HETEROCYCLES

Short Paper | Regular issue | Vol. 87, No. 11, 2013, pp. 2361-2368
Received, 26th August, 2013, Accepted, 25th September, 2013, Published online, 30th September, 2013.
DOI: 10.3987/COM-13-12819

■ Synthesis of Dihydrooxepins by the Cycloaddition of 2-Amino-4,5-dihydro-3-furancarbonitriles with Dimethyl Acetylenedicarboxylate

Fumi Okabe, Hiroshi Maruoka,* Eiichi Masumoto, Toshihiro Fujioka, and Kenji Yamagata

Faculty of Pharmaceutical Sciences, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

Abstract

A facile and efficient synthesis of dihydrooxepins is described. Treatment of readily available 2-amino-4,5-dihydro-3-furancarbonitriles with dimethyl acetylenedicarboxylate at room temperature caused smoothly a cycloaddition reaction, followed by a ring expansion, giving the corresponding dihydrooxepin derivatives.

Seven-membered oxacycles represent an important area of organic chemistry. A remarkable diversity of natural products contains a seven-membered oxacycle in their molecular architecture.1 Therefore, interest in the synthesis of seven-membered oxacycles has steadily increased in recent years because of their occurrence in natural products2 and pharmacological applications.3 Notably, a majority of the natural products are from marine sources. Examples of their occurrence in nature range from the monocyclic zoapatanol,4 isolaurepinnacin,5 crambescidin acid,6 and lobatrienetriol7 to the highly complex ciguatoxin8 (Figure 1). Reported pharmacological investigations on these structures showed that they have antiviral,6 antifungal,7 and ion-channel blocking8a activities. In view of the interest and challenges these molecules present as potential synthetic targets, Elliot9 and Hoberg1b have treated the synthesis of medium ring oxacycles previously in reviews.

In this context, there have been many attempts to develop alternative methods for the synthesis of oxepin derivatives.10 For example, Nicolaou and coworkers reported the synthesis of dihydrooxepins from dihydrofurans and dimethyl acetylenedicarboxylate (DMAD) through a cycloaddition/ring expansion.11 Wamhoff and coworkers12 also discussed the cycloaddition reaction13 of heterocyclic enamines with DMAD. In the course of our studies on heterocyclic β-enaminonitriles, we have previously reported the cycloaddition reaction of heterocyclic β-enaminonitriles with DMAD.14 For these reasons, we have been interested in the development of the methods for the synthesis of dihydrooxepin derivatives. As part of our current studies on the development of new routes in heterocyclic synthesis,15 we herein wish to report a facile and efficient method for preparing new dihydrooxepin derivatives by the reaction of 2-amino-4,5-dihydro-3-furancarbonitriles as one of versatile starting materials and DMAD.

On the basis of the above experimental results together with some literature reports, we focused attention on the extension of our studies to synthesize functionalized dihydrooxepin derivatives, which might have useful biological and therapeutic activities. Thus, we hypothesized that if a cycloaddition reaction of 2-amino-4,5-dihydro-3-furancarbonitriles 1a−i with DMAD could undergo readily under an appropriate reaction condition, the synthesis of dihydrooxepin derivatives 2a−i would be possible by a ring expansion of the intermediate cycloadducts A (Scheme 1).

The starting materials, 2-(cyclic amino)-substituted 4,5-dihydro-3-furancarbonitriles 1a−i, were easily prepared according to our previous investigation.16 Having optimized the cycloaddition/ring expansion reaction parameters, we then carried out several experiments on dihydrofuran 1a, testing different reaction conditions, for example, time, solvent, and substrate/DMAD molar ratio (Table 1). Best result was obtained when 1a was treated with DMAD in DMSO as the solvent (entry 4). Indeed, the cycloaddition/ring expansion reaction proceeded smoothly at room temperature for 24 h, giving the desired dihydrooxepins 2a−i in moderate to good yields (Table 2).

