Chalcogenopyranones from disodium chalcogenide additions to 1,4‐pentadiyn‐3‐ones. The role of enol ethers as intermediates

Enol ethers 9 are formed as a mixture of E ‐ and Z ‐isomers from the addition of ethanol to 1,4‐pentadiyn‐3‐ones 2 in sodium ethoxide/ethanol. The enol ethers react with disodium chalcogenides to give the corresponding 2,6‐disubstituted chalcogenopyranones 1 bearing alkyl, aryl, or heteroaryl substituents in high yield as the only heterocyclic product of reaction. Diynones 2 react with disodium chalcogenides to give mixtures of products in which the chalcogenopyranones 1 are minor components and the dihydrochalcogenophenes 3 are the major products. The addition of hydrogen sulfide to diynone 2b in ethanol gives a product mixture nearly identical to that observed for the addition of disodium sulfide in sodium ethoxide in ethanol to 2b . Intermediates for the addition of hydrogen chalcogenides and disodium chalcogenides to both 2 and 9 are described, which lead to the heterocyclic products.