( 1 R , 2 S , 5 R )-2-Isopropyl-5-methylcyclohexyl 4-Aminobutyrate Hydrochloride

The title ester (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl 4-aminobutyrate hydrochloride was obtained in 96% yield via Steglich esterification. The structure of the target compound was established by FTIR, HR-MS, 1H-NMR, 13C-NMR spectral analysis, and single crystal X-ray diffraction study. Single crystals of the title ester suitable for X-ray investigation were obtained by slow evaporation of the methanolic solution at room temperature. The purity of compound was assessed using HPLC coupled to mass spectrometry.


Introduction
Since the identification, cloning, and characterization of the transient receptor potential (TRP) ion channels, special attention has been focused on terpenes and their derivatives as against/antagonist of aforementioned pharmacological targets.Cyclic terpene alcohol menthol is one of TRP modulators possessing antinociceptive and local anesthetic effects [1,2].Among four pairs of optical isomers (−)-menthol-also known as l-menthol with (1R,2S,5R) configuration-occurs most widely in the nature and has the greatest cooling activity [3,4].Besides binding to TRP channels, l-menthol was found to act as positive allosteric modulators of γ-aminobutyric acid (GABA) A receptors [5].
Based on the foregoing, the combination of l-menthol residues with GABA into one molecule is reasonable to enhance the effect of each component.Thus, the current note is devoted to the detailed description and determination of ester structure based on menthol and GABA-(1R,2S,5R)-2-isopropyl-5-methylcyclohexyl 4-aminobutyrate hydrochloride.

