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NMR study of inversion isomerism in some isoxazolidines

Carbon‐13 NMR spectra were recorded for two series of isoxazolidines, 3 (a 5/5 fused system) and 4 (a 6/5 fused system), over the temperature range − 100 to +60°C. Whereas all the compounds in series 3 existed solely as one isomer throughout this temperature range, some compounds in series 4 showed the presence of two isomers of unequal population at low temperatures. The major isomer was shown to be the trans conformer, which is in equilibrium with the minor isomer (cis conformer) through a relatively slow nitrogen inversion. The monosubstituted isoxazolidines 4 having endo substituents at either C‐2 or C‐3 and 2,3‐di‐ and 2,2,3‐trisubstituted compounds showed the presence of only one isomer throughout the temperature range. The barriers to nitrogen inversion were determined by band shape analysis and were in the range 65.2–‐69.0 kj mol−1.