Is Mn(I) More Promising Than Fe(II)-A Comparison of Mn vs Fe Complexes for Olefin Metathesis

Olefin metathesis is one of the most significant transformations in organic chemistry and is an excellent example for efficient homogeneous catalysis. Although most currently used catalysts are primarily based on 4d and 5d metals, cycloaddition and cycloreversion reactions can also be attributed to first-row transition metals, such as Fe. Surprisingly, the potential of Mn(I)-based catalysts for olefin metathesis has been unexplored despite their prominence in homogeneous catalysis and their diagonal relationship to Ru(II). In the present study, we have investigated the prospective capabilities of Mn complexes for cycloaddition and reversion reactions using density functional theory. Therefore, we have initially compared the literature known iron model systems and their isoelectronic Mn counterparts regarding their reactivity and electronic structure. Next, we constructed potential Mn complexes derived from synthetically accessible species, including carbonyl ligands and obeying octahedral geometry. Based on thermodynamic parameters and the calculation of electronic descriptors, we were able to validate the isodiagonal relationship. Our study serves as guidance for the experimental chemist.

Directionality of Halogen-Bonds: Insights from 2D Energy Decomposition Analysis

Halogen bonds are typically observed to have a linear arrangement with a 180° angle between the nucleophile and the halogen bond acceptor X-R. This linearity is commonly explained using the σ-hole model, although there have been alternative explanations involving exchange repulsion forces. We employ two-dimensional Distortion/Interaction and Energy Decomposition Analysis to examine the archetypal H3 N⋯X2 halogen bond systems. Our results indicate that although halogen bonds are predominantly electrostatic, their directionality is largely due to decreased Pauli repulsion in linear configurations as opposed to angled ones in the I2 and Br2 systems. As we move to the smaller halogens, Cl2 and F2 , the influence of Pauli repulsion diminishes, and the energy surface is shaped by orbital interactions and electrostatic forces. These results support the role of exchange repulsion forces in influencing the directionality of strong halogen bonds. Additionally, we demonstrate that the 2D Energy Decomposition Analysis is a useful tool for enhancing our understanding of the nature of potential energy surfaces in noncovalent interactions.

How cycloalkane fusion enhances the cycloaddition reactivity of dibenzocyclooctynes

Dibenzoannulated cyclooctynes have emerged as valuable compounds for bioorthogonal reactions. They are commonly used in combination with azides in strain-promoted 1,3-dipolar cycloadditions. They are typically, however, unreactive towards 3,6-disubstituted tetrazines in inverse electron-demand Diels-Alder cycloadditions. Recently a dibenzoannulated bicyclo[6.1.0]nonyne derivative (DMBO) with a cyclopropane fused to the cyclooctyne core was described, which showed surprising reactivity towards tetrazines. To elucidate the unusual reactivity of DMBO, we performed density functional theory calculations and revealed that a tub-like structure in the transition state results in a much lower activation barrier than in the absence of cyclopropane fusion. The same transition state geometry is found for different cycloalkanes fused to the cyclooctyne core albeit higher activation barriers are observed for increased ring sizes. This conformation is energetically unfavored for previously known dibenzoannulated cyclooctynes and allows tetrazines and azides to approach DMBO from the face rather than the edge, a trajectory that was hitherto not observed for this class of activated dieno- and dipolarophiles.

"Holographic" Autostereoscopic Displays: A Perspective on Their Technology and Potential Impact in Chemistry

The three-dimensional arrangement of atoms in molecules is essential for understanding their properties and behavior. Traditional 2D representations and digital 3D models presented on 2D media often fall short in conveying the complexity of molecular structures. Autostereoscopic displays, often marketed as "holographic" displays, pose a potential solution to this challenge. These displays, with their multi-view and single-view configurations, promise to advance chemistry education and research by offering accurate 3D representations with depth and parallax. In this perspective, I delve into the possibilities and limitations of autostereoscopic displays in chemistry, discussing the underlying technology and potential applications, from research to teaching and science communication. Multi-view autostereoscopic displays excel in facilitating collaborative work by enabling multiple viewers to simultaneously perceive the same 3D structure from different angles. However, they currently suffer from low resolution and high cost, which could limit their immediate widespread adoption. Conversely, single-view autostereoscopic displays with eye-tracking, while limited to one viewer at a time, provide higher resolution at a lower cost, thus suggesting that they might become the technology of the future given the balance of price to performance. Despite current limitations, autostereoscopic displays possess undeniable potential for shaping the future of chemistry education and research.

To Bond or Not to Bond: Metal-Metal Interaction in Heterobimetallic Rare-Earth Metal-Silver Complexes

The reaction of 2,4-tBu2-6-(PPh2)PhOH (HOArP) with silver(I) triflate in a 3:1 molar ratio gave the mononuclear coinage metal complex (HOArP-κP)3AgIOTf (1). Treatment of HOArP with LnIII[N(SiMe3)2]3 (Ln = La, Sm, Y, Yb) in a 3:1 molar ratio yielded the mononuclear rare-earth metal complexes LnIII(OArP-κ2O,P)3 (2-Ln). The heterobimetallic rare-earth metal-silver complexes LnIII(OTf)(μ-OArP-1κ1O,2κ1P)3AgI (3-Ln) were prepared from monometallic precursors by reactions of equimolar amounts of 1 with LnIII[N(SiMe3)2]3 or 2-Ln with silver(I) triflate, respectively. The compounds were characterized by NMR, ultraviolet-visible (UV-vis), and infrared (IR) spectroscopy, single-crystal X-ray diffraction, elemental analysis, and the effective magnetic moments of the paramagnetic complexes were determined via the Evans NMR method. Computational studies were conducted on 3-La and 3-Y.