Results and Discussion
(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl 4-aminobutyrate hydrochloride was synthesized via Steglich esterification with N,N -dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) as a catalyst in dichloromethane, as shown in Scheme 1. Synthesized ester was isolated in 96% yield as white solid soluble in methanol and dimethyl sulfoxide and fully characterized by 1 H-NMR, FTIR-spectroscopy and FAB-, ESI-mass spectrometry.Additionally, the HPLC analysis was carried out to determine the purity of the title compound.For this purpose, reversed-phase HPLC method with isocratic elution of methanol:ammonium formate buffer was applied.The observed retention time for the ester was 2.676 min with 98% of purity.
The FAB-MS spectra of the title ester displays the protonated molecular ion peak [M + H] + at m/z 242.The HRMS (ESI-TOF) revealed an ion peak of the compound at m/z 242.2134 [M + H] + , thus suggesting a molecular formula C14H28NO2 (calc.242.2120).The FTIR spectra of ester exhibits absorption bands of N-H bonds (3021 cm −1 ), C=O ester groups (1721 cm −1 ), C-O at 1151-1201 cm −1 , and alkyl C-H.The 1 H-NMR spectral data contain resonance signals described by their chemical shift, integration, and multiplicity that are in full agreement with the presented molecular formula.The 1 H NMR spectrum of synthesized compound contains the proton H-1 of cyclohexane ring resonated at δ 4.53-4.59ppm as a triplet of doublets.The methyl group at C-5 is observed as a doublet at δ 0.67 ppm with SSCC J = 6.53 Hz.Signals of axial and equatorial ring protons are also presented in the 1 H-NMR spectrum, their position and multiplicity correspond to similar signals in the spectrum of l-menthol.Thus, according to 1 H-NMR analysis, the initial configuration of l-menthol is preserved.
Additionally l-configuration of menthol was confirmed by X-ray diffraction analysis (see Figure 1).Single crystals of the title compound suitable for X-ray investigation were obtained by slow evaporation of the methanolic solution at room temperature.According to the X-ray diffraction data, the synthesized compound is a chloride of organic cation.The positive charge of the cation is localized on the amino group as indicated by three hydrogen atoms that were found from the electron density difference map; in addition, this fact is proved by elongation of N1-C14 bond to 1.486(4) Å compared with the average value of 1.469 Å [6].The title compound crystallizes in non-centrosymmetric space group indicating the presence of only The FAB-MS spectra of the title ester displays the protonated molecular ion peak [M + H] + at m/z 242.The HRMS (ESI-TOF) revealed an ion peak of the compound at m/z 242.2134 [M + H] + , thus suggesting a molecular formula C 14 H 28 NO 2 (calc.242.2120).The FTIR spectra of ester exhibits absorption bands of N-H bonds (3021 cm −1 ), C=O ester groups (1721 cm −1 ), C-O at 1151-1201 cm −1 , and alkyl C-H.The 1 H-NMR spectral data contain resonance signals described by their chemical shift, integration, and multiplicity that are in full agreement with the presented molecular formula.The 1 H-NMR spectrum of synthesized compound contains the proton H-1 of cyclohexane ring resonated at δ 4.53-4.59ppm as a triplet of doublets.The methyl group at C-5 is observed as a doublet at δ 0.67 ppm with SSCC J = 6.53 Hz.Signals of axial and equatorial ring protons are also presented in the 1 H-NMR spectrum, their position and multiplicity correspond to similar signals in the spectrum of l-menthol.Thus, according to 1 H-NMR analysis, the initial configuration of l-menthol is preserved.
Additionally l-configuration of menthol was confirmed by X-ray diffraction analysis (see Figure 1).Single crystals of the title compound suitable for X-ray investigation were obtained by slow evaporation of the methanolic solution at room temperature.
HPLC method with isocratic elution of methanol:ammonium formate buffer was applied.The observed retention time for the ester was 2.676 min with 98% of purity.
The FAB-MS spectra of the title ester displays the protonated molecular ion peak [M + H] + at m/z 242.The HRMS (ESI-TOF) revealed an ion peak of the compound at m/z 242.2134 [M + H] + , thus suggesting a molecular formula C14H28NO2 (calc.242.2120).The FTIR spectra of ester exhibits absorption bands of N-H bonds (3021 cm −1 ), C=O ester groups (1721 cm −1 ), C-O at 1151-1201 cm −1 , and alkyl C-H.The 1 H-NMR spectral data contain resonance signals described by their chemical shift, integration, and multiplicity that are in full agreement with the presented molecular formula.The 1 H NMR spectrum of synthesized compound contains the proton H-1 of cyclohexane ring resonated at δ 4.53-4.59ppm as a triplet of doublets.The methyl group at C-5 is observed as a doublet at δ 0.67 ppm with SSCC J = 6.53 Hz.Signals of axial and equatorial ring protons are also presented in the 1 H-NMR spectrum, their position and multiplicity correspond to similar signals in the spectrum of l-menthol.Thus, according to 1 H-NMR analysis, the initial configuration of l-menthol is preserved.
Additionally l-configuration of menthol was confirmed by X-ray diffraction analysis (see Figure 1).Single crystals of the title compound suitable for X-ray investigation were obtained by slow evaporation of the methanolic solution at room temperature.According to the X-ray diffraction data, the synthesized compound is a chloride of organic cation.The positive charge of the cation is localized on the amino group as indicated by three hydrogen atoms that were found from the electron density difference map; in addition, this fact is proved by elongation of N1-C14 bond to 1.486(4) Å compared with the average value of 1.469 Å [6].The title compound crystallizes in non-centrosymmetric space group indicating the presence of only All atoms are represented by 50% probability ellipsoids.
According to the X-ray diffraction data, the synthesized compound is a chloride of organic cation.The positive charge of the cation is localized on the amino group as indicated by three hydrogen atoms that were found from the electron density difference map; in addition, this fact is proved by elongation of N1-C14 bond to 1.486(4) Å compared with the average value of 1.469 Å [6].The title compound crystallizes in non-centrosymmetric space group indicating the presence of only one enantiomer in the crystal.Configuration of chiral centers at C1, C2, and C5 atoms (R, S, and R, respectively) was simultaneously determined by calculating the Flack parameter (−0.04 (10)).The saturated cycle adopts the chair conformation (puckering parameters are [7]

General Information
The following chemicals were used as obtained from their commercial suppliers: l-menthol, DMAP, GABA (Acros Organics, Geel, Belgium; Darmstadt, Germany), DCC, di-tert-butyl dicarbonate (TCI, Philadelphia, PA, USA).Boc-protected GABA was not obtained commercially and has been synthesized according to the literature procedure [8].Structure of the obtained compound was established by 1 H-NMR spectroscopy on a AVANCE DRX 500 (500 MHz) instrument (Bruker, Davis, CA, USA) and by 13 C-NMR spectroscopy on Varian-Mercury 400 spectrometer (Varian Inc., Palo Alto, CA, USA) using DMSO-d6 as a solvent 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 spectrum was measured with a

General Information
The following chemicals were used as obtained from their commercial suppliers: l-menthol, DMAP, GABA (Acros Organics, Geel, Belgium; Darmstadt, Germany), DCC, di-tert-butyl dicarbonate (TCI, Philadelphia, PA, USA).Boc-protected GABA was not obtained commercially and has been synthesized according to the literature procedure [8].Structure of the obtained compound was established by 1 H-NMR spectroscopy on a AVANCE DRX 500 (500 MHz) instrument (Bruker, Davis, CA, USA) and by 13 C-NMR spectroscopy on Varian-Mercury 400 spectrometer (Varian Inc., Palo Alto, CA, USA) using DMSO-d 6 as a solvent 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 spectrum was 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 F 254 ) plates (Darmstadt, Germany).