Gold-Catalyzed Cyclization of Alkynyl Heteroarenes with a Norbornene Bridge

Abstract

In this study, gold(III)-catalyzed cyclization reactions of alkynyl norbornene derivatives were developed to provide benzofused (hetero)arenes with substitutions at the benzocyclic ring. The combination of Au(OAc)₃ and AgNTf₂ allowed for the annulation of (hetero)arenes by intramolecular hydro(hetero)arylation followed by retro-Diels–Alder reactions to afford fully conjugated benzofused (hetero)arenes in yields higher than those obtained by our previously developed Pd(OAc)₂-catalyzed protocol. Furthermore, when Au(OAc)₃ was replaced with AuCl₃, (hetero)arenes with the norbornene bridge installed at the more electron-rich and sterically demanding position underwent a rearrangement to afford the corresponding regioisomeric products in a divergent manner. The results indicate that the rigid norbornene ring creates a steric effect, in addition to placing two reactive sites in close proximity and serving as an acetylene synthon. This illustrates the importance of steric effects in addition to the electronic effects of the catalyst and the substrate on the rearrangement.

Key words

heterocycles - alkynes - cyclization - gold catalysis - norbornene

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