HETEROCYCLES

Paper | Special issue | Vol. 86, No. 1, 2012, pp. 305-315
Received, 12th April, 2012, Accepted, 18th May, 2012, Published online, 25th May, 2012.
DOI: 10.3987/COM-12-S(N)10

■ Synthesis and Redox Behavior of Cyanovinyl-Substituted 2H-Cyclohepta[b]furan-2-ones

Taku Shoji,* Junya Higashi, Shunji Ito, Mitsunori Oda, Masafumi Yasunami, and Noboru Morita

Department of Chemistry, Faculty of Science, Shinshu University, Asahi, Matsumoto 390-8621, Japan

Abstract

Cyanovinyl-substituted 2H-cyclohepta[b]furan-2-ones 3, 4, 5 and 7 were prepared by the Wittig, Knoevenagel and electrophilic substitution reaction, respectively. The intramolecular charge-transfer (ICT) interactions between the 2H-cyclohepta[b]furan-2-one ring and cyanovinyl acceptor moiety were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of these novel compounds was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Significant color changes in these compounds were observed under electrochemical reduction conditions by visible spectroscopy.

INTRODUCTION
2H-Cyclohepta[b]furan-2-one is known as a heteroazulene, a versatile precursor for azulene derivatives.1 Although unusual reactivity of the compound including its derivatives has already been revealed by many　research groups,2 there are few derivatives for the application to the advanced materials with potentially useful electronic properties.
Recently, we have reported the reaction of mono-, bis-, and tris(1-azulenylethynyl and 2H-cyclohepta[b]furan-2-on-3-ylethynyl)benzene and thiophene derivatives with tetracyanoethylene (TCNE) to give the tetracyanobutadiene (TCBD) substituted chromophores in excellent yield. We have also revealed that these new chromophores exhibit multistep reduction waves on CV and DPV. Moreover, significant color changes are also observed by the electrochemical reduction of these new derivatives.3,4 However, synthesis and properties of cyanovinyl-substituted, which is unit of the TCBD, 2H-cyclohepta[b]furan-2-ones were never investigated. To clarify the substituent effect toward the spectroscopic and electrochemical property is an important clue to the design of organic materials. Herein, we describe the synthesis of mono-, di-, tricyanovinyl-2H-cyclohepta[b]furan-2-one derivatives by the Wittig, Knoevenagel and electrophilic substitution reaction, respectively. Electronic properties of the new 3-cyanovinyl-2H-cyclohepta[b]furan-2-one derivatives obtained by the reaction are characterized by CV, DPV, and absorption spectroscopy.

RESULTS AND DISCUSSION
Synthesis: Prior to the cyanovinylation of 2H-cyclohepta[b]furan-2-one, the key intermediate formyl derivative 2 was prepared by Vilsmeier formylation of 1, which can readily be prepared by the decarboxylation of 5-isopropyl-3-methoxycarbonyl-2H-cyclohepta[b]furan-2-one with 80% sulfuric acid.2 The reaction of 1 with POCl3 in DMF afforded 1 in 94 % yield, as similar with the literature (Scheme 1).5 We applied the Wittig6 and Knoevenagel7 reaction of 2 for the preparation of the corresponding cyanovinyl derivatives. Reaction of 2 with bromoacetonitrile in the presence of triphenylphosphine in mixed solvent of DMSO/Et3N gave 3 (47% yield) and 4 (51% yield), respectively. Although the reaction afforded a mixture of 3 and 4, these were easily separated by reversed-phase chromatography (ODS gel with 80% MeOH). For the purpose of preparation for dicyanovinyl derivative, we examined similar Wittig reaction of 2 using the 2-bromomalononitrile. However, reaction of 2 with 2-bromomalononitrile formed an insoluble complex mixture. This result might be attributable to the instability of compound in this condition. Thus, synthesis of 5 was established by the reaction of 2 with malononitrile under Knoevenagel condition. The yield of the products in this reaction was directly affected by the solvent and amount of the reagent. The reaction of 2 with equimolar amounts of malononitrile in the presence of pyridine in CH2Cl2 afforded the dicyanovinyl derivative 5 in 51% yield as sole product. The reaction of 2 with the 2 equiv. of malononitrile in CH2Cl2 gave 5 in 10% yield, along with 2-aminozulene derivative 6 in 34% yield, respectively. When the solvent was changed to EtOH, compound 6 was obtained in 83% yield as sole product. In the early days of troponoid chemistry, Nozoe et al. reported the reaction between 2H-cyclohepta[b]furan-2-one and malononitrile in EtOH gave 2-amino-1-cyanoazulene derivatives in high yield.8 Therefore, the reaction between 2 and malononitrile should be proceeded through the similar reaction mechanism reported by Nozoe et al.

