Prebiotic Glycosylation of Uracil with Electron-Donating Substituents

Following the suggestion that nucleoside analogues having their nucleobases joined to ribose via a carbon-carbon bond might easily arise prebiotically, the glycosylation of uracil carrying electron-donating substituents (Me, OH, OCH3, NH2) at its 5 or 6 positions was investigated. Of these, only 6-aminouracil gave glycosylated products in greater than 50% yield under simulated prebiotic conditions. The reaction provided four products, three of which were purified by preparative HPLC. The structure of the isolated compounds was determined by high-resolution mass spectrometry and NMR spectroscopy. The glycosylation products were, as expected, C-nucleosides, with the sugar having either a pyranose or a furanose structure, with the ratio depending on the precise conditions, implying reversible addition. Interestingly, the 6-aminouracil riboside displays two hydrogen bonding patterns, the acceptor-donor-acceptor pattern of uridine itself and (upon 180 degrees rotation) the acceptor-donor-donor hydrogen bonding pattern. The second, in an artificially expanded genetic information system, is trivially called V and pairs with a purine analogue that presents the complementary donor-acceptor-acceptor hydrogen bonding pattern, trivially called J. Key Words: Prebiotic chemistry-Uracil-Glycosylation-C-glycoside-Artificially expanded genetic information systems. Astrobiology 15, 301-306.