Résumé : 2-t-Butyl-4,5-didehydropyrimidine, generated by oxidation of 3-amino-5-t-butyl-3H-v-triazolo[4,5-d] pyrimidine, was allowed to react with a variety of reagents. Trapping experiments with furan and two tetra-cyclones gave the expected adducts in low to moderate yields. On treatment with anthracene and 1,3-cyclohexadiene, complex mixtures were obtained from which the adducts could not be isolated. Cycloaddition of phenyl azide to the intermediate yielded 3-phenyl-5-t-butyl-3H-v-triazolo[4,5-d]pyrimidine as the major product together with the unexpected 2-t-butyl-9H-pyrimido[4,5-b]indole in lesser amount. The structure of these two compounds was established by comparison with authentic specimens whose synthesis is described. Cyclo-addition also occurred with 2,3,5-tri-O-benzoyl-β-D-ribofuranosyl azide to give an 8-azanucleoside in low yield. Oxidation of the precursor in ethanol gave solely 4-ethoxy-2-t-butylpyrimidine. Oxidation in the presence of iodine, in dichloromethane or benzene, afforded products arising from attack on the solvent, i.e. 4-chloro-5-iodo-2-t-butylpyrimidine and 5-iodo-4-phenyl-2-t-butylpyrimidine respectively. In addition, 5-iodo-2-t-butyl-4(3H)-pyrimidinone was obtained in both cases. Mechanisms for these reactions are proposed. The electronic structure of 4,5-didehydropyrimidine has been calculated by an ab initio 3-21G quantum chemical method. Both the Molecular Electrostatic Potential and the Fukui function give a very reasonable account of the strong orientation effect observed in the additions to 2-t-butyl-4,5-didehydropyrimidine.