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.