2.1. Chemistry

All chemicals, reagents and solvents were purchased from commercial sources and used without further purification. Reactions were checked by thin-layer chromatography (TLC) on precoated silica gel plates (Qingdao Marine Chemical Factory, GF₂₅₄); spots were visualized by UV at 254 nm. Melting points are determined on a XT4MP apparatus (Taike Corp., Beijing, China) and are not corrected. ¹H NMR and ¹³C NMR spectra were recorded on Bruker AV-600 or AV-300 MHz instruments using DMSO-d₆ and CDCl₃ as solvent. Chemical shifts are reported in parts per million (δ) downfield from the signal of tetramethylsilane (TMS) as internal standards. Coupling constants are reported in Hz. The multiplicity is defined by s (singlet), d (doublet), t (triplet) or m (multiplet). High resolution mass spectra (HRMS) were obtained on an Agilent 1260-6221 TOF mass spectrometry.