Catalytic Transformation of Biomass into Sustainable Carbocycles: Recent Advances, Prospects, and Challenges

Organic compounds bearing one or more carbocycles in their molecular structure have a discernible presence in all major classes of organic products of industrial significance. However, sourcing carbocyclic compounds from exhaustible, anthropogenic carbon (e. g., petroleum) raises serious concerns about sustainability in the chemical industries. This review discusses recent advances in the renewable synthesis of carbocyclic compounds from biomass components following catalytic pathways. The mechanistic insights, process optimizations, green metrics, and alternative synthetic strategies of carbocyclic compounds have been detailed. Moreover, the renewable syntheses of carbocycles have been assessed against their existing synthetic routes from petroleum for better perspectives on their sustainability and technological preparedness. This work will assist the researchers in acquiring updated information on the sustainable synthesis of carbocyclic compounds from various biomass components, comprehending the research gaps, and developing superior synthetic processes for their commercial production.

Targeted Photoactivatable Green-Emitting BODIPY Based on Directed Photooxidation-Induced Activation and its Application to Live Dynamic Super-Resolution Microscopy

Photoactivatable fluorescent probes are valuable tools in bioimaging for tracking cells down to single molecules and for single molecule localization microscopy. For the latter application, green emitting dyes are in demand. We herein developed an efficient green-emitting photoactivatable furanyl-BODIPY (PFB) and we established a new mechanism of photoactivation called Directed Photooxidation Induced Activation (DPIA) where the furan is photo-oxidized in a directed manner by the singlet oxygen produced by the probe. The efficient photoconverter (93-fold fluorescence enhancement at 510 nm, 49 % yield conversion) is functionalizable and allowed targeting of several subcellular structures and organelles, which were photoactivated in live cells. Finally, we demonstrated the potential of PFB in super-resolution imaging by performing PhotoActivated Localization Microscopy (PALM) in live cells.

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.

Regioselective Photoredox Catalyzed Cycloadditions of Acyclic Carbonyl Ylides

A photoredox catalyzed [3 + 2] dipolar cycloaddition between acyclic carbonyl ylides generated from α-cyano epoxides and dipolarophiles is described. This method, influenced by anionic charge localization and temperature control, enabled the synthesis of regioselective functionalized cyclic ethers. By leveraging different dipolarophiles, Lewis acid mediated activation afforded either furan or hydroxy-dihydronaphthalene scaffolds. A direct synthesis of lignan natural products isodiphyllin and diphyllin is achieved by exploiting the nitrile's reactivity as a directing handle for the desired regioisomer.

Exploring the Potential of Al(III) Photosensitizers for Energy Transfer Reactions

Three homoleptic Al(III) complexes (Al1-Al3) with different degrees of methylation at the 2-pyridylpyrrolide ligand were systematically tested for their function as photosensitizers (PS) in two types of energy transfer reactions. First, in the generation of reactive singlet oxygen (1O2), and second, in the isomerization of (E)- to (Z)-stilbene. 1O2 was directly evidenced by its characteristic NIR emission at around 1276 nm and indirectly by the reaction with an organic substrate [e.g. 2,5-diphenylfuran (DPF)] using in situ UV/vis spectroscopy. In a previous study, the presence of additional methyl groups was found to be beneficial for the photocatalytic reduction of CO2 to CO, but here Al1 without any methyl groups exhibits superior performance. To rationalize this behavior, a combination of photophysical experiments (absorption, emission and excited state lifetimes) together with photostability measurements and scalar-relativistic time-dependent density functional theory calculations was applied. As a result, Al1 exhibited the highest emission quantum yield (64%), the longest emission lifetime (8.7 ns) and the best photostability under the reaction conditions required for the energy transfer reactions (e.g. in aerated chloroform). Moreover, Al1 provided the highest rate constant (0.043 min-1) for the photocatalytic oxygenation of DPF, outperforming even noble metal-based competitors such as [Ru(bpy)3]2+. Finally, its superior photostability enabled a long-term test (7 h), in which Al1 was successfully recycled seven times, underlining the high potential of this new class of earth-abundant PSs.

