Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

Photochemical Homodimerization as a Key Step in Natural Product Synthesis

Photochemical dimerizations constitute a relatively simple but important class of bimolecular reactions. Nature produces several classes of dimeric natural products, some of which appear to be arise from radical or excited-state coupling reactions that could be photochemically initiated. This review focuses on natural products syntheses that exploit these photochemical dimerization reactions; selected total syntheses are discussed as examples, with particular attention given to the photochemical key step and its stereoselectivity.

Total Synthesis of (±)-Rhazinilam via Red-Light-Driven Zinc(II)porphyrin-Catalyzed Radical Cyclization of N-Substituted Pyrrole

A red-light-driven radical cyclization strategy has been developed for the concise synthesis of (±)-rhazinilam. The transformation, catalyzed by [5,15-bis(pentafluorophenyl)-10,20-diphenylporphinato]zinc(II), enables the efficient formation of a tetrahydroindolizine core from an N-acyloxyphthalimide-substituted pyrrole, providing a key intermediate en route to the target alkaloid.

Deconstructive Functionalization of Cyclic Ketones via Electrochemical Interrupted Dowd-Beckwith Reaction

Deconstructive functionalization of cyclic molecules has recently emerged as a prominent strategy to access unique architectures that are challenging to prepare through traditional methods. While significant progress has been made in the deconstructive functionalization of cyclic alcohols and amines, the strategies for deconstructing cyclic ketones remain largely unexplored. The Dowd-Beckwith reaction, a ring-expansion of cyclic ketones, is a powerful method for synthesizing medium- and large-ring compounds. Herein, we developed the first interrupted Dowd-Beckwith (IDB) reaction for highly regioselective deconstructive functionalization of cyclic ketones using electricity as the sole oxidant. This protocol is widely applicable for the deconstruction of small, medium-sized, and macrocyclic ketones bearing a diverse range of functional groups. Remarkably, various naturally occurring complex cyclic ketones were successfully deconstructed into acyclic molecules, which are difficult to access by known strategies. The method was applied to the asymmetric synthesis of planococcol, citrilol acetate, maconelliol, and its derivatives. Furthermore, the functional groups incorporated during the transformation provided versatile handles for subsequent synthetic modifications. Mechanistic experiments and computational studies support an oxidative radical-polar crossover followed by deconstructive functionalization.

Bioinspired Stereoselective Total Synthesis of the Caged Sesquiterpenoid Daphnepapytone A

The first total synthesis of the novel caged sesquiterpenoid daphnepapytone A is disclosed. Key reactions include a Pauson-Khand cycloaddition to provide oleodaphone, a bioinspired photoinduced [2 + 2] cycloaddition to forge the cyclobutane-containing caged skeleton, and a C-H oxidation and reduction protocol to generate daphnepapytone A. Finally, the 17-step synthetic sequence is shortened to 4 steps in protecting group-free and exclusively stereoselective fashion.

Oxa-π, σ-Methane Rearrangement Approach for Epoxide Synthesis

Epoxides are significant chemicals that are utilized across various fields. Here, we describe an unprecedented photochemical rearrangement approach for synthesizing a diverse collection of epoxides enabled by energy transfer catalysis under visible light conditions. The process enables the easy preparation of α-amino-substituted epoxide derivatives with a broad substrate scope, functional group tolerance, and mild reaction conditions. Furthermore, this photorearrangement has also been applied in complex architectures, and the epoxides could be easily transferred to amino alcohol derivatives. Overall, this oxa-π, σ-methane rearrangement provides a complementary strategy to the existing methods of photochemical rearrangement through energy transfer catalysis.

Investigation of the photochemical behavior of allomaltol-containing 2-aminothiazole derivatives

In the present communication, a comprehensive study of the photochemical behavior of terarylenes with a 2-aminothiazole bridge unit and an allomaltol substituent was performed. It was shown that for the considered substrates, two types of photoprocesses can be realized. By controlling the structure of the starting material, the photoprocess can be directed toward the 6π-electrocyclization of the 1,3,5-hexatriene system or the ESIPT-induced contraction of the pyranone ring. Relying on data from DFT calculations and UV-vis spectroscopy, the key features of the observed phototransformations were discussed. The investigated thiazole-containing terarylenes were utilized as a scaffold for the construction of photoacid generators (PAGs). Based on the performed research, a synthetic methodology was developed leading to series of 23 novel polycyclic photoproducts and a set of thiazole-containing salts with a quinoxaline part.

Synthesis of γ-Iodo-allylic Diboronic Esters via Atom Transfer Radical Addition of (Diborylmethyl)iodide to Alkynes

Herein, we disclose a manganese-catalyzed approach that enables the direct iododiborylcarbofunctionalization of alkynes with a (diboronmethyl)iodide under mild conditions. This approach grants access to a range of structurally unique and synthetically useful γ-iodine-substituted gem-bis(boronate)s, which have hitherto been inaccessible. This atom-economical strategy displays excellent functional-group tolerance, encompasses a wide range of applicable substrates, and demonstrates a high Z:E selectivity (up to 96:4).

Xanthochrysanthones A-C, Novel Lobster-Shape Cinnamoyltriketone Dimers with Antiviral Activities from Xanthostemon chrysanthus

Three pairs of enantiomeric cinnamoyltriketone dimers, (+)- and (-)-xanthochrysanthones A-C [(+)- and (-)-1-3)], were recognized and obtained from Xanthostemon chrysanthus by employing the building blocks-based molecular network (BBMN) strategy. Compounds 1-3 featured two new types of lobster-shape carbon skeletons with unprecedented spiro[cyclohexane-1,2'-xanthene] or benzo[7,8]oxocino[4,5-b]chromene tetracyclic ring systems. Their structures with absolute configurations were established by comprehensive spectroscopic analyses, X-ray crystallography, and ECD calculations. Compounds 1-3 represent the first example of phloroglucinol derivatives that are biogenetically constructed by a De Mayo reaction followed by Michael addition. Additionally, compounds 1-3 exhibited significant antiviral activity against respiratory syncytial virus (RSV). Time-of-addition assays indicated that compound 3 specifically acts on the early stages of the virus replication process in RSV.

