Synthesis of a 4-acylcyclohexa-2,5-dienone: 3,4-dihydro-3,3,8a-trimethyl-naphthalene-1,6(2H,8aH)-dione
Abstract
The synthesis is reported of 3,4-dihydro-3,3,8a-trimethylnaphthalene-1,6(2H,8aH)-dione (11; R = Me). This is the first isolated 4-acylcyclohexa-2,5-dienone, a class of compounds postulated particularly as intermediates in the Fries and photo-Fries rearrangements. The dienone undergoes very easy acyl cleavage in the presence of bases or dilute acids, to form a phenolic acid (18). Similar cleavage, or a retro-Fries migration of the acyl group from the 4-position to the dienone ring oxygen, has prevented isolation of 4-acetyl-4-methylcyclohexa-2,5-dienone. Concentrated aqueous sulphuric acid causes rearrangement of the dienone to 3,4-dihydro-8-hydroxy-3,3,5-trimethylnaphtbale-1 (2H)-one (13; R = Me), via recyclisation of (18). In trifluoroacetic acid solutions the same product (13; R = Me) is formed from the dienone by direct, formal dienone–phenol rearrangement.