Large-Scale Flow Photochemical Synthesis of Functionalized trans-Cyclooctenes Using Sulfonated Silica Gel

Trans-cyclooctene-a Swiss army knife for bioorthogonal chemistry: exploring the synthesis, reactivity, and applications in biomedical breakthroughs

BACKGROUND

Trans-cyclooctenes (TCOs) are highly strained alkenes with remarkable reactivity towards tetrazines (Tzs) in inverse electron-demand Diels-Alder reactions. Since their discovery as bioorthogonal reaction partners, novel TCO derivatives have been developed to improve their reactivity, stability, and hydrophilicity, thus expanding their utility in diverse applications.

MAIN BODY

TCOs have garnered significant interest for their applications in biomedical settings. In chemical biology, TCOs serve as tools for bioconjugation, enabling the precise labeling and manipulation of biomolecules. Moreover, their role in nuclear medicine is substantial, with TCOs employed in the radiolabeling of peptides and other biomolecules. This has led to their utilization in pretargeted nuclear imaging and therapy, where they function as both bioorthogonal tags and radiotracers, facilitating targeted disease diagnosis and treatment. Beyond these applications, TCOs have been used in targeted cancer therapy through a "click-to-release" approach, in which they act as key components to selectively deliver therapeutic agents to cancer cells, thereby enhancing treatment efficacy while minimizing off-target effects. However, the search for a suitable TCO scaffold with an appropriate balance between stability and reactivity remains a challenge.

CONCLUSIONS

This review paper provides a comprehensive overview of the current state of knowledge regarding the synthesis of TCOs, and its challenges, and their development throughout the years. We describe their wide ranging applications as radiolabeled prosthetic groups for radiolabeling, as bioorthogonal tags for pretargeted imaging and therapy, and targeted drug delivery, with the aim of showcasing the versatility and potential of TCOs as valuable tools in advancing biomedical research and applications.

Advances in the Synthesis of Bioorthogonal Reagents: s-Tetrazines, 1,2,4-Triazines, Cyclooctynes, Heterocycloheptynes, and trans-Cyclooctenes

Aligned with the increasing importance of bioorthogonal chemistry has been an increasing demand for more potent, affordable, multifunctional, and programmable bioorthogonal reagents. More advanced synthetic chemistry techniques, including transition-metal-catalyzed cross-coupling reactions, C-H activation, photoinduced chemistry, and continuous flow chemistry, have been employed in synthesizing novel bioorthogonal reagents for universal purposes. We discuss herein recent developments regarding the synthesis of popular bioorthogonal reagents, with a focus on s-tetrazines, 1,2,4-triazines, trans-cyclooctenes, cyclooctynes, hetero-cycloheptynes, and -trans-cycloheptenes. This review aims to summarize and discuss the most representative synthetic approaches of these reagents and their derivatives that are useful in bioorthogonal chemistry. The preparation of these molecules and their derivatives utilizes both classical approaches as well as the latest organic chemistry methodologies.

Readily Accessible Strained Difunctionalized trans-Cyclooctenes with Fast Click and Release Capabilities

The click reaction between a functionalized trans-cyclooctene (TCO) and a tetrazine (Tz) is a compelling method for bioorthogonal conjugation in combination with payload releasing capabilities. However, the synthesis of difunctionalized TCOs remains challenging. As a result, these compounds are poorly accessible, which impedes the development of novel applications. In this work, the scalable and accessible synthesis of a new bioorthogonal difunctionalized TCO is reported in only four single selective high yielding steps starting from commercially available compounds. The TCO-Tz click reaction was assessed and revealed excellent kinetic rates and subsequently payload release was shown with various functionalized derivatives. Tetrazine triggered release of carbonate and carbamate payloads was demonstrated up to 100 % release efficiency and local drug release was shown in a cellular toxicity study which revealed a >20-fold increase in cytotoxicity.

Oxidative Desymmetrization Enables the Concise Synthesis of a trans-Cyclooctene Linker for Bioorthogonal Bond Cleavage

Modified trans-cyclooctenes (TCO) are capable of highly efficient molecular manipulations in biological environments, driven by the bioorthogonal reaction with tetrazines (Tz). The development of click-cleavable TCO has fueled the field of in vivo chemistry and enabled the design of therapeutic strategies that have already started to enter the clinic. A key element for most of these approaches is the implementation of a cleavable TCO linker. So far, only one member of this class has been developed, a compound that requires a high synthetic effort, mainly to fulfill the multilayered demands on its chemical structure. To tackle this limitation, we developed a dioxolane-fused cleavable TCO linker (dcTCO) that can be prepared in only five steps by applying an oxidative desymmetrization to achieve diastereoselective introduction of the required functionalities. Based on investigation of the structure, reaction kinetics, stability, and hydrophilicity of dcTCO, we demonstrate its bioorthogonal application in the design of a caged prodrug that can be activated by in-situ Tz-triggered cleavage to achieve a remarkable >1000-fold increase in cytotoxicity.

