12 PHOTOREARRANGEMENT OF 3-HYDROXYFLAVONES
Details
A novel photorearrangement of 3-hydroxyflavones (I) leading to 3-aryl-3-hydroxy-1, 2-indandiones (II) is described. Irradiation of 3-hydroxy-2'-methoxyflavone (Ia) in benzene-isopropyl alcohol with ultraviolet light (>290nm) gave a crystalline isomer, 3-hydroxy-3-(2'-methoxy-phenyl)-1, 2-indandione (IIa), the structure of which was confirmed by a synthesis of its phenylenediamine adduct (IIIa). Under similar conditions, 3-hydroxyflavone (Ib), 3-hydroxy-4'-methoxyflavone (Ic) and 3-hydroxy-5, 7, 3', 4'-tetramethoxyflavone (Id) gave corresponding 3-aryl-3-hydroxy-1, 2-indandiones, IIb, IIc and IId, respectively, which were characterized as o-phenylenediamine adducts, IIIb, IIIc and IIId. Quercetin (Ie) was recovered unchanged under these conditions. On irradiation under nitrogen 3-methoxyflavone (VI) gave a dehydrocyclization product VII. Possible mechanisms for this photorearrangement are discussed in term of the formation of 3-aryl-2, 3-epoxy-2-hydroxy-1-indanone intermediate IX.