Tricyanovinyl-derivative 7 was synthesized by the electrophilic substitution reaction of 1 with TCNE (Scheme 2). TCNE is known as a strong organic electron acceptor, and the high reactivity of TCNE toward nucleophiles or electron-rich reagents is frequently used to introduce strong acceptor moieties into organic molecules.9 Previously, Hafner et al. reported the reaction between azulene and TCNE, giving 1-(1,2,2-tricyanoethenyl)azulene via CT complex between azulene and TCNE.10 We also reported the reaction of 1,6-di-tert-butylazulene with TCNE afforded the corresponding tricyanovinyl-derivative in excellent yield.11 Similar with azulene derivatives, the reaction of 1 with TCNE in ethyl acetate at 50 °C yielded the expected tricyanovinyl-derivative 7 in 80% yield.

Properties: The new compounds 2–7 were fully characterized by the spectral data as shown in the Experimental Section. Mass spectra of 2–7 ionized by ESI showed the correct molecular ion peaks, respectively. The stereochemistry for vinyl moiety of 3 and 4 was confirmed by NOE experiment. The characteristic stretching vibration band of the carbonyl group of 2–7 was observed at ν=1742–1773 cm-1 in their IR spectra, except for azulene derivative 6. Compounds 3–7 also exhibited characteristic C≡N stretching at ν=2197–2218 cm-1 in their IR spectra. These results are consistent with the structure of these products.
The UV/Vis spectra of cyanovinyl derivatives 3, 4, 5 and 7 in CH2Cl2 exhibited strong absorptions in the visible region, which may be attributable to the charge-transfer (CT) absorption band from the 2H-cyclohepta[b]furan-2-one ring to the substituted cyanovinyl moiety. Compound 3 exhibited two absorptions at λ=420 (sh) and 436 nm, respectively. Compound 4 showed broad absorptions at λ=426 nm. The longest wavelength absorption of 3 and 4 showed a bathochromic shift of 47 and 37 nm compared to that of 1 (λ=389 nm), probably due to expansion of the π-conjugation system by the cyanovinyl moiety. In compound 5, relatively strong absorptions was observed at λ=471 nm, which exhibited a further bathochromic shift compared to those of 3 and 4. Compound 7 also showed a strong absorption at λ=507 nm, compared to that of 5. The longest absorption of 3, 4, 5 and 7 displayed a bathochromic shift as the number of cyano group increased. This indicates that electron-withdrawing property of cyano group reduces the HOMO-LUMO gap of the molecule.

Solvatochromism is observed as a characteristic feature of dipolar molecules.12 A noticeable spectral feature of 7 is the presence of distinct absorption band at 507 nm in CH2Cl2, which blue-shifts to 494 nm in less polar hexane, suggesting the intramolecular CT nature of this band (Figure 1). It is assumed that the first excited-state has a larger dipole moment compared with that in the ground state due to the intramolecular CT character from the 2H-cyclohepta[b]furan-2-one to the tricyanovinyl unit as illustrated in Scheme 2. To elucidate the nature of the absorption bands of 7, density functional theory (DFT) calculation at the B3LYP/6-31G** level was carried out on optimized structure of 7.13 The frontier Kohn–Sham orbitals of 7 are shown in Figure 2. Judging from a comparison between the experimental and the theoretical UV/Vis spectra, the ICT of 7 originated from the HOMO located on 2H-cyclohepta[b]furan-2-one to the LUMO that was mainly located on the tricyanovinyl moiety.

