2.1 General Information
Unless otherwise noted, reagents and organic solvents were obtained from chemical suppliers in reagent grade quality and used without further purification. All reactions were monitored by thin-layer chromatography (TLC) with Haiyang GF254 silica gel plates. Flash column chromatography was carried out using 100-200 mesh silica gel at increased pressure. For visualization, TLC plates were either placed under ultraviolet light, or stained with iodine vapor, or acidic vanillin. NMR spectra were recorded on a 400 MHz spectrometer.