Scandium-catalyzed chemoselective carbene insertion into N-H over S-H: access to o-alkylamine-diaryl disulfides

Herein, we report a scandium-catalyzed chemoselective carbene insertion into a N-H bond over a S-H bond with disulfide formation. This reaction represents the first example of the synthesis of o-alkylamine-diaryl disulfides through the N-alkylation of o-aminobenzenethiol, while also undergoing oxidative coupling to form a S-S bond. Control experiments explain the chemo-selectivity of this rare-earth-metal Lewis acid-induced catalysis by a carbene outer-sphere nucleophilic addition mechanism. This method holds tremendous potential as a valuable tool for functionalizing advanced-synthetic-intermediates, offering numerous applications in medicinal and materials chemistry.

Homocouplings of Sodium Arenesulfinates: Selective Access to Symmetric Diaryl Sulfides and Diaryl Disulfides

Symmetrical diaryl sulfides and diaryl disulfides have been efficiently and selectively constructed via the homocoupling of sodium arenesulfinates. The selectivity of products relied on the different reaction systems: symmetrical diaryl sulfides were predominately obtained under the Pd(OAc)2 catalysis, whereas symmetrical diaryl sulfides were exclusively yielded in the presence of the reductive Fe/HCl system.

Dynamic Nucleophilic Aromatic Substitution of Tetrazines

A dynamic nucleophilic aromatic substitution of tetrazines (SN Tz) is presented herein. It combines all the advantages of dynamic covalent chemistry with the versatility of the tetrazine moiety. Indeed, libraries of compounds or sophisticated molecular structures can be easily obtained, which are susceptible to post-functionalization by inverse electron demand Diels-Alder (IEDDA) reaction, which also locks the exchange. Additionally, the structures obtained can be disassembled upon the application of the right stimulus, either UV irradiation or a suitable chemical reagent. Moreover, SN Tz is compatible with the imine chemistry of anilines. The high potential of this methodology has been proved by building two responsive supramolecular systems: A macrocycle that displays a light-induced release of acetylcholine; and a truncated [4+6] tetrahedral shape-persistent fluorescent cage, which is disassembled by thiols unless it is post-stabilized by IEDDA.

Redox Potential Tuning of s-Tetrazine by Substitution of Electron-Withdrawing/Donating Groups for Organic Electrode Materials

Herein, we tune the redox potential of 3,6-diphenyl-1,2,4,5-tetrazine (DPT) by introducing various electron-donating/withdrawing groups (methoxy, t-butyl, H, F, and trifluoromethyl) into its two peripheral benzene rings for use as electrode material in a Li-ion cell. By both the theoretical DFT calculations and the practical cyclic voltammetry (CV) measurements, it is shown that the redox potentials (E_1/2) of the 1,2,4,5-tetrazines (s-tetrazines) have a strong correlation with the Hammett constant of the substituents. In Li-ion coin cells, the discharge voltages of the s-tetrazine electrodes are successfully tuned depending on the electron-donating/withdrawing capabilities of the substituents. Furthermore, it is found that the heterogeneous electron transfer rate (k₀) of the s-tetrazine molecules and Li-ion diffusivity (D_Li) in the s-tetrazine electrodes are much faster than conventional electrode active materials.

Versatile one-pot synthesis of gold nanoclusters and nanoparticles using 3,6-(dipyridin-2-yl)-(1,2,4,5)-tetrazine

A one-pot synthesis of gold nano-objects is described by simply mixing a gold salt (HAuCl₄), dodecanethiol and 3,6-di-2-pyridyl-1,2,4,5-tetrazine. When a large excess of thiol is used, gold nanoclusters of 2 nm are obtained in a large amount and with a narrow size distribution. The reaction mechanism was investigated by absorption and emission spectroscopies and shows the in situ formation of dihydrotetrazine acting as the reductant of Au(iii) to make Au(0). Au nanoclusters were isolated from the molecular precursors by HPLC. The nature of the ligands stabilizing Au nanoclusters was investigated by various techniques such as mass spectrometry, SEM-EDS, XPS and NMR. Thiol and tetrazine are shown to play both the role of ligand stabilizing the clusters. Finally, when a much smaller amount of thiol is used, a mixture of Au nanoclusters and Au nanoparticles of 10–15 nm, sometimes aggregated into clusters of 50 nm is obtained. The formation of larger nanoobjects is explained by the lower amount of thiol available to block the growth at the early stage as shown by UV-vis absorption monitoring.