Electrochemistry: To clarify the electrochemical behavior of cyanovinyl derivatives 3, 4, 5 and 7, redox potentials of these products were measured by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Measurements were carried out with a standard three-electrode configuration.
Tetraethylammonium perchlorate (0.1 M) in benzonitrile was used as a supporting electrolyte with platinum wire auxiliary and platinum disk working electrodes, respectively. All measurements were carried out under an argon atmosphere, and potentials were related to an Ag/Ag+ reference electrode and Fc/Fc+ as an internal reference, which discharges at +0.15 V. The redox potentials are summarized in Table 1.

Electrochemical reduction of 3 exhibited irreversible reduction wave at –1.44 V upon DPV. This result indicates the formation of unstable radical anionic states in the compound. Electrochemical oxidation of 3 also showed irreversible oxidation wave at +1.02 V due to the generation of radical cationic species upon DPV. Irreversible reduction and oxidation waves were observed in 4 at –1.48 V and +1.03 V upon DPV, respectively. Electrochemical reduction and oxidation of 5 also showed irreversible reduction and oxidation waves at –1.07 V and +1.25 V, respectively, owing to the generation of instable radical anionic and cationic species. Electrochemical reduction of 7 showed a reversible reduction wave at the half wave potential of –0.75 V by CV (Figure 3), probably due to the formation of a stabilized radical anion species. The electrochemical reduction also exhibited irreversible waves at –1.72 and –1.80 V by DPV, probably due to the reduction of the substituted 2H-cyclohepta[b]furan-2-one ring. The first reduction potential of 7 was less negative compared with those of 3, 4, and 5. These results reflect that the cyano-substituent on the vinyl moiety directly exerts for decrease of LUMO level of the molecule.

To examine the color changes during the electrochemical reactions, the spectral changes of cyanovinyl derivatives 3, 4, 5 and 7 were monitored by visible spectroscopy. Constant-current reduction was applied to solutions of 3, 4, 5 and 7, with a platinum mesh as the working electrode and a wire counter electrode, and visible spectra were measured in benzonitrile containing Et4NClO4 (0.1 M) as a supporting electrolyte at room temperature under electrochemical reduction conditions.
The colors of the solutions of 3 and 4 changed from orange to pale yellow during the electrochemical reduction. These changes were accompanied by increases in the broad absorption bands up to the near-infrared region. Reverse oxidation of these orange-colored solutions did not regenerate the visible spectra of the orange-colored 3 and 4, according to irreversibility upon CV. When the UV/Vis spectra of 5 were measured under electrochemical reduction conditions, the absorption of 5 at λ=470 nm in the visible region also gradually decreased, along with color-change from orange to yellow color. However, reverse oxidation did not regenerate the absorptions of 5, similar with 3 and 4. The longest absorption of 7 at λ= 507 nm gradually decreased and thus the color of the solution gradually changed from red to yellow during electrochemical reduction (Figure 4).

Reverse oxidation of the yellow-colored solution did not regenerate the spectrum of 7, although good reversibility was observed in the cyclic voltammogram. Recently, we reported the TCBD-substituted 2H-cyclohepta[b]furan-2-one exhibited a reversible color change between electrochemical reduction and oxidation.4 Thus, these irreversibility of color change suggest that cyanovinyl moiety is less stable toward the electrochemical reaction, compared to the TCBD group.
In summary, cyanovinyl-substituted 2H-cyclohepta[b]furan-2-one 3, 4, 5 and 7 were prepared by the Wittig, Knoevenagel and electrophilic substitution reaction, respectively. 2-Amino-1-cyanoazulene derivative 6 was also obtained by the reaction of 2 with malononitrile in EtOH. Analysis by CV and DPV showed that compound 7 exhibit reversible reduction wave, although compounds 3, 4 and 5 showed irreversible waves. Although the compounds 3, 4, 5 and 7 did not exhibit the reversible color change during the electrochemical reduction, significant color changes were observed during the electrochemical reduction. In attempts to improve the electrochromic properties, preparation of novel 2H-cyclohepta[b]furan-2-one-substituted multivalent charge-transfer chromophores with different π-electron cores is now in progress in our laboratory.

