Synthesis of C-4'truncated phosphonated carbocyclic 2'-oxa-3'-azanucleosides as antiviral agents.

A new template of C-4'-truncated phosphonated nucleosides (TPCOANs) has been obtained in good yields according to two different routes which exploit the reactivity of a phosphonated nitrone. The one-step procedure based on the 1,3-dipolar cycloaddition of a phosphonated nitrone with vinyl nucleobases leads to the unnatural alpha-nucleosides as the main adducts. On the other hand, the target beta-anomers have been obtained in high yield by a two-step procedure based on the 1,3-dipolar cycloaddition of a phosphonated nitrone with vinyl acetate followed by nucleosidation reaction. The reactivity of the phosphonated nitrone has been investigated trough quantum mechanical DFT calculations at the B3LYP/D95+(d,p) theory level. Preliminary biological assays show that beta-anomers of TPCOANs are able to inhibit the reverse transcriptase of different retroviruses at concentrations in the nanomolar range, with a potency comparable with that of tenofovir.