Dynamic H-1 NMR study of aryl-nitrogen single bond and carbon-carbon double bond rotational energy barriers in two highly functionalized pyranopyrimidines

The reactive I:I intermediate produced in the reaction between 2,6-dimethyl phenyl isocyanide and dimethyl acetylenedicarboxylate was trapped by N,N ' -dimethylbarbituric acid to yield the isomeric products dimethyl 7-(2,6-dim ethylphenylamino)-1,3-dimethyl-2,4-dioxo-4H-pyrano[3,2-d]pyrimidine-5,6-dic arboxylat and dimethyl (E)-2-((2,6-dimethylphenylamino)-(1,3-dimethyl-2,4,6 -trioxo-pyrimidine-5-ylidene)-methyl)-but-2-enedioate in a nearly 1:1 ratio and an overall yield of 85%. Dynamic effects were observed in the H-1 NMR spectra of these compounds and were attributed to restricted rotation aroun d the aryl-nitrogen single bonds and the polarized carbon-carbon double bon d.