Unraveling the Bürgi-Dunitz Angle with Precision: The Power of a Two-Dimensional Energy Decomposition Analysis

Understanding the geometrical preferences in chemical reactions is crucial for advancing the field of organic chemistry and improving synthetic strategies. One such preference, the Bürgi-Dunitz angle, is central to nucleophilic addition reactions involving carbonyl groups. This study successfully employs a novel two-dimensional Distortion-Interaction/Activation-Strain Model in combination with a two-dimensional Energy Decomposition Analysis to investigate the origins of the Bürgi-Dunitz angle in the addition reaction of CN- to (CH3)2C═O. We constructed a 2D potential energy surface defined by the distance between the nucleophile and carbonylic carbon atom and by the attack angle, followed by an in-depth exploration of energy components, including strain and interaction energy. Our analysis reveals that the Bürgi-Dunitz angle emerges from a delicate balance between two key factors: strain energy and interaction energy. High strain energy, as a result of the carbonyl compound distorting to avoid Pauli repulsion, is encountered at high angles, thus setting the upper bound. On the other hand, interaction energy is shaped by a dominant Pauli repulsion when the angles are lower. This work emphasizes the value of the 2D Energy Decomposition Analysis as a refined tool, offering both quantitative and qualitative insights into chemical reactivity and selectivity.

Orthogonal Inverse-Electron-Demand Cycloaddition Reactions Controlled by Frontier Molecular Orbital Interactions

Chemoselective pairs of bioorthogonal reactants enable the simultaneous labeling of several biomolecules. Here, we access orthogonal click reactions by exploiting differences in frontier molecular orbital interaction energies in transition states. We establish that five-membered cyclic dienes are inert to isonitriles but readily react with strained alkynes, while tetrazines with bulky substituents readily react with isonitriles. Strained alkynes show an opposite reactivity pattern. The approach was demonstrated by orthogonally labeling two proteins with different fluorophores.

Substituent Effects in Bioorthogonal Diels-Alder Reactions of 1,2,4,5-Tetrazines

1,2,4,5-Tetrazines are increasingly used as reactants in bioorthogonal chemistry due to their high reactivity in Diels-Alder reactions with various dienophiles. Substituents in the 3- and 6-positions of the tetrazine scaffold are known to have a significant impact on the rate of cycloadditions; this is commonly explained on the basis of frontier molecular orbital theory. In contrast, we show that reactivity differences between commonly used classes of tetrazines are not controlled by frontier molecular orbital interactions. In particular, we demonstrate that mono-substituted tetrazines show high reactivity due to decreased Pauli repulsion, which leads to a more asynchronous approach associated with reduced distortion energy. This follows the recent Vermeeren-Hamlin-Bickelhaupt model of reactivity increase in asymmetric Diels-Alder reactions. In addition, we reveal that ethylene is not a good model compound for other alkenes in Diels-Alder reactions.

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.

Synthesis and conformational analysis of a potentially super-armed glucuronidation donor

The concise synthesis of a potentially “super-armed” glucuronidation donor is reported. The α-anomer was crystallized and analyzed by single crystal X-ray diffraction. The pyranose ring was found to be in a twist-boat conformation in the solid state. To confirm the relevance of this finding for the solution state, and explain the failure of analysis by NMR, DFT calculations were performed. They revealed the twist-boat to be the dominant among a group of several possible conformers at ambient temperature.

Graphical abstract

Cycloaddition Reactivities Analyzed by Energy Decomposition Analyses and the Frontier Molecular Orbital Model

This Account describes our quest to understand and predict organic reactivity, a principal goal of physical and theoretical organic chemistry. The focus is on the development and testing of models for the prediction of cycloaddition reactivities and selectivities. We describe the involvement of the Houk group, and other groups, in the evolution of theoretical models that can achieve ever greater accuracy as well as provide practical heuristic models for understanding and prediction.Is the venerable frontier molecular orbital (FMO) model, the basis of Kenichi Fukui's 1981 Nobel Prize, still useful, or must it be replaced with more advanced models? In particular, models such as Conceptual Density Functional, the Pauli Exclusion Model, and the recent popularity of Electrostatic Potential Plots and Dispersion Energies have not only added to our understanding, but they have also created uncertainty about whether the simple FMO heuristic model has a place in 21st century discussions. This Account addresses this issue and asserts the value of the FMO model.Beginning with brief descriptions of selected models for cycloaddition reactivity starting with early donor-acceptor (nucleophile-electrophile) charge-transfer concepts, this Account reviews Fukui's frontier molecular orbital model, Salem and Klopman's orbital, electrostatic and Pauli repulsion model, the conceptual DFT model by Parr and later by Domingo and others, the recent Houk and Bickelhaupt Distortion/Interaction Activation Strain model, and the Bickelhaupt-Hamlin's Pauli-repulsion lowering model.Computations and analyses of four well-studied Diels-Alder cycloadditions, both normal and inverse electron-demand types, are presented. Most were studied earlier in our published work but are presented here with new insights from calculations with modern methods. Depending on the types of substrates (cycloaddends), the dominant factors controlling reactivity can be orbital interactions, electrostatics and polarization, or Pauli repulsion and dispersion effects, or a combination of all of these.By comparing orbital interactions, especially the frontier molecular orbital interactions, with the other factors that influence reactivity, we show why the FMO model is such a powerful─and theoretically meaningful─heuristic for understanding and predicting reactivity. We also present a method to understand Pauli repulsion effects on activation barriers, ρ(1.1). The use of a new reaction coordinate, the extent of Pauli repulsion along the reaction path, is advocated to emphasize the role of repulsive occupied orbital interactions on reactivity.Fukui's frontier molecular orbital model is effective because FMO interactions parallel all the quantities that influence reactivity. The FMO model continues to provide a practical model to understand and guide experiments.

Mechanistic Insights into the Reaction of Amidines with 1,2,3-Triazines and 1,2,3,5-Tetrazines

1,2,3-Triazines and 1,2,3,5-tetrazines react rapidly, efficiently, and selectively with amidines to form pyrimidines/1,3,5-triazines, exhibiting an orthogonal reactivity with 1,2,4,5-tetrazine-based conjugation chemistry. Whereas the mechanism of the reaction of the isomeric 1,2,4-triazines and 1,2,4,5-tetrazines with alkenes is well understood, the mechanism of the 1,2,3-triazine/1,2,3,5-tetrazine-amidine reaction as well as its intrinsic reactivity remains underexplored. By using ¹⁵N-labeling, kinetic investigations, and kinetic isotope effect studies, complemented by extensive computational investigations, we show that this reaction proceeds through an addition/N₂ elimination/cyclization pathway, rather than the generally expected concerted or stepwise Diels-Alder/retro Diels-Alder sequence. The rate-limiting step in this transformation is the initial nucleophilic attack of an amidine on azine C4, with a subsequent energetically favored N₂ elimination step compared with a disfavored stepwise formation of a Diels-Alder cycloadduct. The proposed reaction mechanism is in agreement with experimental and computational results, which explains the observed reactivity of 1,2,3-triazines and 1,2,3,5-tetrazines with amidines.

