3‐Oxidopyridinium Ions Are Versatile Bioorthogonal Dipoles for Use in Cycloadditions with Cyclooctynes

Abstract

‐oxidopyridinium ions are water stable and soluble heteroaromatic betaines that behave as latent dipoles and undergo a wide variety of cycloadditions. Research into the cycloaddition reactions of 3‐oxidopyridiniums was spearheaded by Alan R. Katritzky and collaborators from the early 1970s until the late 1980s, but they have yet to be used for bioorthogonal applications. Herein we report that 3‐oxidopyridiniums can readily react with 4‐dibenzocyclooctynol (DIBO), a common bioorthogonal handle, in a [3+2] cycloaddition. The mechanism was investigated by altering the electronics of the reaction by changing the substituent on the 5 position of the pyridinium. Electron‐donating 5‐substituents have been shown to significantly increase the rate of the reaction, with bimolecular rate constants ranging from 0.003 M‐1s‐1 with 5‐trifluoromethyl‐N‐methyl‐3‐oxidopyridinium to 1.07 M‐1s‐1 with 5‐amino‐N‐methyl‐3‐oxidopyridinium. 3‐oxidopyridiniums’ appreciable cycloaddition rates and compatibility with bioorthogonally relevant environments give them the potential to be used in a variety of bioconjugation applications.