Molecular folding governs switchable singlet oxygen photoproduction in porphyrin-decorated bistable rotaxanes

Rotaxanes are mechanically interlocked molecules where a ring (macrocycle) is threaded onto a linear molecule (thread). The position of the macrocycle on different stations on the thread can be controlled in response to external stimuli, making rotaxanes applicable as molecular switches. Here we show that bistable rotaxanes based on the combination of a Zn(II) tetraphenylporphyrin photosensitizer, attached to the macrocycle, and a black-hole-quencher, attached to the thread, are capable of singlet oxygen production which can be switched on/off by the addition of base/acid. However, we found that only a sufficiently long linker between both stations on the thread enabled switchability, and that the direction of switching was inversed with regard to the original design. This unexpected behavior was attributed to intramolecular folding of the rotaxanes, as indicated by extensive theoretical calculations. This evidences the importance to take into account the conformational flexibility of large molecular structures when designing functional switchable systems.

Photooxidative Reaction of β-Oxoamides with Amines for the Synthesis of Pyrrolin-4-ones under External Photocatalyst-Free Conditions

The incorporation of oxygen atoms from air under aerobic conditions plays an important role in organic synthesis. Herein, Brønsted acids are found to be a two-in-one strategic catalyst to transform enamines from β-oxoamides and amines to pyrrolin-4-ones without an external photocatalyst under visible-light conditions. The Brønsted acid can inhibit the C-C bond fragmentation of the [2 + 2] adduct from enamine and 1O2, but most importantly, it can form photosensitizers with enamine and pyrrolin-4-one product by acidochromism to promote the 1O2 generation.

Bioconversion of Furanic Compounds by Chlorella vulgaris-Unveiling Biotechnological Potentials

Lignocellulosic biomass is abundant on Earth, and there are multiple acidic pretreatment options to separate the cellulose, hemicellulose, and lignin fraction. By doing so, the fermentation inhibitors 5-Hydroxymethylfurfural (HMF) and furfural (FF) are produced in varying concentrations depending on the hydrolyzed substrate. In this study, the impact of these furanic compounds on Chlorella vulgaris growth and photosynthetic activity was analyzed. Both compounds led to a prolonged lag phase in Chlorella vulgaris growth. While the photosynthetic yield Y(II) was not significantly influenced in cultivations containing HMF, FF significantly reduced Y(II). The conversion of 5-Hydroxymethylfurfural and furfural to 5-Hydroxymethyl-2-Furoic Acid and 2-Furoic Acid was observed. In total, 100% of HMF and FF was converted in photoautotrophic and mixotrophic Chlorella vulgaris cultivations. The results demonstrate that Chlorella vulgaris is, as of now, the first known microalgal species converting furanic compounds.

Expanding Diterpene Complexity and Diversity via Photoinduced Ring Distortions

Natural products and their semi-synthetic derivatives undoubtedly constitute an important source of therapeutic agents. Their importance lies in their own origin and evolution, since they have great chemical diversity, biochemical specificity, and pharmacological properties. Currently, there is a renewed interest in the development of methodologies capable of efficiently modifying the chemical structure of these bioactive platforms. In this work, the photoderivatization of the diterpene solidagenone was performed using a complexity-to-diversity-oriented approach. By exploring [2+2]-photocycloaddition, photoinduced-hydrogen abstraction, and photoxygenation reactions, a set of solidagenone derivatives was obtained, showing different ring fusions, side chain rearrangements, and modifications of the original furan ring's substitution pattern. The derivatives obtained were characterised by NMR methodologies. To evaluate the structural diversity of the labdane-derived compounds, their physicochemical properties, structural similarity, and chemical space were analysed. These results suggest that photochemical reactions are a useful tool for performing ring distortion transformations, generating derivatives of natural compounds with wide diversity, structural complexity, and with potential biological properties.

