Side reaction during phosphitylation of 5'-O-dimethoxytrityl-N2-isobutyryl-2'-deoxyguanosine was observed. As a suitable dG synthon, 5'-O-dimethoxytrityl-N2-dimethylaminomethyl-ene-3'-H-phosphonate was developed and used, along with 3'-H-phosphonates of 5'-O-dimethoxytrityl derivatives of dT, N4--benzoyl-dC and N6-dimethylaminoethylidene-dA, in automated synthesis of several oligodeoxyribonucleotides containing 30-40 bases.