HOMOALLYL AND CYCLOPROPYLCARBINYL CARBONIUM-ION FORMATIONS UNDER STRONGLY BASIC CONDITIONS

Treatment of trons-2,8-dibromocyclooctanone ethylene ketal with sodium hydroxide in refluxing methanol produced 2,7-cyclooctadienone ethylene ketal in yields of 71-79% and a mixture of 2-(2-cycloheptenyl)-2-methoxy-1,3-dioxolane (15) and 2-(exo-7-bicyclo[4.1.0]heptyl)-2-methoxy-1,3-dioxolane (16). The structures of the ortho ester side products were deduced from spectral and chemical methods. It was shown that the precursor of 15 was 2-bromocyclooct-7-enone ethylene ketal (18), and it was postulated that the intermediate which went on to 16 was 8-bromobicyclo[4.2.0]octan-7-one ethylene ketal (25). For the conversion of trans-2,7-dibromo-cycloheptanone ethylene ketal into the corresponding diene ketal, 2-(2-cyclohexenyl)-2-methoxy-1,3-dioxolane was found as a side product but not 2-(exo-6-bicyclo[3.1.0]hexyl)-2-methoxy-1,3-dioxoiane. No rearrangement products were found with eliminations involving the ethylene ketals of cts-2,6-dibromo-4,4-dimethylcyclohexanone or meso-3,5-dibromo-4-heptanone. The involvement of intermediate carbonium ions is discussed. © 1979, American Chemical Society. All rights reserved.