“…The C5 position of pyrimidine bases is often utilized as a modification site of oligonucleotides (ONs) because a substituent at the C5 position does not interfere with Watson‐Crick base pairing. Therefore, ONs containing C5‐modified pyrimidine bases are used with various nucleic‐acid‐based technologies employing biomolecules such as antisense DNAs (Moriguchi, Sasaki, Suzuki, & Shinozuka, ; Wagner et al., ), enzymes (Åström, Williams, & Strömberg, ; Hollenstein, Hipolito, Lam, & Perrin, ), and DNA probes (Hagiwara et al., ; Obayashi et al., ). Among the methods used to introduce substituents into the C5 position of pyrimidine bases, postsynthetic modification is one of the most efficient.…”