An alternative route from 1,3-disubstituted barbituric acids to biologically relevant oxazolo[5,4-d]pyrimidine-5,7-diones was developed that features sulfonyl-azide-free (SAFE) diazo transfer and Rh₂(esp)₂-catalyzed cycloaddition of the resulting 5-diazobarbituric acids with aliphatic and aromatic nitriles. Besides being shorter compared to the previously described approaches, the method allows introduction of alkyl substituents at the 1,3-oxazole ring of the fused heterocyclic system.