Elemental analyses and IR, 1H NMR, 13C NMR, and MS spectra of 2a−i are consistent with their assigned structures (see experimental section). For example, the IR spectrum of 2a reveals a band at 2202 cm-1 due to a conjugated cyano group and two bands at 1716 and 1663 cm-1 attributable to two ester carbonyl groups. The 1H NMR spectrum of 2a in DMSO-d6 exhibits two three-proton singlets at δ 3.52 and 3.67 assignable to the two methyl ester protons. The 13C NMR spectrum of 2a in DMSO-d6 shows a signal at δ 75.9 because of the C-3 carbon, a signal at δ 99.2 because of the C-5 carbon, a signal at δ 145.5 because of the C-4 carbon, two signals at δ 164.6 and 168.1 because of the two ester carbonyl carbons, and a signal at δ 173.2 because of the C-2 carbon.
In conclusion, we have demonstrated a facile synthesis of new dihydrooxepin derivatives. A key step is the generation of an intermediate 2-oxabicyclo[3.2.0]hept-6-enes under the mild reaction condition through a cycloaddition reaction of 2-(cyclic amino)-substituted 4,5-dihydro-3-furancarbonitriles and DMAD. This methodology offers significant advantages with regard to the simplicity of operation. Functionalized dihydrooxepin derivatives are important building blocks in organic synthesis and for the preparation of biologically active compounds with interest in medicinal chemistry.

