Nitrogen-containing heterocycles occupy the most important place among the numerous biologically active compounds.1 A considerable proportion of them are cyclic imides. These include the widely used barbiturates2,3 as well as many other compounds, for example, the psychoactive buspirone4 and its derivatives,5 antitumor agents azonafide and amonafide6 (Fig. 1), angiogenic inhibitors,7 and many others.8,9

Figure 1.
figure 1

Biologically active cyclic imides.

Full size image

The biological activity of 1,2-benzoxazine derivatives has been studied relatively little. Moreover, such substances are generally poorly represented in the literature, and few methods for their synthesis can be found.10,11,12,13,14,15 1,2-Benzoxazines obtained by such methods often act as starting reagents for subsequent reactions due to their high reactivity.10,16,17 The development of practical methods for their synthesis remains an important and interesting synthetic problem.

Previously, our laboratory developed a simple method for the synthesis of 3-hydroxyminopyridine-2,6(1H,3H)-diones 2 from 5-alkoxy-1,2-oxazin-6-ones 1 (Scheme 1),18 which, in turn, can be accessed by alkylation of 5-hydroxy-1,2-oxazin-6-ones synthesized from the corresponding aldehydes and esters of nitroacetic acid.19,20 We presumed that, in the case of using 2-fluorobenzaldehydes, an intramolecular nucleophilic substitution reaction would be possible since such a strong acceptor as the hydroximino-pyridinedione ring should well promote such a process.

scheme 1

Scheme 1

The starting compounds 1a–d were synthesized according to a previously described method18 by condensation of 2-fluorobenzaldehydes 3a–d with nitroacetic esters 4a,b followed by alkylation (Scheme 2). It should be noted that, due to the difficulty of rotation, the methyl groups of isopropyl esters 1a–c are nonequivalent under the conditions of analysis by NMR spectroscopy, which was previously observed for ortho-substituted derivatives of 5-alkoxy-1,2-oxazin-6-one.20 For the same reason, the doubling of the signals of the ethoxy group was noted. Four new compounds 1a–d with a fluorine atom in the ortho position of the phenyl substituent at the C-4 atom of the 1,2-oxazine ring were obtained.

scheme 2

Scheme 2

For the synthesis of 3-hydroxyminopyridine-2,6(1H,3H)-dione derivatives, the obtained compounds 1a–d were treated with 3 equiv of the primary amine in PhMe (Scheme 3), following the previously described method.18 This resulted in the formation of colored yellow or orange compounds in the course of the reaction, in the 1H NMR spectra of which no signal of the OH group could be observed. In addition, the 13C NMR spectra did not contain multiplet signals characteristic of compounds containing fluorine atoms. Therefore, it can be unambiguously concluded that during the reaction a nucleophilic substitution of the fluorine atom by the hydroxyimine group occurred resulting in the formation of a new 1,2-oxazine ring (structures 5a–f). This result was also confirmed by mass spectrometry data. Quite unexpected for us was the ease of obtaining the recyclization products already at room temperature. Isolation and purification of compounds 5a–f involved extraction with dilute HCl to remove excess amine followed by purification by simple flash chromatography.

scheme 3

Scheme 3

All compounds were characterized by 1H and 13C NMR and high-resolution mass spectrometry. The yields of compounds 5a–f somewhat decreased when acceptor substituents were introduced into the aromatic moiety of the tricyclic framework. Moreover, the nature of the used ester and primary aliphatic amine did not significantly affect the yield. However, just as in our previous study,18 we failed to synthesize aniline derivatives.

To conclude, we have developed a simple procedure for the synthesis of pyrido[3,4-c][1,2]benzoxazine-2,4(3H)-diones from 2-fluorobenzaldehydes and esters of nitro-acetic acid. Compounds of this class are rarely described in the literature, and their synthesis by known methods is a nontrivial task. In total, six new potentially biologically active compounds have been obtained and characterized.

