Claisen rearrangement of tropolone ethers. Part III

Abstract

Pairs of allyl and prop-2-ynyl ethers derived from three 3-substituted tropolones have been prepared and structurally distinguished, and their thermal isomerisation products have been identified. Rearrangements of the 7-substituted tropolone ethers generally proceed via conventional [3,3] sigmatropic migration, but the 3-substituted isomers afford products resulting from both [3,3] and apparent [3,7] sigmatropic shifts, the proportion of each depending on the nature of the substituent. Kinetic evidence suggesting the operation of a direct [3,7] sigmatropic transfer has been obtained in the case of 3-bromo-2-(prop-2-ynyloxy)tropone.