General, Divergent Platform for Diastereoselective Synthesis of trans-Cyclooctenes with High Reactivity and Favorable Physiochemical Properties*

trans-Cyclooctenes (TCOs) are essential partners in the fastest known bioorthogonal reactions, but current synthetic methods are limited by poor diastereoselectivity. Especially hard to access are hydrophilic TCOs with favorable physicochemical properties for live cell or in vivo experiments. Described is a new class of TCOs, "a-TCOs", prepared in high yield by stereocontrolled 1,2-additions of nucleophiles to trans-cyclooct-4-enone, which itself was prepared on a large scale in two steps from 1,5-cyclooctadiene. Computational transition-state models rationalize the diastereoselectivity of 1,2-additions to deliver a-TCO products, which were also shown to be more reactive than standard TCOs and less hydrophobic than even a trans-oxocene analogue. Illustrating the favorable physicochemical properties of a-TCOs, a fluorescent TAMRA derivative in live HeLa cells was shown to be cell-permeable through intracellular Diels-Alder chemistry and to wash out more rapidly than other TCOs.

A Cleavable C2-Symmetric trans-Cyclooctene Enables Fast and Complete Bioorthogonal Disassembly of Molecular Probes

Bioorthogonal chemistry is bridging the divide between static chemical connectivity and the dynamic physiologic regulation of molecular state, enabling in situ transformations that drive multiple technologies. In spite of maturing mechanistic understanding and new bioorthogonal bond-cleavage reactions, the broader goal of molecular ON/OFF control has been limited by the inability of existing systems to achieve both fast (i.e., seconds to minutes, not hours) and complete (i.e., >99%) cleavage. To attain the stringent performance characteristics needed for high fidelity molecular inactivation, we have designed and synthesized a new C2-symmetric trans-cyclooctene linker (C2TCO) that exhibits excellent biological stability and can be rapidly and completely cleaved with functionalized alkyl-, aryl-, and H-tetrazines, irrespective of click orientation. By incorporation of C2TCO into fluorescent molecular probes, we demonstrate highly efficient extracellular and intracellular bioorthogonal disassembly via omnidirectional tetrazine-triggered cleavage.

Synthetic methodology towards allylic trans-cyclooctene-ethers enables modification of carbohydrates: bioorthogonal manipulation of the lac repressor

The inverse electron-demand Diels-Alder (IEDDA) pyridazine elimination is one of the key bioorthogonal bond-breaking reactions. In this reaction trans-cyclooctene (TCO) serves as a tetrazine responsive caging moiety for amines, carboxylic acids and alcohols. One issue to date has been the lack of synthetic methods towards TCO ethers from functionalized (aliphatic) alcohols, thereby restricting bioorthogonal utilization. Two novel reagents were developed to enable controlled formation of cis-cyclooctene (CCO) ethers, followed by optimized photochemical isomerization to obtain TCO ethers. The method was exemplified by the controlled bioorthogonal activation of the lac operon system in E. coli using a TCO-ether-modified carbohydrate inducer.

Flow Photochemical Syntheses of trans-Cyclooctenes and trans-Cycloheptenes Driven by Metal Complexation

trans-Cyclooctenes and trans-cycloheptenes have long been the subject of physical organic study, but the broader application had been limited by synthetic accessibility. This account describes the development of a general, flow photochemical method for the preparative synthesis of trans-cycloalkene derivatives. Here, photoisom erization takes place in a closed-loop flow reactor where the reaction mixture is continuously cycled through Ag(I) on silica gel. Selective complexation of the trans-isomer by Ag(I) during flow drives an otherwise unfavorable isomeric ratio toward the trans-isomer. Analogous photoreactions under batch-conditions are low yielding, and flow chemistry is necessary in order to obtain trans-cycloalkenes in preparatively useful yields. The applications of the method to bioorthogonal chemistry and stereospecific transannulation chemistry are described.