PRACTICAL SYNTHESIS OF 2'-5'-LINKED OLIGOADENYLATES (2-5A OLIGOMERS)

A general, facile synthesis of 2'-5'-linked oligonucleotides (2-5A oligomers) has been achieved based on the second-order regioselective protection of adenosine, one-pot formation of the 2'-5' internucleotide linkage, and O-selective phosphorylation of N-unblocked nucleosides. Standard t-butyldimethylsilylation of 5'-O-p-methoxytrityladenosine followed by careful recrystallization from a mixture of triethylamine, methanol, ethyl acetate and ether (4:4:5:100 v/v) gives the 3',5'-di-O-protected adenosine in high yield. Magnesium alkoxide-mediated condensation of the 2'-O-free adenosine with o-chlorophenyl p-nitrophenyl phosphorochloridate followed by 2',3'-di-O-t-butyldimethylsilyladenosine produces the N-free and fully O-protected adenylyl(2'-5')adenosine. The resulting adenylyl dimer, after removal of the 5'-O-trityl protector, is elongated to the protected trimeric compound through a similar reaction sequence. Deprotection of the product furnishes the 2-5A core. Condensation of the 5'-O-detritylated core and bis(2,2,2-trichloroethyl) phosphorochloridite assisted by 2,6-lutidine and subsequent oxidation with aqueous iodine produces, after deblocking, 2-5A 5'-monophosphate (p5'A2'p5'pA2'p5'A). The 2-5A 5'-monophosphate is converted into 2-5A 5'-triphosphate (ppp5'A2'p5'A2'p5'A) by reaction with N,N'-carbonyldiimidazole in the presence of triethylamine followed by tributylammonium diphosphate. This procedure allows ready synthesis of 2-5A oligomers and related compounds on a multigram scale.