Melting points are uncorrected. IR spectra were recorded with a Hitachi 26-50 spectrometer, NMR spectra with a Varían EM 360A spectrometer, and 13C NMR with a JEOL JNM FX100 spectrometer (solvent, deuteriochloroform; tetramethylsilane as internal standard), and UV spectra with a Shimazu UV 365 spectrometer. GLC analysis was performed by using a Hitachi 164 gas chromatograph with a FID detector. Cyclic voltammograms of substrates were obtained in 0.1 M n-Bu4NC104/CH3CN solution (vs. SCE; scan rate, 200 mV/s; Hokuto Denko Ltd., Potentiostat/galvanostat HA-301). The light source was a Xenon lamp ( > 460 nm) through a glass filter (Y-48, HOYA Co.).Irradiations were carried out through a 2-mm slit while passing oxygen.Diphenylisobenzofuran (DPBF, Tokyo Kasei) was used as received. Its purity was confirmed by the lack of a carbonyl absorption in the IR spectrum, and GLC analysis showed that no trace of o-dibenzoylbenzene (DBB) was present. 2-Methyl-2-pentene (2-MP, Wako Pure Chemicals) was distilled from sodium hydride to remove alcoholic impurities, and its purity was confirmed by GLC analysis.Pyran 1 was prepared as follows: 200 mL of ethanol solution of 4-(formylmethylidene)-2,6-diphenyl-4íí-pyran14 (1.8 mmol) and MiV-dimethylhydrazine (18 mmol) was refluxed for 30 min under an nitrogen atmosphere. The deposited precipitates after cooling were collected and recrystallized from ethanol: 84% yield; mp 279-280 °C; UV Xmax 406 nm (log e 4.53); NMR S 2.93 (s, 6H), 5.83 (d, 1 ), 6.41 (d, 1 H), 6.79 (d, 1 ), 7.42-8.0 (m, 11 H);13C NMR 43.27 (q), 101.51 (d), 108.03 (d), 112.19 (d), 150.3 (s),