The rearrangement of chrysanthemyl alcohol in fluorosulphuric acid

Solution of chrysanthemyl alcohol [1; 2,2-dimethyl-3-(2-methylprop-1 -enyl)cyclopropylmethanol] in fluorosulphuric acid-sulphur dioxide at –78 °C resulted in the formation of the 2-methyl-2(3-oxa-bicyclo[3,1,0]hex-4-yl)-ethan-2-ylium ion (4). On quenching, this species underwent extensive decomposition, yielding a complex mixture of products, of which the main components were 2,2,5,5-tetramethyl-3-vinyltetrahydrofuran (5), 3,3-dimethyl-5-(2-methylprop-2-enyl)tetrahydro-furan (6), 6,6-dimethyl-5-isopropyl-2,3-dihydropyran (7), 3,3-dimethyl-5-(2-methylprop-1-enyl)-tetrahydrofuran (8), and 2,2,6-trimethyl-3,5-heptadien-1-ol (9). Routes to the formation of these products via the cyclobutonium ions derived from (4) are discussed. The reason why extensive rearrangement occurs during quenching, in contrast with the more usual formation of unrearranged products, is also discussed.