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Oxidative rearrangement of 2′-hydroxychalcones having no substituent at the 3′- and 5′-positions with thallium(III) nitrate in methanol

Oxidation of 2′-hydroxychalcones with no substituents at the 3′-and 5′-positions with thallium(III) nitrate (TTN) in methanol has been studied in detail and the following results obtained. (1) 2′-Hydroxy-4,6′-dimethoxychalcones (1b and 2b) have increased reactivity over their methyl ethers as a result of participation of the 6′-methoxy oxygen atom; the reactivity of 2′-hydroxy-4,4′-dimethoxychalcone 3b however was decreased. (2) The reactions were greatly affected by the substituents on the A and B rings and formed the corresponding 1,2-diaryl-3,3-dimethoxypropan-1-ones (acetal) and/or 2-(α-methoxybenzyl)coumaranones (coumaranone): the oxidative rearrangement was greatly accelerated by methoxy groups at the 4- and/or 2-positions to give an acetal as a main product. (3) Formation of the coumaranone was observed only when the 2′-hydroxychalcones had a methoxy group at the 6′-position and a B ring with weaker electron-donating nature. (4) The ratio of the coumaranones formed in the reaction of 2 with no substituent at the 4′-position was lower than that of 1 which formed quickly a cyclic TTN complex between the 2′-hydroxy and the neighbouring carbonyl groups. Only the reaction of 2′-hydroxy-6′-methoxychalcone 2a afforded the acetal and coumaranone together with a large amount of thallium compounds which were converted into a mixture of the corresponding isoflavone and aurone by treatment with hydrochloric acid. From these results, the mechanism of the reaction was proposed as shown in Schemes 2 and 3.