Applications of Norrish type I and II reactions in the total synthesis of natural products: a review

Ultrasound-assisted green synthesis of functionalised xanthene derivatives: Advancing sustainable sonochemical strategies

Xanthenes are an important class of heterocycles in medicinal chemistry due to their diverse pharmacological properties. These tricyclic aromatic compounds, characterised by a dibenzo[b,e]pyran core with an oxygen atom at their central position, have gained significant attention for their extensive applications. Beyond pharmaceuticals, xanthenes are widely used in textiles, food industries, electro-optical devices, dyes, and bioimaging agents. Xanthene derivatives, particularly 9-substituted xanthenes, exhibit a wide range of biological activities, including antiparasitic, antibacterial, antileishmanial, cytotoxic, neuroprotective, and photophysical effects, making them valuable in drug discovery. The xanthene scaffold is present in various bioactive natural compounds such as mulgravanols A and B, hermannol, (+)-myrtucommulone D, homapanicones A and B, blumeaxanthene II, and acrotrione. Clinically relevant xanthene-based drugs include propantheline bromide (antimuscarinic), methantheline (antispasmodic), and phloxine B (photosensitiser in antimicrobial therapy). Thus, various synthetic approaches have been developed for the construction of xanthenes, with ultrasound-assisted green methodologies gaining prominence. Ultrasound technique offers advantages over conventional methods, including higher yields, faster reaction rates, and improved selectivity under milder conditions. This review comprehensively explores the ultrasound-assisted synthesis of functionalised xanthene derivatives as an eco-friendly alternative. To the best of our knowledge, this is the first in-depth review focusing on the green methodology under ultrasound irradiation.

Energy-Transfer-Enabled 1,4-Amino Migration and C-O Diradical Recombination for Norrish-Yang-Type Epoxidation

An energy-transfer-enabled photochemical strategy merges 1,4-nitrogen migration with Norrish-Yang-type epoxidation to achieve skeletal editing of molecular frameworks. This approach bypasses classical di-π-methane rearrangements, providing an oxidant-free and atom-economical route to epoxides via controlled C-O diradical recombination. The protocol accommodates >40 diverse substrates, including arenes, heterocycles, and bioactive motifs, enabling late-stage functionalization of complex architectures. Scalability is demonstrated through gram-scale synthesis (84% yield) and one-pot cascades. Mechanistic studies reveal a triplet energy-transfer pathway distinct from radical chain processes, with nitrogen migration directing regioselective diradical recombination.

Polyethyleneketones with Controlled Spacer Units: Synthesis, Characterization, and Photodegradation

In this study, a new synthetic method for structurally controlled polyethyleneketones, a photodegradable polyethylene-like polymer, has been developed. Telechelic Zn-polyethylene was prepared from α,ω-diene, diethylzinc, and ethylene, which was allowed to react with diacid chlorides to give polyethyleneketones with defined spacing between the neighboring ketone functionalities. The photodegradation of these polymers under UV light demonstrated a degradation rate dependency on the spacer units between the carbonyl groups. The ones with longer-spaced ketones (6-18 carbons) showed efficient degradation, while shorter-spaced ketones (3-5 carbons) exhibited a slower degradation process. Structural analysis by solid-state FT-IR and DSC suggested the existence of carbonyl-carbonyl interactions in the shorter-spaced polymers, which are presumed to interfere with photodegradation via the Norrish reaction.

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.

Light-Driven Regioselective Deoxygenation of Carbohydrate Lactones for 2-Deoxy Sugar Precursor Synthesis

A sustainable method for the synthesis of 2-deoxy lactones as direct precursors to 2-deoxy sugars via regioselective UV-light-driven dealkyloxylation of carbohydrate-derived lactones is detailed. This catalyst- and additive-free protocol utilizes light irradiation, providing high step economy and functional group compatibility. This environmentally friendly and straightforward approach enhances the synthetic toolbox for 2-deoxy sugars, which is vital in numerous biologically active molecules and drug candidates.

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.

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.