Exploring Macrocyclic Chemical Space: Strategies and Technologies for Drug Discovery

Macrocycles have emerged as significant therapeutic candidates in drug discovery due to their unique capacity to target complex and traditionally inaccessible biological interfaces. Their structurally constrained three-dimensional configurations facilitate high-affinity interactions with challenging targets, notably protein-protein interfaces. However, despite their potential, the synthesis and optimization of macrocyclic compounds present considerable challenges related to structural complexity, synthetic accessibility, and the attainment of favorable drug-like properties, particularly cell permeability and oral bioavailability. Recent advancements in synthetic methodologies have expanded the chemical space accessible to macrocycles, enabling the creation of structurally diverse and pharmacologically active compounds. Concurrent developments in computational strategies have further enhanced macrocycle design, providing valuable insights into structural optimization and predicting molecular properties essential for therapeutic efficacy. Additionally, a deeper understanding of macrocycles' conformational adaptability, especially their ability to internally shield polar functionalities to improve membrane permeability, has significantly informed their rational design. This review discusses recent innovations in synthetic and computational methodologies that have advanced macrocycle drug discovery over the past five years. It emphasizes the importance of integrating these strategies to overcome existing challenges, illustrating how their synergy expands the therapeutic potential and chemical diversity of macrocycles. Selected case studies underscore the practical impact of these integrated approaches, highlighting promising therapeutic applications across diverse biomedical targets.

Total Synthesis of Polycyclic Natural Products via Photoenolization/Diels-Alder Reaction

ConspectusPolycyclic ring systems represent the most common structural features of drug molecules and natural products. Chemical synthesis of complex polycyclic molecules with multiple stereogenic centers, especially quaternary carbon stereocenters, has been a significant challenge in the field of total synthesis. Due to the low reactivities of the substrates and congested chemical environments, the efficient establishment of polycyclic rings and enantioselective construction of quaternary carbon stereocenters are still ongoing challenges. In our laboratory, we are devoted to developing new methodologies and strategies for the total syntheses of bioactive polycyclic natural products and the exploration of their biological potentials. The photoenolization/Diels-Alder (PEDA) reaction has been recognized as a powerful strategy to increase synthetic efficiency and address the aforementioned issues. Over the past several years, our group systematically reinvestigated this reaction in terms of its reactivity and stereoselectivity and developed a unique dinuclear metal-promoted reaction process for constructing fused or spiro polycyclic rings bearing quaternary carbon stereocenters. During the course of this investigation, we have come to realize how to rationally design the synthetic route based on the PEDA reaction and successfully implement the synthetic projects.In this Account, we summarize our endeavors and journeys in the development and application of the PEDA reaction to the total synthesis of topologically complex natural products in order to draw attention to its broad utility and encourage further uptake. In the first part, we provide the details on the investigation of the PEDA reaction to address the issues of reactivity, diastereoselectivity, and enantioselectivity. An enantioselective PEDA reaction involving Ti(Oi-Pr)4 and TADDOL-type ligands was developed. This reaction enables the sterically bulky dienophiles to interact with the transient photoenolized hydroxy-o-quinodimethanes, delivering a wide range of polycyclic rings with single or vicinal quaternary carbon stereocenters in good yields with excellent enantioselectivities. In the second part, we showcase the synthetic potential of PEDA reaction in total synthesis of natural products. The fused tricyclic ring systems, bearing gem-dimethyl groups or quaternary carbon stereocenters located at the ring junction, were efficiently constructed by Ti(Oi-Pr)4-promoted PEDA reactions, which enabled the syntheses of three different types of natural products, including aromatic polyketides (anthrabenzoxocinones, fasamycins/naphthacemycins, and benastatins), meroterpenoid (oncocalyxone B), and halenaquinones (xestoquinone, adociaquinones A and B). To access structurally more complex triterpenoids, namely, perovskones and hydrangenones, the asymmetric PEDA reaction was developed to build a tricyclic ring along with three contiguous quaternary carbon stereocenters. The asymmetric PEDA reaction was also applied to achieve the total synthesis of aryltetralin lactone lignans. Furthermore, an intramolecular PEDA reaction provides a new pathway for the rapid construction of highly congested hydrophenanthrene with a quaternary carbon stereocenter, facilitating the total synthesis of five hasubanan alkaloids. We anticipate that the development of the PEDA reaction will inspire future innovations and progressions in asymmetric photo reactions, and its synthetic potential will be expanded by further applications in the total synthesis of complex natural and drug molecules.

Polymers and light: a love-hate relationship

The study of the interaction between polymers and light has significantly bloomed over the past few years in various fundamental research and applications. The relationship between polymers and light can be beneficial (we refer to this as "love") or be destructive (we refer to this as "hate"). It is important to understand the nature of both these love and hate relationships between polymers and light to apply these concepts in various future systems, to surpass performance of existing materials, or to mitigate some problems associated with polymers. Therefore, this perspective highlights both the photophilic (e.g., photopolymerization, rate modulation, temporal/spatial control, drug delivery, waste management, photo functionalization, and photo-enhanced depolymerization) and photophobic (e.g., photodegradation, discoloration, optical density, and loss of functionality) nature of polymers.

Tuning the Efficiency of Iridium(III) Complexes for Energy Transfer (EnT) Catalysis through Ligand Design

Photoactive Iridium(III) complexes are a popular synthetic tool. The impact of ligand design on their photoredox properties has been widely studied, but similar approaches to develop more potent photosensitizers are still absent. We report herein the preparation, characterization and catalytic application of a new family of Iridium(III) complexes that proved superior to their widely-used commercial peers. The best results were observed when naphthyl pendants were installed to the ligands, which could stabilize the triplet intermediates involved in energy-transfer reactions via radical-π dispersion interactions.

