Synthesis of mono-functionalized S-diazocines via intramolecular Baeyer-Mills reactions

Cascade Radical Pathway-Enabled Nitrogen-Sulfur Coupling: Access to Isothiazolo[3,4-b]-meso-tetraarylporphyrins

A catalyst-free radical-mediated domino strategy for the construction of isothiazolo[3,4-b]-meso-tetraarylporphyrins was developed. During the course of the reaction, 2-benzothioylamino-3-thioformyl-meso-tetraarylporphyrins generated in situ after the addition of Lawesson's reagent to a solution of 2-benzoylamino-3-formyl-meso-tetraarylporphyrins in refluxing toluene underwent a homolytic cleavage to produce nitrogen-sulfur radicals. Subsequently, the formation of a new N-S bond through an intramolecular cascade radical coupling provided direct access to novel β-isothiazole-fused porphyrins.

Development of an Indole-Amide-Based Photoswitchable Cannabinoid Receptor Subtype 1 (CB1R) "Cis-On" Agonist

Activation of the human cannabinoid receptor type 1 (hCB₁R) with high spatiotemporal control is useful to study processes involved in different pathologies related to nociception, metabolic alterations, and neurological disorders. To synthesize new agonist ligands for hCB₁R, we have designed different classes of photoswitchable molecules based on an indole core. The modifications made to the central core have allowed us to understand the molecular characteristics necessary to design an agonist with optimal pharmacological properties. Compound 27a shows high affinity for CB₁R (K_i (cis-form) = 0.18 μM), with a marked difference in affinity with respect to its inactive "trans-off" form (CB₁R K_i trans/cis ratio = 5.4). The novel compounds were evaluated by radioligand binding studies, receptor internalization, sensor receptor activation (GRABeCB2.0), Western blots for analysis of ERK1/2 activation, NanoBiT βarr2 recruitment, and calcium mobilization assays, respectively. The data show that the novel agonist 27a is a candidate for studying the optical modulation of cannabinoid receptors (CBRs), serving as a new molecular tool for investigating the involvement of hCB₁R in disorders associated with the endocannabinoid system.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Triazonine-based bistable photoswitches: synthesis, characterization and photochromic properties

We disclose here dibenzotriazonines as a new class of nine-membered cyclic azobenzenes displaying a nitrogen function in the saturated ring chain. The specific features of these compounds are (i) a preferred E-configuration, (ii) high bi-directional photoswitching and (iii) good thermal stability of both E- and Z-forms.

Substituted nitrogen-bridged diazocines

Novel nitrogen-bridged diazocines (triazocines) were synthesized that carry a formyl or an acetyl group at the CH₂NR-bridge and bromo- or iodo-substituents at the distant phenyl ring. The photophysical properties were investigated in acetonitrile and water. As compared to previous approaches the yields of the intramolecular azo cyclizations were increased (from ≈40 to 60%) using an oxidative approach starting from the corresponding aniline precursors. The Z→E photoconversion yields in acetonitrile are 80-85% and the thermal half-lives of the metastable E configurations are 31-74 min. Particularly, the high photoconversion yields (≈70%) of the water-soluble diazocines are noteworthy, which makes them promising candidates for applications in photopharmacology. The halogen substituents allow further functionalization via cross-coupling reactions.

Advances in Application of Azobenzene as a Trigger in Biomedicine: Molecular Design and Spontaneous Assembly

Azobenzene is a well-known derivative of stimulus-responsive molecular switches and has shown superior performance as a functional material in biomedical applications. The results of multiple studies have led to the development of light/hypoxia-responsive azobenzene for biomedical use. In recent years, long-wavelength-responsive azobenzene has been developed. Matching the longer wavelength absorption and hypoxia-response characteristics of the azobenzene switch unit to the bio-optical window results in a large and effective stimulus response. In addition, azobenzene has been used as a hypoxia-sensitive connector via biological cleavage under appropriate stimulus conditions. This has resulted in on/off state switching of properties such as pharmacology and fluorescence activity. Herein, recent advances in the design and fabrication of azobenzene as a trigger in biomedicine are summarized.

Targeted Cancer Therapy Using Compounds Activated by Light

Cancer chemotherapy is affected by a modest selectivity and toxic side effects of pharmacological interventions. Among novel approaches to overcome this limitation and to bring to therapy more potent and selective agents is the use of light for selective activation of anticancer compounds. In this review, we focus on the anticancer applications of two light-activated approaches still in the experimental phase: photoremovable protecting groups ("photocages") and photoswitches. We describe the structural considerations behind the development of novel compounds and the plethora of assays used to confirm whether the photochemical and pharmacological properties are meeting the stringent criteria for an efficient in vivo light-dependent activation. Despite its immense potential, light activation brings many challenges, and the complexity of the task is very demanding. Currently, we are still deeply in the phase of pharmacological tools, but the vivid research and rapid development bring the light of hope for potential clinical use.