EXPERIMENTAL
General: Melting points were determined with a Yanagimoto MPS3 micro melting apparatus and are uncorrected. Mass spectra were obtained with a Bruker APEX II instrument. IR and UV/Vis spectra were measured with a Shimadzu FTIR-8100M and a Shimadzu UV-2550 spectrophotometer, respectively. 1H and 13C NMR spectra were recorded with a Bruker AVANCE 400 spectrometer (400 and 100 MHz). Voltammetry measurements were carried out with a BAS 100B/W electrochemical workstation equipped with Pt working and auxiliary electrodes, and a reference electrode formed from Ag/AgNO3 (0.01 M) in acetonitrile containing tetrabutylammonium perchlorate (0.1 M). Elemental analyses were performed at the Research and Analytical Center for Giant Molecules, Graduate School of Science, Tohoku University.

3-Formyl-5-isopropyl-2H-cyclohepta[b]furan-2-one (2): A mixture of phosphoryl chloride (2.32 g, 15.0 mmol) and DMF (10 mL) was added at room temperature to a solution of 1 (1.02 g, 5.43 mmol) in DMF (20 mL). The resulting solution was stirred at the same temperature for 2 h. The reaction mixture was poured into water and extracted with hexane. The organic layer was washed with brine, dried over MgSO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CH2Cl2 to give 2 (1.10 g, 94 %) as orange crystals. Mp 169.5−170.5 °C (MeOH); HRMS (ESI): Calcd for C13H12O3 + Na+ [M + Na]+ 239.0679. Found: 239.0677; IR (KBr disk): νmax = 2966 (w), 2932 (w), 2872 (w), 2820 (w), 2756 (w), 1773 (s), 1746 (s), 1653 (s), 1582 (s), 1524 (m), 1485 (s), 1466 (s), 1437 (m), 1404 (m), 1375 (m), 1298 (m), 1277 (s), 1242 (m), 1200 (m), 1092 (w), 1042 (w), 1011 (w), 943 (w), 891 (w), 816 (m), 760 (w), 746 (w), 725 (w), 654 (w), 637 (m) cm-1; UV/Vis (CH2Cl2): λmax (log ε) = 254 (4.29), 283 (4.40), 427 nm (4.42); 1H NMR (400 MHz, CDCl3): δH = 10.05 (s, 1H, CHO), 9.04 (d, 1H, J = 1.6 Hz, H-4), 7.60 (t, 1H, J = 9.2 Hz, H-7), 7.55 (dd, 1H, J = 9.2, 1.6 Hz, H-8), 7.42 (dt, 1H, J = 9.2, 1.6, 1.6 Hz, H-4), 3.13 (sept, 1H, J = 6.8 Hz, i-Pr), 1.37 (d, 6H, J = 6.8 Hz, i-Pr) ppm; 13C NMR (100 MHz, CDCl3): δC=186.09, 168.72, 165.50, 159.13, 151.71, 136.88, 136.30, 129.11, 119.94, 101.88, 39.73, 23.57 ppm; Anal. Calcd for C13H12O3·1/10H2O: C, 71.61; H, 5.64. Found: C, 71.86; H, 5.69.