Uncovering the Key Role of Distortion in Bioorthogonal Tetrazine Tools That Defy the Reactivity/Stability Trade-Off

The tetrazine/trans-cyclooctene ligation stands out from the bioorthogonal toolbox due to its exceptional reaction kinetics, enabling multiple molecular technologies in vitro and in living systems. Highly reactive 2-pyridyl-substituted tetrazines have become state of the art for time-critical processes and selective reactions at very low concentrations. It is widely accepted that the enhanced reactivity of these chemical tools is attributed to the electron-withdrawing effect of the heteroaryl substituent. In contrast, we show that the observed reaction rates are way too high to be explained on this basis. Computational investigation of this phenomenon revealed that distortion of the tetrazine caused by intramolecular repulsive N–N interaction plays a key role in accelerating the cycloaddition step. We show that the limited stability of tetrazines in biological media strongly correlates with the electron-withdrawing effect of the substituent, while intramolecular repulsion increases the reactivity without reducing the stability. These fundamental insights reveal thus far overlooked mechanistic aspects that govern the reactivity/stability trade-off for tetrazines in physiologically relevant environments, thereby providing a new strategy that may facilitate the rational design of these bioorthogonal tools.

Synergistic Experimental and Computational Investigation of the Bioorthogonal Reactivity of Substituted Aryltetrazines

Tetrazines (Tz) have been applied as bioorthogonal agents for various biomedical applications, including pretargeted imaging approaches. In radioimmunoimaging, pretargeting increases the target-to-background ratio while simultaneously reducing the radiation burden. We have recently reported a strategy to directly ¹⁸F-label highly reactive tetrazines based on a 3-(3-fluorophenyl)-Tz core structure. Herein, we report a kinetic study on this versatile scaffold. A library of 40 different tetrazines was prepared, fully characterized, and investigated with an emphasis on second-order rate constants for the reaction with trans-cyclooctene (TCO). Our results reveal the effects of various substitution patterns and moreover demonstrate the importance of measuring reactivities in the solvent of interest, as click rates in different solvents do not necessarily correlate well. In particular, we report that tetrazines modified in the 2-position of the phenyl substituent show high intrinsic reactivity toward TCO, which is diminished in aqueous systems by unfavorable solvent effects. The obtained results enable the prediction of the bioorthogonal reactivity and thereby facilitate the development of the next generation of substituted aryltetrazines for in vivo applications.

Origin of Increased Reactivity in Rhenium-Mediated Cycloadditions of Tetrazines

Pyridyl tetrazines coordinated to metals like rhenium have been shown to be more reactive in [4 + 2] cycloadditions than their uncomplexed counterparts. Using density functional theory calculations, we found a more favorable interaction energy caused by stronger orbital interactions as the origin of this increased reactivity. Additionally, the high regioselectivity is due to a greater degree of charge stabilization in the transition state, leading to the major product.

Direct Cu-mediated aromatic 18F-labeling of highly reactive tetrazines for pretargeted bioorthogonal PET imaging

Pretargeted imaging can be used to visualize and quantify slow-accumulating targeting vectors with short-lived radionuclides such as fluorine-18 - the most popular clinically applied Positron Emission Tomography (PET) radionuclide. Pretargeting results in higher target-to-background ratios compared to conventional imaging approaches using long-lived radionuclides. Currently, the tetrazine ligation is the most popular bioorthogonal reaction for pretargeted imaging, but a direct 18F-labeling strategy for highly reactive tetrazines, which would be highly beneficial if not essential for clinical translation, has thus far not been reported. In this work, a simple, scalable and reliable direct 18F-labeling procedure has been developed. We initially studied the applicability of different leaving groups and labeling methods to develop this procedure. The copper-mediated 18F-labeling exploiting stannane precursors showed the most promising results. This approach was then successfully applied to a set of tetrazines, including highly reactive H-tetrazines, suitable for pretargeted PET imaging. The labeling succeeded in radiochemical yields (RCYs) of up to approx. 25%. The new procedure was then applied to develop a pretargeting tetrazine-based imaging agent. The tracer was synthesized in a satisfactory RCY of ca. 10%, with a molar activity of 134 ± 22 GBq μmol-1 and a radiochemical purity of >99%. Further evaluation showed that the tracer displayed favorable characteristics (target-to-background ratios and clearance) that may qualify it for future clinical translation.

Origins of Endo Selectivity in Diels-Alder Reactions of Cyclic Allene Dienophiles

Strained cyclic allenes, first discovered in 1966 by Wittig and co-workers, have recently emerged as valuable synthetic building blocks. Previous experimental investigations, and computations reported here, demonstrate that the Diels-Alder reactions of furans and pyrroles with 1,2-cyclohexadiene and oxa- and azaheterocyclic analogs proceed with endo selectivity. This endo selectivity gives the adduct with the allylic saturated carbon of the cyclic allene endo to the diene carbons. The selectivity is very general and useful in synthetic applications. Our computational study establishes the origins of this endo selectivity. We analyze the helical frontier molecular orbitals of strained cyclic allenes and show how secondary orbital and electrostatic effects influence stereoselectivity. The LUMO of carbon-3 of the allene (C-3 is not involved in primary orbital interactions) interacts in a stabilizing fashion with the HOMO of the diene in such a way that the carbon of the cyclic allene attached to C-1 favors the endo position in the transition state. The furan LUMO, allene HOMO interaction reinforces this preference. These mechanistic studies are expected to prompt the further use of long-avoided strained cyclic allenes in chemical synthesis.

The Influence of Substitution on Thiol-Induced Oxanorbornadiene Fragmentation

Oxanorbornadienes (ONDs) undergo facile Michael addition with thiols and then fragment by a retro-Diels-Alder (rDA) reaction, a unique two-step sequence among electrophilic cleavable linkages. The rDA reaction rate was explored as a function of the furan structure, with substituents at the 2- and 5-positions found to be the most influential and the fragmentation rate to be inversely correlated with electron-withdrawing ability. Density functional theory calculations provided an excellent correlation with the experimentally measured OND rDA rates.

