Conformational preferences of the aldol adducts of oxadiazinones. 1H NMR spectroscopy and computational studies of N4-methyl and N4-isopropyloxadiazinones

The conformational properties of the aldol adducts of some N4-isopropyl-oxadiazinones have been investigated by 1H NMR spectroscopy and computational studies. An earlier study of the syn-aldol adducts of N4-methyl-oxadiazinone 2 led to the conclusion that the solution and solid state conformation of these compounds involve syn-parallel arrangement of the C2- and N3-carbonyls of the oxadiazinones. However, the synthesis and asymmetric aldol reactions of an N3-hydrocinnamoyl-N4-isopropyl-oxadiazinone 4 has yielded aldol adducts 5a-e in which the orientation of the C2- and N3-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 N4-d6-isopropyl group further confirmed this assertion. Computational studies support the conclusion that solution state conformation of the N4-methyl and N4-isopropyl-oxadiazinones involves anti-parallel carbonyls in contrast to the solid state evidence of the X-ray crystallographic data of oxadiazinone 2.

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