Research on the metabolites and key metabolic enzymes of allocryptopine in chicken liver microsomes via stable isotope tracing technology

Allocryptopine (ALL), a principal active component of the novel veterinary medicine Bopu Powder®, has gained widespread application in the poultry farming sector for the effective management of Escherichia coli (E. coli) diarrhea. In order to explore the metabolites and the pivotal enzymes associated with ALL, this study was conducted employing an in vitro chicken liver microsomal incubation. The metabolites of ALL were analyzed and identified by combining isotope tracing technology with the application of mass spectrometry fragmentation patterns. The key metabolic enzymes involved in the biotransformation of ALL were explored using the CYP450 recombinant enzyme method, which facilitated the identification of the enzymes contributing to ALL's metabolic pathway. The liver microsomal metabolism investigation revealed a total of five metabolites, with the predominant being M2 (harmol or 3-hydroxy-4-methoxy-6-methyl-5,7,8,15-tetrahydro-[1,3]dioxolo[4',5':4,5]benzo[1,2-g]benzo[c]azecin-14(6 H)-one). The recombinant enzyme analysis conclusively identified CYP2D6 as the pivotal CYP450 isoenzyme that plays a central role in the metabolic pathway of the principal ALL metabolite, M2. This research not only expands our comprehension of the biotransformation process of ALL but also provides significant scientific evidence for the clinical safety of ALL, which was of great importance for guiding the application of ALL in the field of veterinary medicine.

Visible-Light-Induced Secondary Benzylic C(sp3)-H Functionalization for Nucleophilic Substitution: An Intermolecular C-X (C-N, C-C, and C-Br) Bond Forming Reaction

Exploring new synthetic methods that harness visible light represents a breakthrough in organic synthesis. Amides, amines, nitriles, and halides are essential functional groups and serve as key building blocks in the synthesis of complex molecules in organic and medicinal chemistry. This study introduces a novel intermolecular benzylic C-X (C-N, C-C, and C-Br) bond formation via photoredox benzylic C(sp3)-H bonds. This methodology enables the synthesis of secondary amides, nitriles, and halides through reacting secondary benzylic substrates with readily accessible reagents including BocNH2, benzamide, acetamide, TMSCN, and TBAB. This approach displays the potential of being sustainable and efficient to afford amidation, cyanation, and halogenation products in good yields.

Temperature-Dependent Photoreactivity of 2-Azidomethylbenzophenone: Insights into the Triplet Imine Biradical Pathway

Photoenols, formed through photoinduced intramolecular H atom abstraction in o-alkyl-substituted arylketones, typically have limited utility as reactive intermediates owing to fast reversion to the starting material. Herein, we introduced an azido group on the o-alkyl substituent to render the photoreaction irreversible. Irradiation of 2-azidomethylbenzophenone (1) in methanol yielded 2-(hydroxy(phenyl)methyl)benzonitrile (2). Laser flash photolysis of 1 revealed the formation of biradical 3Br1 followed by intersystem crossing to photoenols Z-3 (τ ∼ 3.3 μs) and E-3 (τ > 45 μs), both of which reverted to 1. Alternatively, 3Br1 could lose N2 to form 3Br2 (not detected), which decays to 2. In cryogenic argon matrices, irradiation of 1 yielded nitrene 31N and 2 but no photoenols, likely because Z-3 regenerated 1. Both ESR spectroscopy and absorption analysis in methyltetrahydrofuran (80 K) confirmed 31N formation. Upon prolonged irradiation, the absorbance of 31N decreased, whereas that of 3 remained unchanged and that of 2 increased. Thus, TK of 1 is proposed to form 3Br1 via H atom abstraction, with subsequent intersystem crossing to 3 competing with the loss of N2 to generate 3Br2. DFT calculations revealed a small energy gap (∼2 kcal/mol) between the triplet and singlet configurations of Br2, supporting a mechanism in which 3Br2 intersystem crosses to yield 2.

Recent advances in 4CzIPN-mediated functionalizations with acyl precursors: single and dual photocatalytic systems

4CzIPN (1,2,3,5-tetrakis(carbazole-9-yl)-4,6-dicyanobenzene) has emerged as a key metal-free photocatalyst for sustainable organic synthesis. Due to its unique design enabling high photoluminescence quantum yield, thermally activated delayed fluorescence (TADF) and long excited state lifetime, 4CzIPN facilitates diverse reactions, such as C-C and C-X bond formation reactions, under mild reaction conditions. This review highlights its application in decarboxylation, acylation and cyclisation reactions involving α-keto acids, carboxylic acids and aldehydes in a single catalytic system, as well as the combination of a dual catalytic system with transition metals to enhance selectivity and scope. 4CzIPN contributes to the advancement of sustainable chemistry by enabling green, efficient and scalable reactions and this review covers studies published between 2020 and 2024.

Radical Brook rearrangement: past, present, and future

The Brook rearrangement has emerged as one of the most pivotal transformations in organic chemistry, with broad applications spanning organic synthesis, drug design, and materials science. Since its discovery in the 1950s, the anion-mediated Brook rearrangement has been extensively studied, laying the groundwork for the development of numerous innovative reactions. In contrast, the radical Brook rearrangement has garnered comparatively less attention, primarily due to the challenges associated with the controlled generation of alkoxyl radicals under mild conditions. However, recent advancements in visible-light catalysis and transition-metal catalysis have positioned the radical Brook rearrangement as a promising alternative synthetic strategy in organic synthesis. Despite these developments, significant limitations and challenges remain, warranting further investigation. This review provides an overview of the radical Brook rearrangement, tracing its development from past to present, and offers perspectives on future directions in the field to inspire the creation of novel synthetic tools based on this transformation.

Identification of 5-amino-1,3,4-thiadiazole appended isatins as bioactive small molecules with polypharmacological activities

The identification of heterocyclic small molecules that cover unexplored chemical space is of great importance for the development of new small-molecule therapeutics. In this study, we synthesized a series of 5-amino-1,3,4-thiadiazoles appended isatins (UZ-1-20) that exhibited polypharmacological properties, as evaluated in a cell-painting assay assessing induced cellular morphological changes. A high hit rate ranging from 55% to 80% was observed for the tested compounds at varied concentrations. The most active compounds showed significant activity in inducing cellular morphological changes with a measured induction value of more than 30% and shared a high biological profiling similarity with an antifungal agent itraconazole and a chemokine receptor inhibitor. The synthesized compounds exhibited moderate to good antiproliferative activity against tested cancer cell lines in the MTT assay. Molecular docking studies were performed to theoretically probe and compare the binding modes between the most active UZ compounds and ITZ or BI-6901, respectively. Additionally, ADMET analysis indicated favorable pharmacokinetic parameters including good oral bioavailability, balanced hydrophilicity, and minimal toxicity. Overall, the findings in this study highlight the potential of developing the aminothiadiazole appended isatins as bioactive agents.

