AN EFFICIENT SYNTHETIC STRATEGY FOR THE FUNCTIONALIZATION OF 9-RIBOSYL PURINE (NEBULARINE)

In the last decades, many research groups have focused their attention to the preparation of new modified nucleosides and nucleotides to expand the pool of molecules with potential antineoplastic, antihypertensive and antiviral activities. In this context, efforts have been directed to the synthesis of sugar and/or base-modified nucleosides. Many nucleobase analogues exist and several nucleoside analogues have been employed against cancer and viral diseases. In addition, base modified nucleosides often show fluorescent properties, and can be used as fluorescent probes for the analysis of DNA and RNA structures as well as for analysing the interaction of DNA and RNA with binding proteins. Purine bases and nucleosides bearing a C or N-substituent at C2 and C6 positions represent an important class of compounds possessing a broad spectrum of biological effects including cytostatic, antiviral, antibacterial as well as receptor modulation activity. The reactivity imparted to purines and related nucleosides by halogenation at C6 and C2 positions has opened the way to the construction of new libraries of C6 and C2 modified nucleosides generally through direct aromatic nucleophilic substitution (SNAr), or metal-mediated cross-coupling processes. We have recently reported on the reactivity of 9-ribosyl purine (nebularine) N1-oxide demonstrating that its C6(C2)-N1-O- nitrone moiety can react with dipolarophiles and with Grignard reagents leading to their addition on the C6 or C2 carbons of the purine base. In particular, we observed that the sugar-protected nebularine N1-oxide can regioselectively react with Grignard reagents at the more electrophilic C6 position leading to an adduct, which re-aromatized by treatment with Ac2O/pyridine, furnishing the C6-substituted purine nucleoside in high yields. We have also shown that a second alkyl/aryl substituent can be introduced at C2 by a similar strategy, that is, by the addition of a Grignard reagent to the C6- substituted nebularine N1-oxides through the opening/reclosing of the pyrimidine ring induced by the Grignard reagent itself. Through this approach, we have synthesized new collections of 2,6-dialkyl(aryl)purine nucleosides. A slightly modified synthetic procedure allowed us also to insert pyridinyl residues at the C6 purine position to obtain the few explored C6-pyridinyl nucleosides. The last are appealing nucleoside analogues because the presence of a nitrogen atom in the C6 residue can potentially alter the hydrogen-bonding capabilities of the nucleoside as well as promote its coordination to biologically important metals, such as platinum and ruthenium. We have discovered that they can be easily accessed after bromine-magnesium exchange between bromopyridines and iPrMgCl