Modular alkene synthesis from carboxylic acids, alcohols and alkanes via integrated photocatalysis

Alkenes serve as versatile building blocks in diverse organic transformations. Despite notable advancements in olefination methods, a general strategy for the direct conversion of carboxylic acids, alcohols and alkanes into alkenes remains a formidable challenge owing to their inherent reactivity disparities. Here we demonstrate an integrated photochemical strategy that facilitates a one-pot conversion of these fundamental building blocks into alkenes through a sequential C(sp3)-C(sp3) bond formation-fragmentation process, utilizing an easily accessible and recyclable phenyl vinyl ketone as the 'olefination reagent'. This practical method not only offers an unparalleled paradigm for accessing value-added alkenes from abundant and inexpensive starting materials but also showcases its versatility through various complex scenarios, including late-stage on-demand olefination of multifunctional molecules, chain homologation of acids and concise syntheses of bioactive molecules. Moreover, initiating from carboxylic acids, alcohols and alkanes, this protocol presents a complementary approach to traditional olefination methods, making it a highly valuable addition to the research toolkit for alkene synthesis.

Photocatalyst-free, visible-light-induced regio- and stereoselective synthesis of phosphorylated enamines from N-allenamides via [1,3]-sulfonyl shift at room temperature

Herein, we report the first visible-light-induced strategy for the rapid synthesis of densely functionalized α- and γ-phosphorylated β-sulfonyl enamines in a regio- and stereoselective manner from N-sulfonyl allenamides and H-phosphine oxides. The transformation displays a broad substrate scope, while operating at room temperature under photocatalyst- and additive-free conditions. In this atom-economical process, either terminal or substituted N-sulfonyl allenamides trigger an unprecedented N-to-C [1,3]-sulfonyl shift, relying on a dual radical allyl resonance and α-heteroatom effect in its triplet excited state. A plausible reaction mechanism is proposed which was supported by the outcomes of theoretical approaches based on Density Functional Theory (DFT) calculations.

Photoinduced [3 + 2] Cycloadditions of Aryl Cyclopropyl Ketones with Alkynes and Alkenes

The five-membered ring skeleton is one of the most pivotal in the area of pharmaceutical and natural products. [3 + 2] cycloadditions of cyclopropyl and unsaturated compounds are a highly efficient and atom-economical way to build a five-member compound. The previous works about the kind of [3 + 2] cycloadditions usually utilized metal or organic small molecule catalysts. However, an ideal [3 + 2] cycloaddition reaction that smoothly happens without any additives and catalysts under mild conditions is underdeveloped. Hence, we report [3 + 2] cycloadditions of aryl cyclopropyl without any additives and catalysts under purple LED. In this method, a broad scope of cyclopropyl, alkyne, and alkene was very compatible, especially drug derivatives ibuprofen and Ioxoprofen, to obtain the corresponding cycloaddition product with a good yield up to 93%.

Photodegradation of biodegradable plastics in aquatic environments: Current understanding and challenges

Direct and indirect photolysis are important abiotic processes in aquatic environments through which plastics can be transformed physically and chemically. Transport of biodegradable plastics in water is influenced by vertical mixing and turbulent flow, which make biodegradable plastics remain susceptible to sunlight and photolysis despite their high density. In general, biodegradable plastics are composed of ester containing polymers (e.g., poly(butylene succinate), polyhydroxyalkanoate, and polylactic acid), whereas non-biodegradable plastics are composed of long chains of saturated aliphatic hydrocarbons in their backbones (e.g., polyethylene, polypropylene, and polystyrene). Based on the reviewed knowledge and discussion, we may hypothesize that 1) direct photolysis is more pronounced for non-biodegradation than for biodegradable plastics, 2) smaller plastics such as micro/nano-plastics are more prone to photodegradation and photo-transformation by direct and indirect photolysis, 3) the production rate of reactive oxygen species (ROS) on the surface of biodegradable plastics is higher than that of non-biodegradable plastics, 4) the photodegradation of biodegradable plastics may be promoted by ROS produced from biodegradable plastics themselves, and 5) the subsequent reactions of ROS are more active on biodegradable plastics than non-biodegradable plastics. Moreover, micro/nanoplastics derived from biodegradable plastics serve as more effective carriers of organic pollutants than those from non-biodegradable plastics and thus biodegradable plastics may not necessarily be more ecofriendly than non-biodegradable plastics. However, biodegradable plastics have been largely unexplored from the viewpoint of direct or indirect photolysis. Roles of reactive oxygen species originating from biodegradable plastics should be further explored for comprehensively understanding the photodegradation of biodegradable plastics.

