Synthesis of O- and N-Heterocycles by Au(I)-Catalyzed Allene Cyclizations

An efficient and mild intramolecular exo-hydrofunctionalization of allenyl carbamates, alcohols and indoles to give pyrrolidines, piperidines, tetrahydrofurans, tetrahydropyrans, and fused indoles under Au(I)-Ag(I) catalytic conditions is reported. Use of a sterically hindered o-biphenyl ligand is crucial for the success of the reaction. The methodology is effective for both protected γ,δ-allenyl amines and alcohols and tolerates substituents at internal and terminal allenyl carbon atoms. An interesting effect of the counteranion was observed: the use of AgOTf in the Au-catalyzed hydroalkoxylation of 2,2-diphenyl-γ-allenyl alcohol led to a 1:1 ratio of 5-exo/6-endo hydroalkoxylation products while almost only the 5-exo cyclization product was formed using AgOTs.