Nitrogen-15 NMR study of some barbituric acid derivatives

The 15N chemical shifts and the one‐, two‐ and three‐bond 15N‐1H spin‐spin coupling constants for ten unenriched barbituric acid derivatives have been measured at 10.14 MHz in dimethyl sulphoxide‐d6. The measured 15N chemical shifts are in three distinct ranges: −270 to −280 ppm for urea‐type nitrogens, approximately −264 ppm for amide‐type nitrogens and between −226 and −236 ppm for imide‐type nitrogens, in accord with the gradually increasing possibility for delocalization of the nitrogen lone‐pair electrons. The 2 ppm shielding effect of the side‐chain hydroxy group, together with the 5 ppm shielding effect of the N‐methylation on the ring nitrogens, allows the identification of the diastereoisomers of 5‐allyl‐5‐(2‐hydroxypropyl)‐1‐methyl‐2,4,6(1H,3H,5H)‐pyrimidinetrione. The three‐bond coupling constant of 1.5 ± 0.2 Hz found in five compounds is a unique example of pure trans‐type spin‐spin coupling over an NCONH path. This study shows that particular attention must be paid to sample preparation and measuring conditions in order to achieve structurally significant nitrogen chemical shift and coupling information.