(2E)-3-(5-Isopropyl-2-oxo-2H-cyclohepta[b]furan-3-yl)propenenitrile (3) and (2Z)-3-(5-Isopropyl-2- oxo-2H-cyclohepta[b]furan-3-yl)propenenitrile (4): A mixture of triphenylphosphine (1.21 g, 4.62 mmol) and bromoacetonitrile (505 mg, 4.21 mmol) in toluene (7 mL) was added at room temperature to a solution of 2 (438 mg, 2.03 mmol) and triethylamine (2 mL) in DMSO (6 mL). The resulting solution was stirred at the same temperature for 15 h under an Ar atmosphere. The reaction mixture was poured into water and extracted with EtOAc. The organic layer was washed with brine, dried over MgSO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CH2Cl2 and reversed-phase chromatography (ODS gel, 80% MeOH/H2O) to give 3 (227 mg, 47 %) as red crystals and 4 (246 mg, 51 %) as red crystals, respectively.
3: Mp 149.0−152.0 °C (MeOH); HRMS (ESI): Calcd for C15H13NO2 + Na+ [M + Na]+ 262.0838. Found: 262.0837; IR (KBr disk): νmax = 2959 (w), 2932 (w), 2907 (w), 2868 (w), 2212 (m), 1749 (s), 1720 (m), 1603 (s), 1585 (m), 1520 (s), 1497 (s), 1431 (w), 1408 (w), 1350 (w), 1325 (w), 1306 (w), 1261 (w), 1252 (w), 1236 (w), 964 (w), 824 (w), 800 (w), 743 (m) cm-1; UV/Vis (CH2Cl2): λmax (log ε) = 240 (4.32), 277 (4.28), 286 (4.29), 307 (3.91), 321 (3.79), 420 sh (4.39), 436 (4.41) nm; 1H NMR (400 MHz, CDCl3): δH = 7.43 (d, 1H, J = 1.2 Hz, H-4), 7.39 (d, 1H, J = 16.0 Hz, H-Vinyl), 7.23 (dd, 1H, J = 11.2, 9.2 Hz, H-7), 7.15 (dd, 1H, J = 9.2, 1.2 Hz, H-8), 7.05 (dd, 1H, J = 11.2, 1.2 Hz, H-6), 6.54 (d, 1H, J = 16.0 Hz, H-Vinyl), 2.97 (sept, 1H, J = 6.8 Hz, i-Pr), 1.33 (d, 6H, J = 6.8 Hz, i-Pr) ppm; 13C NMR (100 MHz, CDCl3): δC = 166.33, 159.61, 157.36, 147.65, 136.71, 134.41, 133.89, 123.49, 119.58, 116.52, 102.41, 96.24, 39.49, 23.21 ppm; Anal. Calcd for C15H13NO2: C, 75.30; H, 5.48; N, 5.85. Found: C, 75.01; H, 5.50; N, 5.81.
4: Mp 105.0−107.0 °C (MeOH); HRMS (ESI): Calcd for C15H13NO2 + Na+ [M + Na]+ 262.0838. Found: 262.0838; IR (KBr disk): νmax = 2980 (w), 2958 (w), 2936 (w), 2906 (w), 2867 (w), 2212 (m), 1742 (s), 1608 (m), 1586 (m), 1520 (m), 1498 (m), 1374 (w), 1316 (w), 1283 (m), 1260 (w), 1219 (w), 793 (w), 740 (w), 731 (w), 712 (w), 649 (w) cm-1; UV/Vis (CH2Cl2): λmax (log ε)=240 (4.41), 272 (4.21), 307 sh (3.71), 319 (3.56), 426 (4.30) nm; 1H NMR (400 MHz, CDCl3): δH = 7.41 (d, 1H, J = 1.2 Hz, H-4), 7.25 (dd, 1H, J = 11.2, 9.2 Hz, H-7), 7.17 (dd, 1H, J = 9.2, 1.2 Hz, H-8), 7.16 (d, 1H, J = 12.0 Hz, H-Vinyl), 7.07 (dd, 1H, J = 11.2, 1.2 Hz, H-6), 5.57 (dd, 1H, J = 12.0 Hz, H-Vinyl), 2.99 (sept, 1H, J = 6.8 Hz, i-Pr), 1.35 (d, 6H, J = 6.8 Hz, i-Pr) ppm; 13C NMR (100 MHz, CDCl3): δC=167.06, 159.63, 158.07, 147.46, 138.71, 134.06, 133.94, 126.40, 117.21, 116.11, 103.22, 97.24, 39.55, 23.31 ppm; Anal. Calcd for C15H13NO2·1/4H2O: C, 73.91; H, 5.58; N, 5.75. Found: C, 73.76; H, 5.51; N, 5.88.