Progress of Metal Nanomaterial Controllable Preparation by Photoreduction

Metal nanoparticles (NPs) are widely used in biomedicine, catalysis, environment, electronics, and other fields, which is closely related to its structural form. For this purpose, researchers have been looking for a simple, green, and controllable way to mass produce metal nanomaterials with desired characteristics (shape, size, stability, etc.). Due to the surface plasmon resonance (SPR) effect of metal nanoparticles, photoreduction method can control the morphology of metal nanoparticles well, which is also simple, large-scalable, and energy-saving. This review provides an overview of the photoreduction method for the synthesis of metal nanoparticles and discusses the factors such as the light source, pH value, reagents, and temperature on the morphology of the nanoparticles. Finally, the challenges and development trends in the controlled preparation of nanomaterials are proposed.

Acyl-caged rhodamines: photo-controlled and self-calibrated generation of acetyl radicals for neural function recovery in early AD mice

The biological function of radicals is a broad continuum from signaling to killing. Yet, biomedical exploitation of radicals is largely restricted to the theme of healing-by-killing. To explore their potential in healing-by-signaling, robust radical generation methods are warranted. Acyl radicals are endogenous, exhibit facile chemistry and elicit matrix-dependent biological outcomes. Their implications in health and disease remain untapped, primarily due to the lack of a robust generation method with spatiotemporal specificity. Fusing the Norrish chemistry into the xanthene scaffold, we developed a novel general and modular molecular design strategy for photo-triggered generation of acyl radicals, i.e., acyl-caged rhodamine (ACR). A notable feature of ACR is the simultaneous release of a fluorescent probe for cell redox homeostasis allowing real-time monitoring of the biological outcome of acyl radicals. With a donor of the endogenous acetyl radical (ACR575a), we showcased its capability in precise and continuous modulation of the cell redox homeostasis from signaling to stress, and induction of a local oxidative burst to promote differentiation of neural stem cells (NSCs). Upon intracerebral-injection of ACR575a and subsequent fiber-optical activation, early AD mice exhibited enhanced differentiation of NSCs toward neurons, reduced formation of Aβ plaques, and significantly improved cognitive abilities, including learning and memory.

Photo-Responsive Phase-Separating Fluorescent Molecules for Intracellular Protein Delivery

Cellular membranes, including the plasma and endosome membranes, are barriers to outside proteins. Various vehicles have been devised to deliver proteins across the plasma membrane, but in many cases, the payload gets trapped in the endosome. Here we designed a photo-responsive phase-separating fluorescent molecule (PPFM) with a molecular weight of 666.8 daltons. The PPFM compound condensates as fluorescent droplets in the aqueous solution by liquid-liquid phase separation (LLPS), which disintegrate upon photoirradiation with a 405 nm light-emitting diode (LED) lamp within 20 min or a 405 nm laser within 3 min. The PPFM coacervates recruit a wide range of peptides and proteins and deliver them into mammalian cells. Photolysis disperses the payload from condensates into the cytosolic space. Altogether, a type of small molecules that are photo-responsive and phase separating are discovered; their coacervates can serve as transmembrane vehicles for intracellular delivery of proteins, whereas photo illumination triggers the cytosolic distribution of the payload.

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.

