Cross-linked nucleoside and nucleotide using same

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

• where Base represents a purin-9-yl group that may have any one or more substituents selected from an a group, or a 2-oxo-1,2-dihydropyrimidin-1-yl group that may have any one or more substituents selected from the α group, the α group consisting of a hydroxy group, a hydroxy group protected by a protecting group for nucleic acid synthesis, linear alkyl groups having 1 to 6 carbon atoms, linear alkoxy groups having 1 to 6 carbon atoms, a mercapto group, a mercapto group protected by a protecting group for nucleic acid synthesis, linear alkylthio groups having 1 to 6 carbon atoms, an amino group, linear alkylamino groups having 1 to 6 carbon atoms, an amino group protected by a protecting group for nucleic acid synthesis, and halogen atoms;
• R2 and R3 each independently represent a hydrogen atom, a hydroxy group protecting group for nucleic acid synthesis, an alkyl group having 1 to 7 carbon atoms that may form a branch or a ring, an alkenyl group having 2 to 7 carbon atoms that may form a branch or a ring, an aryl group having 3 to 10 carbon atoms that may have any one or more substituents selected from the α group and may have a heteroatom, an aralkyl group with an aryl moiety having 3 to 12 carbon atoms that may have any one or more substituents selected from the α group and may have a heteroatom, an acyl group that may have any one or more substituents selected from the α group, a silyl group that may have any one or more substituents selected from the α group, a phosphate group that may have any one or more substituents selected from the α group, a phosphate group protected by a protecting group for nucleic acid synthesis, or —P(R4)R5, where R4 and R5 each independently represent a hydroxy group, a hydroxy group protected by a protecting group for nucleic acid synthesis, a mercapto group, a mercapto group protected by a protecting group for nucleic acid synthesis, an amino group, an alkoxy group having 1 to 6 carbon atoms, an alkylthio group having 1 to 6 carbon atoms, a cyanoalkoxy group having 1 to 6 carbon atoms, or a dialkylamino group that has an alkyl group having 1 to 6 carbon atoms;
• X1 is an alkylene group having 1 to 5 carbon atoms, an alkenylene group having 2 to 5 carbon atoms, or —Y1—(CH2)n—, —(CH2)n—Y1— or —(CH2)l—Y1—(CH2)m—, where Y1 is a sulfonyl group, a sulfonamide group, an amide group, an ester group, or a carbonyl group, n is an integer of 1 to 5, 1 and m are positive integers, and the sum of l and m is 2 to 5;
• and
• X2 is an oxygen atom, a sulfur atom, —NH—, or a methylene group.