Synthesis of Aldehyde‐Linked Nucleotides and DNA and Their Bioconjugations with Lysine and Peptides through Reductive Amination

5‐(5‐Formylthienyl)‐, 5‐(4‐formylphenyl)‐ and 5‐(2‐fluoro‐5‐formylphenyl)cytosine 2′‐deoxyribonucleoside mono‐ ( dC^RMP ) and triphosphates ( dC^RTP ) were prepared by aqueous Suzuki–Miyaura cross‐coupling of 5‐iodocytosine nucleotides with the corresponding formylarylboronic acids. The dC^RTP s were excellent substrates for DNA polymerases and were incorporated into DNA by primer extension or PCR. Reductive aminations of the model dC^RMP s with lysine or lysine‐containing tripeptide were studied and optimized. In aqueous phosphate buffer (pH 6.7) the yields of the reductive aminations with tripeptide III were up to 25 %. Bioconjugation of an aldehyde‐containing DNA with a lysine‐containing tripeptide was achieved through reductive amination in yields of up to 90 % in aqueous phosphate buffer.