2-Isoxazolidineethanols: An NMR study of the effect of intramolecular H-bonding on the population of nitrogen invertomers and inversion process

A series of (βR,5R)- and (βR,5S)-2,5-disubstituted isoxazolidines: 5-(substituent)-β-phenyl-2-isoxazolidineethanols, have been prepared by asymmetric nitrone cycloaddition reactions and their NMR spectra recorded over a wide range of temperatures. The spectra at low temperatures indicate the presence of the (βR,5S) diasteromer almost exclusively as a single invertomer having trans disposition of the substituents at N(2) and C(5), while the (βR,5R) diasteromer remained as a mixture of two interconverting invertomers in deuterated chloroform. The effect of H-bonding - intramolecular in CDCl₃ and intermolecular in CD₃OD - on the population ratio of the invertomers and nitrogen inversion process has been investigated. The nitrogen inversion barriers are determined using complete line-shape analysis, and their dependence on solvent is discussed. Due to steric factor the trans-invertomers are found to be more stable than their cis counterparts.