Stereoselective Synthesis of Highly Functionalized Arabinosyl Nucleosides through Application of an N-Nitro Protecting Group

2′-Deoxy-2′,5-disubstituted arabinosyl uridine derivatives bearing a halogen (Cl, Br or I) at C2′ and an ethynyl group at C5 have been synthesized in 6 steps from 2′,3′,5′-tri-O-acetyl-5-iodo-uridine in overall yields of 61% (compound 3, Cl), 47% (compound 4, Br), and 19% (compound 5, I). Stabilization of a 2′-O-triflyl leaving group intermediate to overcome spontaneous intramolecular 2,2′-anhydro uridine formation was pivotal to the synthesis. Specifically, to favor SN2 reaction with a halogen nucleophile over intramolecular cyclization, the nucleophilicity of O-2 oxygen was reduced by incorporation of an adjacent electron withdrawing nitro substituent at N-3. The introduction of the 3-N-nitro group proceeded rapidly (nitronium trifluoroacetate, 1 min) and in quantitative yield. A one-pot method to remove the 3-N-nitro group by reductive nitration (zinc metal in acetic acid, 5 min) and the silyl protecting groups of the alkyne and 3′,5′ hydroxyls (fluoride reagent, 16 h) was established as the final synthetic step. This application of the 3-N-nitro protecting group addresses the significant shortfalls of the conventional approach to synthesis of 2′ modified nucleosides, wherein condensation of a 2′ modified sugar fragment with a pyrimidine base provides poor stereocontrol of N-glycosylation, low yields and incompatibility with 2′ iodo sugars.