Melting point apparatus

All of the reagents were obtained from commercial sources and used without further purification unless stated otherwise. Thin-layer chromatography (TLC) on the silica gel plates GF254 (200–300 mm; Qingdao Haiyang Chemical Co., Ltd., Qingdao, China) with UV light illumination was used to monitor chemical reactions. ¹H NMR (500 MHz) and ¹³C NMR (100 MHz) spectra were measured by Avance spectrometer (Bruker, Varian Mercury, Billerica, MA). Chemical shifts were reported in δ values (ppm) with tetramethylsilane as the internal standard. High-resolution mass spectrometry (HRMS) was performed using the Thermo Scientific^™ Exactive^™ Plus mass spectrometer (Thermo, Waltham, MA). The melting points were recorded with a Mettler Toledo melting point apparatus. The purity of all the target compounds was determined by high-performance liquid chromatography (HPLC) on a Waters Acquity machine with a BEH C18 column (1.7 µm, 50 × 2.1 mm); mobile phase A = water (containing 0.1% formic acid) and mobile phase B = acetonitrile; the flow rate was 0.25 mL/min.

The solvents including methanol, n-hexane, ethanol, chloroform, ethyl acetate, n-butanol, acetone, and petroleum ether were obtained from Merck, KGaA, Darmstadt, Germany. Besides, nitric acid, sulphuric acid, hydrochloric acid, and perchloric acid were purchased from (Sigma Aldrich Chemical, Steinheim, Germany). Aluminium-coated plates for TLC (silica gel 60 F254) and silica gel with mesh size 230–400 for column chromatography were purchased from Merck, KGaA, Darmstadt, Germany. NMR spectral analyses were done using AZ-300, Bruker spectrometers, Fällanden, Switzerland), FTIR analyses were performed using PerkinElmer Spectrum 3 Tri-Range FT-IR Spectrometer, Llantrisant, United Kingdom, whereas HPLC measurements were done using (Model LC-10AT, Shimadzu, Kyoto, Japan). The melting point is measured using the melting point apparatus (Mettler Toledo, Greifensee, Switzerland). The used rotary vacuum evaporator was Heidolph, model Laborata 4000, Schwabach, Germany; meanwhile, a UV lamp 254 and 365 nm (Fisher Scientific Ltd., Pandan Crescent, UE Tech Park, Singapore) was used for UV detection, while a UV/Visible spectrophotometer (Hitachi-UV-3200, Tokyo, Japan) was used for UV measurements.