Furan Dearomatization: A Route to Diverse Fluoroalkyl/Aryl Triazoles

The 5-fluoroalkyl-1,2,3-triazoles, serving as a pivotal element in medicinal chemistry, hold substantial research significance. In this work, we developed a furan dearomatization reaction for the synthesis of various 5-fluoroalkyl-1,2,3-triazoles, which contains -CF3, -CF2H, -CF2CF3, -CF2CF2CF3, -CF2CO2Et, and -C6F5. This methodology relies on the intermolecular [3 + 2] cycloaddition/furan ring-opening triggered by α-fluoroalkyl furfuryl cation with azides to stereoselectively synthesize a series of (E)-fluoroalkyl enone triazoles. The reaction proceeds without metal participation, exhibits excellent substrate tolerance, and has excellent synthetic utility.

Aerobic Catalytic Cross-Dehydrogenative Coupling of Furans with Indoles Provides Access to Fluorophores with Large Stokes Shift

Sustainability in chemical processes is a crucial aspect in contemporary chemistry with sustainable catalysis as a vital parameter of the same. There has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions. Furan is a versatile heterocycle of natural origin used for multiple applications. However, it has scarcely been used in cross-dehydrogenative coupling. In this work, we have explored the cross-dehydrogentive coupling of furans with indoles using commonly available, inexpensive FeCl3  ⋅ 6H2 O (<0.25 $/g) as catalyst in the presence of so called 'ultimate oxidant' - oxygen, without the need for any external ligand or additive. The reactions were found to be scalable and to work even under partially aqueous conditions. This makes the reaction highly economical, practical, operationally simple and sustainable. The methodology provides direct access to π-conjugated short oligomers consisting of furan, thiophene and indole. These compounds were found to show interesting fluorescence properties with remarkably large Stokes shift (up to 205 nm). Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by nucleophilic trapping by furan.

New Type Annihilator of π-Expanded Diketopyrrolopyrrole for Robust Photostable NIR-Excitable Triplet-Triplet Annihilation Upconversion

Near-infrared light excitable triplet-triplet annihilation upconversion (NIR TTA-UC) materials have attracted interest in a variety of emerging applications such as photoredox catalysis, optogenetics, and stereoscopic 3D printing. Currently, the practical application of NIR TTA-UC materials requires substantial improvement in photostability. Here, we found that the new annihilator of π-expanded diketopyrrolopyrrole (π-DPP) cannot activate oxygen to generate superoxide anion via photoinduced electron transfer, and its electron-deficient characteristics prevent the singlet oxygen-mediated [2 + 2] cycloaddition reaction; thus, π-DPP exhibited superior resistance to photobleaching. In conjunction with the NIR photosensitizer PdTNP, the upconversion efficiency of π-DPP is as high as 8.9%, which is eight times of the previously reported PdPc/Furan-DPP. Importantly, after polystyrene film encapsulation, less than 10% photobleaching was observed for this PdTNP/π-DPP-based NIR TTA-UC material after four hours of intensive NIR light exposure. These findings provide a type of annihilator with extraordinary photostability, facilitating the development of NIR TTA-UC materials for practical photonics.

Polaronic state of conducting oligomer as a new approach to design non-lieaner optical materials: A case study of oligofurans

The geometric, electronic and nonlinear optical properties of neutral and polaron based oligofurans are studied comparatively. We have reported the role of polaron to trigger the nonlinear optical response of oligofurans (nFu). The polaron based oligomers show excellent opto-electronic properties. The effect of polaron on nFu* chains is measured by electronic properties i.e (ionization energy, electron affinity, band gap) and global reactivity descriptors like softness, hardness and chemical potential than their neutral counterpart. An interesting trends of reactivity descriptors have been observed. Lower band gaps (EH-L = 4.66 and 4.41 eV) are observed for polaronic systems as compared to their neutral counterpart. On the other hand, the TD-DFT study further demonstrated that, as the size of chain increases, the absorption maxima (λmax) also increases with significant reduction in excitation energies (ΔE). Furthermore, the nonlinear optical response is confirmed through the linear polarizability (αo), static first order hyperpolarizability (βo) and dynamic (frequency denepndent) hyperpolarizability. Electric filed induced second harmonic generation (EFISHG) and electro-optic pockle effect (EOPE) at 532 nm and 1064 nm, commonly used lasers frequencies have also been employed. Our results showed that the maximum hyperpolarizabilities are observed for polaron based 7Fu* and 9Fu* i.e 1.3 × 104, and 3.1 × 104 au. This study concluded that these polaron based organic polymers (nFu*) are useful as an efficient NLO material with vast applications in different fields.

