Synthesis and Conformation of Pentopyranoside Nucleoside Phosphonates.
In contrast to natural nucleosides, where the nucleobase is positioned at the anomeric center, we report the synthesis of pentopyranoside nucleosides with a phosphonate functionality at the 1'-anomeric oxygen. Starting from l-arabinose, key functionalized l- glycero- and l- erythro-pentopyranose carbohydrate synthons were prepared and further elaborated into the final six-membered ring nucleosides via nucleobase incorporation and phosphonomethylation reactions. NMR analysis demonstrated that these nucleoside phosphonates exist in solution as conformers predominantly adopting a chair structure in which the base moiety is equatorially positioned. Such conformation prevents unfavorable 1,3-diaxial steric and electronic interactions. Notably, the stereochemical outcome of the Vorbrüggen glycosylation step utilized en route to the thymine analogue clearly suggests the absence of anchimeric assistance, as opposed to what is usually observed during nucleoside synthesis using protected furanose precursors. The finding that the diphosphates of the compounds developed in this study are recognized by DNA polymerases is important in view of the future selection of artificial genetic systems and dedicated polymerases as well as applications in therapy.