Conformational preferences of the aldol adducts of oxadiazinones. 1H NMR spectroscopy and computational studies of N 4-methyl and N 4-isopropyloxadiazinones

Abstract

The conformational properties of the aldol adducts of some N 4-isopropyl-oxadiazinones have been investigated by 1H NMR spectroscopy and computational studies. An earlier study of the syn-aldol adducts of N 4-methyl-oxadiazinone 2 led to the conclusion that the solution and solid state conformation of these compounds involve syn-parallel arrangement of the C 2- and N 3-carbonyls of the oxadiazinones. However, the synthesis and asymmetric aldol reactions of an N 3-hydrocinnamoyl-N 4-isopropyl-oxadiazinone 4 has yielded aldol adducts 5a– e in which the orientation of the C 2- and N 3-carbonyls are most likely in the anti-parallel arrangement. These aldol adducts have been studied by 1H NMR spectroscopy and the shielding aspect observed clearly suggests the presence of the anti-parallel arrangement. The installment of a N 4- d 6-isopropyl group further confirmed this assertion. Computational studies support the conclusion that solution state conformation of the N 4-methyl and N 4-isopropyl-oxadiazinones involves anti-parallel carbonyls in contrast to the solid state evidence of the X-ray crystallographic data of oxadiazinone 2 .