Synthesis of Neocaesalpin A, AA, and Nominal Neocaesalpin K

The first total synthesis of heavily oxidized cassane-type diterpenoid neocaesalpin A (1) is disclosed. At the heart of the synthesis lies an intermolecular Diels-Alder reaction that rapidly assembles the target framework from commercial materials. A carefully orchestrated sequence of oxidations secured the desired oxygenation pattern. Late-stage release of the characteristic butenolide occurred through a novel mercury(II)-mediated furan oxidation. Successful extension of the route allowed preparation of neocaesalpin AA (2) as well as nominal neocaesalpin K (3) and suggested structural revision of neocaesalpin K, leading to the hypothesis that the two are likely the same natural product with correct assignment as 2.

Design of Furan-Based Acceptors for Organic Photovoltaics

We explore a series of furan-based non-fullerene acceptors and report their optoelectronic properties, solid-state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced band gap and red-shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels-Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to most non-fullerene acceptors, and the photostability depends strongly on the molecular structure. Placement of furans next to the acceptor termini leads to better photostability, well-balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker offers the best photostability and power conversion efficiency (>14 %), outperforming all furan-based acceptors reported to date and all indacenodithiophene-based acceptors. Our findings show the possibility of photostable furan-based alternatives to the currently omnipresent thiophene-based photovoltaic materials.

The Photochemical Mediated Ring Contraction of 4H-1,2,6-Thiadiazines To Afford 1,2,5-Thiadiazol-3(2H)-one 1-Oxides

1,2,6-Thiadiazines treated with visible light and 3O2 under ambient conditions are converted into difficult-to-access 1,2,5-thiadiazole 1-oxides (35 examples, yields of 39-100%). Experimental and theoretical studies reveal that 1,2,6-thiadiazines act as triplet photosensitizers that produce 1O2 and then undergo a chemoselective [3 + 2] cycloaddition to give an endoperoxide that ring contracts with selective carbon atom excision and complete atom economy. The reaction was optimized under both batch and continuous-flow conditions and is also efficient in green solvents.

Ex Situ Generation of 18O2 and 17O2 from Endoperoxides for *O-Labeling and Mechanistic Studies of Oxidations by Dioxygen

Near-stoichiometric amounts of ¹⁸O₂ and ¹⁷O₂ were generated ex situ from endoperoxides in a two-chamber glassware to oxidize various substrates. This strategy gave [*O₂]endoperoxides, [*O₁]quinones, [*O₁]phenols, and [*O_x]artemisin in moderate to good yields and high isotopic enrichments (up to 84%) at affordable costs. Moreover, mass spectrometry and ¹⁷O NMR of the [*O]products provided valuable information about the chemical mechanisms involved.

Dual-Color Photoconvertible Fluorescent Probes Based on Directed Photooxidation Induced Conversion for Bioimaging

We herein present a new concept to produce dual-color photoconvertible probes based on a mechanism called Directed Photooxidation Induced Conversion (DPIC). As a support of this mechanism, styryl-coumarins (SCs) bearing Aromatic Singlet Oxygen Reactive Moieties (ASORMs) like furan and pyrrole have been synthesized. SCs are bright fluorophores, which undergo a hypsochromic conversion upon visible light irradiation due to directed photooxidation of the ASORM that leads to the disruption of conjugation. SC-P, a yellow emitting probe bearing a pyrrole moiety, converts to a stable blue emitting coumarin with a 68 nm shift allowing the photoconversion and tracking of lipid droplet in live cells. This new approach might pave the way to a new generation of photoconvertible dyes for advanced bioimaging applications.

