Ligand-controlled chemoselectivity in gold-catalyzed cascade cyclization of 1,4-diene-tethered 2-alkynylbenzaldehydes

Abstract

A method to chemo- and stereoselectively assemble polycyclic bridged pyrrolidines and azepines which relies on the gold(I)-catalyzed cascade annulation of skipped 1,4-diene-tethered 2-alkynylbenzaldehydes is described. The divergence in product selectivity was accomplished by controlling the reaction pathway of the surmised Au-bound benzopyrylium species, generated from 6-endo-dig oxycyclization of the substrate, and by changing the steric nature of the ligand of the metal complex. With BrettPhosAuNTf₂ (BrettPhos = dicyclohexyl(2′,4′,6′-triisopropyl-3,6-dimethoxy-[1,1′-biphenyl]-2-yl)phosphine) as the catalyst, the ensuing [3 + 2] cycloaddition/cyclopropanation cascade of the metal-bound cyclic oxonium intermediate was found to occur to afford the cyclopropane-fused bridged-pyrrolidine product. The presence of SIMesAuNTf₂ (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) as the catalyst, on the other hand, was observed to result in an Au-bound benzopyrylium intermediate undergoing a sequential [3 + 2] cycloaddition followed by selective C(sp³)–H bond insertion in the ensuing gold carbenoid species to give the cyclopropane-bridged-azepine ring system. The synthetic utility of the divergent catalytic protocol was demonstrated by the late-stage modification of a series of structurally complex natural products and drug molecules under mild reaction conditions.