Rhodium(i)-catalyzed dearomative cycloisomerization of 1H-indole N-tethered o-alkynylphenyl nitrones and application to spirocyclic pyrido[1,2-a]indole synthesis

Abstract

A synthetic method for the synthesis of indoline-fused polycyclic tropane scaffolds that relies on the rhodium(I)-catalyzed highly regio- and diastereoselective 6-exo-dig cyclization/internal redox(IR)/azomethine ylide formation/dearomative [3 + 2] cycloaddition of 1H-indole N-tethered o-alkynylphenyl nitrones at a low catalyst loading of 1 mol% is described. The efficient dealkylative spirocyclization of the N-hexacyclic ring systems into their 4H,6H′-spiro[isoquinoline-3,9′-pyrido[1,2-a]indole]-4,6′-dione derivatives on further treatment with ammonium cerium(IV) nitrate (CAN) under mild oxidative reaction conditions is also disclosed.