Furonaphthoquinones are promising skeletons for anticancer drug molecules. In particular, methoxylated furonaphthoquinones are characteristic constituents of Tabebuia plants. In this research, we synthesized the furonaphthoquinones by effective one-pot cascade reactions of 3-phenyliodonio-1,2,4-trioxo-1,2,3,4-tetrahydronaphthalenides with 3-butyn-2-ol in the presence of palladium and cuprous catalysts via Sonogashira coupling and intramolecular cyclization. Furthermore, we demonstrated that the synthetic furonaphthoquinones showed moderate cytotoxicity against human leukemia U937 and HL-60 cells. Our work highlights the importance of furonaphthoquinones as antileukemic agents.Key words furonaphthoquinone; Tabebuia plant; Sonogashira coupling; antileukemic activity Tabebuia plants belong to the Bignoniaceae family, which is commonly found in North and South America. The plants have been used for the treatment of cancer as folk medicines, Pau d'Arco, Ipé Roxo, Taheebo, and Lapacho. 1) In 1982, 2-(1-hydroxyethyl) naphtho [2,3-b] furan-4,9-dione FN0-ol and 2-acetylnaphtho[2,3-b] furan-4,9-dione FN0-one were isolated from Tabebuia cassinoides. Both these furonaphthoquinones showed significant activity against human oral epidermoid carcinoma KB cells.2) Subsequently, FN0-one was investigated as an anticancer agent in vitro, and it was found that this quinone shows moderate cytotoxicity against various cancer cell lines, including human oral squamous carcinoma HSC-2, HSC-3, HSC-4, human prostate carcinoma DU-145, human melanoma C8161, human lung adenocarcinoma A549, and human epithelial carcinoma HeLa cells.3-6) Previous phytochemical investigations of Tabebuia plants resulted in the isolation and characterization of a variety of furonaphthoquinones bearing a methoxy group in the benzene ring.7-9) Thus, these methoxylated furonaphthoquinones were confirmed to be characteristic constituents of Tabebuia plants.As described above, furonaphthoquinones are considered interesting natural compounds in biochemical and pharmacological fields. Herein, we report the detailed synthesis of these furonaphthoquinones and their antileukemic activity.
Results and DiscussionSome of naturally occurring furonaphthoquinones bear a 1-hydroxyethyl group at the C2-position. Kobayashi et al. reported a synthesis method of furonaphthoquinones based on sequential coupling/ring closure reactions.10) However, the reaction with intact 3-butyn-2-ol was unsuccessful. Thus, we attempted to improve their method, with the aim of making it useful for the synthesis of a wide variety of the compounds having a hydroxy group. 2-(1-Hydroxyethyl)-furonaphthoquinones such as FN0-ol are envisaged to be formed through the in situ generation of iodonaphthoquinones from phenyliodonium ylides in the presence of cuprous oxide (Cu 2 O), followed by Sonogashira coupling with acetylene and subsequent intramolecular cyclization. The phenyliodonium ylide 3-phenyliodonio-1,2,4-trioxo-1,2,3,4-tetrahydronaphthalenide IY0, which would serve as a suitable precursor for the su...