OXIDATIVE REARRANGEMENT OF 2'-HYDROXYCHALCONES HAVING NO SUBSTITUENT AT THE 3'-POSITION AND 5'-POSITION 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 stud ied in detail and the following results obtained, (1) 2'-Hydroxy-4,6'- dimethoxychalcones (1b and 2b) have increased reactivity over their me thyl ethers as a result of participation of the 6'-methoxy oxygen atom ; the reactivity of 2'-hydroxy-4,4'-dimethoxychalcone 3b however was d ecreased, (2) The reactions were greatly affected by the substituents on the A and B rings and formed the corresponding 1,2-diaryl-3,3-dimet hoxypropan-1-ones (acetal) and/or 2-(alpha-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 mai n product, (3) Formation of the coumaranone was observed only when the 2'-hydroxychalcones had a methoxy group at the 6'-position and a B ri ng with weaker electron-donating nature, (4) The ratio of the coumaran ones formed in the reaction of 2 with no substituent at the 4'-positio n was lower than that of 1 which formed quickly a cyclic TTN complex b etween the 2'-hydroxy and the neighbouring carbonyl groups, Only the r eaction of 2'-hydroxy-6'-methoxychalcone 2a afforded the acetal and co umaranone together with a large amount of thallium compounds which wer e 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.