Amidite for synthesizing modified nucleic acid and method for synthesizing modified nucleic acid

A method for synthesizing a modified DNA by a treatment of a modified solid support to which a nucleotide or a nucleoside is added comprising:

• (i) repeating a series of the following sequential steps (a) to (d) which is conducted in order of (a), (b), (c) and (d) or in order of (a), (c), (b) and (d) and which results in an addition of one nucleotide to the modified solid support until a sequence of a final modified DNA is obtained:
• (a) coupling a selected amidite having a 5′-hydroxyl protective group to a 5′-hydroxyl group of a nucleoside or a nucleotide on the modified solid support to form a phosphite triester linkage between the nucleoside structure in the selected amidite and the nucleoside or the nucleotide on the modified solid support;
• (b) optionally blocking unreacted hydroxyl groups of the nucleoside or the nucleotide on the modified solid support;
• (c) oxidizing the phosphite triester linkage to form a phosphotriester linkage;
• (d) removing the 5′-hydroxyl protective group derived from the selected amidite to generate a 5′-hydroxyl group for coupling a subsequent amidite as a selected amidite in subsequent (a);
• and
• (ii) removing the final modified DNA from the solid support;
• wherein the selected amidites applied in (a) to (d) include at least one or more amidites having the General Formula (I);
• wherein the modified DNA obtained is converted into an unmodified DNA from which—Y—Z of the amidite having the General Formula (I) has been removed in concentrated ammonia water from X of the amidite having the General Formula (I):
• wherein X represents a base selected from the group consisting of 9-adeninyl, 9-guaninyl and 1-cytosinyl,
• wherein Y is represented by the General Formula (IV) or the General Formula (V):
• wherein Z is represented by the General Formula (II) or the General Formula (III):
• wherein Q represents a substituent selected from the group consisting of a hydrogen atom, a hydroxyl group and a hydroxyl group protected by a protective group, and
• wherein Y bonds to an amino group of X that is located at position 2 of 9-adeninyl, at position 2 of 9-guaninyl or at position 4 of 1-cytosinyl.