In addition, the oligonucleotide protecting groups could be removed using a mild base (e.g., 50 mM potassium carbonate methanol solution). Thus, the deprotection time and temperature were significantly reduced compared to the conventional conditions (28% NH 3 aq., 55 ☌, 17 h). All the protecting groups were removed by treating the oligonucleotides with 40% aqueous methylamine at room temperature for 2 h. Various basic solutions were investigated for protecting group removal. The resultant phosphoramidites were then successfully incorporated into oligonucleotides bearing a 3′-amino linker. Therefore, we designed Fmoc-protected phosphoramidites for the synthesis of base-labile oligonucleotides modified with a 3′-amino linker. However, chemically modified nucleosides, which are unstable under basic conditions, cannot be incorporated into oligonucleotides using the conventional method entailing the preparation of oligonucleotides bearing a 3′-amino linker. Oligonucleotides with an amino linker at the 3′-end are useful for the preparation of conjugated oligonucleotides.
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