Silver-Catalyzed tert-Butyl 3‑Oxopent-4-ynoate π‑Cyclizations: Controlling the Ring SizeHydroxypyrone or Pulvinone Formationby Counterion and Additive Optimization

tert-Butyl 2,5-diaryl-3-oxopent-4-ynoates, obtained from arylacetylenes and the acid chloride of tert-butyl 2-phenylmalonate, represent strongly enolized β-ketoesters. Their CC bonds were activated by Ag­(I) salts so that de-tert-butylating π-cyclizations occurred. The latter followed a 6-endo-dig mode giving 3,6-diaryl-4-hydroxy-2-pyrones, or a 5-exo-dig mode giving (Z)-configured 2-aryl-4-(arylmethyli­dene)­tetronic acids (“pulvinones”). Perfectly selective pyrone formations were induced by AgSbF₆ in methanol and equally selective pulvinone formations by Ag₂CO₃ and DABCO in acetonitrile.