Au(I)-Catalyzed Cycloisomerization Reaction of Amide- or Ester-Tethered 1,6-Enynes to Bicyclo[3.2.0]hept-6-en-2-ones

The cycloisomerization reaction of N-allyl-2-(2′-arylethyne-2-yl)amides or allylic 2-alkynoates in the presence of cationic triphenylphosphinegold(I) produced bicyclo[3.2.0]hept-6-en-2-ones under mild conditions. Computational investigations with a density functional theory (DFT) revealed that a stepwise 6-endo-dig cyclization of 1,6-enyne systems, followed by a skeletal rearrangement to form bicyclo[3.2.0]hept-6-en-2-one species represents the most probable reaction pathway. This can be further supported by employing N-(1′,1′-dimethyl-1-propen-3-yl)-2-(1′′-phenylethyne-2′′-yl)amide as a substrate, which exclusively produced 4-phenyl-5-(prop-1-en-2-yl)-5,6-dihydropyridino-2(1H)-one unambiguously via 6-endo-dig cyclization. Theoretical calculations suggest both an aryl group at the terminal alkyne and a keto carbonyl at the C(5) position of the 1,6-enyne system play a complementary role for the reactivity.