Thermal Claisen rearrangement studies on 4,6- and 2,4-diacetylresorcinol bisallyl ethers: observation of loss or [1,5] sigmatropic shift of acetyl groups

Abstract

Thermal Claisen rearrangement of 4,6-diacetylresorcinol bisallyl ether (2) in N,N-dimethylaniline gave a mixture of readily characterised products. While no rearrangement occurred with lower boiling solvents (benzene and dioxane), higher boiling solvents (diphenyl ether and glycerol) gave rise to more complex rearrangement and a lowering of the yields of isolable products. Trifluoroacetic acid both at room temperature and 60 °C effected either partial or total deallylation but no rearrangement. Product formation has been rationalised in terms of symmetry allowed sigmatropic [3,3] allyl, [1,5] acetyl or [1,5] H shifts followed by allyl or acetyl group loss; the latter is a novel observation. The acetyl group, most probably eliminated as a cation, effected both O-acylation and nuclear acylation of the substrates. Claisen rearrangement of the bromo and nitro derivatives of compound (2) in which the ortho and para positions are blocked, gave products arising from bromo and nitro group elimination. Rearrangement of 2,4-diacetylresorcinol bisallyl ether (18) in N,N-dimethylaniline occurred similarly.