Synthesis of new transglycosidically tethered 5'-nucleotides constrained to a highly biologically relevant profile

6-azido and 6-azidomethyl uracil nucleosides

Azido nucleosides have been utilized for click reactions, metabolic incorporation into cellular DNA, and fluorescent imaging of live cells. Two classes of 6-azido modified uracil nucleosides; one with azido group directly attached to uracil ring and second with azido group attached via methylene linker are described. The 6-azido-2'-deoxyuridine (6-AdU) was prepared in 55% overall yield by lithiation-based regioselective C6-iodination of silyl protected 2'-deoxyuridine followed by treatment with sodium azide and deprotection with TBAF. Lithiation-based C6-alkylation of the protected uridine with methyl iodide followed by the oxidation of the 6-methyl product with selenium dioxide and the subsequent mesylation and azidation of the resulting 6-hydroxymethyl group gave after deprotection 6-azidomethyluridine (6-AmU) in 61% overall yield. Direct lithiation-based C6-hydroxymethylation followed by mesylation/azidation sequence and deprotection provided 6-AmU or 6-azidomethyl-2'-deoxyuridine (6-AmdU). Yields for the lithiation-based regioselective C6-iodination and alkylation were higher for uridine than 2'-deoxyuridine derivatives and they appear to be less dependent on the sugar protection group used. Strain promoted click reactions of 6-AdU and 6-AmdU with symmetrically fused cyclopropyl cyclooctyne (OCT) provided fluorescent triazoles. DFT-calculated dihedral angles and energy differences for the favored anti and syn conformation of 6-AdU and 6-AmdU versus their C5 azido counterparts are discussed.

Alpha-phosphono lactone analogues of cytidine and cytosine arabinoside diphosphates: synthesis via ring closing metathesis

alpha-Phosphono lactone derivatives of the nucleosides cytidine and cytosine arabinoside have been prepared from the corresponding nucleoside aldehydes. The stereochemical outcome of allylation reactions with these aldehydes was found to be dependent upon both the choice of protecting groups for the 2'- and 3'-hydroxyl groups and, to some extent, the nature of the Lewis acid catalyst employed. Ultimately, conditions were found that favored either the 5'R or 5'S diastereomer from different cytidine aldehydes, and gave some stereoselectivity in additions to an aldehyde derivative of ara-C. The resulting homoallylic alcohols were used as substrates in attempted Knovenagel and Horner-Wadsworth-Emmons condensations, but elimination was found to predominate over lactone formation under the conditions employed. The desired alpha-phosphono lactones could be prepared through a reaction sequence that included ring-closing metathesis on acrylate esters of the homoallylic alcohols, followed by reduction of the resulting alpha,beta-unsaturated lactones and carbon-phosphorus bond formation on enolates generated from the saturated lactones.