2-(5-Isopropyl-2-oxo-2H-cyclohepta[b]furan-3-ylmethylene)malononitrile (5): A mixture of malononitrile (78 mg, 1.18 mmol) and pyridine (1 mL) was added at room temperature to a solution of 1 (234 mg, 1.08 mmol) in CH2Cl2 (5 mL). The resulting solution was stirred at the same temperature for 24 h under an Ar atmosphere. The reaction mixture was poured into water and extracted with CHCl3. The organic layer was washed with brine, dried over MgSO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CH2Cl2/EtOAc (20 : 1) to give 5 (135 mg, 51 %) as orange crystals. Mp 183.0−186.0 °C (CH2Cl2); HRMS (ESI): Calcd for C16H12N2O2 + Na+ [M + Na]+ 287.0791. Found: 287.0790; IR (KBr disk): νmax = 3013 (w), 2961 (w), 2929 (w), 2871 (w), 2218 (m, CN), 2210 (m, CN), 1758 (s), 1588 (m), 1563 (m), 1523 (w), 1477 (s), 1440 (w), 1348 (m), 1312 (w), 1286 (m), 1259 (m), 1218 (m), 936 (m), 814 (w), 793 (w), 760 (w), 739 (w), 720 (w), 604 (w) cm-1; UV/Vis (CH2Cl2): λmax (log ε) = 248 sh (4.36), 252 (4.40), 282 (4.12), 301 sh (3.89), 323 (3.83), 332 (3.81), 334 sh (3.81), 337 (3.83), 471 (4.54) nm; 1H NMR (400 MHz, CDCl3): δH = 7.77 (s, 1H, H-Vinyl), 7.75 (d, 1H, J = 1.2 Hz, H-4), 7.60 (dd, 1H, J = 10.8, 9.6 Hz, H-7), 7.51 (dd, 1H, J = 9.6, 1.2 Hz, H-8), 7.42 (dd, 1H, J = 10.8, 1.2 Hz, H-6), 3.14 (sept, 1H, J = 6.8 Hz, i-Pr), 1.41 (d, 6H, J = 6.8 Hz, i-Pr) ppm; 13C NMR (100 MHz, CDCl3): δC = 164.73, 164.18, 149.55, 147.88, 137.69, 136.26, 128.36, 120.09, 115.15, 113.21, 101.62, 100.52, 79.21, 40.15, 23.50 ppm; Anal. Calcd for C16H12N2O2: C, 72.72; H, 4.58; N, 10.60. Found: C, 72.68; H, 4.83; N, 10.31.

2-(2-Amino-3-cyano-7-isopropyl-azulen-1-ylmethylene)malononitrile (6): Malononitrile (133 mg, 2.01 mmol) in pyridine (2 mL) was added at room temperature to a solution of 2 (215 mg, 1.00 mmol) in EtOH (5 mL). The resulting solution was stirred at the same temperature for 24 h under an Ar atmosphere. The reaction mixture was poured into water and extracted with CH2Cl2. The organic layer was washed with brine, dried over MgSO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CH2Cl2/EtOAc (20 : 1) to give 6 (235 mg, 83 %) as brown crystals. Mp 260.0−263.0 °C (decomp.); HRMS (ESI): Calcd for C18H14N4 + Na+ [M + Na]+ 309.1111. Found: 309.1111; IR (KBr disk): νmax = 3464 (m), 3314 (w), 3061 (m), 2972 (w), 2958 (m), 2932 (w), 2870 (w), 2214 (s), 2197 (s), 1654 (s), 1588 (s), 1538 (s), 1509 (s), 1476 (m), 1448 (m), 1437 (s), 1414 (m), 1385 (m), 1366 (w), 1348 (w), 1329 (m), 1302 (m), 1284 (m), 1254 (m), 1222 (m), 1190 (w), 1021 (w), 898 (w), 795 (m), 777 (m), 662 (w), 544 (w), 531 (w), 523(w), 459 (w) cm-1; UV/Vis (CH2Cl2): λmax (log ε) = 256 (4.37), 292 (4.17), 318 (4.15), 351 sh (4.76), 365 (4.92), 399 (3.90), 425 (3.79), 446 sh (3.47), 516 sh (2.67), 559 sh (2.40) nm; 1H NMR (400 MHz, CDCl3): δH = 8.67 (s, 1H, H-Vinyl), 8.45 (d, 1H, J = 1.6 Hz, H-4), 8.43 (dd, 1H, J = 10.0, 1.6 Hz, H-8), 7.69 (dd, 1H, J = 10.0, 1.6 Hz, H-6), 7.64 (t, 1H, J = 10.0 Hz, H-7), 5.79 (bs, 2H, NH2), 3.23 (sept, 1H, J = 6.8 Hz, i-Pr), 1.44 (d, 6H, J = 6.8 Hz, i-Pr) ppm; 13C NMR (100 MHz, CDCl3): δC = 161.42, 160.60, 155.00, 150.51, 139.27, 138.90, 136.32, 134.25, 134.12, 132.75, 118.67, 117.13, 115.09, 89.98, 88.65, 39.34, 24.66 ppm; Anal. Calcd for C18H14N4: C, 75.50; H, 4.93; N, 19.57. Found: C, 75.30; H, 5.20; N, 19.52.

