radical cyclization of 2b was found to be particularly successful in the selective synthesis of the seven-membered trans-amino alcohol 8b. Preparation of the enantiomerically pure hexahydroazepine-containing fragment was achieved through the enantioselective enzymatic acetylation of racemic alcohol 9, employing the immobilized lipase from Pseudomonas sp. The benzophenone fragment was prepared in short steps through a biomimetic oxidative anthraquinone ring cleavage starting from commercially available natural chrysophanic acid 15c. This reaction proceeded via [4 + 2]-cycloaddition of singlet oxygen to anthracene derivative 17c, followed by Baeyer−Villiger-type rearrangement of the resulting hydroperoxide to afford the benzophenone derivatives 22 and 23.