Easy synthesis of gold nanoclusters stabilized by thiols and tetrazines.

Bifunctional Reagents for Formylglycine Conjugation: Pitfalls and Breakthroughs

Formylglycine-generating enzymes specifically oxidize cysteine within the consensus sequence CxPxR to Cα -formylglycine (FGly). This noncanonical electrophilic amino acid can subsequently be addressed selectively by bioorthogonal hydrazino-iso-Pictet-Spengler (HIPS) or Knoevenagel ligation to attach payloads like fluorophores or drugs to proteins to obtain a defined payload-to-protein ratio. However, the disadvantages of these conjugation techniques include the need for a large excess of conjugation building block, comparably low reaction rates and limited stability of FGly-containing proteins. Therefore, functionalized clickable HIPS and tandem Knoevenagel building blocks were synthesized, conjugated to small proteins (DARPins) and subsequently linked to strained alkyne-containing payloads for protein labeling. This procedure allowed the selective bioconjugation of one or two DBCO-carrying payloads with nearly stoichiometric amounts at low concentrations. Furthermore, an azide-modified tandem Knoevenagel building block enabled the synthesis of branched PEG linkers and the conjugation of two fluorophores, resulting in an improved signal-to-noise ratio in live-cell fluorescence-imaging experiments targeting the EGF receptor.

Metabolism of a Bioorthogonal PET Tracer Candidate [19F/18F]SiFA-Tetrazine in Mouse Liver Microsomes: Biotransformation Pathways and Defluorination Investigated by UHPLC-HRMS

Organofluorosilicon based ¹⁸F-radiolabeling is an efficient method for incorporating fluorine-18 into ¹⁸F-radiopharmaceuticals for positron emission tomography (PET) by ¹⁹F/¹⁸F isotopic exchange (IE). The first PET radiopharmaceutical, ¹⁸F-SiFAlin-TATE, radiolabeled with a silicon-based [¹⁸F]fluoride acceptor (SiFA), namely, a para-substituted di-tert-butyl[¹⁸F]fluorosilylbenzene, has entered clinical trials, and is paving the way for other potential [¹⁸F]SiFA-labeled radiopharmaceuticals for diagnostic use. In this study, we report the in vitro metabolism of an oxime-linked SiFA tetrazine (SiFA–Tz), a new PET-radiotracer candidate, recently evaluated for pretargeted PET imaging and macromolecule labeling. Metabolism of SiFA–Tz was studied in mouse liver microsomes (MLM) for elucidating its major biotransformation pathways. Nontargeted screening by ultrahigh performance liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS) was utilized for detection of unknown metabolites. The oxime bond between the SiFA and Tz groups forms two geometric (E/Z) isomers, which underwent the same biotransformations, but unexpectedly with different kinetics. In total, nine proposed metabolites of SiFA–Tz from phase I and II reactions were detected, five of which were defluorinated in MLMs, elucidating the metabolic pathway leading to previously reported defluorination of [¹⁸F]SiFA–Tz in vivo. Based on the HRMS studies a biotransformation pathway is proposed: hydroxylation (+O) to tert-butyl group adjacent to the silicon, followed by oxidative defluorination (+OH/-F) cleaving the fluorine off the silicon. Interestingly, eight proposed metabolites of a reduced dihydrotetrazine analogue, SiFA–H₂Tz, from phase I and II reactions were additionally detected. To the best of our knowledge, this is the first reported comprehensive investigation of enzyme mediated metabolic pathway of tetrazines and para-substituted di-tert-butylfluorosilylbenzene fluoride acceptors, providing novel structural information on the biotransformation and fragmentation patterns of radiotracers bearing these structural motifs. By investigating the metabolism preceding defluorination, structurally optimized new SiFA compounds can be designed for expanding the portfolio of efficient ¹⁹F/¹⁸F isotopic exchange labeling probes for PET imaging.

s-Tetrazine Dyes: A Facile Generation of Photoredox Organocatalysts for Routine Oxidations

A series of organic dyes derived from s-tetrazine have been synthesized, and their photophysical and electrochemical properties are systematically investigated. Testing these compounds as photoredox catalysts in a model oxidative C-S bond cleavage of thioethers has led us to identify new classes of active s-tetrazines. Moreover, some of them can be formed in situ from commercially available 3,6-dichlorotetrazine, making this photocatalyzed C-S bond functionalization simple and highly practical.

