Synthesis of 8-0-Methylfusarubin, 8-0-Methylanhydrofusarubin, Fusarubin and Anhydrofusarubin

Abstract

Pyranonaphthoquinones are a group of secondary metabolites in naphthoquinone family which were isolated from diverse natural sources. Fusarubin (1), anhydrofusarubin (2) and 8-O-methylfusarubin (3) are pyranonaphthoquinone natural products which were isolated from various strain of fungi. In 2010, Rukachaisirikul and co-workers reported the isolation and biological activity of 1 and 2 from sea-fan derived fungi Fusarium spp. PSU- F14 and PSU-F135. Additionally, compound 3 was isolated from a seagrass-derived fungus Pestalotiopsis sp. PSU-ES180. These three pyranonaphthoquinones exhibited good cytotoxic activity against human breast (MCF-7) cancer cells with IC50 ranges of 0.9 to 9.8 μM compared with doxorubicin standard drug (IC50 = 2.18 μM). 8-O-Methylanhydro fusarubin (4), an analogue of 8-O-methylfusarubin (3), was first isolated from F. oxysporum from roots of diseased citrus tree by the Tatum group in 1985. To date, the biological activity of 4 as well as the synthesis of 1, 3 and 4 have not been reported. This work involves the syntheses of compounds 1-4 to provide sufficient materials for further study of their cytotoxic activities. Our synthetic approach relied on the key Diels-Alder reaction to construct naphthalene core and intramolecular acetalization to form pyran ring. The syntheses of 1-4 utilized the common naphthoquinone intermediate 8, which could be elaborated from the known pentaalkoxynaphthalene 6. Naphthalene 6 can be prepared in 6 steps starting from commercially available 5-bromovanillin (5) following a protocol reported by Green and co-workers. The naphthoquinone skeleton was generated via Vilsmeier-Haack formylation of naphthalene 6 and hydroxymethylene-directed selective oxidation using diacetoxyiodobenzene to provide the desired naphthoquinone 7 in 3 steps. Acetonylation of 7 via pyridinium ylide furnished the key naphthoquinone intermediate 8 in 2 steps. The synthesis of 8-O-methylfusarubin (3) was completed via acid-promoted acetalization of acetonyl naphthoquinone 8. Alternatively, compound 3 could be achieved in 3 steps from naphthoquinone 7 via one-pot acetalization. 8-O-Methylanhydrofusarubin (4) can be prepared from acid-catalyzed dehydration of 3. The syntheses of 1 and 2 were accomplished via manipulation of the oxidation state of naphthoquinone precursor 8. Selective quinone reduction of 8 and subsequent protection of the hydroquinone moiety with ethoxymethyl (EOM) protecting groups gave EOM ether 9. Removal of benzyl (Bn) protecting group followed by naphthol oxidation generated new naphthoquinone core 10 in 2 steps. Fusarubin (1) was then synthesized via global deprotection and acetalization of 10 under acidic conditions. Lastly, dehydration of 1 furnished anhydrofusarubin (2). The syntheses of 1-4 were achieved in 16, 17, 12 and 13 steps from 5-bromovanillin with 3, 2, 13 and 9 overall yields, respectively. The cytotoxic activity against MCF-7 breast cancer cells of four synthetic compounds were evaluated using REMA, MTT and 3D cancer spheroid assays. Synthetic 3 displayed highest cytotoxicity against MCF-7 cells among the four compounds. Additionally, compounds 1-4 exhibited good cytotoxic activity against five human cancer (C33A, HeLa, SiHa, HCT116 and HepG2) cell lines with IC, ranges of 4.73 >22.5 μM.