Covalent analogues of DNA base-pairs and triplets IV. Synthesis of trisubstituted benzenes bearing purine and/or pyrimidine rings by cyclotrimerization of 6-ethynylpurines and/or 5-ethynyl-1,3-dimethyluracil

Ni-Catalyzed cyclotrimerizations of 6-ethynylpurines 3 or 5-ethynyl-1,3-dimethyluracil (4) afforded the 1,2,4-tris(purin-6-yl)benzenes 7 or 1,2,4-tris(1,3-dimetyhyluracil-5-yl)benzene (9), respectively. The symmetrical 1,3,5-tris(purin-6-yl)benzenes 8 were also formed as minor products in very low yields. Co-cyclotrimerization of 9-benzyl-6-ethynylpurine (3a) with 4 afforded the tris( purinyl) benzene 7a as a major product along with 1,2-bis(9-benzylpurin-6-yl)-4-(1,3-dimethyluracil-5-yl)benzene (10) and a complex mixture of other derivatives and isomers. Compounds 7-10 are analogues of Hoogsteen base-triplets.