Cyclotrimerization of 6-ethynylpurines. Synthesis of 1,2,4- and 1,3,5-tris(purin-6-yl)benzenes as novel Hoogsteen-triplet analogues

Cyclotrimerization reactions of the 9-protected 6-ethynylpurines 4a,b in the presence of various transition metal catalysts were studied. The best results were obtained with Ni(COD)2 or Ni(COD)2/PPh3 to obtain the 1,2,4- and 1,3,5-tris(purin-6-yl)benzenes 5a,b and 6a,b in moderate to good yields (in a 4:1 to 10:1 ratio). This method is suitable for the practical synthesis of the unsymmetrical 1,2,4-tris(purin-6-yl)benzenes 5a–c, while the symmetrical 1,3,5-tris(purin-6-yl)benzenes 6a,b are generally formed as minor products.