Structural and Conformational Studies on Carboxamides of 5,6-Diaminouracils—Precursors of Biologically Active Xanthine Derivatives

8-Arylethynylxanthine derivatives are potent, selective adenosine A(2A) receptor antagonists, which represent (potential) therapeutics for Parkinson’s disease, Alzheimer’s dementia, and the immunotherapy of cancer. 6-Amino-5-amidouracil derivatives are important precursors for the synthesis of such xanthines. We noticed an unexpected duplication of NMR signals in many of these uracil derivatives. Here, we present a detailed analytical study of structurally diverse 6-amino-5-carboxamidouracils employing dynamic and two-dimensional NMR spectroscopy, density functional theory calculations, and X-ray analysis to explain the unexpected properties of these valuable drug intermediates.