ALKYNE REACTIONS WITH THE CYCLOPALLADATED 8-METHYLQUINOLINE LIGAND - SYNTHESIS OF NOVEL HETEROCYCLIC-COMPOUNDS WITH A BRIDGEHEAD NITROGEN

The stepwise reaction of cyclopalladated 8-methylquinoline (compound 1, obtained via intramolecular metalation of 8-methylquinoline by Pd(II)) with 2 equiv of various internal alkynes affords 4H-indolo[2,1,7-cde]quinolizines 3 through simultaneous Pd-mediated C-C and C-N bond formation. This reaction displays a high selectivity provided that the first alkyne reacting with 1 bears two electron-withdrawing substituents such as -CF3 or -CO2Me and that the second alkyne is also substituted by electrophilic substituents. Varying the reaction conditions allowed us to isolate organometallic intermediates, whose nature shed some light upon the mechanism of the formation of 3. Several deviations from the synthesis of 3 were encountered when different alkynes were reacted with 1, e.g., with ethyl-3-phenylpropynoate a four-substituted alpha-pyrone was synthesized through C-O activation of one of the ester functions.