Recent Advances in Metal-Catalyzed Approaches for the Synthesis of Quinazoline Derivatives

Quinazolines are an important class of heterocyclic compounds that have proven their significance, especially in the field of organic synthesis and medicinal chemistry because of their wide range of biological and pharmacological properties. Thus, numerous synthetic methods have been developed for the synthesis of quinazolines and their derivatives. This review article briefly outlines the new synthetic methods for compounds containing the quinazoline scaffold employing transition metal-catalyzed reactions.

Cascade Reactions Catalyzed by Gold Hybrid Nanoparticles Generate CO Gas Against Periodontitis in Diabetes

The treatment of diabetic periodontitis poses a significant challenge due to the presence of local inflammation characterized by excessive glucose concentration, bacterial infection, and high oxidative stress. Herein, mesoporous silica nanoparticles (MSN) are embellished with gold nanoparticles (Au NPs) and loaded with manganese carbonyl to prepare a carbon monoxide (CO) enhanced multienzyme cooperative hybrid nanoplatform (MSN-Au@CO). The Glucose-like oxidase activity of Au NPs catalyzes the oxidation of glucose to hydrogen peroxide (H2O2) and gluconic acid，and then converts H2O2 to hydroxyl radicals (•OH) by peroxidase-like activity to destroy bacteria. Moreover, CO production in response to H2O2, together with Au NPs exhibited a synergistic anti-inflammatory effect in macrophages challenged by lipopolysaccharides. The underlying mechanism can be the induction of nuclear factor erythroid 2-related factor 2 to reduce reactive oxygen species, and inhibition of nuclear factor kappa-B signaling to diminish inflammatory response. Importantly, the antibacterial and anti-inflammation effects of MSN-Au@CO are validated in diabetic rats with ligature-induced periodontitis by showing decreased periodontal bone loss with good biocompatibility. To summarize, MSN-Au@CO is fabricate to utilize glucose-activated cascade reaction to eliminate bacteria, and synergize with gas therapy to regulate the immune microenvironment, offering a potential direction for the treatment of diabetic periodontitis.

Integrating Enzymes with Supramolecular Polymers for Recyclable Photobiocatalytic Catalysis

Chemical modifications of enzymes excel in the realm of enzyme engineering due to its directness, robustness, and efficiency; however, challenges persist in devising versatile and effective strategies. In this study, we introduce a supramolecular modification methodology that amalgamates a supramolecular polymer with Candida antarctica lipase B (CalB) to create supramolecular enzymes (SupEnzyme). This approach features the straightforward preparation of a supramolecular amphiphilic polymer (β-CD@SMA), which was subsequently conjugated to the enzyme, resulting in a SupEnzyme capable of self-assembly into supramolecular nanoparticles. The resulting SupEnzyme nanoparticles can form micron-scale supramolecular aggregates through supramolecular and electrostatic interactions with guest entities, thus enhancing catalyst recycling. Remarkably, these aggregates maintain 80 % activity after seven cycles, outperforming Novozym 435. Additionally, they can effectively initiate photobiocatalytic cascade reactions using guest photocatalysts. As a consequence, our SupEnzyme methodology exhibits noteworthy adaptability in enzyme modification, presenting a versatile platform for various polymer, enzyme, and biocompatible catalyst pairings, with potential applications in the fields of chemistry and biology.

MBene with Redox-Active Terminal Groups for an Energy-Dense Cascade Aqueous Battery

Two-dimensional (2D) transition metal borides (MBenes), new members of the 2D materials family, hold great promise for use in the electrocatalytic and energy storage fields because of their high specific area, high chemical activity, and fast charge carrier mobility. Although various types of MBenes are reported, layered MBenes featuring redox-active terminal groups for high energy output are not yet produced. A facile and energy-efficient method for synthesizing MBenes equipped with redox-active terminal groups for cascade Zn||I2 batteries is presented. Layered MBenes have ordered metal vacancies and ─Br terminal groups, enabling the sequential reactions of I-/I0 and Br-/Br0. The I2-hosting MBene-Br cathode results in a specific energy as high as 485.8 Wh kg-1 at 899.7 W kg-1 and a specific power as high as 6007.7 W kg-1 at 180.2 Wh kg-1, far exceeding the best records for Zn||I2 batteries. The results of this study demonstrate that the challenges of MBene synthesis can be overcome and reveal an efficient path for producing high-performance redox-active electrode materials for energy-dense cascade aqueous batteries.