2-Cyano-3-(5-isopropyl-2-oxo-2H-cyclohepta[b]furan-3-yl)-but-2-enedinitrile (7): To a solution of 1 (218 mg, 1.16 mmol) in EtOAc (4 mL) was added TCNE (168 mg, 1.25 mmol). The resulting mixture was stirred at 50 °C for 2 h under an Ar atmosphere. The solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel with CH2Cl2/EtOAc (10 : 1) and Bio-Beads with CH2Cl2 to give 7 (268 mg, 80 %) as red crystals. Mp 150.0−152.0 °C (CH2Cl2); HRMS (ESI): Calcd for C17H11N3O2 + Na [M + Na]+ 312.0743. Found: 312.0744; IR (KBr disk): νmax = 2978 (w), 2961 (w), 2928 (w), 2226 (m), 2208 (w), 1767 (s), 1578 (m), 1518 (s), 1479 (s), 1439 (s), 1406 (m), 1366 (w), 1340 (m), 1317 (m), 1306 (m), 1277 (s), 1259 (m), 1238 (m), 1205 (m), 1138 (w), 1090 (w), 1022 (w), 939 (w), 912 (w), 806 (m), 766 (m), 721 (w), 673 (w), 559 (w), 500 (w) cm-1; UV-Vis (CH2Cl2): λmax (log ε) = 242 (4.33), 264 (4.30), 283 sh (4.14), 300 sh (3.96), 320 sh (3.62), 362 (3.67), 507 (4.66) nm; UV-Vis (hexane): λmax (log ε) = 242 (4.22), 260 (4.19), 283 sh (3.96), 315 sh (3.55), 345 (3.41), 362 (3.46), 494 (4.46) nm; 1H NMR (400 MHz, CDCl3): δH = 8.04 (d, 1H, J = 1.6 Hz, H-4), 7.79 (dd, 1H, J = 10.4, 9.2 Hz, H-7), 7.70 (dd, 1H, J = 9.2, 1.2 Hz, H-8), 7.62 (dd, 1H, J = 10.4, 1.2 Hz, H-6), 3.19 (sept, 1H, J = 6.8 Hz, i-Pr), 1.43 (d, 6H, J = 6.8 Hz, i-Pr) ppm; 13C NMR (100 MHz, CDCl3): δC = 166.26, 162.24, 159.79, 149.31, 139.29, 138.22, 130.21, 128.61, 122.00, 112.78, 112.17, 111.11, 97.28, 89.34, 40.34, 23.68 ppm; Anal. Calcd for C17H11N3O2: C, 70.58; H, 3.83; N, 14.53. Found: C, 70.45; H, 4.10; N, 14.49.

ACKNOWLEDGEMENTS
This work was partially supported by a Grant-in-Aid for Research Activity Start-up (Grant 22850007 to T.S.) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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13. The B3LYP/6-31G** time-dependence density functional calculations were performed with Spartan’10, Wavefunction, Irvine, CA.
14. Redox potentials were measured by CV and DPV [in V vs. Ag/AgNO3, 1mM in benzonitrile containing Et4NClO4 (0.1 M), Pt electrode (i.d. 1.6 mm), scan rate 100 mVs–1, and Fc/Fc+ = +0.15 V]. In the case of reversible waves, redox potentials measured by CV are presented. The peak potentials measured by DPV are shown in parentheses.