Nuclear magnetic resonance study of the protonation of 2,6-dimethyl-γ-pyrone and related bases in superacid systems

Abstract

¹ H and ¹³C n.m.r. spectra indicate that 2,6-dimethyl-4H-pyran-4-one in solution in HSO₃F–SbF₅(1 : 1) is doubly protonated at the exocyclic oxygen atom. The sulphur and nitrogen analogues, 2,6-dimethyl-4H-thiopyran-4-one, and 2,6-dimethyl-N-methyl-4-pyridone, respectively, behave similarly. Other related compounds show ¹H and ¹³C n.m.r. spectra in support of our interpretation.