HETEROCYCLES

Communication | Regular issue | Vol. 83, No. 10, 2011, pp. 2271-2274
Received, 11th July, 2011, Accepted, 11th August, 2011, Published online, 18th August, 2011.
DOI: 10.3987/COM-11-12303

■ The [2+2] Cycloaddition Reaction of Ethynylated 2H-Cyclohepta[b]furan-2-ones with 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone

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

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

Abstract

Ethynylated 2H-cyclohepta[b]furan-2-ones reacted with 2,3-dichrolo-5,6-dicyano-1,4-benzoquinone (DDQ) in a formal [2+2] cycloaddition reaction to afford the corresponding DDQ-adducts 10–15 in good yields.

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is well known as an oxidizing reagent by the formal dehydrogenation reaction in organic synthesis.1 Recently, Diederich2 and Trofimov3 have reported [2+2] cycloaddition reaction of DDQ with various alkyne derivatives to give the corresponding cycloadducts. They have also reported the unusual reactivities and properties of the DQQ-adducts, such as rearrangement of the carbon skeleton,2,4 intramolecular charge-transfer absorption and third-order optical nonlinearities.2 These results show the DQQ-adducts have a potential as a new series of advanced organic materials.
2H-Cyclohepta[b]furan-2-one is known as a heteroazulene, a versatile precursor for azulene derivatives.5 However, there are few derivatives for the application to the advanced materials with potentially useful electronic properties. For the extension of this chemistry, we have focused our studies on the synthesis of 2H-cyclohepta[b]furan-2-one-substituted tetracyanobutadiene (TCBD) derivatives by the [2+2] cycloaddition reaction of the corresponding alkynes with tetracyanoethylene.6 The study revealed the TCBD derivatives are featured by a strong ICT absorption in their UV/Vis spectra. Arylethynyl-2H-cyclohepta[b]furan-2-ones 1–9 are also expected to show high reactivity with DDQ toward the [2+2] cycloaddition reaction to afford the novel DDQ-adducts due to the electron-donating nature of the 2H-cyclohepta[b]furan-2-one moieties. Moreover, the DDQ-adducts with 2H-cyclohepta[b]furan-2-one substituents are also expected to show the third-order optical nonlinearities.
Herein, we describe the synthesis of the DDQ-adducts with 2H-cyclohepta[b]furan-2-one substituents by the formal [2+2] cycloaddition reaction of arylethynyl-2H-cyclohepta[b]furan-2-ones 1–9 with DDQ.
The arylethynyl-2H-cyclohepta[b]furan-2-ones 1–9 were prepared by Sonogashira–Hagihara reaction, according to the procedure reported by us, recently.5 To the synthesis of novel DDQ-adducts, [2+2] cycloaddition reaction of ethynyl substituted 2H-cyclohepta[b]furan-2-ones 1–9 with DDQ was examined according to the procedure described in the literature.2,3 General experimental procedure was shown in the section of REFERENCE AND NOTES.7 The yield of the products was summarized in Table 1.

The reaction of 1 with DDQ in refluxing ethyl acetate yielded 10 in 83% yield. Acetylene derivatives 2 and 3 were also readily reacted with DDQ to afford the corresponding cycloadducts 11 and 12 in 70% and 84% yields, respectively, as similar to the reaction of 1 with DDQ. In the reaction of acetylene derivatives with TCNE, the [2+2] cycloaddition with an alkyne moiety afford the cyclobutene intermediate, following to the ring-opening reaction of the cyclobutene derivative gives the thermodynamically stable TCBDs.5c,6 Stability of cyclobutene in DDQ-adducts might be attributable to the structural effect of fused cyclohexene ring. To investigate the effect of the substituent on the benzene ring, the reaction of 4, 5 and 6, which possess electron-withdrawing and electron-donating aryl groups, respectively, on the acetylene terminal, were examined under similar reaction conditions. Although the cycloaddition reaction of 4 with DDQ afforded the corresponding product 13 in 74% yield, reaction of 5 and 6 gave inseparable complex mixtures instead of the desired cycloaddition products. Furthermore, compounds 5 and 6 did not react with DDQ at room temperature and starting materials were completely recovered. Similar with 5 and 6, ferrocene-substituted alkyne 7 did not generate the presumed cycloaddition product by the reaction with DDQ in ethyl acetate at the refluxing temperature. These results indicate the highly electron-donating aryl group on the acetylene terminal should be prevented the desired [2+2] cycloaddition reaction of acetylene moiety with DDQ. The [2+2] cycloaddition reaction of thiophene-substituted alkyne 8 with DDQ also afforded the corresponding cycloadduct 14 in 76% yield, under the similar reaction conditions. The reaction of symmetrically substituted alkyne 9 with DDQ yielded the corresponding cycloadduct 15 in 59% yield. These novel DDQ adducts 10–15 are obtained as stable crystals and can be storable in the crystalline state under ordinary conditions.

These new compounds 10–15 were fully characterized by the spectral data. Mass spectra of 10–15 ionized by ESI showed the correct molecular ion peaks observed as [M + Na]+ ion peaks. The characteristic stretching vibration band of the C≡N moieties of 10–15 was observed at ν = 2191–2225 cm-1 on their IR spectra, instead of the characteristic stretching band for C≡C triple bond of the starting acetylene derivatives. The UV/Vis spectra of 3 and 12 in dichloromethane are shown in Figure 1. The absorption maxima of the acetylene derivative 3 showed a relatively strong absorption band at 426 nm.66 On the other hand, the DDQ-adduct 12 (λmax = 418 nm) showed hypsochromic shift by about 8 nm, compared with that of 3. The blue-shift of 12 might be ascribed to the less effective conjugation owing to the low planarity between 2H-cyclohepta[b]furan-2-one and aryl moieties.

In conclusion, the novel DDQ-adducts with 2H-cyclohepta[b]furan-2-one substituents 10–15 were synthesized by the [2+2] cycloaddition reaction of the arylethynyl-2H-cyclohepta[b]furan-2-ones 1–9 with DDQ in good yields, although the electron-rich arene-substituted 5–7 did not afford the presumed cycloaddition products. To evaluate the value of the novel DDQ-adducts, reactivities, spectroscopic, electrochemical and theoretical aspects of these new compounds are currently progress in our laboratory.

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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7. To a solution of 1–9 in AcOEt was added 1.1–1.6 equiv. of DDQ. The resulting mixture was refluxed for 1–18 h under an Ar atmosphere. The solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel to give corresponding DDQ-adducts 10–15 (The yield of the products is shown in Table 1).