CATION-STABILIZING AUXILIARIES IN POLYENE CYCLIZATIONS .4. THE FLUORINE ATOM AS A CATION-STABILIZING AUXILIARY IN BIOMIMETIC POLYENE CYCLIZATIONS .1. BACKGROUND AND EXPLORATORY EXPERIMENTS

With the aim of testing the fluorine atom as a potential cation-stabilizing (C-S) auxiliary for enhancing polyene cyclizations, new methodology has been developed for the stereoselective synthesis of polyenic cyclization substrates in which the vinyl H of an E-trisubstituted olefinic bond is replaced by F. Thus the substrate 5 was produced, as suggested by formulas 13-24, the Claisen rearrangements 15 --> 17 and 18 --> 19 being the key steps for producing the vinyl fluoride moiety. SnCl4-catalyzed cyclization of substrate 5 gave a relatively high yield of a complex mixture of tetracyclic materials indicating that, in principle, the fluorine atom can serve effectively as a C-S auxiliary. The major product of the cyclization was proved by X-ray crystallography to be 25, resulting from dehydrofluorination of compound 6. A fluorine-containing substance was isolated in low yield, and it most likely was the compound 6, because when treated with the Ohsawa-Oishi reagent (Na/K, crown ether, toluene) it was converted into a product which appeared to be identical with compound 2. That this reduction is synthetically useful and does, indeed, proceed stereoselectively with retention of configuration has been confirmed unequivocally in another example described in the next paper of this series.