Synthesis of 6- and 8-alkynylated purines and their ribonucleosides by the coupling of halopurines with alkynes.

Coupling reaction of 6-iodo- or 6-chloropurine with alkynes in the presence of (PPh3) 2PdCl2-CuI catalyst in triethylamine to give 6-alkynylated purines was achieved by the use of dipolar aprotic solvent as co-solvent. Under the reaction conditions, ribonucleoside as well as its tri-O-acetate of 6-chloropurine also gave the corresponding alkynylated products in high yields. Though similar reaction for 8-bromo-derivatives of adenine and guanine gave poor yields, the 8-alkynylated free bases could be obtained by acid hydrolysis of the alkynylated ribonucleosides. 6-Alkynylated purines exhibited moderate to weak cytokinin activity in Amaranthus test.