Encapsulation of Enzymes into Hydrophilic and Biocompatible Metal Azolate Framework: Improved Functions of Biocatalyst in Cascade Reactions and its Sensing Applications

Multiple enzyme-triggered cascade biocatalytic reactions are vital in vivo or vitro, considering the basic biofunction preservation in living organisms and signals transduction for biosensing platforms. Encapsulation of such enzymes into carrier endows a sheltering effect and can boost catalytic performance, although the selection and preparation of an appropriate carrier is still a concern. Herein, focusing on MAF-7, a category of metal azolate framework (MAF) with superiority against the topologically identical ZIF-8, this enzyme@MAF system can ameliorate the sustainability of encapsulating natural enzymes into carriers. The proposed biocatalyst composite AChE@ChOx@MAF-7/hemin is constructed via one-pot in situ coprecipitation method. Subsequently, MAF-7 is demonstrated to exhibit an excellent capacity of the carrier and protection against external factors in the counterpart of ZIF-8 through encapsulated and free enzymes. In addition, detections for specific substrates or inhibitors with favorable sensitivity are accomplished, indicating that the properties above expectation of different aspects of the established platform are successfully realized. This biofunctional composite based on MAF-7 can definitely provide a potential approach for optimization of cascade reaction and enzyme encapsulation.

"Multi-in-One" Yolk-Shell Structured Nanoplatform Inducing Pyroptosis and Antitumor Immune Response Through Cascade Reactions

Pyroptosis, a new mode of regulatory cell death, holds a promising prospect in tumor therapy. The occurrence of pyroptosis can trigger the release of damage-associated molecular patterns (DAMPs) and activate the antitumor immune response. Moreover, enhancing intracellular reactive oxygen species (ROS) generation can effectively induce pyroptosis. Herein, an integrated nanoplatform (hCZAG) based on zeolitic imidazolate framework-8 (ZIF-8) with Cu2+ and Zn2+ as active nodes and glucose oxidase (GOx) loading is constructed to evoke pyroptosis. GOx can effectively elevate intracellular hydrogen peroxide (H2 O2 ) levels to regulate the unfavorable tumor microenvironment (TME). Cu2+ can be reduced to Cu+ by endogenous overexpressed GSH and both Cu2+ and Cu+ can exert Fenton-like activity to promote ROS generation and amplify oxidative stress. In addition, the accumulation of Cu2+ leads to the aggregation of lipoylated dihydrolipoamide S-acetyltransferase (DLAT), thus resulting in cuproptosis. Notably, the outburst of ROS induced by hCZAG activates Caspase-1 proteins, leads to the cleavage of gasdermin D (GSDMD), and induces pyroptosis. Pyroptosis further elicits an adaptive immune response, leading to immunogenic cell death (ICD). This study provides effective strategies for triggering pyroptosis-mediated immunotherapy and achieving improved therapeutic effects.

Targeting and Microenvironment-Activated Nanoreactor for Diabetic Chronic Wound Healing via Multienzyme Cascade Reactions

The development of cell-like nanoreactors with the ability to initiate biocatalytic cascades under special conditions holds tremendous potential for therapeutic applications. Herein, conformationally gated nanoreactors that respond to the acidic microenvironment of infected diabetic wounds were developed by cucur[8]bituril (CB[8])-based supramolecular assembly. The bioinspired nanoreactors exhibit not only self-regulated permeability and selectivity to control internal enzyme activities by substance exchange but also distinct binding specificities toward Gram-positive and Gram-negative bacteria via noncovalent modification with different ligands. The encapsulation of glucose oxidase (GOx), Fe3O4 nanozyme, and l-arginine (l-Arg) into the nanocarriers enables intelligent activation of multienzyme cascade reactions upon glucose (Glu) uptake to produce gluconic acid (GA) and hydrogen peroxide (H2O2), which is further converted into highly toxic hydroxyl radicals (·OH) for selective antibacterial activity. Moreover, acidic H2O2 promotes the oxidization of l-Arg, leading to the release of nitric oxide (NO). Consequently, this nanoreactor provides a multifunctional and synergistic platform for diabetic chronic wound healing by combining enzyme dynamic therapy with NO gas therapy to combat bacterial infections and inflammation under high blood Glu levels.

