FLUORO ANALOGA OF CYCLOHEPTADIENE PHEROMONES OF MARINE BROWN-ALGAE - EXCEPTIONALLY FACILE [3,3]-SIGMATROPIC REARRANGEMENT OF A TRANS-DI(ALKENYL)-SUBSTITUTED GEMINAL DIFLUOROCYCLOPROPANE

The 3,3-difluorocyclopropane 1,2-dicarbonyl compounds 9a and 9b are obtained by addition of difluorocarbene to methyl cinnamate or trans-stilbene followed by oxidative degradation of the phenyl rings(s) (Scheme 2). Compound 9b is a versatile building block for the synthesis of the di(alkenyl)-substituted geminal difluorocyclopropane 5 or the chrysanthemum ester 10b. Cyclopropane 5 is unstable at room temperature and rearranges to the 3,3-difluorocyclohepta-1,4-diene 4, an analog of the algal pheromone dictyotene (1; Scheme 3). The kinetic parameters of the [3,3]-sigmatropic rearrangement are determined. The data support the view that a geminal difluoro group within a cyclopropane weakens the distal bond.