On the prebiotic synthesis of ribonucleotides: photoanomerisation of cytosine nucleosides and nucleotides revisited.

Recent work has emphasised the importance of D-ribose aminooxazoline 1 in the synthesis of cytidine ribonucleosides under potentially prebiotic conditions. Upon treatment with cyanoacetylene, 1 is transformed into alpha-D-cytidine (alpha-2), and if an efficient means of anomerising this nucleoside or a derivative thereof were to be found, then the synthesis of one of the key beta-D-nucleosides required to make RNA would be realised. Photoanomerisation of alpha-2 has previously been described, but the yield was extremely low. Therefore, the present study was initiated to determine whether this low yield was the result of a low conversion or competing reaction pathways.