Silyl Tether-Assisted Photooxygenation of Electron-Deficient Enaminoesters: Direct Access to Oxamate Formation

Photooxygenation reactions of electron-deficient enaminoesters bearing an oxophilic silyl tether at the α-position of the nitrogen atom using methylene blue (MB) were explored to develop a mild and efficient photochemical strategy for oxidative C-C double bond cleavage reactions via singlet oxygen (¹O₂). Photochemically generated ¹O₂, through energy transfer from the triplet excited state of MB (³MB*) to molecular oxygen (³O₂), was added across a C-C double bond moiety of enaminoesters to form perepoxides, which rearranged to form dioxetane intermediates. The cycloreversion of the formed dioxetane via both C-C and O-O bond cleavage processes led to the formation of oxamates. Importantly, contrary to alkyl group tether-substituted electron-deficient enaminoesters that typically disfavor photooxygenation, the silyl tether-substituted analogues undergo this photochemical transformation efficiently with the assistance of a silyl tether, which facilitates formation of the perepoxide. The observations in this study provide useful information about photosensitized oxygenation reactions of unsaturated C-C bonds, and, moreover, this photochemical strategy can be utilized as a mild and feasible method for the preparation of diversely functionalized carbonyl compounds including oxamates.

Multiphotocatalyst Cascades: From Furans to Fused Butyrolactones and Substituted Cyclopentanones

High value oxygenated polycycles have been rapidly and efficiently accessed from simple precursors in one pot processes. The reported methodology relies on a new and mild method for butenolide synthesis mediated by thiols. The initial photooxygenation and butenolide synthesis have been merged with subsequent photoredox reactions to achieve rare dual-photocatalyst cascades affording various fused butyrolactones. Ground state Lewis acid activity for methylene blue has been unveiled and then exploited in the synthesis of substituted cyclopentanones.

Development of a modular photoreactor for the upscaling of continuous flow photochemistry

The upscaling of biphasic photochemical reactions is challenging because of the inherent constraints of liquid-gas mixing and light penetration. Using semi-permeable coaxial flow chemistry within a modular photoreactor, the photooxidation of the platform chemical furfural was scaled up to produce routinely 29 gram per day of biobased building block hydroxybutenolide, a precursor to acrylate alternatives.

Dual Roles of Azide: Dearomative Dimerization of Furfuryl Azides

A dearomative dimerization of furfuryl azides for the construction of furfuryl triazoles is developed. As a rare leaving group, azide is capable of initiating the generation of a furfuryl cation under the Lewis acid-catalyzed conditions, followed by reacting with the other azide to realize an intermolecular [3 + 2] cycloaddition/furan ring-opening cascade. By extending the reaction time, a fragmentation reaction of resulting furfuryl triazoles occurs to afford 1H-triazoles in high yield. Control studies demonstrated that key furfuryl cations also can be obtained from furfuryl triazoles. Furthermore, a chemoselective cross-cycloaddition can be achieved between furfuryl azides and a benzyl azide.

Lewis-Acid-Catalyzed Regioselective Construction of Diversely Functionalized Polycyclic Fused Furans

A novel, facile, and efficient Lewis-acid-catalyzed [4 + 1] annulation protocol for the construction of functionalized polycyclic-fused furans is developed. This methodology is free of transition metals and ligands and provides a rapid synthetic route to divergently orientated polycyclic furans in good yields. In addition, this protocol can also be used to synthesize multisubstituted furans.

A Photosensitized Singlet Oxygen (1O2) Toolbox for Bio-Organic Applications: Tailoring 1O2 Generation for DNA and Protein Labelling, Targeting and Biosensing

Singlet oxygen (1O2) is the excited state of ground, triplet state, molecular oxygen (O2). Photosensitized 1O2 has been extensively studied as one of the reactive oxygen species (ROS), responsible for damage of cellular components (protein, DNA, lipids). On the other hand, its generation has been exploited in organic synthesis, as well as in photodynamic therapy for the treatment of various forms of cancer. The aim of this review is to highlight the versatility of 1O2, discussing the main bioorganic applications reported over the past decades, which rely on its production. After a brief introduction on the photosensitized production of 1O2, we will describe the main aspects involving the biologically relevant damage that can accompany an uncontrolled, aspecific generation of this ROS. We then discuss in more detail a series of biological applications featuring 1O2 generation, including protein and DNA labelling, cross-linking and biosensing. Finally, we will highlight the methodologies available to tailor 1O2 generation, in order to accomplish the proposed bioorganic transformations while avoiding, at the same time, collateral damage related to an untamed production of this reactive species.

