Reaction of cytidine nucleotides with cyanoacetylene:: Support for the intermediacy of nucleoside-2′,3′-cyclic phosphates in the prebiotic synthesis of RNA

A robust and prebiotically plausible synthesis of RNA is a key requirement of the RNA World hypothesis, but, to dote, no such synthesis has been demonstrated. Monomer synthesis strategies involving attachment of preformed nucleobases to sugars have failed, and, even if activated 5'-nucleotides could be made, the hydrolysis of these intermediates in water makes their efficient oligomerisation appear unlikely. We recently reported a synthesis of cytidine-2',3'-cyclic phosphate 1 (C > p) in which the nucleobase was assembled in stages on a sugar-phosphate template. However, 2',3'-cyclic nucleotides (N > p's) also undergo hydrolysis, in this case giving a mixture of the 2'- and 3'-monophosphates. This hydrolysis has previously been seen as making the, otherwise promising, oligomerisation of N > p's seem as unlikely as that of the 5'-activated nucleotides. We now find that cyanoacetylene, the reagent used for the second stage of nucleobase assembly in the synthesis of C > p, also reverses the effect of the hydrolysis by driving efficient cyclisotion of C2'p and C3'p back to C > p. Excess cyanoacetylene also derivatises the nucleobase, but this modification is reversible at neutral pH These findings significantly strengthen the case for N > p's in a prebiotic synthesis of RNA.