NMR and computational studies on tautomerism of 3-hydroxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one

The tautomeric system of 3-hydroxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one 1 has been investigated by NMR spectroscopy between 224 and 298 K. At all temperatures an endocyclic enol tautomer was the major isomer; however, at low temperatures two other enol isomers were found. Conformational search of the potential energy surfaces of all tautomers of cyclohexenone 1 was also carried out. Extensive calculations were performed for two triketones and four cis-endocyclic double bond enol tautomers with the lowest energies. Syntheses of 3-methoxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one 2 and 2-benzoyl-3-hydroxycyclohex-2-en-1-one 3 were carried out to analyze the features of thienyl group rotation and structural differences with a symmetrical substituent, respectively.