Lewis Acid Mediated Claisen-Type Rearrangement of Aryl Dienyl Ethers

The Claisen rearrangement and its applications to organic synthesis have been studied for a long time.l In recent years, this rearrangement has been applied in stereocontrolled natural product synthesis2 and asymmetric i n d ~ c t i o n . ~ The term "Claisen rearrangement", which originally denoted the rearrangement of allyl aryl ethers to 0or p-allylphenols, has now been extended to analogous rearrangements of allyl vinyl ethers (IrelandClaisen rearrangement),3b,4 N-allylenamines and N-allylanilines (aza-Claisen rearrangement),3av5 and allyl vinyl sulfides (thio-Claisen rearrangement)! Experimental and theoretical studies of these reactions have been carried out.' Several methods have been used to induce these rearrangements, including therma1,l acid catalysis: anion catalysis (especially for the Ireland-Claisen rearrangement): and transition metal cataly~is .~ Only one examplelo of the thermal rearrangement of an aryl 2,4-pentadienyl ether has been reported, and with low selectivity, giving a mixture of [5,5] and [3,3] rearrangement products (Table I, entry 1). The regioselective dienylation of an aromatic ring could be very useful in organic synthesis,l' and we have therefore explored the regioselectivity of the rearrangement of aryl 2,4-pentadienyl ethers in the presence of Lewis acids.