Oligonucleotide Conjugation by Tyrosine‐Click Reaction.

N,O‐Diacetyl tyrosine and 3‐(4‐hydroxy‐phenyl)‐propanoic acid were converted into phosphoramidite derivatives and introduced to the 5′‐end of oligonucleotides. The resulting oligonucleotides exhibiting a 4‐hydroxyphenyl alkyl group were conjugated with 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione as a model of a Y‐click reaction. The reaction is fast (<1 h) and efficient (>90 %). A partial side reaction occurred at the C8 position of deoxyguanosine in single‐stranded oligonucleotides. This side reaction does not take place on the G quadruplex and double‐stranded oligonucleotides and when N2 position of guanine is protected. This Y‐click reaction could be applied to 1,2,4‐triazoline‐3,5‐dione derivatives displaying a fluorescent dye, a carbohydrate or orthogonal functions (alkyne, azide, ketone or maleimide) for the labelling of oligonucleotides. [ABSTRACT FROM AUTHOR]