[4 + 2] and [2 + 4] cycloaddition reactions on single- and double-stranded DNA: a dual-reactive nucleoside.

Here we report dual reactivity of diene-modified duplex DNA containing 5-(1,3-butadienyl)-2'-deoxyuridine "BDdU". Normal-electron demand [4 + 2] cycloaddition proceeded upon addition of a maleimide, whereas inverse-electron demand [2 + 4] cycloaddition occurred upon addition of a tetrazine to give a novel, photoswitchable product.