Experimental

1H and 13C NMR spectra were acquired on a Bruker Avance III spectrometer (700 and 176 MHz, respectively); 13C NMR spectra were also recorded on Bruker Avance III 800 with a cryoprobe (201 MHz) and Bruker Fourier 300 (75 MHz) spectrometers. DMSO-d6 was used as the solvent, TMS or the residual solvent signal (2.50 ppm for 1H nuclei, 39.5 ppm for 13C nuclei) served as internal standards. The assignment of signals in the 13C NMR spectra of compounds 1a–d was carried out on the basis of the data obtained previously for related compounds.18 Highresolution mass spectra were recorded on an AB Sciex TripleTOF 5600+ mass spectrometer using electrospray ionization. Melting points were determined on an SMP 30 apparatus. Merck Kieselgel 60 silica gel was used for chromatography.

Reagents supplied by Acros Organics were used without additional purification; freshly distilled solvents were used for the reactions. Isopropyl nitroacetate (4a) was obtained according to a literature procedure.21

Synthesis of compounds 1a–d (General method). Isopropyl nitroacetate (4a) or methyl nitroacetate (4b) (6.6 mmol) was added to a solution of the corresponding 2-fluorobenzaldehyde 3a–d (3 mmol) and Et2NH (548 mg, 7.5 mmol) in MeOH or i-PrOH (10 ml) (for the synthesis of compounds 1a–c and 1d, respectively). The resulting solution was stirred for 7 days, and the solvent was evaporated to a final volume of 2–3 ml. Et2O (20 ml) was added to the residue, and the resulting mixture was kept in a refrigerator at 0°C for 12 h. The formed precipitate was filtered off, dissolved in CHCl3 (50 ml), and washed with 1% aqueous HCl (3×20 ml). The organic layer was dried over anhydrous Na2SO4, and the solvent was evaporated, the residue was dissolved in CH2Cl2 (20 ml). [Et3O][BF4] (684 mg, 3.6 mmol), Et3N (606 mg, 6 mmol) were added to the obtained solution, and the resulting mixture was stirred for 12 h. EtOAc (100 ml) was added to the resulting mixture. The solution was successively washed with saturated aqueous NaHCO3 solution (30 ml), saturated aqueous KCl solution (2×30 ml), dried over anhydrous Na2SO4, and evaporated to dryness. The residue was purified by flash chromatography (eluent hexane–EtOAc, 10:1).

Isopropyl 5-ethoxy-4-(2-fluorophenyl)-6-oxo-6H-1,2-oxazine-3-carboxylate (1a). Yield 510 mg (53%), white powder, mp 81–83°C. 1H NMR spectrum, δ, ppm (J, Hz): 0.95 (3H, br. d, J = 5.0) and 1.04 (3H, br. d, J = 5.0, OCH(CH3)2); 1.13 (3H, t, J = 7.0, OCH2CH3); 4.42 (2H, 2 signals (Δδ = 2.0 Hz) q, J = 6.9, OCH2CH3); 4.88 (1H, sept, J = 6.2, OCH(CH3)2); 7.29–7.35 (2H, m, H Ar); 7.35–7.39 (1H, m, H Ar); 7.50–7.56 (1H, m, H Ar). 13C NMR spectrum (75 MHz), δ, ppm (J, Hz): 15.2 (OCH2CH3); 20.6, 20.8 (OCH(CH3)2); 69.7 (OCH(CH3)2); 70.9 (OCH2CH3); 115.5 (d, J = 20.9, C Ar); 117.5 (d, J = 16.4, C Ar); 118.4 (C-4); 124.5 (d, J = 3.7, C Ar); 130.8 (d, J = 2.2, C Ar); 131.8 (d, J = 8.2, C Ar); 148.0 (C-3); 151.8 (C-5); 159.1 (d, J = 246.6, C Ar); 159.9 (C=O); 160.4 (COOi-Pr). Found, m/z: 322.1085 [M+H]+. C16H17FNO5. Calculated, m/z: 322.1085.

