Interconversions of heterocycles containing S,N, and O

On heating with hydrogen sulfide in the presence of sulfuric acid, tetrahydropyran-4-ones (III) and 4-piperidones (II) are converted to the corresponding thiopyranones (I). The anticipated III are obtained by heating I or II in dilute sulfuric acid solution in the presence of mercuric sulfate. Analogously, I and II in aqueous methylamine solution are converted to II;2,3,6-trimethyl-4-pyrone (VII) and 1, 2, 3, 6-tetramethyl-4-pyridonen (VIII) are also interconvertible.