Methyl 2-deoxy-3-O-[2-(formylamino)ethyl]-5-O-trityl-D-erythro- pentofuranoside (4) was obtained in a 3-O alkylation reaction by treatment with 2-chloroethylamine in DMF. Compound 4 afforded alpha nucleosides as the main products when condensed with uracils under the Vorbrüggen conditions. The nucleosides were deblocked by treatment with 80% acetic acid and subsequently with sodium methoxide in methanol.