Internal labeling of oligonucleotide probes by Diels–Alder cycloaddition

A new method of adding fluorescent labels to the middle of oligonucleotides is reported. Diels–Alder cycloaddition was used to add five fluorescent maleimides to an oligonucleotide containing a 2′-deoxyuridine modified at the 5-position with a spaced furan. This is a new approach to internal oligonucleotide chemistry that opens up a large range of possibilities for further conjugation.