Genetic Incorporation of a Thioxanthone-Containing Amino Acid for the Design of Artificial Photoenzymes

Genetically encodable photosensitizers allow the design of artificial photoenzymes to expand the scope of abiological reactions. Herein, we report the genetic incorporation of a thioxanthone-containing amino acid into a protein scaffold via an engineered pyrrolysyl-tRNA/pyrrolysyl-tRNA synthetase pair. The designer enzyme was engineered to catalyze a dearomative [2+2] cycloaddition reaction in high yields (up to>99 % yield) with excellent enantioselectivity (up to 98 : 2 e.r.). This work provides a robust and facile method for photoenzyme design and lays the foundation for the development of further photoenzymatic reactions.

Deacylative Homolysis of Ketone C(sp3)-C(sp2) Bonds: Streamlining Natural Product Transformations

The homolytic cleavage of C-C bonds adjacent to specific functional groups has lately emerged as a versatile approach for molecular diversification. Despite the ubiquity and synthetic utility of ketones, radical fragmentation of their α-C-C bonds has proven to be a formidable challenge. Here, we present a broadly applicable deacylative strategy designed to homolytically cleave aliphatic ketones of various complexities, including transformations of cycloalkanones into carboxylic acids tethered to C-centered free radicals that can be engaged in diverse radical-based processes. The method involves ketone activation through treatment with hydrogen peroxide, yielding gem-dihydroperoxides. Subsequent single-electron-transfer reduction mediated by a low-valent metal complex generates alkyl radicals that can be captured selectively with a radicophile of choice, including through catalytic cross-coupling. The logic of our deacylative functionalization is exemplified by the total synthesis of 14 natural products, one analogue, and two drugs starting from readily available natural products, showcasing its transformative power in complex settings. This approach obviates the need for complex reagents and allows the controlled conversion of ketones to reconstructed products, making the process highly applicable across a spectrum of domains.

BrCF2CN for photocatalytic cyanodifluoromethylation

Considering the unique electronic properties of the CF2 and the CN groups, the CF2CN group has significant potential in drug and agrochemical development, as well as material sciences. However, incorporating a CF2CN group remains a considerable challenge. In this work, we disclose a use of bromodifluoroacetonitrile (BrCF2CN), a cost-effective and readily available reagent, as a radical source for cyanodifluoromethylation of alkyl alkenes, aryl alkenes, alkynes, and (hetero)arenes under photocatalytic conditions. This protocol demonstrates an exceptionally broad substrate scope and remarkable tolerance to various functional groups. Notably, the cyanodifluoromethylation of alkynes predominantly provides sterically hindered alkenes, a thermodynamically unfavorable outcome, and (hetero)arene C-H bonds are directly amenable to cyanodifluoromethylation without pre-functionalization.

Structurally Diverse Nitrogen-Rich Scaffolds via Continuous Photo-Click Reactions

Continuous flow technology was exploited for the effective generation of nitrile imines via photolysis of substituted aryl tetrazoles. The resulting photo-click process rapidly affords advanced nitrogen-rich scaffolds upon the subsequent trapping of the reactive dipole with alkenes, alkynes, and benzylic amines. Crucially, this approach uncovers the differential reactivity for ether vs amine tethers, thus providing facile and scalable access to underexplored medicinally relevant heterocyclic entities.

Organic Two-Photon-Absorbing Photosensitizers Can Overcome Competing Light Absorption in Organic Photocatalysis

Conventional organic photocatalysis typically relies on ultraviolet and short-wavelength visible photons as the energy source. However, this approach often suffers from competing light absorption by reactants, products, intermediates, and co-catalysts, leading to reduced quantum efficiency and side reactions. To address this issue, we developed novel organic two-photon-absorbing (TPA) photosensitizers capable of functioning under deep red and near-infrared light irradiation. Three model reactions including cyclization, Sonogashira Csp2-Csp cross-coupling, and Csp2-N cross-coupling reactions were selected to compare the performance of the new photosensitizers under both blue (427 nm) and deep red (660 nm) light irradiation. The obtained results unambiguously prove that for reactions involving blue light-absorbing reactants, products, and/or co-catalysts, deep red light source resulted in better performance than blue light when utilizing our TPA photosensitizers. This work highlights the potential of our metal-free TPA photosensitizers as a sustainable and effective solution to mitigate the competing light absorption issue in photocatalysis, not only expanding the scope of organic photocatalysts but also reducing reliance on expensive Ru/Ir/Os-based photosensitizers.

Photochemical reactions of biomass derived platform chemicals

Platform chemicals obtained from biomass will play an important role in chemical industry. Already existing compounds or not yet established chemicals are produced from this renewable feedstock. Using photochemical reactions as sustainable method for the conversion of matter furthermore permits to develop processes that are interesting from the ecological and economical point of view. Furans or levoglucosenone are thus obtained from carbohydrate containing biomass. Photochemical rearrangements, photooxygenation reactions or photocatalytic radical reactions can be carried out with such compounds. Also, sugars such pentoses or hexoses can be more easily transformed into heterocyclic target compounds when such photochemical reactions are used. Lignin is an important source for aromatic compounds such as vanillin. Photocycloaddition of these compounds with alkenes or the use light supported multicomponent reactions yield interesting target molecules. Dyes, surfactants or compounds possessing a high degree of molecular diversity and complexity have been synthesized with photochemical key steps. Alkenes as platform chemicals are also produced by fermentation processes, for example, with cyanobacteria using biological photosynthesis. Such alkenes as well as terpenes may further be transformed in photochemical reactions yielding, for example, precursors of jet fuels.

