A no. of model substrates, e.g., I (X = Br, iodo) (anti/cis), were submitted to Zn-promoted fragmentation and the compn. of the resulting mixt. of isomeric fluorodienes was detd. The exocyclic reaction center, bearing the electrofugal leaving group, undergoes almost complete stereochem. randomization, while ring opening proceeds stereoselectively. The stereochem. results are related to changes in electron configuration occurring during the reaction. [on SciFinder (R)]