Ferrocene-modified purines as potential electrochemical markers:: Synthesis, crystal structures, electrochemistry and cytostatic activity of (ferrocenylethynyl)- and (ferrocenylethyl)purines

Palladium-catalyzed Sonoga-shira cross-coupling reactions of halopurines 9-benzyl-6-chloropurine (2a), 9-benzyl-8-bromoadenine (2b), and 9-benzyl-2-chloroadenine (2c) with ethynylferrocene (1) gave the corresponding (ferrocenylethynyl)purines 3a-c in moderate to good yields. Catalytic hydrogenation of these alkynes over Pd/C afforded the respective saturated [2(ferrocenyl)ethyl]purines 4 a-c. The crystal structures 3a, 3b, 4a and 4b as determined by X-ray diffraction show interesting solid-state interactions, markedly different for purines 3a and 4a on one hand and adenines 3b and 4b that possess a free amino group on the other. Electrochemistry of electrochemically labelled purines 3 and 4 has been studied by voltammetry and cyclic voltammetry on platinum disc electrode and the experimental oxidation potentials were confirmed and explained by ionization potentials from theoretical DFT calculations. Several compounds of this series exhibited a considerable cytostatic effect.