A study of conformational behaviour in some hexahydro-2H-isoxazolo[2,3-a] pyridines

A study of the effects of substituents on the conformational behaviour of a series of hexahydro-2H-isoxazolo[2,3-a]pyridines (a 6/5 fused ring system), prepared via nitrone cycloaddition reaction of 4-butyloxycarbonyl-3,4,5,6- tetrahydropyridine-1-oxide with mono-and disubstituted alkenes, has been carried out. Some of these bicyclic cycloaddition products show the presence of two isomers (cis-and trans-fused system), equilibrating via relatively slow nitrogen inversion process, while the rest exist solely as the trans-invertomer. Stereochemistry of the ring fusion was determined by NMR spectral analyses. The effect of substituents and solvents-CDCl₃ toluene-d6, and CD ₃OD-on the population ratio of the invertomers and inversion barriers has been investigated. The nitrogen inversion barriers, determined using complete line-shape analysis, are in the range 62.1-72.3 kJ mol^-1.