NMR study of the anomeric effect and nitrogen inversion in some isoxazolidines

A series of isoxazolidines with various alkyl substituents at N and an ethoxy group at C(5) have been synthesised and their NMR spectra recorded over a range of temperatures. The NMR spectra at low temperature indicate the presence of two isomers due to slow nitrogen inversion. The nitrogen inversion barriers are determined using complete band shape analysis and are found to be in the range 59.3-65.6 kJ mol^-1. The strong anomeric effect exerted by the C(5) ethoxy group locks the substituent in the pseudoaxial orientation. The cis isomers are found to be more stable than the trans isomers.