Synthesis of propargylamine: pioneering a green path with non-conventional KA2 coupling approach

The KA2 coupling reaction is a well-explored and versatile method for forming C-C bonds in synthetic chemistry. It is composed of ketone, amine, and alkyne, which play a major role in the synthesis of propargylamines, known for their diverse biological activities and are used in treating neurogenetical disorders. The KA2 coupling is highly challenging due to the low reactivity of ketimines toward nucleophilic attacks with metal acetylide intermediates formed by activating the C-H bond of the alkyne. Despite predominant studies conducted on thermal conditions for KA2 coupling reactions, green and sustainable approaches like non-conventional methods still have a lot to achieve. This review article provides a comprehensive introduction to the non-conventional approach in the KA2 coupling reaction, outlining its mechanisms and exploring future aspects.

Light-Initiated Four-Step Domino-Multicomponent Synthesis of Functionalized Alkylidenecyclobutanes

A four-step domino-multicomponent reaction (domino-MCR) is described for the synthesis of functionalized E-alkylidenecyclobutanes from 4-hydroxy-2-methylcyclopent-2-enone derivatives and three other simple reagents. The domino-MCR is accomplished in a single protocol, comprising a tandem photochemical [2 + 2]-cycloaddition/Norrish-I/γ-H transfer reaction followed by an acetal protection and an allylic substitution reaction. In parallel, a consecutive process has been established with distinct photochemical and nonradiative sequences. An intramolecular version of these reactions provides access to complex fused-bicyclic alkylidenecyclobutanes.

Photoredox-Catalyzed Markovnikov Hydroamination of Alkenes with Azoles

A visible-light induced intermolecular hydroamination of alkenes with azoles is reported, delivering pharmaceutically valuable N-benzyl azoles in high yields with excellent Markovnikov selectivity. Mechanistic studies suggest that the process is initiated by the energy transfer of the excited photocatalyst with alkenes, followed by the single electron reduction, protonation, and subsequent single electron oxidation to afford the key alkyl carbocation intermediate. This protocol exhibits advantages of broad functional group tolerance, excellent atom economy, high efficiency, and mild reaction conditions.

Abdel-Maksoud M, Fatima S, Almutairi S, Kiani BH, Hashem AH. Anabasis setifera leaf extract from arid habitat: A treasure trove of bioactive phytochemicals with potent antimicrobial, anticancer, and antioxidant properties

The main objective of this study was to evaluate the biological activities of Anabasis setifera extract, including its antimicrobial, anticancer, and antioxidant properties. In the current study, Anabasis setifera leaves extract was evaluated for antimicrobial, anticancer, antioxidant activities and phytochemical analyses. Ethyl acetate extract of Anabasis setifera (EA-AS) exhibited promising antimicrobial activity toward Escherichia coli, Staphylococcus aureus, Salmonella typhimurium, Bacillus subtilis, Candida albicans, Aspergillus brasiliensis, Aspergillus fumigatus with MICs 62.5, 125, 62.5, 31.25, 62.5, 125 and 125 μg/mL respectively. Moreover, EA-AS showed anticancer activity at safe concentrations, where IC50 were 36.4 and 44 μg/mL toward Hep-G2 and MCF-7 cancerous cell lines. EA-AS was found to contain 55 significant compounds identified through gas chromatography mass spectrophotometry (GCMS). The most abundant compounds were 1,4-dimethoxy-6,7,8,9-tetrahydro-5-benzocycloheptenone (26.04%), hexa-2,4-diyn-1-ylbenzene (8.40%), dihydrobenzo[b]fluoranthene (6.10%), ethanone, 1-[2,3-dihydro-2-(1-methylethenyl)-5-benzofuranyl (6.10%), and valerenol (4.08%). GC mass analysis confirmed the antioxidant properties of AS by detecting several compounds with antioxidant activity, including hexa-2,4-diyn-1-ylbenzene, nerolidol, spathulenol, -naphthalenem ethanol, decahydro-4-trimethyl-8-methylene, hexadecenoic acid, tremetone, desmethoxyencecalin, heptadecyn-1-ol, thunbergol, hexadecanol, dotriacontane, taylorione, ligulatin, retinoic acid, and falcarinol. The analysis of EA-AS reveals that it is a rich source of valuable phytochemicals: total Phenolic Content: a promising 4,264 μg/mL /, suggesting substantial biological and pharmacological potential. Total tannin content: 391.17 μg/mL, indicating potential applications in industries like nutraceuticals, pharmaceuticals, and cosmetics. Total flavonoid content exceptionally high at 5,163 μg/mL, while the total alkaloid content measured 1,036.26 μg/mL. Additionally, EA-AS demonstrated antioxidant activity with an EC50 of 30.6 μg/mL. In conclusion, the comprehensive analysis of the EA-AS reveals its immense potential as a rich source of valuable phytochemicals with diverse bioactivities, warranting further in-depth studies to unlock its full pharmaceutical and commercial prospects. Our results suggest substantial biological and pharmacological prospects for EA-AS as a promising antimicrobial, anticancer, and potent antioxidant.

Multicomponent Construction of Tertiary Alkylamines by Photoredox/Nickel-Catalyzed Aminoalkylation of Organohalides

Tertiary alkylamines are privileged structural motifs widely present in natural products, pharmaceutical agents, and bioactive molecules, and their efficient synthesis has been a longstanding goal in organic chemistry. The functionalization of α-amino radicals derived from abundant precursors represents an emerging approach to accessing alkylamines, but application of this strategy to obtain tertiary alkylamines remains challenging. Here, we show that dual photoredox/nickel catalysis enables aminoalkylation of organohalides (sp2- and sp3-hybridized) in combination with secondary alkylamines and aldehydes. The multicomponent process proceeds through selective generation of α-amino radicals from the reduction of in situ-generated iminium ions by photoredox catalysis, followed by nickel-catalyzed cross-coupling to build a wide array of functionally diverse tertiary alkylamines. This strategy could also be extended to unprecedented four-component reactions and their asymmetric variants to deliver enantioenriched α-aryl-substituted γ-amino acid derivatives. Taken together, this work offers a streamlined synthetic route to aliphatic tertiary amines.

DFT study of electron donor-acceptor (EDA) complexes: mechanism exploration and theoretical prediction

Organic synthesis methods initiated by visible light have received increasing attention from synthetic chemists. Reactions initiated by EDA complexes do not require the use of toxic or expensive photoredox catalysts, unlike traditional photoreaction processes. However, this kind of reaction requires a particular structure for the substrate, so it is important to study the detailed and systematic reaction mechanism for its design. EDA complexes of substituted 1H-indole and substituted benzyl bromide derivatives were studied by density functional theory (DFT). The difference between EDA complexes with substituents of different kinds and locations were compared by theoretical study and a new EDA complex was predicted.

