Claisen rearrangement of prenyl ethers of isomeric acetylnaphthols and bisprenyl ethers of 4,6- and 2,4-diacetylresorcinols

Abstract

Claisen rearrangements of the 3-methylbut-2-enyl(prenyl) ethers 3 and 9 of 2-acetyl-1-hydroxyand 1-acetyl-2-hydroxynaphthalenes 1 and 2 and the bis(3-methylbut-2-enyl) ethers 12 and 18 of 2,4-and 4,6-diacetylresorcinols 11 and 17 have been studied under a variety of thermal and catalytic conditions. 2-Acetyl-4-(3- methylbut-2-enyl) naphthalene-1-ol 4 was the sole product on rearrangement of compound 3, in DMA or neat. Under catalytic conditions 3,4-dihydro-2,2-dimethyl-2H-naphtho[1,2-b]pyran 7 was obtained in poor yield. An isomeric pyran, 1,2-dihydro-3,3-dimethyl-3H-naphtho[2,1-b]pyran 10(25–43%) was obtained under both thermal and catalytic conditions from 9. The rearrangement of 12 under thermal conditions (DMA and neat) furnished 3-acetyl-2,4-dihydroxy5-(3-methylbut-2-enyl)acetophenone 13 and 3,8-diisopropylbenzo[1,2-b: 3,4-b′]difuran 14. Pd^II mediated rearrangement of 12 gave only the partially deprenylated ether, 3-acetyl-4-hydroxy-2-(3-methylbut-2-enyloxy)acetophenone 15(29%). While the isomeric ether 18 gave the monoprenyl ether, 5-acetyl-4-hydroxy-2-(3-methylbut-2-enyloxy)acetophenone 19 in quantitative yield (95%). The rearrangement products were characterised and their formation rationalised in terms of allowed sigmatropic shifts ([3,3] prenyl and [1,5] H) followed by loss of prenyl or acetyl groups. The reactions of prenyl ethers are both comparable to those of the corresponding allyl ethers and consistent with the greater fixed double-bond character of the naphthalene system compared to that of benzene. Prenyl derivatives of benzene and naphthalene containing ortho-hydroxyacetophenone units were formed which have potential in synthesis.