Lipophilicity and Click Reactivity Determine the Performance of Bioorthogonal Tetrazine Tools in Pretargeted In Vivo Chemistry

The development of highly selective and fast biocompatible reactions for ligation and cleavage has paved the way for new diagnostic and therapeutic applications of pretargeted in vivo chemistry. The concept of bioorthogonal pretargeting has attracted considerable interest, in particular for the targeted delivery of radionuclides and drugs. In nuclear medicine, pretargeting can provide increased target-to-background ratios at early time-points compared to traditional approaches. This reduces the radiation burden to healthy tissue and, depending on the selected radionuclide, enables better imaging contrast or higher therapeutic efficiency. Moreover, bioorthogonally triggered cleavage of pretargeted antibody-drug conjugates represents an emerging strategy to achieve controlled release and locally increased drug concentrations. The toolbox of bioorthogonal reactions has significantly expanded in the past decade, with the tetrazine ligation being the fastest and one of the most versatile in vivo chemistries. Progress in the field, however, relies heavily on the development and evaluation of (radio)labeled compounds, preventing the use of compound libraries for systematic studies. The rational design of tetrazine probes and triggers has thus been impeded by the limited understanding of the impact of structural parameters on the in vivo ligation performance. In this work, we describe the development of a pretargeted blocking assay that allows for the investigation of the in vivo fate of a structurally diverse library of 45 unlabeled tetrazines and their capability to reach and react with pretargeted trans-cyclooctene (TCO)-modified antibodies in tumor-bearing mice. This study enabled us to assess the correlation of click reactivity and lipophilicity of tetrazines with their in vivo performance. In particular, high rate constants (>50 000 M-1 s-1) for the reaction with TCO and low calculated logD 7.4 values (below -3) of the tetrazine were identified as strong indicators for successful pretargeting. Radiolabeling gave access to a set of selected 18F-labeled tetrazines, including highly reactive scaffolds, which were used in pretargeted PET imaging studies to confirm the results from the blocking study. These insights thus enable the rational design of tetrazine probes for in vivo application and will thereby assist the clinical translation of bioorthogonal pretargeting.

Computational Exploration of Ambiphilic Reactivity of Azides and Sustmann's Paradigmatic Parabola

We examine the theoretical underpinnings of the seminal discoveries by Reiner Sustmann about the ambiphilic nature of Huisgen’s phenyl azide cycloadditions. Density functional calculations with ωB97X-D and B2PLYP-D3 reproduce the experimental data and provide insights into ambiphilic control of reactivity. Distortion/interaction-activation strain and energy decomposition analyses show why Sustmann’s use of dipolarophile ionization potential is such a powerful predictor of reactivity. We add to Sustmann’s data set several modern distortion-accelerated dipolarophiles used in bioorthogonal chemistry to show how these fit into the orbital energy criteria that are often used to understand cycloaddition reactivity. We show why such a simple indicator of reactivity is a powerful predictor of reaction rates that are actually controlled by a combination of distortion energies, charge transfer, closed-shell repulsion, polarization, and electrostatic effects.

How the Lewis Base F- Catalyzes the 1,3-Dipolar Cycloaddition between Carbon Dioxide and Nitrilimines

The mechanism of the Lewis base F^– catalyzed 1,3-dipolar cycloaddition between CO₂ and nitrilimines is interrogated using DFT calculations. F^– activates the nitrilimine, not CO₂ as proposed in the literature, and imparts a significant rate enhancement for the cycloaddition. The origin of this catalysis is in the strength of the primary orbital interactions between the reactants. The Lewis base activated nitrilimine–F^– has high-lying filled FMOs. The smaller FMO-LUMO gap promotes a rapid nucleophilic attack and overall cycloaddition with CO₂.

Computational generation of an annotated gigalibrary of synthesizable, composite peptidic macrocycles

Peptidomimetic macrocycles have the potential to regulate challenging therapeutic targets. Structures of this type having precise shapes and drug-like character are particularly coveted, but are relatively difficult to synthesize. Our laboratory has developed robust methods that integrate small-peptide units into designed scaffolds. These methods create macrocycles and embed condensed heterocycles to diversify outcomes and improve pharmacological properties. The hypothetical scope of the methodology is vast and far outpaces the capacity of our experimental format. We now describe a computational rendering of our methodology that creates an in silico three-dimensional library of composite peptidic macrocycles. Our open-source platform, CPMG (Composite Peptide Macrocycle Generator), has algorithmically generated a library of 2,020,794,198 macrocycles that can result from the multistep reaction sequences we have developed. Structures are generated based on predicted site reactivity and filtered on the basis of physical and three-dimensional properties to identify maximally diverse compounds for prioritization. For conformational analyses, we also introduce ConfBuster++, an RDKit port of the open-source software ConfBuster, which allows facile integration with CPMG and ready parallelization for better scalability. Our approach deeply probes ligand space accessible via our synthetic methodology and provides a resource for large-scale virtual screening.

Live Monitoring of Strain-Promoted Azide Alkyne Cycloadditions in Complex Reaction Environments by Inline ATR-IR Spectroscopy

The strain-promoted azide alkyne cycloaddition (SPAAC) is a powerful tool for forming covalent bonds between molecules even under physiological conditions, and therefore found broad application in fields ranging from biological chemistry and biomedical research to materials sciences. For many applications, knowledge about reaction kinetics of these ligations is of utmost importance. Kinetics are commonly assessed and studied by NMR measurements. However, these experiments are limited in terms of temperature and restricted to deuterated solvents. By using an inline ATR-IR probe we show that the cycloaddition of azides and alkynes can be monitored in aqueous and even complex biological fluids enabling the investigation of reaction kinetics in various solvents and even human blood plasma under controlled conditions in low reaction volumes.

