8452a diode array spectrophotometer

¹H NMR and ¹³C NMR spectra were recorded using Bruker ARX-400 (Bruker, Billerica, MA, USA) at 400 MHz for ¹H and 100.6 MHz for ¹³C, Bruker Biospin Avance III 400 at 400 MHz for ¹H and 100.6 MHz for ¹³C, Bruker Ascend™-400 at 400 MHz for ¹H and 100.6 MHz for ¹³C or in a Bruker AVANCE™-600 AT 600 MHz for ¹H and 150 MHz for ¹³C spectrometers. Mass spectra were performed in a mass spectrometer with electronic impact ionization (70 eV) Thermo Scientific DSQ II Single Quadrupole GC/MS with Focus GC. The HRMS (Electrospray Ionization Time of Flight, ESI-TOF) mass spectra (MS) were performed on a High Resolution Mass Spectrometer Orbitrap, Q-Exactive (Thermo Fisher Scientific, Waltham, MA, USA) Electrospray Ionization (H-ESI-II), coupled to a Liquid Chromatographer uHPLC Ultimate 2000 Dionex (Thermo Fisher Scientific, Waltham, MA, USA). Matrix-Assisted Laser Desorption/Ionization (MALDI) experiments were performed on a 4700 Proteomics Analyzer MALDI-TOF mass spectrometer. UV–Vis absorption spectra were registered on a Hewlett-Packard 8452A Diode Array spectrophotometer and an AGILENT 8453 spectrophotometer (Santa Clara, CA, USA).

The NMR spectra were recorded at 25 °C with a Varian Inova 500 spectrometer. The ¹H NMR spectra were acquired using standard Varian library pulse programs. The 2D spectra (COSY, TOCSY, HSQC) were acquired using 1K data points, 256 increments, and a relaxation delay of 1.5 s.
The mass spectra were recorded with a Mariner Perseptive Biosystem ESI-MS.
The UV-visible spectra were recorded with an Agilent 8452A diode array spectrophotometer. The CD spectra were performed on a JASCO model J-810 spectropolarimeter.
The 4a, 4b, and EUK-108 stock solutions were prepared in EtOH/water (1:1). The final EtOH concentration did not exceed 5% and did not affect the measurements. The spectra of freshly prepared solutions were recorded at 25 °C.