Isopropyl 4-(5-chloro-2-fluorophenyl)-5-ethoxy-6-oxo-6H-1,2-oxazine-3-carboxylate (1b). Yield 652 mg (61%), white powder, mp 62–64°C. 1H NMR spectrum, δ, ppm (J, Hz): 1.03 (3H, br. d, J = 5.5) and 1.08 (3H, br. d, J = 5.5, CH(CH3)2); 1.15 (3H, t, J = 7.0, CH2CH3); 4.47 (2H, q, J = 7.0, CH2CH3); 4.94 (1H, sept, J = 6.2, CH(CH3)2); 7.41 (1H, t, J = 9.1, H Ar); 7.48 (1H, dd, J = 5.9, J = 2.7, H Ar); 7.57–7.63 (1H, m, H Ar). 13C NMR spectrum (75 MHz), δ, ppm (J, Hz): 15.2 (OCH2CH3); 20.7, 20.8 (OCH(CH3)2); 70.0 (OCH(CH3)2); 71.1 (OCH2CH3); 116.8 (C-4); 117.5 (d, J = 23.1, C Ar); 119.5 (d, J = 18.6, C Ar); 128.2 (d, J = 3.0, C Ar); 130.4 (d, J = 2.2, C Ar); 131.4 (d, J = 8.2, C Ar); 148.2 (C-3); 151.0 (C-5); 157.9 (d, J = 267.5, C Ar); 159.7 (C=O); 160.1 (COOi-Pr). Found, m/z: 356.0703 [M+H]+. C16H16ClFNO5. Calculated, m/z: 356.0696.

Isopropyl 4-(4-bromo-2-fluorophenyl)-5-ethoxy-6-oxo-6H-1,2-oxazine-3-carboxylate (1c). Yield 575 mg (48%), beige powder, mp 65–67°C. 1H NMR spectrum, δ, ppm (J, Hz): 1.02 (3H, br. d, J = 5.1) and 1.08 (3H, br. d, J = 5.1, OCH(CH3)2); 1.14 (3H, t, J = 7.0, OCH2CH3); 4.45 (2H, 2 signals (Δδ = 1.7 Hz) q, J = 6.9, OCH2CH3); 4.92 (1H, sept, J = 6.2, OCH(CH3)2); 7.34 (1H, t, J = 8.0, H Ar); 7.56 (1H, dd, J = 8.2, J = 1.7, H Ar); 7.73 (1H, dd, J = 9.5, J = 1.7, H Ar). 13C NMR spectrum (75 MHz), δ, ppm (J, Hz): 15.3 (OCH2CH3); 20.7, 20.9 (OCH(CH3)2); 69.9 (OCH(CH3)2); 71.1 (OCH2CH3); 117.1 (d, J = 16.4, C Ar); 117.3 (C-4); 119.2 (d, J = 24.6, C Ar); 123.3 (d, J = 9.7, C Ar); 127.8 (d, J = 3.7, C Ar); 132.3 (d, J = 3.0, C Ar); 148.1 (C-3); 151.1 (C-5); 158.1 (d, J = 251.1, C Ar); 159.8 (C=O); 160.2 (COOi-Pr). Found, m/z: 400.0194 [M+H]+. C16H16BrFNO5. Calculated, m/z: 400.0190.

