Asymmetric auxiliary group

A method for synthesis of a nucleoside 3′-phosphoramidite derivative, comprising reacting an optionally protected nucleoside with a 2-chloro-1,3,2-oxazaphospholidine derivative of a chiral reagent or a salt thereof, wherein the chiral reagent has the following chemical formula (I),

• wherein:
• G1 is a hydrogen atom, a nitro group, a halogen atom, a cyano group, or a group of formula (II), (III) or (V), G2 is a group of formula (V),
• wherein G21 to G23 are independently a hydrogen atom, a nitro group, a halogen atom, a cyano group or C1-3 alkyl group,
• wherein G31 to G33 are independently C1-4 alkyl group, C1-4 alkoxy group, C6-14 aryl group, C7-14 aralkyl group, C1-4 alkyl C6-14 aryl group, C1-4 alkoxy C6-14 aryl group, or C6-14 aryl C1-4 alkyl group,
• wherein G51 to G53 are independently a hydrogen atom, a nitro group, a halogen atom, a cyano group, C1-3 alkyl group or C1-3 alkyloxy group, and
• G3 and G4 are independently a hydrogen atom, C1-3 alkyl group, C6-14 aryl group, or G3 and G4 are taken together to form a heteroatom-containing ring that has 3 to 16 carbon atoms.