The stems of a number of Dendrobium species (Orchidaceae) have been used as a precious health food and nutrient [1]. Previous studies on the chemical constituents of the genus led to the isolation of a series of diverse compounds such as alkaloids, fluorenones, sesquiterpenoids, bibenzyls, and phenanthrenes, in which some compounds were found to possess antioxidant, antitumor, and antimutagenic activities [2–6]. D. longicornu Lindl is distributed in Nepal, Sikkim, Bhutan, India, Vietnam, and the southwestern part of China [7]. Previously, bibenzyls, phenanthrenes, and lignan glycosides were isolated [8]. In the course of our search for bioactive natural products from medicinal plants in Yunnan of China, we isolated eight mono-aromatic compounds from D. longicornu [9]. Further investigation on the plant extract led to the isolation of five bibenzyls, two 9,10-dihydrophenanthrenes, and a phenanthraquinone.

The extracts of D. longicornu were prepared as described in the previous report [9]. The EtOAc extract (26 g) was applied to a silica gel column, eluting with petroleum ether containing increasing amounts of acetone to obtain six fractions. Fraction 5 (9 g) was purified on column chromatography (silica gel, CHCl3–acetone 10:1) to afford two subfractions. The first subfraction (4 g) was subjected to repeated column chromatography, first on silica gel (CHCl3–acetone 20:1) and then on Sephadex LH-20 (MeOH), to obtain 1 (7 mg), 2 (20 mg), 3 (8 mg), and 4 (9 mg). The second subfraction (3 g) was subjected to repeated column chromatography similarly, first on Sephadex LH-20 (MeOH), and then on preparative TLC (petroleum ether–EtOAc 2:1, CHCl3–MeOH 15:1), to afford 5 (45 mg), 6 (9 mg), and 7 (6 mg). The petroleum ether extract (30 g) was applied to a silica gel column, eluting with petroleum ether containing increasing amounts of EtOAc to obtain seven fractions. Fraction 4 (1 g) was purified on Sephadex LH-20 (MeOH), and then on preparative TLC (petroleum ether–acetone 5:1) to afford 8 (10 mg).

The structures of isolates were identified by a combination of spectroscopic methods (MS, PMR, 13C NMR, and 2D NMR) and comparisons with the literature data as 3,4'-dihydroxy-3',4,5-trimethoxybibenzyl (1) [10], 3,3'-dihydroxy-4,5dimethoxybibenzyl (2) [11], cannabistilbene II (3) [12], batatasin III (4) [13], aloifol I (5) [11], coelonin (6) [14], hircinol (7) [15], and ephemeranthoquinone (8) [16]. Compounds 1–3 were discovered from the Dendrobium genus for the first time, and 6–8 were originally isolated from the plant. Compound 1 was isolated before as a diacetate from the heartwood of Combretum apiculatum [10]; this was the first time that the compound was obtained in its original form.