Reaction of deoxynucleosides 3'-O-(methylthiomethyl)acetal (1) with N-iodosuccinimide (NIS) and diphenylphosphinic acid furnished the corresponding 3'-O-(diphenylphosphinyloxy)methyl acetal (2). Condensation of the altter with 3'-protected deoxynucleoside (5) in the presence of trimethylsilyl trifluomethanesulfonate (TMSOTf) afforded the (3'→ 5') methylene acetal linked d(B1-m-B2) dimers (6). In addition, the coupling of donor 2 (B=T or C) with 3'-protected dimer 9 (T-m-T or C-m-T) yielded corresponding trimers 10 (T-m-T-m-T, C-m-T-m-T and T-m-C-m-T).

Key words: FORMALDEHYDE ACETAL, DIMER, NUCLEOSIDE, TRIMER, DEOXYRIBONUCLEOSIDE, SYNTHESIS METHODS, OLIGONUCLEOTIDE