Gold(I)-Catalysed Hydroarylation of Lactam-Derived Enynes as an Entry to Tetrahydrobenzo[g]quinolines

The gold(I)-catalysed cyclization of N-tosyl-protected 5-benzyl-6-((trimethylsilyl)ethynyl)-1,2,3,4-tetrahydropyridines, prepared by the Sonogashira coupling of lactam-derived enol triflates, provides tetrahydrobenzo[g]quinolines whose skeleton represents a recurrent motif in natural compounds. The Au(I)-catalysed reaction is carried out with (C6F5)3PAuCl/AgNTf2 as the catalyst system and proceeds via a 6-exo-dig cyclization to form an exocyclic double bond, which eventually isomerises to the aromatic tetrahydrobenzo[g]quinoline. The mode of cyclization is discussed and supported by DFT calculations.