NMR spectra were recorded at 400 MHz, using Agilent/Varian Inova two-channel spectrometer (Agilent Technologies, CA, USA). Proton chemical shifts are reported in ppm (δ) downfield from tetramethylsilane (TMS) or in reference to the solvent CDCl3 (δ 7.26). Coupling constants (J) are reported in hertz (Hz).