A new synthetic strategy towards S, N, and O-heterocyclooctynes facilitates bioconjugation using multifunctional handles

Over the past two decades, the introduction of bioorthogonal reactions have transformed the ways in which chemoselective labelling, isolation, imaging, and drug delivery are carried out in a complex biological milieu. A key feature of a good bioorthogonal probe is the ease with which it can be attached to a target compound through bioconjugation. This paper describes the expansion of the utility of a class of unique S, N, and O-containing heterocyclooctynes (SNO-OCTs), which show chemoselective reactivity with Type I and II dipoles and exhibit divergent reactivities in response to electronic tuning of the alkyne. Currently, bioconjugation of SNO-OCTs to a desired target is achieved through an inconvenient aryl- or amide-linker at the sulfamate nitrogen. Herein, a new synthetic approach towards general SNO-OCT scaffolds is demonstrated that enables installation of functional handles at both propargylic carbons of the heterocycloalkyne. This capability increases the utility of SNO-OCTs as labeling reagents through the design of bifunctional bioorthogonal probes with expanded capabilities. NMR kinetics also revealed up to six-fold improvement in cycloaddition rates of the second-generation analogs compared to first-generation SNO-OCTs.