Protect the 3'-OH group of one of the nucleosides to prevent it from reacting prematurely. This is often done by converting it into a protecting group like a dimethoxytrityl (DMT) group.
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N-[5'-O-(4,4'-dimethoxytrityl)-3'-amino-3'-deoxythymidin-3'-yl]-O-(3'-O-acetylthymidin-5'-yl)-methanephosphonamidate
Adenosine, N-benzoyl-5'-O-[bis(4-methoxyphenyl)phenylmethyl]-, 3'-benzoate