EXPERIMENTAL
All melting points are uncorrected. The IR spectra were recorded on a JASCO FT/IR-4100 spectrometer. The 1H and 13C NMR spectra were measured with a JEOL JNM-A500 spectrometer at 500.00 and 125.65 MHz, respectively. The 1H and 13C chemical shifts (δ) are reported in parts per million (ppm) relative to TMS as internal standard. Positive FAB MS spectra were obtained on a JEOL JMS-700T spectrometer. Elemental analyses were performed on YANACO MT-6 CHN analyzer. The substrate 1a−i were prepared in this laboratory according to procedure reported in literature.16
General procedure for the preparation of dihydrooxepins 2a−i from 1a−i and DMAD. A solution of 1a−i (5 mmol) and DMAD (1.07 g, 7.5 mmol) in DMSO (5 mL) was stirred at rt for 24 h. After removal of the solvent in vacuo, cold water was added to the residue. The precipitate was collected by filtration, dried, and recrystallized from an appropriate solvent to give 2a−i.
Dimethyl 5-cyano-6,7-dihydro-2-(4-morpholinyl)-3,4-oxepindicarboxylate (2a): Colorless prisms (1.61 g, 80%), mp 231−232 °C (MeOH/CHCl3); IR (KBr): 2202 (CN), 1716, 1663 cm-1 (CO); 1H NMR (DMSO-d6): δ 2.49−2.50 (m, 2H, 6-H), 3.40−3.43 (m, 4H, 2NCH2), 3.52 (s, 3H, OCH3), 3.66−3.68 (m, 4H, 2OCH2), 3.67 (s, 3H, OCH3), 4.57−4.60 (m, 2H, 7-H); 13C NMR (DMSO-d6): δ 33.1 (C-6), 49.0 (2NCH2), 50.4, 52.1 (OCH3), 65.6 (OCH2), 69.9 (C-7), 75.9 (C-3), 99.2 (C-5), 119.2 (CN), 145.5 (C-4), 164.6, 168.1 (CO), 173.2 (C-2); MS: m/z 323 [M+H]+. Anal. Calcd for C15H18N2O6: C, 55.90; H, 5.63; N, 8.69. Found: C, 55.77; H, 5.60; N, 8.71.
Dimethyl 5-cyano-6,7-dihydro-2-(1-piperidinyl)-3,4-oxepindicarboxylate (2b): Colorless prisms (1.14 g, 71%) (dec.) 161−162 °C (Et2O); IR (KBr): 2211 (CN), 1732, 1656 cm-1 (CO); 1H NMR (DMSO-d6): δ 1.55−1.64 (m, 6H, 3CH2), 2.77 (t, J = 5.2 Hz, 2H, 6-H), 3.35−3.38 (m, 4H, 2NCH2), 3.50 (s, 3H, OCH3), 3.67 (s, 3H, OCH3), 4.57 (t, J = 5.2 Hz, 2H, 7-H); 13C NMR (DMSO-d6): δ 23.3 (piperidine C-4), 25.3 (piperidine C-3 and -5), 33.1 (C-6), 49.7 (2NCH2), 50.3, 52.0 (OCH3), 69.3 (C-7), 75.3 (C-3), 98.1 (C-5), 119.3 (CN), 145.9 (C-4), 164.8, 168.2 (CO), 173.1 (C-2); MS: m/z 321 [M+H]+. Anal. Calcd for C16H20N2O5: C, 59.99; H, 6.29; N, 8.74. Found: C, 59.82; H, 6.28; N, 8.71.
Dimethyl 5-cyano-6,7-dihydro-2-(1-pyrrolidinyl)-3,4-oxepindicarboxylate (2c): Colorless prisms (1.08 g, 70%) (dec.) 181−182 °C (CH2Cl2/petroleum ether); IR (KBr): 2198 (CN), 1720, 1665 cm-1 (CO); 1H NMR (DMSO-d6): δ 1.86−1.90 (m, 4H, 2CH2), 2.73 (t, J = 4.9 Hz, 2H, 6-H), 3.43 (br s, 4H, 2NCH2), 3.50 (s, 3H, OCH3), 3.68 (s, 3H, OCH3), 4.61 (t, J = 4.9 Hz, 2H, 7-H); 13C NMR (DMSO-d6): δ 24.4 (2CH2), 32.9 (C-6), 49.8 (2NCH2), 50.1, 52.0 (OCH3), 70.3 (C-7), 76.1 (C-3), 95.9 (C-5), 119.6 (CN), 146.2 (C-4), 164.1, 168.2 (CO), 171.4 (C-2); MS: m/z 307 [M+H]+. Anal. Calcd for C15H18N2O5: C, 58.82; H, 5.92; N, 9.15. Found: C, 58.55; H, 5.93; N, 9.05.
