Esquire 6000 spectrometer

Column chromatography was conducted using silica gel (SiO₂, 200–300 µm mesh; Qingdao Haiyang Chemical Co., Ltd., Qingdao, China) and Sephadex LH-20 (20–100 µm; Pharmacia, Uppsala, Sweden) as packing materials. Silica GF₂₅₄ (10–40 mm) for TLC was supplied by the Qingdao Marine Chemical Factory, Qingdao, China. All TLC spots were visualized under UV light (254 nm) and stained with a 10% H₂SO₄ solution in ethanol, followed by heating. Optical rotations were recorded with a 341 polarimeter (Perkin-Elmer, Waltham, MA, USA) in a 1 dm cell. The UV spectra were measured on a UV-260 spectrophotometer (Shimadzu, Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto, Japan) and the IR spectra were obtained on a NEXUS 670 FT-IR spectrometer (Nicolet, Madison, WI, USA). Nuclear magnetic resonance spectra were recorded on INOVA-400 (Varian, Palo Alto, CA, USA) and AVANCE III-400 (Bruker, Billerica, MA, USA) spectrometers. Chemical shifts were given on a δ (ppm) scale using tetramethylsilane as the internal standard. High-resolution electrospray ionization (ESI) mass spectrometry (MS) was carried out on a APEX II mass spectrometer (Bruker Daltonics, Billerica, MA, USA) and ESI-MS spectra were determined on a Bruker Daltonics Esquire 6000 spectrometer. The ABTS and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) used were obtained from Aladdin Industrial Co. (Shanghai, China).