Direct photochemical intramolecular [4 + 2] cycloadditions of dehydrosecodine-type substrates for the synthesis of the iboga-type scaffold and divergent [2 + 2] cycloadditions employing micro-flow system

Photocyclisation reactions offer a convenient and versatile method for constructing complex polycyclic scaffolds, particularly in the synthesis of natural products. While the [2 + 2] photocycloaddition reaction is well-established and extensively reported, the [4 + 2] counterpart via direct photochemical means remains challenging and relatively unexplored. In this work, we devised the rapid assembly of the iboga-type scaffold through photochemical intramolecular Diels-Alder reaction using a common biomimetic dehydrosecodine-type intermediate having vinyl indole and dihydropyridine (DHP) sub-units. Exploiting a micro-flow system, the medicinally important iboga-type scaffold was obtained up to 77% yield under mild, neutral conditions at room temperature. This study demonstrated the site-selective activation of the DHP moiety by direct UV-LED irradiation, eliminating the need for external photocatalysts or photosensitisers and showing good tolerance to a wide range of stabilised dehydrosecodine-type substrates. By adjusting the spatial arrangement of the DHP ring and the vinyl indole group, this versatile photochemical approach efficiently facilitates both [4 + 2] and [2 + 2] cyclisations, assembling architecturally complex multicyclic scaffolds. Precise photoactivation of the DHP subunit, generating short-lived biradical species, enabled the new way of harnessing the hidden but innately pre-encoded reactivity of the polyunsaturated dehydrosecodine-type intermediate. These photo-mediated [4 + 2] cyclisation and divergent [2 + 2] cycloadditions are distinct from biosynthetic processes, which are mainly mediated through concerted thermal cycloadditions.

A new synthesis of indolo[2,3-b]quinolines from 3-acetyl-N-alkyl-2-chloroindoles with 2-aminobenzophenone

A new synthesis of N-alkyl- and 11-phenyl-modified indolo[2,3-b]quinolines was achieved via PEG-400-promoted and visible light-induced one-step reaction of 3-acetyl-N-alkyl-2-chloroindoles with 2-aminobenzophenone in 40% methanol aqueous solution. The merits of the protocol include cost efficiency, convenience, and eco- and user-friendliness.

Visible-light-induced redox-neutral difunctionalization of alkenes and alkynes

The twelve principles of green chemistry illuminate the pathway in the direction of sustainable and eco-friendly synthesis, marking a fundamental shift in synthetic organic chemistry paradigms. In this realm, harnessing the power of visible light for the difunctionalization of various skeletons without employing any external oxidant or reductant, specifically termed as redox-neutral difunctionalization, has attracted tremendous interest from synthetic organic chemists due to its low cost, easy availability and environmentally friendly nature in contrast to traditional metal-catalyzed difunctionalizations. This review presents an overview of recent updates on visible-light-induced redox-neutral difunctionalization reactions with literature coverage up to May 2024.

Organic Reactions Enabled by Mechanical Force-Induced Single Electron Transfer

Mechanochemical reactions, achieved through milling, grinding, or other mechanical actions, have emerged as a solvent-free alternative to traditional solution-based chemistry. Mechanochemistry not only provides the opportunity to eliminate bulk solvent use, reducing waste generation, but also unveils a new reaction strategy which enables the realization of reactions previously inaccessible in solution. While the majority of organic reactions facilitated by mechanical force traditionally follow two-electron transfer pathways similar to their solution-based counterparts, the field of mechanochemically induced single-electron transfer (SET) reactions has witnessed rapid development. This review outlines examples of mechanochemical reactions facilitated by the SET process, focusing on the reagents that initiate SET, thereby positioning mechanochemistry as a burgeoning field within the realm of single-electron chemistry.

Photoredox Catalysis by 21-Thiaporphyrins: A Green and Efficient Approach for C-N Borylation and C-H Arylation

Photoredox catalysis provides a green and sustainable alternative for C-H activation of organic molecules that eludes harsh conditions and use of transition metals. The photocatalytic C-N borylation and C-H arylation mostly depend on the ruthenium and iridium complexes or eosin Y and the use of porphyrin catalysts is still in infancy. A series of novel 21-thiaporphyrins (A2B2 and A3B type) were synthesized having carbazole/phenothiazine moieties at their meso-positions and screened as catalysts for C-N borylation and C-H arylation. This paper demonstrates the 21-thiaporphyrin catalyzed C-N borylation and het-arylation of anilines under visible light. The method utilizes only 0.1 mol % of 21-thiaporphyrin catalyst under blue light for the direct C-N borylation and het-arylation reactions. A variety of substituted anilines were used as source for expensive and unstable aryl diazonium salts in the reactions. The heterobiaryls and aryl boronic esters were obtained in decent yields (up to 88 %). Versatility of the 21-thiaporphyrin catalyst was tested by thiolation and selenylation of anilines under similar conditions. Mechanistic insight was obtained from DFT studies, suggesting that 21-thiaporphyrin undergo an oxidative quenching pathway. The photoredox process catalyzed by 21-thiaporphyrins offers a mild, efficient and metal-free alternative for the formation of C-C, C-S, and C-Se bonds in aryl compounds; it can also be extended to borylation reaction.

Trityl isocyanide as a general reagent for visible light mediated photoredox-catalyzed cyanations

A photoredox catalytic strategy has been developed to enable the functionalization of a variety of commercially available, structurally different radical precursors by the use of a bench-stable isonitrile as an efficient cyanating reagent. Specifically, a radical-based reaction has provided a mild and convenient procedure for the cyanation of primary, secondary and tertiary radicals derived from widely accessible sp3-hybridized carboxylic acids, alcohols and halides under visible light irradiation. The reaction tolerates a variety of functional groups and it represents a complementary method for the cyanation of structurally different scaffolds that show diverse native functionalities, expanding the scope of previously reported methodologies.