Synthesis of Pyrrolidine- and γ-Lactam-Containing Natural Products and Related Compounds from Pyrrole Scaffolds

Polycyclic alkaloid natural products featuring pyrrolidine and pyrrolidinone motifs remain enduring targets of total synthesis endeavors. Pyrrole and its derivatives have been exploited to access many such frameworks, including alkaloids belonging to the Aspidosperma, Stemona, and batzelladine families. In this article, a selection of exemplars that highlight the utility of pyrrole-based approaches to facilitate total syntheses of pyrrolidine- and pyrrolidinone-containing alkaloids and related molecules are showcased.

A flavin-inspired covalent organic framework for photocatalytic alcohol oxidation

Covalent organic frameworks (COFs) offer a number of key properties that predestine them to be used as heterogeneous photocatalysts, including intrinsic porosity, long-range order, and light absorption. Since COFs can be constructed from a practically unlimited library of organic building blocks, these properties can be precisely tuned by choosing suitable linkers. Herein, we report the construction and use of a novel COF (FEAx-COF) photocatalyst, inspired by natural flavin cofactors. We show that the functionality of the alloxazine chromophore incorporated into the COF backbone is retained and study the effects of this heterogenization approach by comparison with similar molecular photocatalysts. We find that the integration of alloxazine chromophores into the framework significantly extends the absorption spectrum into the visible range, allowing for photocatalytic oxidation of benzylic alcohols to aldehydes even with low-energy visible light. In addition, the activity of the heterogeneous COF photocatalyst is less dependent on the chosen solvent, making it more versatile compared to molecular alloxazines. Finally, the use of oxygen as the terminal oxidant renders FEAx-COF a promising and “green” heterogeneous photocatalyst.

In this manuscript, we report the development of a novel alloxazine COF inspired by naturally occurring flavin cofactors for photoredox catalysis.

Singlet Oxygen: Discovery, Chemistry, C60 -Sensitization†

This review article refers to the discovery of excited molecular oxygen, in particular on its lower singlet excited state (¹ Δ_g , ¹ O₂ ). After a short report on singlet oxygen generation, the review is focused on the chemistry of this reactive species. Specifically, the three major reactions of ¹ O₂ with unsaturated organic substrates, namely the [4 + 2] and [2 +2] cycloadditions as well as the ene reaction, are reviewed. The proposed mechanisms of these reactions, through the years, based on experimental and computational work, have been presented. Selected examples of singlet oxygen-synthetic applications are also mentioned. The [60]fullerene and fullereno-materials photosensitized oxidations in homogeneous, as well as in heterogeneous conditions, are also comprehensively discussed. Finally, the self-sensitized photooxidation of open cage fullerenes as well as fullerenes bearing oxidizable groups is reported.

One-Pot Transformation of Furans into 1-Azaspirocyclic Alkaloid Frameworks Induced by Visible Light

High-value 1-azaspirocyclic scaffolds have been made from simple and readily accessible furan precursors in a single operation. The protocol is a one-pot sequence using highly sustainable conditions (oxygen, visible light, and a favored green solvent) that leads to a dramatic increase in molecular complexity. The initial substrates can include functionalities that are suitable for further elaboration; in this way, the pruned polycyclic skeletons of the stemonamine, cylindricine, and lepadiformine natural products were rapidly accessed.

Eosin: a versatile organic dye whose synthetic uses keep expanding

Organic dyes, which absorb light in the visible region of the electromagnetic spectrum, offer a lower cost, greener alternative to precious metals in photocatalysis. In this context, the organic dye eosin's uses are currently expanding at a significant rate. For a long time, its action as an energy transfer agent dominated, more recently, however, there has been a growing interest in its potential as an electron transfer agent. In this short review, we highlight some recent (from 2016 onwards) contributions to the field with a focus on the breadth of the reactions eosin can catalyse.

