Artificial nucleic acid, production method therefor, and use thereof

A compound represented by formula (1) or a salt thereof:

• (wherein:
• “Base” represents an aromatic heterocyclic group which may have a substituent or an aromatic hydrocarbon ring group which may have a substituent;
• A1 represents a linear alkylene group;
• A2 represents a single bond or an alkylene group;
• X represents an alkylene group which may have a substituent or a group having such a structure that at least one methylene group moiety in the alkylene group is substituted by —N(RX)— (wherein RX represents a hydrogen atom or an alkyl group), —O— or —S(═O)k— (wherein k represents 0, 1, or 2);
• R1 and R2 are the same as or different from each other and independently represent a hydrogen atom, an alkyl group which may have a substituent, an alkenyl group which may have a substituent, a cycloalkyl group which may have a substituent, a cycloalkenyl group which may have a substituent, an aryl group which may have a substituent, a protecting group for a hydroxyl group, a phosphino group which has a substituent, a dihydroxyphosphinyl group which may have a substituent, or a hydroxymercaptophosphinyl group which may have a substituent, or R1, R2, two oxygen atoms respectively adjacent to R1 and R2 and carbon atoms at position-3 and position-5 in a furanose together form a ring which may have a substituent;
• and
• R3 represents an amino group which may have a substituent).