Photoswitchable Azo- and Diazocine-Functionalized Derivatives of the VEGFR-2 Inhibitor Axitinib

In this study, we aimed at the application of the concept of photopharmacology to the approved vascular endothelial growth factor receptor (VEGFR)-2 kinase inhibitor axitinib. In a previous study, we found out that the photoisomerization of axitinib’s stilbene-like double bond is unidirectional in aqueous solution due to a competing irreversible [2+2]-cycloaddition. Therefore, we next set out to azologize axitinib by means of incorporating azobenzenes as well as diazocine moieties as photoresponsive elements. Conceptually, diazocines (bridged azobenzenes) show favorable photoswitching properties compared to standard azobenzenes because the thermodynamically stable Z-isomer usually is bioinactive, and back isomerization from the bioactive E-isomer occurs thermally. Here, we report on the development of different sulfur–diazocines and carbon–diazocines attached to the axitinib pharmacophore that allow switching the VEGFR-2 activity reversibly. For the best sulfur–diazocine, we could verify in a VEGFR-2 kinase assay that the Z-isomer is biologically inactive (IC₅₀ >> 10,000 nM), while significant VEGFR-2 inhibition can be observed after irradiation with blue light (405 nm), resulting in an IC₅₀ value of 214 nM. In summary, we could successfully develop reversibly photoswitchable kinase inhibitors that exhibit more than 40-fold differences in biological activities upon irradiation. Moreover, we demonstrate the potential advantage of diazocine photoswitches over standard azobenzenes.

Oxidative Approach Enables Efficient Access to Cyclic Azobenzenes

Azobenzenes are versatile photoswitches that have found widespread use in a variety of fields, ranging from photopharmacology to the material sciences. In addition to regular azobenzenes, the cyclic diazocines have recently emerged. Although diazocines have fascinating conformational and photophysical properties, their use has been limited by their synthetic accessibility. Herein, we present a general, high-yielding protocol that relies on the oxidative cyclization of dianilines. In combination with a modular substrate synthesis, it allows for rapid access to diversely functionalized diazocines on gram scales. Our work systematically explores substituent effects on the photoisomerization and thermal relaxation of diazocines. It will enable their incorporation into a wide variety of functional molecules, unlocking the full potential of these emerging photoswitches. The method can be applied to the synthesis of a new cyclic azobenzene with a nine-membered central ring and distinct properties.

Nitrogen Bridged Diazocines: Photochromes Switching within the Near-Infrared Region with High Quantum Yields in Organic Solvents and in Water

Diazocines are bridged azobenzenes with superior photophysical properties. In contrast to azobenzenes the Z configuration is thermodynamically stable and the E isomer is metastable. We present a new class of nitrogen bridged diazocines with bathochromically shifted switching wavelengths and remarkably high quantum yields (-NH-CH₂- bridged diazocine: Φ_Z→E = 0.57, Φ_E→Z = 0.8). Z to E isomerization is induced by irradiation with blue light, whereas switching back to the Z isomer is accomplished with light in the near-infrared window (up to 740 nm), which is important for medical applications like photopharmacology (deep tissue penetration). Furthermore, substitution at the bridging nitrogen should provide access to widely applicable tricyclic, photoswitchable pharmacophores. The -NAc-CH₂- bridged derivative is soluble in water, and all photophysical properties (conversion rates, quantum yields, and thermal half-lives) are largely retained. Hence, this diazocine is an ideal photoswitch for applications in biochemical systems and in photopharmacology.

Synthesis of functionalized diazocines for application as building blocks in photo- and mechanoresponsive materials

Seven symmetrically 3,3'-substituted diazocines were synthesized. Functional groups include alcohol, azide, amine and vinyl groups, which are suitable for polymer synthesis. Upon irradiation at 385 and 530 nm the diazocines perform a reversible, pincer-type movement switching the 3,3'-distance between 6.1 Å (cis, stable isomer) and 8.2 Å (trans, metastable isomer). Key reactions in the synthesis are an oxidative C-C coupling of 2-nitrotoluenes (75-82% yield) and a reductive ring closure to form the diazocines (56-60% yield). The cyclization of the dinitro compound to the azo compound was improved in yield and reproducibility, by over-reduction to the hydrazine and reoxidation to the azo unit. In contrast to 3,3'- and 4,4'-diaminodiazocine, which have been implemented in macromolecules for conformation switching, our compounds exhibit improved photophysical properties (photostationary states, separation of absorption bands in the cis and trans configuration). Hence they are promising candidates as molecular switches in photo and mechanoresponsive macromolecules and other smart materials.