Concerted [4 + 2] and Stepwise (2 + 2) Cycloadditions of Tetrafluoroethylene with Butadiene: DFT and DLPNO-UCCSD(T) Explorations

Tetrafluoroethylene and butadiene form the 2 + 2 cycloadduct under kinetic control, but the Diels–Alder cycloadduct is formed under thermodynamic control. Borden and Getty showed that the preference for 2 + 2 cycloaddition is due to the necessity for syn-pyramidalization of the two CF₂ groups in the 4 + 2 transition state. We have explored the full potential energy surface for the concerted and stepwise reactions of tetrafluoroethylene and butadiene with density functional theory, DFT (B3LYP and M06-2X), DLPNO-UCCSD(T), and CASSCF-NEVPT2 methods and with the distortion/interaction–activation strain model to explain the energetics of different pathways. The 2 + 2 cycloadduct is formed by an anti-transition state followed by two rotations and a final bond formation transition state. Energetics are compared to the reaction of maleic anhydride and ethylene.

Trans-Cyclooctene-Functionalized PeptoBrushes with Improved Reaction Kinetics of the Tetrazine Ligation for Pretargeted Nuclear Imaging

Tumor targeting using agents with slow pharmacokinetics represents a major challenge in nuclear imaging and targeted radionuclide therapy as they most often result in low imaging contrast and high radiation dose to healthy tissue. To address this challenge, we developed a polymer-based targeting agent that can be used for pretargeted imaging and thus separates tumor accumulation from the imaging step in time. The developed targeting agent is based on polypeptide-graft-polypeptoid polymers (PeptoBrushes) functionalized with trans-cyclooctene (TCO). The complementary ¹¹¹In-labeled imaging agent is a 1,2,4,5-tetrazine derivative, which can react with aforementioned TCO-modified PeptoBrushes in a rapid bioorthogonal ligation. A high degree of TCO loading (up to 30%) was achieved, without altering the physicochemical properties of the polymeric nanoparticle. The highest degree of TCO loading resulted in significantly increased reaction rates (77-fold enhancement) compared to those with small molecule TCO moieties when using lipophilic tetrazines. Based on computer simulations, we hypothesize that this increase is a result of hydrophobic effects and significant rearrangements within the polymer framework, in which hydrophobic patches of TCO moieties are formed. These patches attract lipophilic tetrazines, leading to increased reaction rates in the bioorthogonal ligation. The most reactive system was evaluated as a targeting agent for pretargeted imaging in tumor-bearing mice. After the setup was optimized, sufficient tumor-to-background ratios were achieved as early as 2 h after administration of the tetrazine imaging agent, which further improved at 22 h, enabling clear visualization of CT-26 tumors. These findings show the potential of PeptoBrushes to be used as a pretargeting agent when an optimized dose of polymer is used.

Tetrazine- and trans-cyclooctene-functionalised polypept(o)ides for fast bioorthogonal tetrazine ligation

Tetrazine- and trans-cyclooctene-functionalised polypeptides and polypetoids were prepared by ring-opening polymerisation of N-carboxyanhydrides using the respective functional initiators and shown to react in fast bioorthogonal tetrazine ligations.

Isonitrile-responsive and bioorthogonally removable tetrazine protecting groups

Tetrazylmethyl groups are reported here as bioorthogonal protecting groups that are readily removed by isonitriles, establishing a valuable addition to the dissociative bioorthogonal chemistry and synthetic methodology toolboxes.

In vivo compatible reactions have a broad range of possible applications in chemical biology and the pharmaceutical sciences. Here we report tetrazines that can be removed by exposure to isonitriles under very mild conditions. Tetrazylmethyl derivatives are easily accessible protecting groups for amines and phenols. The isonitrile-induced removal is rapid and near-quantitative. Intriguingly, the deprotection is especially effective with (trimethylsilyl)methyl isocyanide, and serum albumin can catalyze the elimination under physiological conditions. NMR and computational studies revealed that an imine-tautomerization step is often rate limiting, and the unexpected cleavage of the Si–C bond accelerates this step in the case with (trimethylsilyl)methyl isocyanide. Tetrazylmethyl-removal is compatible with use on biomacromolecules, in cellular environments, and in living organisms as demonstrated by cytotoxicity experiments and fluorophore-release studies on proteins and in zebrafish embryos. By combining tetrazylmethyl derivatives with previously reported tetrazine-responsive 3-isocyanopropyl groups, it was possible to liberate two fluorophores in vertebrates from a single bioorthogonal reaction. This chemistry will open new opportunities towards applications involving multiplexed release schemes and is a valuable asset to the growing toolbox of bioorthogonal dissociative reactions.

HPMA-Based Nanoparticles for Fast, Bioorthogonal iEDDA Ligation

Fast and bioorthogonally reacting nanoparticles are attractive tools for biomedical applications such as tumor pretargeting. In this study, we designed an amphiphilic block copolymer system based on HPMA using different strategies to introduce the highly reactive click units 1,2,4,5-tetrazines (Tz) either at the chain end (Tz-CTA) or statistical into the hydrophobic block. This reactive group undergoes a rapid, bioorthogonal inverse electron-demand Diels–Alder reaction (iEDDA) with trans-cyclooctenes (TCO). Subsequently, this polymer platform was used for the preparation of different Tz-covered nanoparticles, such as micelles and colloids. Thereby it was found that the reactivity of the polymeric micelles is comparable to that of the low molar mass tetrazines. The core-cross-linked micelles can be successfully conjugated at rather low concentrations to large biomacromolecules like antibodies, not only in physiological buffer, but also in human blood plasma, which was confirmed by fluorescence correlation spectroscopy (FCS).

Stable, Reactive, and Orthogonal Tetrazines: Dispersion Forces Promote the Cycloaddition with Isonitriles

The isocyano group is a structurally compact bioorthogonal functional group that reacts with tetrazines under physiological conditions. Now it is shown that bulky tetrazine substituents accelerate this cycloaddition. Computational studies suggest that dispersion forces between the isocyano group and the tetrazine substituents in the transition state contribute to the atypical structure-activity relationship. Stable asymmetric tetrazines that react with isonitriles at rate constants as high as 57 L mol-1  s-1 were accessible by combining bulky and electron-withdrawing substituents. Sterically encumbered tetrazines react selectively with isonitriles in the presence of strained alkenes/alkynes, which allows for the orthogonal labeling of three proteins. The established principles will open new opportunities for developing tetrazine reactants with improved characteristics for diverse labeling and release applications with isonitriles.

