Gold(I)-Catalyzed [2 + 2 + 2] Cyclotrimerization of 1,3-Diarylpropargyl Acetals

A gold-nitrone catalyzed [2 + 2 + 2] cyclotrimerization of 1,3-diarylpropargyl acetals into cyclohexylidene products (up to 74% yield) is reported. The trimerization is proposed to proceed through allenic intermediates via gold-catalyzed 1,3-alkoxy rearrangement. The presence of catalytic amounts of different nitrones, tuning of the Au(I) catalyst activity, was essential for controlled regio-/chemoselective cyclotrimerization. A linear nitrone-O-Au(I)-P coordination mode was shown (X-ray analysis) for a catalytic active phosphane-gold-nitrone complex, representing a group of Au(I) catalysts with specific properties.