Process for producing nucleoside

A method of producing a purine nucleoside represented by the formula (I′)

• or a salt thereof, comprising reacting a pyrimidine nucleoside represented by the formula (I)
• or a salt thereof with a purine nucleobase wherein the dissociative hydrogen atom is silylated, which is represented by the formula B′H, in a solvent, in the presence of a Lewis acid:
• wherein
• R1 and R1′ are each independently a hydrogen atom, a hydroxy-protecting group, an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group optionally having substituent(s), an aralkyl group optionally having substituent(s), an acyl group, an aliphatic or aromatic sulfonyl group or a silyl group,
• R2 and R2′ are each independently a hydrogen atom, a hydroxy-protecting group, an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group optionally having substituent(s), an aralkyl group optionally having substituent(s), an acyl group, an aliphatic or aromatic sulfonyl group or a silyl group, or
• R1 and R2 in combination or R1′ and R2′ in combination optionally form a group represented by the formula
• wherein
• R4 in the number of p, R5 in the number of p, R6 and R7 are each independently an alkyl group or an aryl group, and
• p is an integer of 1 to 3,
• R3 is a hydrogen atom, an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group optionally having substituent(s), an aralkyl group optionally having substituent(s), an acyl group, an aliphatic or aromatic sulfonyl group or a functional molecular unit substituent,
• B is a residue derived from a pyrimidine nucleobase optionally having substituent(s),
• B′ is a residue derived from a purine nucleobase optionally having substituent(s),
• m is an integer of 0 to 2, and
• n is an integer of 1 to 3.