Methyl 4-(5-bromo-2-fluorophenyl)-5-ethoxy-6-oxo-6H-1,2-oxazine-3-carboxylate (1d). Yield 568 mg (51%), white powder, mp 98–100°C. 1H NMR spectrum, δ, ppm (J, Hz): 1.14 (3H, t, J = 7.1, OCH2CH3); 3.72 (3H, s, OCH3); 4.47 (2H, q, J = 7.1, OCH2CH3); 7.33 (1H, t, J = 9.3, H Ar); 7.62 (1H, dd, J = 6.3, J = 2.7, H Ar); 7.70 (1H, ddd, J = 8.8, J = 4.6, J = 2.5, H Ar). 13C NMR spectrum (176 MHz), δ, ppm (J, Hz): 15.1 (OCH2CH3); 53.4 (OCH2CH3); 69.9 (O CH3); 115.7 (d, J = 3.0, C Ar); 117.1 (C-4); 117.7 (d, J = 23.1, C Ar); 119.8 (d, J = 17.7, C Ar); 133.2 (d, J = 2.7, C Ar); 134.2 (d, J = 8.7, C Ar); 148.1 (C-3); 150.0 (C-5); 158.2 (d, J = 247.0, C Ar); 160.1 (C=O); 160.8 (COOMe). Found, m/z: 371.9877 [M+H]+. C14H12BrFNO5. Calculated, m/z: 371.9877.

Synthesis of compounds 5a–f (General method). Compound 1a–d (0.5 mmol) was dissolved in PhMe (3 ml), and the corresponding amine (1.5 mmol) was added to the resulting solution. The resulting mixture was stirred at room temperature for 24 h, 1% aqueous HCl (30 ml) was added, and the whole was extracted with EtOAc (3×50 ml). The combined organic layers were washed with saturated aqueous KCl solution, dried over anhydrous Na2SO4, and evaporated. The residue was purified by flash chromatography (eluent hexane–CH2Cl2, 1:1).

3-Benzyl-1-ethoxy-2H-pyrido[3,4-c][1,2]benzoxazine-2,4(3H)-dione (5a). Yield 102 mg (59%), yellow powder, mp 149–151°C. 1H NMR spectrum, δ, ppm (J, Hz): 1.37 (3H, t, J = 7.1, OCH2CH3); 4.25 (2H, q, J = 7.1, OCH2CH3); 5.05 (2H, s, CH2Ph); 7.26 (1H, t, J = 7.1, H Ph); 7.30–7.37 (4H, m, H Ph); 7.39–7.44 (2H, m, H Ar); 7.64 (1H, t, J = 7.7, H Ar); 8.50 (1H, d, J = 8.2, H Ar). 13C NMR spectrum (75 MHz), δ, ppm: 15.4; 43.3; 68.8; 112.9; 113.8; 114.8; 126.2; 127.2; 127.4; 127.5 (2C); 128.3 (2C); 132.8; 136.4; 138.5; 144.6; 153.1; 157.5; 160.4. Found, m/z: 349.1189 [M+H]+. C20H17N2O4. Calculated, m/z: 349.1183.

3-Butyl-1-ethoxy-2H-pyrido[3,4-c][1,2]benzoxazine-2,4(3H)-dione (5b). Yield 97 mg (62%), yellow powder, mp 92–94°C. 1H NMR spectrum, δ, ppm (J, Hz): 0.91 (3H, t, J = 7.3, CH2CH2CH2CH3); 1.32 (2H, ddd, J = 7.6, J = 7.5, J = 7.3, CH2CH2CH2CH3); 1.37 (3H, t, J = 7.1, OCH2CH3); 1.54 (2H, q, J = 7.5, CH2CH2CH2CH3); 3.81–3.86 (2H, m, CH2CH2CH2CH3); 4.25 (2H, q, J = 7.1, OCH2CH3); 7.38–7.42 (2H, m, H Ar); 7.62 (1H, ddd, J = 8.4, J = 7.2, J = 1.5, H Ar); 8.48 (1H, dd, J = 8.4, J = 1.5, H Ar). 13C NMR spectrum (176 MHz), δ, ppm: 13.6; 15.4; 19.6; 29.1; 39.7 (overlaps with DMSO-d6 signal); 68.7; 112.2; 113.7; 114.7; 126.1; 127.3; 132.6; 138.6; 144.4; 153.0; 157.2; 160.3. Found, m/z: 315.1344 [M+H]+. C17H19N2O4. Calculated, m/z: 315.1339.

