Conformational preferences of the syn-pyridinecarboxaldehyde oximes

Abstract
The results of an 1H and 13C nmr study of the three syn-pyridine aldoximes are reported. Observed 6J(Hα,Hpara) values in the 2- and 3-pyridine aldoximes indicate that the oxime group is twisted approximately 20° out of the pyridine ring plane. Stereospecific 5J(Hα,Hmeta) values measured for acetone solutions of 2-pyridine aldoxime indicate that the ratio of the NCCN trans conformation to the NCCN cis conformation is 0.86 ± 0.14. The results of similar measurements on 3-pyridine aldoxime give 0.60 ± 0.10 for the ratio of the C2CCN trans and C2CCN cis conformations. Abinitio molecular orbital calculations are in good qualitative agreement with experiment. Carbon-13 chemical shifts of the oximes are compared with those previously measured for the parent aldehydes.

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