CHELATION CONTROL BY A 2ND NITROGEN ATOM IN FORMAL HETERO-DIELS-ALDERREACTIONS OF N-ARYLIMINES

Lewis acid-catalyzed cyclization of proline-derived N-arylimines 8 giv es the tetracyclic hydro-7,7,11-trimethyl-indolizino[3,4-b]quinolines 17a, b in a ratio, which is highly dependent on the coordination numbe r of the acidic catalyst. Whereas monodentate Bronsted and Lewis acids as well as octahedral bidentate Lewis acids preferably gave the all-t rans product 17b, the all-cis product 17a was obtained by tetrahedral aluminium Lewis acids. In contrast to this chelation-controlled revers al of diastereoselectivity, mixtures of cis/trans o-6,6,10-trimethyl-1 1-azacyclopenta[a]-anthracenes 18b-e, h were isolated from the cycliza tion of the corresponding cyclopentane-derived N-arylimines 16 irrespe ctive of the Lewis acid. Force field calculations support a stepwise i minium ion cyclization under thermodynamic control rather than a conce rted hetero-Diels-Alder mechanism.