Dimethyl 5-cyano-6,7-dihydro-2-(4-morpholinyl)-6-phenyl-3,4-oxepindicarboxylate (2d): Colorless prisms (1.56 g, 77%), mp 202−203 °C (CH2Cl2/petroleum ether); IR (KBr): 2193 (CN), 1725, 1665 cm-1 (CO); 1H NMR: (DMSO-d6): δ 3.33−3.56 (m, 4H, 2NCH2), 3.57 (s, 3H, OCH3), 3.58−3.70 (m, 4H, 2OCH2), 3.70 (s, 3H, OCH3), 4.26 (q, J = 3.8 Hz, 1H, 6-H), 4.50 (dd, J = 7.8, 11.6 Hz, 1H, 7-H), 4.61 (dd, J = 3.8, 11.6 Hz, 1H, 7-H), 7.25−7.27 (m, 2H, aryl H), 7.32−7.36 (m, 1H, aryl H), 7.40−7.43 (m, 2H, aryl H); 13C NMR (DMSO-d6): δ 48.0 (C-6), 49.4 (2NCH2), 50.7, 52.3 (OCH3), 65.8 (2OCH2), 73.2 (C-7), 75.6 (C-3), 101.0 (C-5), 119.0 (CN), 127.9, 128.4, 128.8, 138.3 (C aryl), 146.4 (C-4), 164.7, 168.4 (CO), 172.9 (C-2); MS: m/z 399 [M+H]+. Anal. Calcd for C21H22N2O6 0.4H2O: C, 62.18; H, 5.47; N, 6.91. Found: C, 62.16: H, 5.46; N, 6.67.
Dimethyl 5-cyano-6,7-dihydro-6-phenyl-2-(1-piperidinyl)-3,4-oxepindicarboxylate (2e): Colorless prisms (1.53 g, 77%), mp 155−158 °C (CH2Cl2/petroleum ether); IR (KBr): 2202 (CN), 1723, 1661 cm-1 (CO); 1H NMR (DMSO-d6): δ 1.52−1.63 (m, 6H, 3CH2), 3.34−3.40 (m, 4H, 2NCH2), 3.55 (s, 3H, OCH3), 3.71 (s, 3H, OCH3), 4.24 (dd, J = 3.4, 7.3 Hz, 1H, 6-H), 4.49 (dd, J = 7.3, 11.5 Hz, 1H, 7-H), 4.62 (dd, J = 3.4, 11.5 Hz, 1H, 7-H), 7.22−7.25 (m, 2H, aryl H), 7.31−7.35 (m, 1H, aryl H), 7.39−7.43 (m, 2H, aryl-H); 13C NMR (DMSO-d6): δ 23.3 (piperidine C-4), 25.7 (piperidine C-3 and -5), 48.0 (C-6), 50.1 (2NCH2), 50.6, 52.3 (OCH3), 72.7 (C-7), 75.0 (C-3), 99.5 (C-5), 119.2 (CN), 127.8, 128.4, 128.8, 138.6 (C aryl), 146.9 (C-4), 164.9, 168.5 (CO), 172.7 (C-2); MS: m/z 397 [M+H]+. Anal. Calcd for C22H24N2O5: C, 66.65; H, 6.10; N, 7.07. Found: C, 66.52: H, 6.09; N, 7.00.
Dimethyl 5-cyano-6,7-dihydro-6-phenyl-2-(1-pyrrolidinyl)-3,4-oxepindicarboxylate (2f): Colorless prisms (1.27 g, 66%), mp 207−210 °C (dec.) (CH2Cl2/petroleum ether); IR (KBr): 2107 (CN), 1731, 1677 cm-1 (CO); 1H NMR (DMSO-d6): δ 1.86−1.89 (m, 4H, 2CH2), 3.42−3.60 (m, 4H, 2NCH2), 3.55 (s, 3H, OCH3), 3.71 (s, 3H, OCH3), 4.21 (dd, J = 3.8, 7.8 Hz, 1H, 6-H), 4.54 (dd, J = 7.8, 11.9 Hz, 1H, 7-H), 4.62 (dd, J = 3.8, 11.9 Hz, 1H, 7-H), 7.25−7.27 (m, 2H, aryl H), 7.31−7.35 (m, 1H, aryl H), 7.39−7.42 (m, 2H, aryl H); 13C NMR (DMSO-d6): δ 24.4 (2CH2), 32.9 (C-6), 47.9 (NCH2), 50.4 (OCH3), 51.4 (2NCH2), 52.3 (OCH3) 73.4 (C-7), 75.6 (C-3), 98.0 (C-5), 119.4 (CN), 127.8, 128.4, 128.8, 138.4 (C aryl), 146.9 (C-4), 164.3, 168.5 (CO), 171.5 (C-2); MS: m/z 383 [M+H]+. Anal. Calcd for C21H22N2O5: C, 65.96; H, 5.80; N, 7.33. Found: C, 65.89: H, 5.85; N, 7.30.
