Facile one-pot synthesis of 2,3-dihydro-1H-indolizinium derivatives by rhodium(iii)-catalyzed intramolecular oxidative annulation via C–H activation: application to ficuseptine synthesis

Abstract

Various substituted indolizidinium, quinolizinium and pyrido[1,2-a]azepinium salts synthesized from benzaldehydes (or α,β-unsaturated aldehydes) and alkyne–amines catalyzed by rhodium complexes via C–H activation are demonstrated. The reaction was carried out under mild reaction conditions using Cu(BF₄)₂·6H₂O as oxidant and anion source and inexpensive oxygen as a co-oxidant. A reaction mechanism involving imine formation followed by an ortho C–H activation, alkyne insertion and reductive elimination via a 7-membered rhodacycle is proposed. The present method has been successfully applied to the synthesis of a natural product, ficuseptine.