Synthetic Applications of Flexible SNO-OCT Strained Alkynes and Their Use in Postpolymerization Modifications

A Synthetic Strategy toward S-, N-, and O-Heterocyclooctynes Facilitates Bioconjugation Using Multifunctional Handles

Over the past two decades, the introduction of bioorthogonal reactions has transformed the ways in which chemoselective labeling, 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 type II dipoles and 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 toward general SNO-OCT scaffolds is demonstrated that enables the 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 sixfold improvement in cycloaddition rates of new analogues compared to first-generation SNO-OCTs.

The 1,3-Dipolar Cycloaddition: From Conception to Quantum Chemical Design

The 1,3-dipolar cycloaddition (1,3-DCA) reaction, conceptualized by Rolf Huisgen in 1960, has proven immensely useful in organic, material, and biological chemistry. The uncatalyzed, thermal transformation is generally sluggish and unselective, but the reactivity can be enhanced by means of metal catalysis or by the introduction of either predistortion or electronic tuning of the dipolarophile. These promoted reactions generally go with a much higher reactivity, selectivity, and yields, often at ambient temperatures. The rapid orthogonal reactivity and compatibility with aqueous and physiological conditions positions the 1,3-DCA as an excellent bioorthogonal reaction. Quantum chemical calculations have been critical for providing an understanding of the physical factors that control the reactivity and selectivity of 1,3-DCAs. In silico derived design principles have proven invaluable for the design of new dipolarophiles with tailored reactivity. This review discusses everything from the conception of the 1,3-DCA all the way to the state-of-the-art methods and models used for the quantum chemical design of novel (bioorthogonal) reagents.

Recent Developments and Strategies for Mutually Orthogonal Bioorthogonal Reactions

Over the past decade, several different metal-free bioorthogonal reactions have been developed to enable simultaneous double-click labeling with minimal-to-no competing cross-reactivities; such transformations are termed 'mutually orthogonal'. More recently, several examples of successful triple ligation strategies have also been described. In this minireview, we discuss selected aspects of the development of orthogonal bioorthogonal reactions over the past decade, including general strategies to drive future innovations to achieve simultaneous, mutually orthogonal click reactions in one pot.