3-Benzyl-9-chloro-1-ethoxy-2H-pyrido[3,4-c][1,2]benzoxazine-2,4(3H)-dione (5d). Yield 84 mg (44%), yellow powder, mp 152–154°C. 1H NMR spectrum, δ, ppm (J, Hz): 1.38 (3H, t, J = 7.1, OCH2CH3); 4.31 (2H, q, J = 7.1, OCH2CH3); 5.04 (2H, s, CH2Ph); 7.26 (1H, t, J = 7.1, H Ph); 7.30–7.36 (4H, m, H Ph); 7.48 (1H, d, J = 9.0, H Ar); 7.70 (1H, dd, J = 9.0, J = 2.5, H Ar); 8.47 (1H, d, J = 2.5, H Ar). 13C NMR spectrum (201 MHz), δ, ppm: 15.5; 43.3; 69.3; 111.6; 115.4; 117.0; 126.3; 127.2; 127.6 (2C); 128.3 (2C); 129.5; 132.5; 136.2; 139.2; 144.4; 151.8; 157.2; 160.1. Found, m/z: 383.0800 [M+H]+. C20H16ClN2O4. Calculated, m/z: 383.0793.

3-Benzyl-8-bromo-1-ethoxy-2H-pyrido[3,4-c][1,2]benzoxazine-2,4(3H)-dione (5e). Yield 105 mg (49%), yellow powder, mp 155–157°C. 1H NMR spectrum, δ, ppm (J, Hz): 1.36 (3H, t, J = 7.1, OCH2CH3); 4.27 (2H, q, J = 7.1, OCH2CH3); 5.04 (2H, s, CH2Ph); 7.26 (1H, t, J = 7.0, H Ph); 7.29–7.36 (4H, m, H Ph); 7.60 (1H, dd, J = 8.7, J = 2.0, H Ar); 7.75 (1H, d, J = 1.9, H Ar); 8.40 (1H, d, J = 8.8, H Ar). 13C NMR spectrum (201 MHz), δ, ppm: 15.4; 43.3; 69.1; 112.2; 113.2; 117.6; 125.1; 127.2; 127.5 (2C); 128.3 (2C); 129.0; 129.1; 136.2; 139.0; 144.8; 153.4; 157.3; 160.2. Found, m/z: 427.0293 [M+H]+. C20H16BrN2O4. Calculated, m/z: 427.0288.

3-Benzyl-9-bromo-1-ethoxy-2H-pyrido[3,4-c][1,2]benzoxazine-2,4(3H)-dione (5f). Yield 107 mg (50%), yellow powder, mp 157–159°C. 1H NMR spectrum, δ, ppm (J, Hz): 1.38 (3H, t, J = 7.1, OCH2CH3); 4.30 (2H, q, J = 7.1, OCH2CH3); 5.04 (2H, s, CH2Ph); 7.24–7.28 (1H, m, H Ph); 7.31–7.35 (4H, m, H Ph); 7.40 (1H, d, J = 8.8, H Ar); 7.80 (1H, dd, J = 8.9, J = 2.4, H Ar); 8.62 (1H, d, J = 2.5, H Ar). 13C NMR spectrum (176 MHz), δ, ppm: 15.5; 43.3; 69.3; 111.4; 115.8; 117.2; 117.5; 127.2; 127.6; 128.3; 129.3; 135.2; 136.2; 139.2; 144.5; 152.1; 157.2; 160.1. Found, m/z: 427.0290 [M+H]+. C20H16BrN2O4. Calculated, m/z: 427.0288.

Supplementary information file containing 1H and 13C NMR and high-resolution mass spectra of all synthesized compounds is available at the journal website http://springerlink.bibliotecabuap.elogim.com/journal/10593.

The study was supported financially by the Russian Science Foundation (project No. 20-73-10195).