Abstract A compound having a skeleton of an antitumor quassinoid, bruceantin, was prepared according to the following strategy; 1) construction of ring ABC from ring AB by cyclization of a hindered 1,5-diketone; 2) construction of ring E (ether ring) by opening of a 12,13-epoxide ring followed by attack of oxygen at C-20 toward C-13; 3) formation of ring D (hemiacetal ring) by treatment of a 7α-hydroxy-16-al dimethyl acetal with PPTS.