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

• We report the first experimental study of the tautomerism of a 2-acylcyclohexane-1,3-dione.
• In theoretical calculations, monoenols showed the lowest energy.
• The endocyclic enol is the isomer in higher proportion.

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.

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