Decennary Update on Oxidative-Rearrangement Involving 1,2-Aryl C-C Migration Around Alkenes: Synthetic and Mechanistic Insights

In recent years, numerous methodologies on oxidative rearrangements of alkenes have been investigated, that produce multipurpose synthons and heterocyclic scaffolds of potential applications. The present review focused on recently established methodologies for oxidative transformation via 1,2-aryl migration in alkenes (2013-2023). Special emphasis has been placed on mechanistic pathways to understand the reactivity pattern of different substrates, challenges to enhance selectivity, the key role of different reagents, and effect of different substituents, and how they affect the rearrangement process. Moreover, synthetic limitations and future direction also have been discussed. We believe, this review offers new synthetic and mechanistic insight to develop elegant precursors and approaches to explore the utilization of alkene-based compounds for natural product synthesis and functional materials.

Visible-Light-Photocatalyzed Self-Cyclopropanation Reactions of Dibenzoylmethanes for the Synthesis of Cyclopropanes

A new self-cyclopropanation of 1,3-diphenylpropane-1,3-dione, leading to tetrasubstituted cyclopropane containing three contiguous stereogenic centers with high stereoselectivity, has been achieved through violet-light-emitting diode-irradiated photocatalysis, featuring both cycloaddition and a distinctive rearrangement. Diverging from conventional cyclopropanation pathways, this reaction yields a tetrasubstituted cyclopropane through unprecedented rearrangement and cascade reactions.

Di-π-ethane Rearrangement of Cyano Groups via Energy-Transfer Catalysis

Molecular rearrangement occupies a pivotal position among fundamental transformations in synthetic chemistry. Radical translocation has emerged as a prevalent synthetic tool, efficiently facilitating the migration of diverse functional groups. In contrast, the development of di-π-methane rearrangement remains limited, particularly in terms of the translocation of cyano functional groups. This is primarily attributed to the energetically unfavorable three-membered-ring transition state. Herein, we introduce an unprecedented di-π-ethane rearrangement enabled by energy-transfer catalysis under visible light conditions. This innovative open-shell rearrangement boasts broad tolerance toward a range of functional groups, encompassing even complex drug and natural product derivatives. Overall, the reported di-π-ethane rearrangement represents a complementary strategy to the development of radical translocation enabled by energy-transfer catalysis.

Cage escape governs photoredox reaction rates and quantum yields

Photoredox catalysis relies on light-induced electron transfer leading to a radical pair comprising an oxidized donor and a reduced acceptor in a solvent cage. For productive onward reaction to occur, the oxidized donor and the reduced acceptor must escape from that solvent cage before they undergo spontaneous reverse electron transfer. Here we show the decisive role that cage escape plays in three benchmark photocatalytic reactions, namely, an aerobic hydroxylation, a reductive debromination and an aza-Henry reaction. Using ruthenium(II)- and chromium(III)-based photocatalysts, which provide inherently different cage escape quantum yields, we determined quantitative correlations between the rates of photoredox product formation and the cage escape quantum yields. These findings can be largely rationalized within the framework of Marcus theory for electron transfer.

Photocatalytic systems: reactions, mechanism, and applications

The photocatalytic field revolves around the utilization of photon energy to initiate various chemical reactions using non-adsorbing substrates, through processes such as single electron transfer, energy transfer, or atom transfer. The efficiency of this field depends on the capacity of a light-absorbing metal complex, organic molecule, or substance (commonly referred to as photocatalysts or PCs) to execute these processes. Photoredox techniques utilize photocatalysts, which possess the essential characteristic of functioning as both an oxidizing and a reducing agent upon activation. In addition, it is commonly observed that photocatalysts exhibit optimal performance when irradiated with low-energy light sources, while still retaining their catalytic activity under ambient temperatures. The implementation of photoredox catalysis has resuscitated an array of synthesis realms, including but not limited to radical chemistry and photochemistry, ultimately affording prospects for the development of the reactions. Also, photoredox catalysis is utilized to resolve numerous challenges encountered in medicinal chemistry, as well as natural product synthesis. Moreover, its applications extend across diverse domains encompassing organic chemistry and catalysis. The significance of photoredox catalysts is rooted in their utilization across various fields, including biomedicine, environmental pollution management, and water purification. Of course, recently, research has evaluated photocatalysts in terms of cost, recyclability, and pollution of some photocatalysts and dyes from an environmental point of view. According to these new studies, there is a need for critical studies and reviews on photocatalysts and photocatalytic processes to provide a solution to reduce these limitations. As a future perspective for research on photocatalysts, it is necessary to put the goals of researchers on studies to overcome the limitations of the application and efficiency of photocatalysts to promote their use on a large scale for the development of industrial activities. Given the significant implications of the subject matter, this review seeks to delve into the fundamental tenets of the photocatalyst domain and its associated practical use cases. This review endeavors to demonstrate the prospective of a powerful tool known as photochemical catalysis and elucidate its underlying tenets. Additionally, another goal of this review is to expound upon the various applications of photocatalysts.

Photoreaction products of extract from the fruiting bodies of Polyozellus multiplex

Photochemical reactions are powerful tools for synthesizing organic molecules. The input of energy provided by light offers a means to produce strained and unique molecules that cannot be assembled using thermal protocols, allowing for the production of immense molecular complexity in a single chemical step. Furthermore, unlike thermal reactions, photochemical reactions do not require active reagents such as acids, bases, metals, or enzymes. Photochemical reactions play a central role in green chemistry. This article reports the isolation and structure determination of four new compounds (1-4) from the photoreaction products of the Polyozellus multiplex MeOH ext. The structures of the new compounds were elucidated using MS, IR, comprehensive NMR measurements and microED. The four compounds were formed by deacetylation of polyozellin, the main secondary metabolite of P. multiplex, and addition of singlet oxygen generated by sunlight. To develop drugs for treating Alzheimer's disease (AD) on the basis of the amyloid cascade hypothesis, the compounds (1-4) obtained by photoreaction were evaluated for BACE1 inhibitory activity. The hydrolysates (5 and 6) of polyozellin, the main secondary metabolites of P. multiplex, were also evaluated. The photoreaction products (3 and 4) and hydrolysates (5 and 6) of polyozellin showed BACE1 inhibitory activity (IC50: 2.2, 16.4, 23.3, and 5.3 μM, respectively).

