Oligonucleotide

An oligonucleotide, comprising a nucleotide residue or a nucleoside residue represented by formula (I) at the 5' end thereof, wherein the nucleotide residue or the nucleoside residue binds to an adjacent nucleotide residue through the oxygen atom at position 3 :

• wherein X1 is an oxygen atom or a sulfur atom,
• R1 is formula (II):
• wherein Y1 is a nitrogen atom,
• R5 is a hydrogen atom, halogen, cyano, unsubstituted or substituted lower alkyl, unsubstituted or substituted lower alkenyl, unsubstituted or substituted lower alkynyl, unsubstituted or substituted lower alkoxy, unsubstituted or substituted lower alkylthio, unsubstituted or substituted aryl, an unsubstituted or substituted aromatic heterocyclic group, or —NR10aR10b wherein R10a and R10b may be the same or different, and each is a hydrogen atom or unsubstituted or substituted lower alkyl,
• R6 is a hydrogen atom, halogen, cyano, unsubstituted or substituted lower alkyl, unsubstituted or substituted lower alkenyl, unsubstituted or substituted lower alkynyl, unsubstituted or substituted lower alkoxy, unsubstituted or substituted lower alkylthio, unsubstituted or substituted lower alkanoyl, unsubstituted or substituted aryl, unsubstituted or substituted aralkyl, unsubstituted or substituted aryloxy, unsubstituted or substituted arylthio, unsubstituted or substituted aroyl, an unsubstituted or substituted aromatic heterocyclic group, unsubstituted or substituted aromatic heterocyclic alkyl, unsubstituted or substituted aromatic heterocyclicoxy, unsubstituted or substituted aromatic heterocyclicthio, unsubstituted or substituted aromatic heterocyclic carbonyl, or —NR11aR11b wherein R11a and R11b may be the same or different, and each is a hydrogen atom, unsubstituted or substituted lower alkyl, unsubstituted or substituted lower alkenyl, unsubstituted or substituted lower alkynyl, unsubstituted or substituted aryl, unsubstituted or substituted aralkyl, unsubstituted or substituted aromatic heterocyclic alkyl, unsubstituted or substituted lower alkanoyl, unsubstituted or substituted lower alkylsulfonyl, unsubstituted or substituted aroyl, unsubstituted or substituted arylsulfonyl, an unsubstituted or substituted aromatic heterocyclic group, unsubstituted or substituted aromatic heterocyclic carbonyl, or unsubstituted or substituted aromatic heterocyclic sulfonyl,
• R7 is a hydrogen atom, or
• R1 is formula (III):
• wherein Y2 is a nitrogen atom, R6a is an amino group and R7a is a hydrogen atom, or
• R1 is formula (IV):
• wherein R12 is a hydrogen atom,
• is a double bond,
• Y3 is CR14e wherein R14e is a hydrogen atom, cyano, unsubstituted or substituted lower alkyl, unsubstituted or substituted lower alkenyl, or unsubstituted or substituted lower alkynyl and
• Y4 is CR14f wherein R14f is a hydrogen atom, halogen, cyano, unsubstituted or substituted lower alkyl, unsubstituted or substituted lower alkenyl, unsubstituted or substituted lower alkynyl, unsubstituted or substituted aryl, unsubstituted or substituted aralkyl, an unsubstituted or substituted aromatic heterocyclic group, or unsubstituted or substituted aromatic heterocyclic alkyl, or
• R1 is formula (V):
• wherein R12a is a hydrogen atom, ring A is an aromatic ring or a heteroaromatic ring, n1 is an integer of 0 to 4, R16 is halogen, hydroxy, sulfanyl, nitro, cyano, unsubstituted or substituted lower alkyl, unsubstituted or substituted lower alkenyl, unsubstituted or substituted lower alkynyl, unsubstituted or substituted lower alkoxy, unsubstituted or substituted lower alkylamino, unsubstituted or substituted di-lower alkylamino, provided that when n1 is an integer of 2 to 4, the respective R16's may be the same or different, with the proviso that when ring A is a benzene ring and n1 is 2, R16's are not lower alkoxy,
• R2 is a hydrogen atom, hydroxy, halogen, or unsubstituted or substituted lower alkoxy, and
• R3 is a hydrogen atom or
• wherein n2 is 1.