Competing A^1,3-strain and Ph:Ph diaxial repulsion in oximes and semicarbazones of N-nitroso-<i style="">r</i>-2,<i style="">c</i>-6-diphenylpiperidin-4-ones

Abstract

The effect of A^1,3-strain introduced by N-NO group in N-nitroso-<i style="">r</i>-2,<i style="">c</i>-6-diphenylpiperidin-4-ones on the selectivity in the formation of oximes <b style="">14-18</b> and semicarbazones <b style="">19-22</b> has been studied. In general, the oximes and semicarbazones formed are the <i style="">E</i> isomers, exclusively, except in the cases of the oximation of 3-methyl-N-nitrosopiperidin-4-one and 3,5-dimethyl-N-nitrosopiperidin-4-one which have produced a mixture of <i style="">E</i> and <i style="">Z</i> isomeric oximes. Oximation of 3,5-dimethyl-N-nitrosopiperidin-4-one yields a mixture of two isomers <b style="">17</b> and <b style="">18</b> differing in the relative configurations of methyl group due to epimerization at C5. The configurations and preferred conformations of the oximes <b style="">14-18</b> and semicarbazones <b style="">19-22</b> of N-nitroso-<i style="">r</i>-2,<i style="">c</i>-6-diphenylpiperidin-4-ones have been determined using NMR spectral experiments and are found to prefer twist-boat conformations with an equilibrium between two rotameric states due to restricted rotation around N-NO bond. The semiempirical MO calculations performed on the nitrosamines <b style="">14-22</b> have also shown the preference of twist-boat conformations.