Previous research has identified ribose aminooxazoline as a potential intermediate in the prebiotic synthesis of the pyrimidine nucleotides with remarkable properties. It crystallizes spontaneously from reaction mixtures, with an enhanced enantiomeric excess if initially enantioenriched, which suggests that reservoirs of this compound might have accumulated on the early Earth in an optically pure form. Ribose aminooxazoline can be converted efficiently into α-ribocytidine by way of 2,2'-anhydroribocytidine, although anomerization to β-ribocytidine by ultraviolet irradiation is extremely inefficient. Our previous work demonstrated the synthesis of pyrimidine β-ribonucleotides, but at the cost of ignoring ribose aminooxazoline, using arabinose aminooxazoline instead. Here we describe a long-sought route through ribose aminooxazoline to the pyrimidine β-ribonucleosides and their phosphate derivatives that involves an extraordinarily efficient photoanomerization of α-2-thioribocytidine. In addition to the canonical nucleosides, our synthesis accesses β-2-thioribouridine, a modified nucleoside found in transfer RNA that enables both faster and more-accurate nucleic acid template-copying chemistry.

The nature of the first genetic polymer is the subject of major debate in the origin of life field 1 . Although the common 'RNA world' theory suggests RNA as the first replicable information carrier at the dawn of life, other evidence implies that life may have started with a heterogeneous nucleic acid genetic system including both RNA and DNA 2 . Such a theory streamlines the eventual 'genetic takeover' of homogeneous DNA from RNA as the principal information storage molecule in the central dogma, but requires a selective abiotic synthesis of both RNA and DNA building blocks in the same local primordial geochemical scenario. Herein, we demonstrate a high-yielding, completely stereo-, regio-, and furanosyl-selective prebiotic synthesis of the purine deoxyribonucleosides, 2 deoxyadenosine and deoxyinosine. Our synthesis utilizes key intermediates in the prebiotic synthesis of the canonical pyrimidine ribonucleosides, and we show that, once generated, the pyrimidines persist throughout the synthesis of the purine deoxyribonucleosides, ultimately leading to a mixture of deoxyadenosine, deoxyinosine, cytidine, and uridine. These results support the notion that purine deoxyribonucleosides and pyrimidine ribonucleosides may have coexisted before the emergence of life 3 .