Synthesis of Tetrasubstituted Furan Derivatives by Condensation of 1,3‐Diketones with Cyclic α‐Hydroxy‐β‐oxo Esters

Tetrasubstituted 3‐acylfuran derivatives were obtained by condensation of acetylacetone with alicyclic and heterocyclic α‐hydroxy‐β‐oxo esters under acidic conditions [0.1 equiv. cerium(III) salt in AcOH]. With [ b ]‐annulated 4‐acyl‐3‐hydroxy‐5‐methylfuran‐2‐carboxylates as common intermediates, the reaction proceeded along different pathways depending on the starting materials used: Alicyclic α‐hydroxy‐β‐oxo esters gave cycloalkane‐annulated furans as products. The ester group was lost after saponification and decarboxylation at elevated temperature. A thiopyrano[3,4‐ b ]‐annulated 2 H ‐furan was obtained when starting from a tetrahydrothiopyranone derivative. With piperidones or a tetrahydropyranone as starting materials, the heterocyclic six‐membered ring was cleaved in a retro‐Mannich type reaction and one carbon atom was lost. 4‐Acetyl‐5‐methylfuran‐2‐carboxylates with a 2‐amino or 2‐hydroxyethyl substituent in the 3‐position are obtained in these cases.