Thermal Claisen rearrangement of allyl ethers of isomeric acetylnaphthols

Abstract

Thermal Claisen rearrangement of the allyl ethers (3) and (12) of the isomeric 2-acetyl-1-hydroxy- and 1-acetyl-2-hydroxy-naphthalenes (1) and (2) has been studied by heating each in refluxing N,N-dimethylaniline (DMA) and neat at 185 °C under an N₂ atmosphere. 2-Acetyl-4-allyl-1-hydroxynaphthalene (4), the sole product isolated from (3), was isolated in 56 and 88% yield respectively under the two sets of conditions. The rearrangement of the allyl ether (12) gave 3-acetyl-1-allyl-2-hydroxynaphthalene (13)(18%) and 2-acetoxy-1-allylnaphthalene (14)(23%) in N,N-DMA while in the neat rearrangement it gave (14) as the major product (70%) along with 1-allyl-2-hydroxynaphthalene (17)(20%). The rearrangement of the allyl ether (7), obtained on further allylation of (4), furnished 2,4-diallyl-1-hydroxynaphthalene (8), (15%) and (4)(15%) in DMA while it gave 1-acetoxy-2,4-diallylnaphthalene (9) as the sole product (67%) in the neat rearrangment. The rearrangement of the allyl ether (15), obtained by further allylation of (13), in DMA gave 3-acetyl-1,1-diallylnaphthalen-2(1H)-one (16)(20%) along with (13)(6%). The various rearrangement products have been characterised by their spectroscopic data (UV, IR, ¹H NMR, and EI-MS) and the reactions rationalised by the allowed sigmatropic shifts such as [3,3] allyl, [1,5] acetyl followed by loss of allyl or acetyl groups. The significant differences noticed in the reactions of the isomeric series of allyl ethers are consistent with the greater fixed double-bond character of the naphthalene system compared to that of benzene as well as the positional isomerism that exists in the allyl ethers. The above Claisen rearrangement studies result in the isolation of some interesting naphthylallyl derivatives with orhto-hydroxyacetophenone units which form useful synthetic intermediates for preparing several naphthyl-α- and γ-pyrone derivatives of pharmacological activity.