An NMR study of conformational behaviour and nitrogen inversion process in some 5,5-disubstituted-β-phenyl- 2-isoxazoli-dineethanols

A series of (βR,5R)- and (βR,5S)-2,5,5-trisubstituted isoxazolidines 5-methyl-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 cis disposition of the substituents at N(2) and C(5)-methyl, 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 CD ₃OD - 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.