Selective Cβ-O Bond Cleavage of Chalcone Epoxides Induced by Pyrylium Salt-Sensitized Photoreactions and Dark Reactions withCerium( IV) Salts.-Irradiation of the epoxides (I) using a Hg-Xe lamp and a glass filter ( λ ¿ 390 nm) in the presence of 2,4,6-triphenylpyrylium tetrafluoroborate yields the β-keto aldehydes (II). In methanol ( III), the adducts (IV) are obtained. A single electron transfer-induced ring opening is assumed which is supported by quenching experiments. With Ce(IV) salts as the oxidants, the ring opening occurs in the dark. Thus, (Ia) reacts with (NBu4)2(Ce(NO3)6) in acetonitrile to form the nitrate ester (VI), whereas in methanol (III), the adducts (IV) are the main reaction products. Catalytic amounts of the cerium salt are sufficient for this reaction. -(HASEGAWA, E.; ISHIYAMA, K.; KASHIWAZAKI, H.; HORAGUCHI, T.; SHIMIZU, T.; Tetrahedron Lett. 31 (1990) 28, 4045-4048; Dep. Chem., Fac. Sci., Niigata Univ., Ikarashi, Niigata 950-21, Japan; EN)
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