Lewis or brønsted acid provoked rearrangements in ortho-(1,1-dimethylpropenyl)phenols
摘要:
Treatment of the o-(1,1-dimethylpropenyl)phenol, dihydrolicochalcone A, with a Bronsted acid affords two isomeric 2,2-dimethyl-4H-dihydropyrano[b]benzenes. The rearrangement probably involves a [3,5] charge-accelerated rearrangement. Attempts to provoke the same reaction in licochalcone A only afforded poorly defined degradation products. Treatment of dihydrolicochalcone A as well as licochalcone A with Lewis acids afforded 2,3,3-trimethyldihydro-furano[b] benzenes and 2,2,3-trimethyldihydrofurano[b]benzenes. Proper use of solvents and Lewis acid enables preferential formation of the kinetically favoured product or the thermodynamically more stable product. (C) 1997 Elsevier Science Ltd.