Biomimetic Dearomatization Strategies in the Total Synthesis of Meroterpenoid Natural Products

Natural products are biosynthesized from a limited pool of starting materials via pathways that obey the same chemical logic as textbook organic reactions. Given the structure of a natural product, it is therefore often possible to predict its likely biosynthesis. Although biosynthesis mainly occurs in the highly specific chemical environments of enzymes, the field of biomimetic total synthesis attempts to replicate predisposed pathways using chemical reagents.We have followed several guidelines in our biomimetic approach to total synthesis. The overarching aim is to construct the same skeletal C-C and C-heteroatom bonds and in the same order as our biosynthetic hypothesis. In order to explore the innate reactivity of (bio)synthetic intermediates, the use of protecting groups is avoided or at least minimized. The key step, which is usually a cascade reaction, should be predisposed to selectively generate molecular complexity under substrate control (e.g., cycloadditions, radical cyclizations, carbocation rearrangements). In general, simple reagents and mild conditions are used; many of the total syntheses presented in this Account could be achieved using pre-1980s methodology. We have focused almost exclusively on the synthesis of meroterpenoids, that is, natural products of mixed terpene and aromatic polyketide origin, using commercially available terpenes and electron-rich aromatic compounds as starting materials. Finally, all of the syntheses in this Account involve a dearomatization step as a means to trigger a cascade reaction or to construct stereochemical complexity from a planar, aromatic intermediate.A biomimetic strategy can offer several advantages to a total synthesis project. Most obviously, successful biomimetic syntheses are usually concise and efficient, naturally adhering to the atom, step, and redox economies of synthesis. For example, in this Account, we describe a four-step synthesis of garcibracteatone and a three-step synthesis of nyingchinoid A. It is difficult to imagine shorter, non-biomimetic syntheses of these intricate molecules. Furthermore, biomimetic synthesis gives insight into biosynthesis by revealing the chemical relationships between biosynthetic intermediates. Access to these natural substrates allows collaboration with biochemists to help uncover the function of newly discovered enzymes and elucidate biosynthetic pathways, as demonstrated in our work on the napyradiomycin family. Third, by making biosynthetic connections between natural products, we can sometimes highlight incorrect structural assignments, and herein we discuss structure revisions of siphonodictyal B, rasumatranin D, and furoerioaustralasine. Last, biomimetic synthesis motivates the prediction of "undiscovered natural products" (i.e., missing links in biosynthesis), which inspired the isolation of prenylbruceol A and isobruceol.

A versatile heterogeneous photocatalyst: nanoporous gold powder modified with a zinc(II) phthalocyanine derivative for singlet oxygen [4 + 2] cycloadditions

Nanoporous gold was functionalized with a photosensitizer, a zinc(II) phthalocyanine derivative. Such systems are active for the generation of reactive singlet oxygen which can be used for photocatalytic oxidation reactions. This study aims to demonstrate the versatility of such an approach, in terms of substrates and the employed solvent, only possible for a truly heterogeneous catalytic system. The activity of the hybrid system was studied for [4 + 2] cycloadditions of three different types of dienes and a total of eight substrates in two organic solvents and once in water. The highest activity was measured for 1,3-diphenylisobenzofuran, which is also highest in terms of sensitivity for the reaction with ¹O₂. Trends in conversion could be anticipated based on reported values for the rate constant for the reaction of ¹O₂. In almost all cases, an amplification of the conversion by immobilization of the sensitizer onto nanoporous gold was observed. The limiting case was ergosterol, which was the largest of all substrates with a van-der-Waals radius of about 2.1 nm. Additional factors such as the limited lifetime of ¹O₂ in different solvents as well as the hampered diffusion of the substrates were identified.

Solar Photooxygenations for the Manufacturing of Fine Chemicals-Technologies and Applications

Photooxygenation reactions involving singlet oxygen (1O2) are utilized industrially as a mild and sustainable access to oxygenated products. Due to the usage of organic dyes as photosensitizers, these transformations can be successfully conducted using natural sunlight. Modern solar chemical reactors enable outdoor operations on the demonstration (multigram) to technical (multikilogram) scales and have subsequently been employed for the manufacturing of fine chemicals such as fragrances or biologically active compounds. This review will highlight examples of solar photooxygenations for the manufacturing of industrially relevant target compounds and will discuss current challenges and opportunities of this sustainable methodology.

