*2.1. General*

The reagents were purchased from commercial suppliers (Sigma-Aldrich S.A. and Riedelde Haën, Sofia, Bulgaria) and used as received. A Bruker NEO 400 (400/100 MHz 1H/13C) spectrometer was used for the recording of the NMR spectral data (BAS-IOCCP—Sofia, Bruker, Billerica, MA, USA). All compounds were analyzed in CDCl3 at 400 MHz and 101 MHz for 1H-NMR and 13C-NMR, respectively. Chemical shifts were determined to tetramethylsilane (TMS) (δ = 0.00 ppm) as an internal standard; the coupling constants are given in Hz. Recorded NMR spectra were taken at room temperature (approx. 295 K). Absorbance was measured with a spectrophotometer Camspec M508, Leeds, UK. The high-resolution mass (HRMS) analysis was carried out on a Q Exactive Plus mass spectrometer with a heated electrospray ionization source (HESI-II) (Thermo Fisher Scientific, Inc., Bremen, Germany) coupled with an ultrahigh-performance liquid chromatography (UHPLC) system Dionex Ultimate 3000RSLC (Thermo Fisher Scientific, Inc.) consisting of 6-channel degasser SRD-3600, high-pressure gradient pump HPG-3400RS, autosampler WPS-3000TRS, column compartment TCC-3000RS, and narrow bore Hypersil GOLDTM C18 (2.1 × 50 mm, 1.9 μm) column. For the TLC analysis, precoated 0.2 mm Fluka silica gel 60 plates (Merck KGaA, Darmstadt, Germany) were used.