Dimethyl 5-cyano-6,7-dihydro-2-(4-morpholinyl)-7-phenyl-3,4-oxepindicarboxylate (2g): Pale yellow prisms (1.23 g, 62%), mp 173−174 °C (CH2Cl2/petroleum ether); IR (KBr): 2202 (CN), 1734, 1671 cm-1 (CO); 1H NMR (DMSO-d6): δ 2.91 (dd, J = 2.3, 17.7 Hz, 1H, 6-H), 3.16 (dd, J =10.4, 17.7 Hz, 1H, 6-H), 3.21−3.26 (m, 2H, NCH2), 3.42−3.47 (m, 2H, NCH2), 3.55 (s, 3H, OCH3), 3.59−3.70 (m, 4H, 2OCH2), 3.71 (s, 3H, OCH3), 5.97 (dd, J = 2.3, 10.4 Hz, 1H, 7-H), 7.37−7.45 (m, 3H, aryl H), 7.49−7.51 (m, 2H, aryl H); 13C NMR (DMSO-d6): δ 39.9 (C-6), 49.3 (2NCH2), 50.6, 52.3 (OCH3), 65.5 (OCH2), 77.4 (C-3), 83.1 (C-7), 99.4 (C-5), 119.1 (CN), 126.3, 128.7, 137.9 (C aryl), 145.7 (C-4), 164.6, 168.0 (CO), 171.3 (C-2); MS: m/z 399 [M+H]+. Anal. Calcd for C21H22N2O6: C, 63.31; H, 5.57; N, 7.03. Found: C, 63.18; H, 5.57; N, 6.90.
Dimethyl 5-cyano-6,7-dihydro-7-phenyl-2-(1-piperidinyl)-3,4-oxepindicarboxylate (2h): Pale yellow prisms (1.52 g, 77%), mp 181−185 °C (dec.) (CH2Cl2/petroleum ether); IR (KBr): 2197 (CN), 1726, 1682 cm-1 (CO); 1H NMR (DMSO-d6): δ 1.53−1.60 (m, 6H, 3CH2), 2.50 (dd, J = 2.1, 3.6 Hz, 1H, 6-H), 3.12−3.21 (m, 3H, NCH2, 6-H), 3.38−3.42 (m, 2H, NCH2), 3.54 (s, 3H, OCH3), 3.71 (s, 3H, OCH3), 5.94 (dd, J = 2.1, 10.7 Hz, 1H, 7-H), 7.36−7.45 (m, 3H, aryl H), 7.48−7.51 (m, 2H, aryl H); 13C NMR (DMSO-d6): δ 23.3 (piperidine C-4), 25.3 (piperidine C-3 and -5), 40.0 (C-6), 50.0 (2NCH2), 50.5, 52.3 (OCH3), 76.7 (C-3), 82.2 (C-7), 98.4 (C-5), 119.2 (CN), 126.2, 128.65, 128.71, 138.1 (C aryl), 146.0 (C-4), 164.7, 168.2 (CO), 171.3 (C-2); MS: m/z 397 [M+H]+. Anal. Calcd for C21H22N2O5: C, 66.65; H, 6.10; N, 7.07. Found: C, 66.44: H, 6.15; N, 6.99.
Dimethyl 5-cyano-6,7-dihydro-7-phenyl-2-(1-pyrrolidinyl)-3,4-oxepindicarboxylate (2i): Pale yellow columns (1.32 g, 69%), mp 158−159 °C (CH2Cl2/petroleum ether); IR (KBr): 2205 (CN), 1733, 1676 cm-1 (CO); 1H NMR (DMSO-d6): δ 1.78−1.80 (m, 2H, 2CH2), 1.93−1.94 (m, 2H, 2CH2), 2.87 (dd, J = 2.4, 17.4 Hz, 1H, 6-H), 3.05 (dd, J = 9.9, 17.4 Hz, 1H, 6-H), 3.25 (br s, 2H, NCH2), 3.53 (s, 3H, OCH3), 3.56 (br s, 2H, NCH2), 3.71 (s, 3H, OCH3), 6.00 (dd, J = 2.4, 9.9 Hz, 1H, 7-H), 7.35−7.44 (m, 3H, aryl H), 7.49−7.51 (m, 2H, aryl H); 13C NMR (DMSO-d6): δ 24.4 (2CH2), 39.9 (C-6), 50.0 (2NCH2), 50.3, 52.2 (OCH3), 77.5 (C-3), 83.0 (C-7), 96.3 (C-5), 119.5 (CN), 126.0, 128.5, 128.6, 138.4 (C aryl), 146.4 (C-4), 164.1, 168.1 (CO), 169.4 (C-2); MS: m/z 383 [M+H]+. Anal. Calcd for C21H22N2O5: C, 65.96; H, 5.80; N, 7.33. Found: C, 66.15: H, 5.88; N, 7.35.

ACKNOWLEDGEMENTS
The authors thank Hiroshi Hanazono and Yukiko Iwase for obtaining MS and NMR spectra and Junko Honda for her valuable help with elemental analyses.

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