Synthesis, 17O NMR spectroscopy and structure of 2-trifluoroacetyl-1-methoxycycloalkenes

Among the synthesis of a series of five well-known 2-trifluoroacetyl-1-methoxycycloalkenes derived from cyclopentanone and substituted cyclohexanones, this paper describes the synthesis of three new 2-trifluoroacetyl-1-methoxycycloalkenes derived from cycloheptanone, cyclooctanone and cyclododecanone in 60–68% yield. Subsequently, the 17O NMR chemical shift analysis of the carbonyl and the methoxy groups for these cyclic molecules clearly showed the electron push–pull phenomenon and revealed large and irregular variations of 17O NMR chemical shifts with the ring size. Finally, a more stable conformation of these trifluoroacetyl-containing cycloalkenes was determined by energy minimization calculations using Austin Model 1 (AM1) semi-empirical method and correlations between 17O NMR data and torsion angles or oxygen net charge calculated by AM1 semi-empirical method were performed.