Synthesis of Chiral Thienylpyridines from Naturally Occurring Monoterpenes: Useful Ligands for Cyclometallated Complexes

On treatment with ammonium acetate, α,β-unsaturated ketones or aldehydes can easily undergo condensation with acetylpyridinium salts (Kröhnke reaction). Four "thienylpyridine" ligands, derived from (-)-β-pinene, (+)-camphor, (+)-3-carene and (+)-2-carene, were prepared according to this method. The multistep syntheses to get (1R,5R)-3-methyleneopinone (5), (+)-3-methylenecamphor (10), (-)-3-caren-10-al (15) and (1S,6R)-7,7-dimethyl-3-methylenebicyclo[4.1.0]heptan-2-one (18) are also described.