Guanine-7-oxide

Nucleoside antibiotics possess various biological activities such as antibacterial, antifungal, anticancer, and herbicidal activities. RIKEN scientists contributed to this area of research with two representative antifungal nucleoside antibiotics, blasticidin S and polyoxin. Blasticidin S was the first antibiotic exploited in agriculture worldwide. Meanwhile, the polyoxins discovered by Isono and Suzuki are still used globally as an agricultural antibiotic. In this review article, the research on nucleoside antibiotics mainly done by Isono and his collaborators is summarized from the discovery of polyoxin to subsequent investigations.

Guanine 7-N-oxide (G-7-Ox) was examined for its antiviral activity against 9 viruses based on plaque reduction, neuraminidase activity reduction, a fluorescent antibody technique or ELISA. The following viruses were included in the tests: influenza, Sendai, simian virus 5 (SV5), respiratory syncytial, western equine encephalitis, Japanese encephalitis, vesicular stomatitis, rabies and polio. G-7-Ox showed broad anti-RNA viral activity against all viruses tested, except for poliovirus. Inhibition of persistent SV5 infection by G-7-Ox indicates that its antiviral activity is independent of cytotoxicity.

A full account is given of the first chemical synthesis of the antitumor antibiotic guanine 7-oxide (5) and its 9-substituted derivatives (24a--k and 26). Coupling of appropriate primary amines (17a--e, g--k) with phenacyl bromide (16) produced, after treatment with HCl, the corresponding N-substituted phenacylamine hydrochlorides (18a--e, g--k). A similar phenacylation of 4-amino-l-butanol (21) failed to give the desired compound 18f, so that 21 was heated with 2-bromomethyl-2-phenyl-1,3-dioxolane (20) at 150-155 degrees C for 3h to furnish, after treatment with HCl, the amino ketal hydrochloride 22 in 40% yield. Deketalization of 22 with hot 2 N aqueous HCl afforded 18f in 96% yield. Condensations of the free bases, generated in situ from the hydrochlorides 18a--l and 1N aqueous NaOH, with the chloropyrimidinone 6 were effected in aqueous EtOH at the boiling point for 20 min or at 25-30 degrees C for 3-24h, giving the 6-phenacylamino-4-pyrimidinones 19a-l in 54-90% yields. On treatment with 2N aqueous NaOH at room temperature for 10-60 min, the nitropyrimidinones 19a--k cyclized to provide the 9-substituted guanine 7-oxides 24a--k in 61-98% yields. A similar alkali-treatment of 191 failed to yield guanine 7-oxide (5). However, removal of the 9-(arylmethyl) group from 24i--k was effected with conc. H2SO4 at room temperature for 1-3h in the presence of toluene, producing the target N-oxide 5 in 56-89% yields.