Secondary Orbital Interactions Enhance the Reactivity of Alkynes in Diels-Alder Cycloadditions

We have investigated the inverse electron-demand Diels–Alder reactions of trans-cyclooctene (TCO) and endo-bicyclo[6.1.0]nonyne (BCN) with a 1,2,4,5-tetrazine, a cyclopentadienone, and an ortho-benzoquinone. Tetrazines react significantly faster with TCO compared to BCN because the highest occupied molecular orbital (HOMO) of TCO is significantly higher in energy than the HOMO of BCN and there is less distortion of the tetrazine. Despite the different HOMO energies, TCO and BCN have similar reactivities toward cyclopentadienones, while BCN is significantly more reactive than TCO in the cycloaddition with ortho-benzoquinone. We find that the higher reactivity of BCN compared to TCO with ortho-benzoquinone is due to secondary orbital interactions of the BCN HOMO-1 with the diene LUMO.

2-O-Benzyloxycarbonyl protected glycosyl donors: a revival of carbonate-mediated anchimeric assistance for diastereoselective glycosylation

Benzyloxycarbonyl can be used as participating group for the diastereoselective glycosylation of base-labile products and the synthesis of glycosyl esters.

Chemoselectivity of Tertiary Azides in Strain-Promoted Alkyne-Azide Cycloadditions

The strain‐promoted alkyne‐azide cycloaddition (SPAAC) is the most commonly employed bioorthogonal reaction with applications in a broad range of fields. Over the years, several different cyclooctyne derivatives have been developed and investigated in regard to their reactivity in SPAAC reactions with azides. However, only a few studies examined the influence of structurally diverse azides on reaction kinetics. Herein, we report our investigations of the reactivity of primary, secondary, and tertiary azides with the cyclooctynes BCN and ADIBO applying experimental and computational methods. All azides show similar reaction rates with the sterically non‐demanding cyclooctyne BCN. However, due to the increased steric demand of the dibenzocyclooctyne ADIBO, the reactivity of tertiary azides drops by several orders of magnitude in comparison to primary and secondary azides. We show that this chemoselective behavior of tertiary azides can be exploited to achieve semiorthogonal dual‐labeling without the need for any catalyst using SPAAC exclusively.

Factors Controlling the Diels-Alder Reactivity of Hetero-1,3-Butadienes

We have quantum chemically explored the Diels-Alder reactivities of a systematic series of hetero-1,3-butadienes with ethylene by using density functional theory at the BP86/TZ2P level. Activation strain analyses provided physical insight into the factors controlling the relative cycloaddition reactivity of aza- and oxa-1,3-butadienes. We find that dienes with a terminal heteroatom, such as 2-propen-1-imine (NCCC) or acrolein (OCCC), are less reactive than the archetypal 1,3-butadiene (CCCC), primarily owing to weaker orbital interactions between the more electronegative heteroatoms with ethylene. Thus, the addition of a second heteroatom at the other terminal position (NCCN and OCCO) further reduces the reactivity. However, the introduction of a nitrogen atom in the backbone (CNCC) leads to enhanced reactivity, owing to less Pauli repulsion resulting from polarization of the diene HOMO in CNCC towards the nitrogen atom and away from the terminal carbon atom. The Diels-Alder reactions of ethenyl-diazene (NNCC) and 1,3-diaza-butadiene (NCNC), which contain heteroatoms at both the terminal and backbone positions, are much more reactive due to less activation strain compared to CCCC.

[18F]Fluoroalkyl azides for rapid radiolabeling and (Re)investigation of their potential towards in vivo click chemistry

In recent years, radiofluorinated alkyl azides have been reported for click radiolabeling and pretargeted PET imaging, but only little is known about the biodistribution and metabolism of these compounds. In this work, we present a significantly improved procedure for the synthesis of [¹⁸F]fluoroethyl azide and reinvestigated this radiolabeled probe in detail showing poor stability and very restricted suitability for in vivo application. Therefore, modified low-molecular-weight [¹⁸F]fluoroalkyl azides were developed. Propargyl-tagged endomorphin-1 (as model compound) was successfully radiolabeled in high yield and short reaction time making these probes useful and efficient bioorthogonal tools for rapid radiolabeling. Biodistribution, pharmacokinetics and in vivo stability were studied by preclinical PET/MR scanning and metabolite analysis. The results of this study revealed only limited applicability of [¹⁸F]fluoroalkyl azides for in vivo application.

Acylation-Mediated ‘Kinetic Turn-On’ of 3-Amino-1,2,4,5-tetrazines

The fast and biocompatible ligation of 1,2,4,5-tetrazines with strained alkenes has found numerous applications in biomedical sciences. The reactivity of a 1,2,4,5-tetrazine can generally be tuned by changing its electronic properties by varying the substituents in the 3- and/or 6-position. An increased reactivity of such bioorthogonal probes upon conjugation or attachment to a target molecule has not previously been described. Such an approach would be beneficial, as it would minimize the impact of residual tetrazine reagents and/or impurities. Herein, we describe such a ‘kinetic turn-on’ of 1,2,4,5-tetrazines upon conjugation. On the basis of the significant increase in reactivity following N-acylation predicted by quantum chemical calculations, we prepared 3-aminotetrazines and their corresponding acetylated derivatives. An investigation of the reaction kinetics indeed revealed a remarkable increase in reactivity upon acylation.

Design, Synthesis, and Evaluation of a Low-Molecular-Weight (11)C-Labeled Tetrazine for Pretargeted PET Imaging Applying Bioorthogonal in Vivo Click Chemistry

A low-molecular-weight tetrazine labeled with the short-lived positron emitter carbon-11 was developed as a bioorthogonal PET probe for pretargeted imaging. A method for efficient and fast synthesis of this imaging agent is presented using radiolabeling of a readily available precursor. High reactivity with trans-cyclooctenes was observed and in vivo investigations including PET/MR scanning showed homogeneous biodistribution, good metabolic stability, and rapid excretion in naive mice. These properties are key to the success of bioorthogonal (11)C-PET imaging, which has been shown in a simple pretargeting experiment using TCO-modified mesoporous silica nanoparticles. Overall, this (11)C-labeled tetrazine represents a highly versatile and advantageous chemical tool for bioorthogonal PET imaging and enables pretargeting approaches using carbon-11 for the first time.

