Synthesis of reactive cytidine derivatives as building blocks for cross-linking oligonucleotides

6-Vinylcytidine derivative (1) possessing good Michael acceptor properties has been synthesized through C-6 formylation and subsequent Wittig reaction. In view of introducing the reactive nucleoside into the oligonucleotide sequence, protection of the vinyl group as ethylthio derivative was proved to be effective for the masking and subsequent regeneration of the reactive vinyl moiety.