Recent Advances in the Domino Annulation Reaction of Quinone Imines

Quinone imines are important derivatives of quinones with a wide range of applications in organic synthesis and the pharmaceutical industry. The attack of nucleophilic reagents on quinone imines tends to lead to aromatization of the quinone skeleton, resulting in both the high reactivity and the unique reactivity of quinone imines. The extreme value of quinone imines in the construction of nitrogen- or oxygen-containing heterocycles has attracted widespread attention, and remarkable advances have been reported recently. This review provides an overview of the application of quinone imines in the synthesis of cyclic compounds via the domino annulation reaction.

Visible photons as ideal reagents for the activation of coloured organic compounds

In recent decades, the traceless nature of visible photons has been exploited for the development of efficient synthetic strategies for the photoconversion of colourless compounds, namely, photocatalysis, chromophore activation, and the formation of an electron donor/acceptor (EDA) complex. However, the use of photoreactive coloured organic compounds is the optimal strategy to boost visible photons as ideal reagents in synthetic protocols. In view of such premises, the present review aims to provide its readership with a collection of recent photochemical strategies facilitated via direct light absorption by coloured molecules. The protocols have been classified and presented according to the nature of the intermediate/excited state achieved during the transformation.

Cyclic Amine Synthesis via Catalytic Radical-Polar Crossover Cycloadditions

The rapid assembly of valuable cyclic amine architectures in a single step from simple precursors has been recognized as an ideal platform in term of efficiency and sustainability. Although a vast number of studies regarding cyclic amine synthesis has been reported, new synthetic disconnection approaches are still high in demand. Herein, we report a catalytic radical-polar crossover cycloaddition to cyclic amine synthesis triggered from primary sulfonamide under photoredox condition. This newly developed disconnection, comparable to established synthetic approaches, will allow to construct β, β-disubstituted cyclic amine and β-monosubstituted cyclic amine derivatives efficiently. This study highlights the unique utility of primary sulfonamide as a bifunctional reagent, which acts as a radical precursor and a nucleophile. The open-shell methodology demonstrates broad tolerance to various functional groups, drug derivatives and natural products in an economically and sustainable fashion.

Redox-Neutral Decarboxylative Cross-Coupling of Oxamates with Aryl Bromides

Dual nickel-photoredox-enabled direct synthesis of amides through cross-coupling of cesium oxamates with aryl bromides has been developed. This methodology's key advantages are mild reaction conditions, utilizing organic dye as a photocatalyst, employing readily available starting chemicals as coupling partners, and late-stage carbamoylation of pharmaceutically relevant molecules. DFT studies suggested that the nickel catalytic cycle proceeds via a radical addition pathway prior to the oxidative insertion.

Post-synthetic Rhodium (III) Complexes in Covalent Organic Frameworks for Photothermal Heterogeneous C-H Activation

Although photocatalytic C-H activation has been realized by using heterogeneous catalysts, most of them require high-temperature conditions to provide the energy required for C-H bond breakage. The catalysts with photothermal conversion properties can catalyze this reaction efficiently at room temperature, but so far, these catalysts have been rarely developed. Here, we construct bifunctional catalysts Rh-COF-316 and -318 to combine photosensitive covalent organic frameworks (COFs) and transition-metal catalytic moiety using a post-synthetic approach. The Rh-COF enable the heterogeneous C-H activation reaction by photothermal conversion for the first time, and exhibit excellent yields (up to 98 %) and broad scope of substrates in [4+2] annulation at room temperature, while maintaining the high stability and recyclability. Significantly, this work is the highest yield reported so far in porous materials catalyzing C(sp2)-C(sp2) formation at room temperature. The excellent performances can be attributed to the COF-316, which enhances the photothermal effect (ΔT=50.9 °C), thus accelerating C-H bond activation and the exchange of catalyst with substrates.

Photochemistry in Medicinal Chemistry and Chemical Biology

Photochemistry has emerged as a transformative force in organic chemistry, significantly expanding the chemical space accessible for medicinal chemistry. Light-induced reactions enable the efficient synthesis of intricate organic structures and have found applications throughout the different stages of the drug discovery and development processes. Moreover, photochemical techniques provide innovative solutions in chemical biology, allowing precise spatiotemporal drug activation and targeted delivery. In this Perspective, we highlight the already numerous remarkable applications and the even more promising future of photochemistry in medicinal chemistry and chemical biology.

Total Synthesis of Ganoderma Meroterpenoids Cochlearol B and Its Congeners Driven by Structural Similarity and Biological Homology

Secondary metabolites that have the same biological origin must share some relationship in their biosynthesis. Exploring this relationship has always been a significant task for synthetic biologists. However, from the perspective of synthetic chemists, it is equally important to propose, prove, or refute potential biosynthetic pathways in order to elucidate and understand the biosynthesis of homologous secondary metabolites. In this study, driven by the high structural similarity between the homologous Ganoderma meroterpenoids cochlearol B and ganocin B, two chemically synthetic strategies were designed and investigated sequentially for the synthesis of cochlearol B from ganocin B. These strategies include intramolecular metal-catalyzed hydrogen atom transfer (MHAT) and intramolecular photochemical [2+2] cycloaddition. The aim was to reveal their potential biosynthetic conversion relationship using chemical synthesis methods. As a result, a highly efficient total synthesis of cochlearol B, cochlearol T, cochlearol F, as well as the formal total synthesis of ganocins A-B, and ganocochlearins C-D, has been achieved. Additionally, a novel synthetic approach for the synthesis of 6,6-disubstituted 6H-dibenzo[b,d]pyran and its analogues has been developed through palladium(II)-catalyzed Wacker-type/cross-coupling cascade reactions.

Reversing the stereoselectivity of intramolecular [2+2] photocycloaddition utilizing cucurbit[8]uril as a molecular flask

Macrocyclic hosts, such as cucurbit[8]uril (CB8), can significantly influence the outcomes of chemical reactions involving encapsulated reactive guests. In this study, we demonstrate that CB8 completely reverses the stereoselectivity of intramolecular [2+2] photo-cycloaddition reactions. Notably, it was also found that CB8 can trigger the unreactive diene to be reactive.