Enzymatic and Site-Specific Ligation of Minimal-Size Tetrazines and Triazines to Proteins for Bioconjugation and Live-Cell Imaging

Diels-Alder reactions with inverse electron demand (DA_inv) have emerged as an indispensable tool for bioorthogonal labeling and the manipulation of biomolecules. In this context, reactions between tetrazines and strained dienophiles have received attention because of high reaction rates. Current methods for the DA_inv-mediated functionalization of proteins suffer from slow reactivity, impaired stability, isomerization, or elimination of the incorporated strained dienophiles. We report here a versatile platform for the posttranslational, highly selective, and quantitative modification of proteins with stable dienes. New synthetic access to minimal size tetrazine and triazine derivatives enabled us to synthesize tailored diene substrates for the lipoic acid protein ligase A (LplA) from Escherichia coli, which we employ for the rapid, mild, and quantitative bioconjugation of proteins by DA_inv. The presented method benefits from the minimal tag size for LplA recognition and can be applied to proteins from any source organism. We demonstrate its broad suitability by site-specific in vitro protein labeling and live cell labeling for fluorescence microscopy. With this work we expand the scope of DA_inv bioorthogonal chemistry for site-specific protein labeling, providing additional experimental flexibility for preparing well-defined bioconjugates and addressing biological questions in complex biological environments.

2,2'-Dithiodisuccinic acid-stabilized polyion complex micelles for pH and reduction dual-responsive drug delivery

In this work, a new kind of polyion complex (PIC) micelles with pH and reduction dual responsiveness was developed for effective intracellular drug delivery. The PIC micelles can be readily prepared by mixing a polycationic block polymer, methoxy poly(ethylene glycol)-b-poly(l-lysine) (mPEG-PLL), with a small molecule polyacid, 2, 2'-dithiodisuccinic acid (DTS) in aqueous media. The resultant PIC micelles are of uniform spherical shapes with hydrodynamic radii ranging from 65 to 75 nm based on different feeding ratios of mPEG-PLL and DTS. Interestingly, by using the small molecule polyacid DTS, the obtained PIC micelles show distinct pH-responsive disintegration in acid solution. Meanwhile, the PIC micelles were also assessed to be reduction-responsive due to the presence of disulfide bond in DTS. In view of these stimuli-responsive properties, the potential use of this PIC micelle as smart drug carrier was then investigated. Doxorubicin (DOX), a cationic anticancer drug, was loaded into the PIC micelles with high efficiency. In vitro drug release studies revealed that release of DOX from the PIC micelles was suppressed in neutral solutions due to the stable electrostatic interactions between PIC micelles and DOX, but could be accelerated in acidic solutions or under high GSH condition. Furthermore, flow cytometry and confocal laser scanning microscopy (CLSM) studies indicated that the DOX-loaded PIC micelles could be effectively internalized by MCF-7 human breast cancer cells and release the loaded DOX in intracellular environment. Consequently, the DOX-loaded PIC micelles were capable of inhibiting the proliferation of C26 murine colon cancer and MCF-7 human breast cancer cells in high efficiency, showing similar IC₅₀ values as free DOX. Thus, this biocompatible PIC micelle may be promising for intracellular drug delivery.

Base-controlled Fe(Pc)-catalyzed aerobic oxidation of thiols for the synthesis of S–S and S–P(O) bonds

Base-controlled Fe(Pc)-catalyzed S–S/S–P(O) bond formation.

Synthesis of 2,3-diaza-anthraquinones via the bidentate Lewis acid catalysed inverse electron-demand Diels–Alder (IEDDA) reaction

A new synthesis of aza-anthraquinones and substituted anthraquinones via bidentate Lewis acid catalysis has been developed.