Mining reactive triplet carbonyls in biological systems

Aliphatic triplet carbonyls can be treated as short-lived radicals, since both species share similar reactions such as hydrogen atom abstraction, cyclization, addition, and isomerization. Importantly, enzyme-generated triplet carbonyls excite triplet molecular oxygen to the highly reactive, electrophilic singlet state by resonance energy transfer, which can react with proteins, lipids, and DNA. Carbonyl triplets, singlet oxygen, and radicals are endowed with the potential to trigger both normal and pathological responses. In this paper, we present a short review of easy, fast, and inexpensive preliminary tests for the detection of transient triplet carbonyls in chemical and biological systems. This paper covers direct and indirect methods to look for triplet carbonyls based on their spectral distribution of chemiluminescence, photoproduct analysis, quenching of light emission by conjugated dienes, and enhancement of light emission by the sensitizer 9,10-dibromoanthracence-2-sulfonate ion (DBAS).

Merging the Norrish type I reaction and transition metal catalysis: photo- and Rh-promoted borylation of C-C σ-bonds of aryl ketones

Synthesis of arylboronates via borylation of C-C σ-bonds of aryl ketones was achieved by the combined use of photoenergy and a Rh catalyst. The cooperative system enables α-cleavage of photoexcited ketones to generate aroyl radicals via the Norrish type I reaction, which are successively decarbonylated and borylated with the rhodium catalyst. This work establishes a new catalytic cycle merging the Norrish type I reaction and Rh catalysis and demonstrates the new synthetic utility of aryl ketones as aryl sources for intermolecular arylation reactions.

Visible-Light-Mediated Energy Transfer Enables the Synthesis of β-Lactams via Intramolecular Hydrogen Atom Transfer

The synthesis of 2-azetidinones (β-lactams) from simple acrylamide starting materials by visible-light-mediated energy transfer catalysis is reported. The reaction features a C(sp3 )-H functionalization via a variation of the Norrish-Yang photocyclization involving a carbon-to-carbon 1,5-hydrogen atom transfer (supported by deuterium labelling and DFT calculations) and can be used for the construction of a diverse range of β-lactam products.

Diradicals Photogeneration from Chloroaryl‐Substituted Carboxylic Acids

With the aim of generating new, thermally inaccessible diradicals, potentially able to induce a double‐strand DNA cleavage, the photochemistry of a set of chloroaryl‐substituted carboxylic acids in polar media was investigated. The photoheterolytic cleavage of the Ar−Cl bond occurred in each case to form the corresponding triplet phenyl cations. Under basic conditions, the photorelease of the chloride anion was accompanied by an intramolecular electron‐transfer from the carboxylate group to the aromatic radical cationic site to give a diradical species. This latter intermediate could then undergo CO₂ loss in a structure‐dependent fashion, according to the stability of the resulting diradical, or abstract a hydrogen atom from the medium. In aqueous environment at physiological pH (pH=7.3), both a phenyl cation and a diradical chemistry was observed. The mechanistic scenario and the role of the various intermediates (aryl cations and diradicals) involved in the process was supported by computational analysis.

Synthesis of bioactive natural products and their analogs at room temperature – an update

Sustainability is a concept that is employed to distinguish methods and procedures that can ensure the long-term productivity of the environment as it includes environmental, social, and economic dimensions. New generations can live on this planet with less hazardous substances and minimum requirement of energy for chemical transformations as green chemistry is related to creativity and the development of innovative research. Among the 12 principles of this clean chemistry, the sixth principle is devoted to the “design of energy efficiency” which discloses that less or the minimum amount of energy is required to conduct a specific reaction with optimum productivity. The most successful way to save energy is to construct strategies/methodologies that are capable enough to carry out the chemical transformations at ambient temperature and standard pressure. Hence, the present review wishes to cover the synthesis of bioactive natural products and their derivatives at room temperature. Bioactive secondary metabolites play a crucial role in the drug discovery together with drug development process; chiefly anticancer along with antibiotic molecules is noticeably enriched with molecules of natural origin. Natural sources, structures, and biological activities of natural products are highlighted in this review and it is also aimed to offer an overview of the design and synthesis of bioactive natural products and their analogs at room temperature for the first time efficiently.

A Vinylogous Norrish Reaction as a Strategy for Light-Mediated Ring Expansion

The reactions of bicyclic divinyl ketones display wavelength-dependent changes in product formation. UV irradiation results in the formation of competitive [6,3,5] and [7,3,5] tricyclic unsaturated ketones that subsequently undergo ring...