New Approach in the Application of Conjugated Polymers: The Light-Activated Source of Versatile Singlet Oxygen Molecule

For many years, the research on conjugated polymers (CPs) has been mainly focused on their application in organic electronics. Recent works, however, show that due to the unique optical and photophysical properties of CPs, such as high absorption in UV–Vis or even near-infrared (NIR) region and efficient intra-/intermolecular energy transfer, which can be relatively easily optimized, CPs can be considered as an effective light-activated source of versatile and highly reactive singlet oxygen for medical or catalytic use. The aim of this short review is to present the novel possibilities that lie dormant in those exceptional polymers with the extended system of π-conjugated bonds.

Multi-Photocatalyst Cascades: Merging Singlet Oxygen Photooxygenations with Photoredox Catalysis for the Synthesis of Alkaloid Frameworks

The development of photocascades that rapidly transform simple and readily accessible furan substrates into polycyclic alkaloid frameworks or erythrina natural products is described. Each of the sequences developed makes use of photocatalyzed energy transfer processes, which generate singlet oxygen, to set up the substrates for the second photocatalyzed reaction, wherein electron transfer generates carbon-centered radicals for the cyclizations that give the final complex frameworks. A chemical switch has been developed that can "switch off" one photocatalyst; thus, allowing a second photocatalyst to take over control of the sequence. As a corollary, this strategy represents the first time it has been possible to use multiple photocatalysts in photocascades, and, as such, it expands significantly the reactions that can be included in such cascades and the order in which they can be initiated.

The wavelength-regulated stereodivergent synthesis of (Z)- and (E)-1,4-enediones from phosphonium ylides

The wavelength-regulated, photoredox-catalyzed stereodivergent synthesis of (Z)- and (E)-1,4-enediones from phosphonium ylides is reported.

Synthetic feasibility of oxygen-driven photoisomerizations of alkenes and polyenes

This review describes O₂-dependent photoreactions for possible routes to double-bond isomerizations. E,Z-isomerizations triggered by O₂ and visible light are a new area of potential synthetic interest. The reaction involves the reversible addition of O₂ to form a peroxy intermediate with oxygen evolution and partial regeneration of the compound as its isomer. Targeting of O₂-dependent photoisomerizations also relates to a practical use of visible light, for example the improved light penetration depth for visible as opposed to UV photons in batch sensitized reactions. This review is intended to draw a link between visible-light formation of a peroxy intermediate and its dark degradation with O₂ release for unsaturated compound isomerization. This review should be of interest both to photochemists and synthetic organic chemists, as it ties together mechanistic and synthetic work, drawing attention to an overlooked subject.

Visible-light triggered templated ligation on surface using furan-modified PNAs

Oligonucleotide-templated reactions are frequently exploited for target detection in biosensors and for the construction of DNA-based materials and probes in nanotechnology. However, the translation of the specifically used template chemistry from solution to surfaces, with the final aim of achieving highly selective high-throughput systems, has been difficult to reach and therefore, poorly explored. Here, we show the first example of a visible light-triggered templated ligation on a surface, employing furan-modified peptide nucleic acids (PNAs). Tailored photo-oxidation of the pro-reactive furan moiety is ensured by the simultaneous introduction of a weak photosensitizer as well as a nucleophilic moiety in the reacting PNA strand. This allows one to ensure a localized production of singlet oxygen for furan activation, which is not affected by probe dilution or reducing conditions. Simple white light irradiation in combination with target-induced proximity between reactive functionalities upon recognition of a short 22mer DNA or RNA sequence that functions as a template, allows sensitive detection of nucleic acid targets in a 96 well plate format.

Merging singlet-oxygen induced furan oxidations with organocatalysis: synthesis of enantiopure cyclopentanones and hydrindanes

A general, flexible and highly effective method for the regio-, diastereo- & enantioselective one-pot synthesis of important carbocycles, namely, enantiopure cyclopentanones and their hydrindane congeners is presented.

The reticent tautomer: exploiting the interesting multisite and multitype reactivity of 4-pyrrolin-2-ones

Multisite and multitype reactivities of the highly versatile and valuable synthetic building block 4-pyrrolin-2-one are covered in this review.