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.