Conformational Analysis of (S)-4-(Cyclohexoxycarbonyl)-2-azetidinone

Abstract

The conformational analysis of (S)-4-(cyclohexoxycarbonyl)-2-azetidinone was carried out using quantum mechanical calculations. 1H NMR spectra at different temperatures were used to estimate the relative populations of axial and equatorial chair forms of the cyclohexyl group. Gas-phase energy calculations of the whole molecule rendered two preferred structures, both of them with the cyclohexyl group in equatorial conformation. The azetidinone ring was found to be in a quasi-planar arrangement with endocyclic dihedrals deviating from the ideal planar conformation by 1−2°. Results showed also that the cyclohexoxycarbonyl group is highly flexible and that the stabilization of the molecule is favored by intramolecular interactions between the amide and the ester group. The occurrence of this type of interaction in nonpolar solution was evidenced by FTIR spectroscopy. Additionally, SCRF calculations revealed that solvent effects on the conformational preferences of this compound are small.