32nd International Austrian Winter Symposium : Zell am See, the Netherlands. 20-23 January 2016

A1 68Ga-PSMA PET/CT in staging and restaging of Prostate Cancer Patients: comparative study with 18F-Choline PET/CT

W Langsteger, A Rezaee, W Loidl, HS Geinitz, F Fitz, M Steinmair, G Broinger, L Pallwien-Prettner, M Beheshti

A2 F18 Choline PET – CT: an accurate diagnostic tool for the detection of parathyroid adenoma?

L Imamovic, M Beheshti, G Rendl, D Hackl, O Tsybrovsky, M Steinmair, K Emmanuel, F Moinfar, C Pirich, W Langsteger

A3 [18F]Fluoro-DOPA-PET/CT in the primary diagnosis of medullary thyroid carcinoma

A Bytyqi, G Karanikas, M Mayerhöfer, O Koperek, B Niederle, M Hartenbach

A4 Variations of clinical PET/MR operations: An international survey on the clinical utilization of PET/MRI

T Beyer, K Herrmann, J Czernin

A5 Standard Dixon-based attenuation correction in combined PET/MRI: Reproducibility and the possibility of Lean body mass estimation

I Rausch, P Rust, MD DiFranco, M Lassen, A Stadlbauer, ME Mayerhöfer, M Hartenbach, M Hacker, T Beyer

A6 High resolution digital FDG PET/MRI imaging for assessment of ACL graft viability

K Binzel, R Magnussen, W Wei, MU Knopp, DC Flanigan, C Kaeding, MV Knopp

A7 Using pre-existing hematotoxicity as predictor for severe side effects and number of treatment cycles of Xofigo therapy

A Leisser, M Nejabat, M Hartenbach, G Kramer, M Krainer, M Hacker, A Haug

A8 QDOSE – comprehensive software solution for internal dose assessment

Wencke Lehnert, Karl Schmidt, Sharok Kimiaei, Marcus Bronzel, Andreas Kluge

A9 Clinical impact of Time-of-Flight on next-generation digital PET imaging of Yttrium-90 radioactivity following liver radioembolization

CL Wright, K Binzel, J Zhang, Evan Wuthrick, Piotr Maniawski, MV Knopp

A10 Snakes in patients! Lessons learned from programming active contours for automated organ segmentation

M Blaickner, E Rados, A Huber, M Dulovits, H Kulkarni, S Wiessalla, C Schuchardt, RP Baum, B Knäusl, D Georg

A11 Influence of a genetic polymorphism on brain uptake of the dual ABCB1/ABCG2 substrate [11C]tariquidar

M Bauer, B Wulkersdorfer, W Wadsak, C Philippe, H Haslacher, M Zeitlinger, O Langer

A12 Outcome prediction of temporal lobe epilepsy surgery from P-glycoprotein activity. Pooled analysis of (R)-[11C]-verapamil PET data from two European centres

M Bauer, M Feldmann, R Karch, W Wadsak, M Zeitlinger, MJ Koepp, M-C Asselin, E Pataraia, O Langer

A13 In-vitro and in-vivo characterization of [18F]FE@SNAP and derivatives for the visualization of the melanin concentrating hormone receptor 1

M Zeilinger, C Philippe, M Dumanic, F Pichler, J Pilz, M Hacker, W Wadsak, M Mitterhauser

A14 Reducing time in quality control leads to higher specific radioactivity of short-lived radiotracers

L Nics, B Steiner, M Hacker, M Mitterhauser, W Wadsak

A15 In vitro 11C-erlotinib binding experiments in cancer cell lines with epidermal growth factor receptor mutations

A Traxl, Thomas Wanek, Kushtrim Kryeziu, Severin Mairinger, Johann Stanek, Walter Berger, Claudia Kuntner, Oliver Langer

A16 7-[11C]methyl-6-bromopurine, a PET tracer to measure brain Mrp1 function: radiosynthesis and first PET evaluation in mice

S Mairinger, T Wanek, A Traxl, M Krohn, J Stanek, T Filip, M Sauberer, C Kuntner, J Pahnke, O Langer

A17 18F labeled azidoglucose derivatives as “click” agents for pretargeted PET imaging

D Svatunek, C Denk, M Wilkovitsch, T Wanek, T Filip, C Kuntner-Hannes, J Fröhlich, H Mikula

A18 Bioorthogonal tools for PET imaging: development of radiolabeled 1,2,4,5-Tetrazines

C Denk, D Svatunek, T Wanek, S Mairinger, J Stanek, T Filip, J Fröhlich, H Mikula, C Kuntner-Hannes

A19 Preclinical evaluation of [18F]FE@SUPPY- a new PET-tracer for oncology

T Balber, J Singer, J Fazekas, C Rami-Mark, N Berroterán-Infante, E Jensen-Jarolim, W Wadsak, M Hacker, H Viernstein, M Mitterhauser

A20 Investigation of Small [18F]-Fluoroalkylazides for Rapid Radiolabeling and In Vivo Click Chemistry

C Denk, D Svatunek, B Sohr, H Mikula, J Fröhlich, T Wanek, C Kuntner-Hannes, T Filip

A21 Microfluidic 68Ga-radiolabeling of PSMA-HBED-CC using a flow-through reactor

S Pfaff, C Philippe, M Mitterhauser, M Hartenbach, M Hacker, W Wadsak

A22 Influence of 24-nor-ursodeoxycholic acid on hepatic disposition of [18F]ciprofloxacin measured with positron emission tomography

T Wanek, E Halilbasic, M Visentin, S Mairinger, B Stieger, C Kuntner, M Trauner, O Langer

A23 Automated 18F-flumazenil production using chemically resistant disposable cassettes

P Lam, M Aistleitner, R Eichinger, C Artner

A24 Similarities and differences in the synthesis and quality control of 177Lu-DOTA-TATE, 177Lu -HA-DOTA-TATE and 177Lu-DOTA-PSMA (PSMA-617)

H Eidherr, C Vraka, A Haug, M Mitterhauser, L Nics, M Hartenbach, M Hacker, W Wadsak

A25 68Ga- and 177Lu-labelling of PSMA-617

H Kvaternik, R Müller, D Hausberger, C Zink, RM Aigner

A26 Radiolabelling of liposomes with 67Ga and biodistribution studies after administration by an aerosol inhalation system

U Cossío, M Asensio, A Montes, S Akhtar, Y te Welscher, R van Nostrum, V Gómez-Vallejo, J Llop

A27 Fully automated quantification of DaTscan SPECT: Integration of age and gender differences

F VandeVyver, T Barclay, N Lippens, M Troch

A28 Lesion-to-background ratio in co-registered 18F-FET PET/MR imaging – is it a valuable tool to differentiate between low grade and high grade brain tumor?

