Light-Induced Hetero-Diels-Alder Cycloaddition: A Facile and Selective Photoclick Reaction

2-Napthoquinone-3-methides (oNQMs) generated by efficient photodehydration (Phi = 0.2) of 3-(hydroxymethyl)-2-naphthol undergo facile hetero-Diels-Alder addition (k(D-A) similar to 4 x 10(4) M-1 s(-1)) to electron-rich polarized olefins in an aqueous solution. The resulting photostable benzo[g]chromans are produced in high to quantitative yield. The unreacted oNQM is rapidly hydrated (k(H2O) similar to 145 s(-1)) to regenerate the starting diol. This competition between hydration and cycloaddition makes oNQMs highly selective, since only vinyl ethers and enamines are reactive enough to form the Diels-Alder adduct in an aqueous solution; no cycloaddition was observed with other types of alkenes. To achieve photolabeling or photoligation of two substrates, one is derivatized with a vinyl ether moiety, while 3-(hydroxymethyl)-2-naphthol is attached to the other via an appropriate linker. The light-induced Diels Alder "click" strategy permits the formation of either a permanent or hydrolytically labile linkage. Rapid kinetics of this photoclick reaction (k = 4 x 10(4) M-1 s(-1)) is useful for time-resolved applications. The short lifetime (tau similar to 7 ms in H2O) of the active form of the photoclick reagent prevents its migration from the site of irradiation, thus, allowing for spatial control of the ligation or labeling.