Silica gel

The ¹H and ¹³C NMR spectra were recorded on a Bruker Ascend 600 NMR spectrometer (Bruker, Karlsruhe, Germany; 600 and 150 MHz for the ¹H and ¹³C NMR, respectively). All of the samples were prepared in CD₃Cl and the chemical shifts of the solvent were used as internal references. The chemical shifts (δ) have been given in ppm, and the coupling constants (J) have been presented in Hz. High resolution mass spectrometry (HRMS) experiments were performed using an Agilent 6230 time-of-flight (TOF) mass spectrometer (Agilent, Santa Clara, CA, USA) with an atmospheric pressure chemical ionization (APCI) source. Purifications by column chromatography (CC) were performed over silica gel (40‒63 µm, Grace, Columbia, MD, USA) and MCI-gel CHP 20P (75‒150 µm, Mitsubishi Chemical Co. Ltd, Tokyo, Japan). Thin-layer chromatography (TLC) was conducted on pre-coated TLC Kieselgel 60 F₂₅₄ plates (200 µm thick, Merck KGaA, Darmstadt, Germany). The TLC plates were visualized under ultraviolet (UV) light irradiation at a wavelength of 254 nm (Spectroline^®, New York, NY, USA), as well as being sprayed with a 95 % EtOH solution containing 5 % sulfuric acid, followed by heating at 110 °C.

AR-grade n-hexane, dichloromethane, n-butanol, methanol and HPLC-grade methanol were obtained from Anaqua Chemicals Supply (Houston, TX, USA). MS-grade acetonitrile, methanol and water were purchased from J.T. Baker (Danville, PA, USA), and MS-grade formic acid was provided by Sigma-Aldrich Laboratories, Inc. (St. Louis, MO, USA). AR-grade potassium hydroxide, hydrochloric acid (37%), and diethylamine were purchased from Merck and Advanced Technology & Industrial CO. Ltd. (Hong Kong, China), respectively. Silica gel (75–150 mesh) and ODS (35–70 µm) were provided by Grace (Columbia, MD, USA).