Four methyl α (or β)-D-threo (or erythro)-pentofuranosid-2-ulose p-nitrophenylhydrazone derivatives were oxidized to the corresponding methyl 2-0-acetyl-2-C-p-nitrophenylazopentofuranosides. Upon Zemplén de-0-acetylation, these compounds rearranged regiospecifically to the corresponding methyl 2-aza-2-deoxyhexopyranosid-3-ulose derivatives. The stereochemistry of other existing asymmetric centers was maintained.