Fluoro-Analoga von Cycloheptadien-Pheromonen mariner Braunalgen: ungewöhnlich leicht verlaufende [3,3]-sigmatrope Umlagerung eines trans-di(alkenyl)-substituierten geminalen Difluorocyclopropans

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.