Formation of phostonic acids during the reduction of azidonucleosidephosphonic acids

Reduction of 5′-azido-3′,5′-dideoxy-3′-(phosphorylmethyl)nucleosides, either with H2/Pd/C or with dithiothreitol, leads to cyclic phosphonate esters (phostonic acids) in addition to the desired 5′-amino compounds. The phostonic acids are obtained as the main products when a 20-fold excess of dithiothreitol is used.