L menthol

Thymol of 99.5% purity (Sigma, Steinheim, Germany), 98% carvacrol, 99% eugenol, 98% syringaldehyde (Aldrich, Steinheim, Germany), 99% vanillin and 97% phythol (Sigma-Aldrich, Steinheim, Germany), 99% benzyl alcohol, 96% limonene, 98% α-pinene, 98% β-pinene, 98% D-carvone, 75% camphene, as well as α-fenchene, myrcene, 99.5% L-menthol, and 97% L-borneol (Acros Organics, Geel, Belgium) were used as standard antioxidants. Their 10 mM stock solutions were prepared by dissolving an accurately weighed portion in 5.0 mL of ethanol (rectificate). Less concentrated solutions were obtained with an exact dilution prior to measurements. Other reagents were of chemical purity and used as received.
The commercially available essential oils of various plant materials (Table 1) of different trademarks were studied. Their ten-fold dilution with ethanol was used exactly before the measurements.

The following chemicals and drugs were used as obtained from their commercial suppliers: l-menthol, DMAP, GABA (Acros Organics, Geel, Belgium; Darmstadt, Germany), carvacrol, guaiacol, DCC, di-tert-butyl dicarbonate (TCI, Philadelphia, PA, USA), thymol (Sigma-Aldrich, Schnelldorf, Germany). Boc-protected GABA was not obtained commercially and has been synthesized according to the literature procedure [21 (link)]. Structures of the obtained compound were established by ¹H-NMR spectroscopy on a AVANCE DRX 500 (500 MHz) instrument (Bruker, Davis, CA, USA) using DMSO-d₆ and CDCl₃ as solvents and TMS as an internal standard. FAB mass spectrum was obtained on a VG 70-70EQ mass spectrometer (VG Analytical Ltd., Manchester, UK) equipped with Xe ion gun (8 kV); the sample was mixed with m-nitrobenzyl-alcohol matrix. High-resolution mass spectrometry (HRMS) was performed on a 6530 Accurate Mass quadrupole time of flight (Q-TOF) spectrometer (Agilent, Santa Clara, CA, USA) using ESI (electrospray ionization) coupled to an Agilent 1260 Infinity HPLC system. IR spectra were measured with a Frontier FT-IR spectrometer (Perkin-Elmer, Hopkinton, MA, USA) using KBr pellets. The purity and identity of the compound were monitored by TLC on Merck-made (TLC Silica gel 60 F₂₅₄) plates (Darmstadt, Germany).