3-(1-Cyclohexyl-2-(cyclohexylamino)-4-(4-ethoxyphenyl)-6-oxo-1,6-dihydropyrimidin-5-yl)-1-methylquinoxalin-2(1H)-one

Abstract

The reaction of 3-cyclohexyl-2-(cyclohexylimino)-6-(4-ethoxyphenyl)-5-(4-methyl-3-oxo-3,4-dihydroquinoxalin-2-yl)-2,3-dihydro-4H-1,3-oxazin-4-one with ammonium acetate afforded 3-(1-cyclohexyl-2-(cyclohexylamino)-4-(4-ethoxyphenyl)-6-oxo-1,6-dihydropyrimidin-5-yl)-1-methylquinoxalin-2(1H)-one in a good yield. The compound was fully characterized.

1. Introduction

2-(Pyrimidin-5-yl)quinoxalines are interesting nitrogen heterocycles for medical applications. For example, potential pharmaceutical substances, bearing a 2-(pyrimidin-5-yl)quinoxaline core, were developed for treating pain [1], inflammatory conditions [2], T-cell-mediated autoimmune disease [2], anemia [3], and cancer [2,4] (Figure 1).

Figure 1. Potential pharmaceutical substances, bearing a 2-(pyrimidin-5-yl)quinoxaline core.

As a continuation of our study on the syntheses of quinoxaline-based heterocycles A and B, via reactions of acyl(quinoxalin-2-yl)ketenes C, generated by the thermolysis of [e]-fused 1H-pyrrole-2,3-diones D [5], we investigated the capacity of the conversion of heterocycles A to their aza-analogs E, e.g., pyrimidinylquinoxalines (X = NR″; Y = CR‴₂) (Scheme 1).

Scheme 1. Syntheses of quinoxaline derivatives A and B via reactions of acyl(quinoxalin-2-yl)ketenes C.

2. Results and Discussion

The starting quinoxaline derivative 1 was synthesized according to the previously reported procedure [6] using the solvent-free reaction of [e]-fused 1H-pyrrole-2,3-dione 2 with dicyclohexylcarbodiimide (DCC) (Scheme 2). This transformation was completed in two steps through a one-pot procedure. First, pyrrole-2,3-dione 2 underwent thermal decomposition to form a highly reactive acyl(quinoxalin-2-yl)ketene 3 in situ. Second, ketene 3 was involved in a [4+2] cycloaddition reaction with DCC to afford the desired compound 1.

Scheme 2. Synthesis of the starting quinoxaline derivative 1 [6].

It should be mentioned that compound 1 was chosen for this study, since its closest analogs had shown significant antihypoxic effects in vivo [7] (Figure 2).

Figure 2. Close analog of quinoxaline derivative 1 [7].

The reaction of compound 1 with ammonium acetate afforded pyrimidinylquinoxaline derivative 4 in a good yield (80%) (Scheme 3). No other products were observed through the UPLC-UV-MS of the reaction mixture. The structure of compound 4 was unambiguously confirmed through single crystal X-ray analysis (CCDC Refcode = ZUNYIT, CCDC 2010489 [8]).

Scheme 3. Synthesis of pyrimidinylquinoxaline 4 from quinoxalinyl-1,3-oxazine 1.

3. Materials and Methods

3.1. General Information

¹H and ¹³C NMR spectra (Supplementary Materials) were acquired using a Bruker Avance III 400 HD spectrometer (at 400 and 100 MHz, respectively) in CDCl₃ (stab. with Ag) using the HMDS signal (in ¹H NMR) or solvent residual signal (in ¹³C NMR, 77.00 for CDCl₃) as internal standards. The IR spectrum was recorded using a Perkin–Elmer Spectrum Two spectrometer from a mull in mineral oil. The melting point was measured on a Khimlabpribor PTP apparatus. Elemental analysis was carried out on a Vario MICRO Cube analyzer. The reaction conditions were optimized using UPLC-UV-MS (Waters ACQUITY UPLC I-Class system; Acquity UPLC BEH C18 column; grain size of 1.7 μm; acetonitrile–water as eluents; flow rate of 0.6 mL/min; ACQUITY UPLC PDA eλ Detector (wavelength range of 230–780 nm); Xevo TQD mass detector; electrospray ionization (ESI); positive and negative ion detection; ion source temperature of 150 °C; capillary voltage of 3500–4000 V; cone voltage of 20–70 V; vaporizer temperature of 200 °C). The single crystal X-ray analysis of compound 4 was performed on an Xcalibur Ruby diffractometer (Agilent Technologies). The empirical absorption correction was introduced by the multi-scan method using the SCALE3 ABSPACK algorithm [9]. Using OLEX2 [10], the structure was solved using the SUPERFLIP [11] program and refined by the full-matrix least-squares minimization in the anisotropic approximation for all non-hydrogen atoms with the SHELXL [12] program. Hydrogen atoms bound to carbon were positioned geometrically and refined using a riding model. The hydrogen atom of the NH group was refined freely with isotropic displacement parameters. Starting compound 1 was obtained according to the reported procedure [6] from DCC (purchased from commercial vendors) and 1H-pyrrole-2,3-dione 2 (obtained according to the reported procedure [6] from commercially available reagents). All other solvents and reagents were purchased from commercial vendors and used as received.

3.2. 3-(1-Cyclohexyl-2-(cyclohexylamino)-4-(4-ethoxyphenyl)-6-oxo-1,6-dihydropyrimidin-5-yl)-1-methylquinoxalin-2(1H)-one 4

A mixture of 50 mg (90 µmol) of compound 1, 21 mg (270 µmol) of ammonium acetate, and 6 mg (100 µmol) of acetic acid in 1 mL of ethyl acetate were stirred in a screw-capped vial at 85 °C for 24 h. Then, the reaction mixture was cooled to room temperature. The formed precipitate was filtered off and recrystallized from ethyl acetate to yield the title compound 4 (yield: 40 mg (80%); pale yellow solid; mp 289–291 °C). ¹H NMR (CDCl₃, 400 MHz): δ = 7.77 (m, 1 H), 7.50 (m, 3 H), 7.28 (m, 2 H), 6.73 (m, 2 H), 5.16 (br.s, 1 H), 4.92 (br.s, 1 H), 4.18 (m, 1 H), 3.97 (q, J 6.8 Hz, 2 H), 3.70 (s, 3 H), 2.16–1.12 (m, 25 H) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 162.8, 159.8, 156.9, 154.8, 152.0, 133.6, 133.1, 133.1, 130.4, 130.3, 130.0, 130.0, 123.2, 123.1, 113.9, 113.9, 113.4, 63.3, 50.4, 32.9, 29.8, 29.3, 26.5, 25.7, 24.6, 14.7 ppm. IR (mineral oil): 3366, 1654 cm⁻¹. Anal. Calcd. (%) for C₃₃H₃₉N₅O₃: C 71.58; H 7.10; N 12.65. Found: C 71.92; H 7.13; N 12.51. MS (ESI+): m/z calcd. for C₃₃H₃₉N₅O₃+H⁺: 554.31 [M+H⁺]; found: 554.30. The crystal structure of compound 4 was deposited at the Cambridge Crystallographic Data Centre with the deposition number CCDC 2010489, CCDC Refcode = ZUNYIT [8].