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.

Various substituted indolizidinium, quinolizinium and pyrido[1,2-a]azepinium salts synthesized from benzaldehydes (or α,ß-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.