L Hehenwarter, B Egger, J Holzmannhofer, M Rodrigues-Radischat, C Pirich

A29 [11C]-methionine PET in gliomas - a retrospective data analysis of 166 patients

N Pötsch, I Rausch, D Wilhelm, M Weber, J Furtner, G Karanikas, A Wöhrer, M Mitterhauser, M Hacker, T Traub-Weidinger

A30 18F-Fluorocholine versus 18F-Fluorodeoxyglucose for PET/CT imaging in patients with relapsed or progressive multiple myeloma: a pilot study

T Cassou-Mounat, S Balogova, V Nataf, M Calzada, V Huchet, K Kerrou, J-Y Devaux, M Mohty, L Garderet, J-N Talbot

A31 Prognostic benefit of additional SPECT/CT in sentinel lymph node mapping of breast cancer patients

S Stanzel, G Pregartner, T Schwarz, V Bjelic-Radisic, B Liegl-Atzwanger, R Aigner

A32 Evaluation of diagnostic value of TOF-18F-FDG PET/CT in patients with suspected pancreatic cancer

S Stanzel, F Quehenberger, RM Aigner

A33 New quantification method for diagnosis of primary hyperpatahyroidism lesions and differential diagnosis vs thyropid nodular disease in dynamic scintigraphy

A Koljević Marković, Milica Janković, V Miler Jerković, M Paskaš, G Pupić, R Džodić, D Popović

A34 A rare case of diffuse pancreatic involvement in patient with merkel cell carcinoma detected by 18F-FDG

MC Fornito, D Familiari

A35 TSH-stimulated 18F-FDG PET/CT in the diagnosis of recurrent/metastatic radioiodine-negative differentiated thyroid carcinomas in patients with various thyroglobuline levels

P Koranda, H Polzerová, I Metelková, L Henzlová, R Formánek, E Buriánková, M Kamínek

A36 Breast Dose from lactation following I131 treatment

WH Thomson, C Lewis

A37 A new concept for performing SeHCAT studies with the gamma camera

WH Thomson, J O’Brien, G James, A Notghi

A38 Whole body F-18-FDG-PET and tuberculosis: sensitivity compared to x-ray-CT

H Huber, I Stelzmüller, R Wunn, M Mandl, F Fellner, B Lamprecht, M Gabriel

A39 Emerging role 18F-FDG PET-CT in the diagnosis and follow-up of the infection in heartware ventricular assist system (HVAD)

MC Fornito, G Leonardi

A40 Validation of Poisson resampling software

WH Thomson, J O’Brien, G James

A41 Protection of PET nuclear medicine personnel: problems in satisfying dose limit requirements

J Hudzietzová, J Sabol, M Fülöp

Conformationally Strained trans-Cyclooctene (sTCO) Enables the Rapid Construction of (18)F-PET Probes via Tetrazine Ligation

The bioorthogonal reaction between tetrazines and trans-cyclooctenes is a method for the rapid construction of F-18 probes for PET imaging. Described here is a second generation ¹⁸F-labeling system based on a conformationally strained trans-cyclooctene (sTCO)—a dienophile that is approximately 2 orders of magnitude more reactive than conventional TCO dienophiles. Starting from a readily prepared tosylate precursor, an ¹⁸F labeled sTCO derivative (¹⁸F-sTCO) could be synthesized in 29.3 +/- 5.1% isolated yield and with high specific activity. Tetrazine ligation was carried out with a cyclic RGD-conjugate of a diphenyl-s-tetrazine analogue (RGD-Tz) chosen from a diene class with an excellent combination of fast reactivity and stability both for the diene as well as the Diels-Alder adduct. For both the tetrazine and the sTCO, mini-PEG spacers were included to enhance solubility and improve the in vivo distribution profile of the resulting probe. Extremely fast reactivity (up to 2.86 x 10⁵ M^-1s^-1 at 25 °C in water) has been observed in kinetic studies in the reaction of sTCO with diphenyl-s-tetrazine derivatives. A kinetic study on sTCO diastereomers in 55:45 MeOH:water showed that the syn-diastereomer displayed slightly faster reactivity than the anti-diastereomer. An ¹⁸F-sTCO conjugate with RGD-Tz demonstrated prominent and persistent tumor uptake in vivo with good tumor-to-background contrast. Unlike most radiolabeled RGD peptides, the tumor uptake of this PET agent increased from 5.3 +/- 0.2% ID/g at 1 h post injection (p.i.), to 8.9 +/- 0.5% ID/g at 4 h p.i., providing evidence for prolonged blood circulation. These findings suggest that tetrazine ligations employing ¹⁸F-sTCO should serve as a powerful and general platform for the rapid construction of peptide or protein derived PET agents.

Efficient low-cost preparation of trans-cyclooctenes using a simplified flow setup for photoisomerization

Abstract

Bioorthogonal ligations have emerged as highly versatile chemical tools for biomedical research. The exceptionally fast reaction between 1,2,4,5-tetrazines and trans-cyclooctenes (TCOs), also known as tetrazine ligation, is frequently used in this regard. Growing numbers of applications for the tetrazine ligation led to an increased demand for TCO compounds, whose commercial availability is still very limited. Reported photochemical procedures for the preparation of TCOs using flow chemistry are straightforward and high yielding but require expensive equipment. Within this contribution, we present the construction and characterization of a low-cost flow photoreactor assembled from readily accessible components. Syntheses of all commonly used trans-cyclooctene derivatives were successfully carried out using the described system. We are convinced that the presented system for photoisomerization will promote access to bioorthogonally reactive TCO derivatives.

Graphical abstract

Electronic supplementary material

The online version of this article (doi:10.1007/s00706-016-1668-z) contains supplementary material, which is available to authorized users.