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Prebiotic stereoselective synthesis of purine and noncanonical pyrimidine nucleotide from nucleobases and phosphorylated carbohydrates

According to a current RNA first model for the origin of life, RNA emerged in some form on early Earth to become the first biopolymer to support Darwinism here. Threose nucleic acid (TNA) and other polyelectrolytes are also considered as the possible first Darwinian biopolymer(s). This model is being developed by research pursuing a Discontinuous Synthesis Model (DSM) for the formation of RNA and/or TNA from precursor molecules that might have been available on early Earth from prebiotic reactions, with the goal of making the model less discontinuous. In general, this is done by examining the reactivity of isolated products from proposed steps that generate those products, with increasing complexity of the reaction mixtures in the proposed mineralogical environments. Here, we report that adenine, diaminopurine, and hypoxanthine nucleoside phosphates and a noncanonical pyrimidine nucleoside (zebularine) phosphate can be formed from the direct coupling reaction of cyclic carbohydrate phosphates with the free nucleobases. The reaction is stereoselective, giving only the β-anomer of the nucleotides within detectable limits. For purines, the coupling is also regioselective, giving the N-9 nucleotide for adenine as a major product. In the DSM, phosphorylated carbohydrates are presumed to have been available via reactions explored previously [Krishnamurthy R, Guntha S, Eschenmoser A (2000) Angew Chem Int Ed 39:2281-2285], while nucleobases are presumed to have been available from hydrogen cyanide and other nitrogenous species formed in Earth's primitive atmosphere.

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) phosphorochloridate 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.

SYNTHESIS OF 2'-5',3'-5' LINKED TRIADENYLATES

2'-5',3'-5' Linked triadenylates have been synthesized by direct bisadenylylation of adenosine 2' and 3' hydroxyls with an adenosine 5'-phosphorochloridite followed by oxidation.