CRISPR/Cas-Powered Amplification-Free Detection of Nucleic Acids: Current State of the Art, Challenges, and Futuristic Perspectives

CRISPR/Cas system is becoming an increasingly influential technology that has been repositioned in nucleic acid detection. A preamplification step is usually required to improve the sensitivity of CRISPR/Cas-based detection. The striking biological features of CRISPR/Cas, including programmability, high sensitivity and sequence specificity, and single-base resolution. More strikingly, the target-activated trans-cleavage could act as a biocatalytic signal transductor and amplifier, thereby empowering it to potentially perform nucleic acid detection without a preamplification step. The reports of such work are on the rise, which is not only scientifically significant but also promising for futuristic end-user applications. This review started with the introduction of the detection methods of nucleic acids and the CRISPR/Cas-based diagnostics (CRISPR-Dx). Next, we objectively discussed the pros and cons of preamplification steps for CRISPR-Dx. We then illustrated and highlighted the recently developed strategies for CRISPR/Cas-powered amplification-free detection that can be realized through the uses of ultralocalized reactors, cascade reactions, ultrasensitive detection systems, or others. Lastly, the challenges and futuristic perspectives were proposed. It can be expected that this work not only makes the researchers better understand the current strategies for this emerging field, but also provides insight for designing novel CRISPR-Dx without a preamplification step to win practicable use in the near future.

Thermally Activated Geometrical Regioselective E→Z Isomerization-Enabled Cascade Sequences of Conjugated Dienals: Experimental and DFT Studies

The geometrical regioselective E→Z isomerization of a conjugated alkene under thermal activation pose a challenge due to microscopic reversibility. Herein we report that such reversibility issues can be circumvented by integrating E→Z isomerization with subsequent cyclization cascade, particularly in the absence of commonly employed light, acids, or metal-catalysts. Thus, linearly conjugated dienals in a mixture of toluene-alcohol (2 : 1) solvents or only with alcohol at 60-70 °C can be converted to γ-alkoxybutenolides in moderate to good yields. The intermediary 2Z,4E-isomer can be isolated, which includes the first example of isolating the regioselective isomerization product under thermal conditions. Density functional theory (DFT) studies have been employed to shed light on the feasibility of geometrical alkene isomerization and ensuing cascade sequences. It has been observed that the regioselective 2E,4E→2Z,4E isomerization of dienal is a thermodynamically facile (ΔG <0) process. Structural elucidation further reveals that the presence of a certain charge transfer and a non-covalent interaction may be the primary reasons for the enhanced stability of the 2Z,4E-isomer. The thermodynamic plausibility of the subsequent cascade reaction from the Z-isomer to the anticipated product in the presence of a polar protic solvent (here MeOH) is also explicated. Out of the two probable pathways, the "hemiacetal pathway" involving a relay proton transfer is kinetically more feasible due to the diminished activation barrier than the "conjugate addition pathway".

Endogenous H2O2 Self-Replenishment and Sustainable Cascades Enhance the Efficacy of Sonodynamic Therapy

Purpose

Sonodynamic therapy (SDT), with its high tissue penetration and noninvasive advantages, represents an emerging approach to eradicating solid tumors. However, the outcomes of SDT are typically hampered by the low oxygen content and immunosuppression in the tumor microenvironment (TME). Accordingly, we constructed a cascade nanoplatform to regulate the TME and improve the anti-tumor efficiency of SDT.

Methods

In this study, we rationally design cascade nanoplatform by incorporating immunostimulant hyaluronic acid (HA) and sonosensitizer chlorin e6 (Ce6) on the polydopamine nanocarrier that is pre-doped with platinum nanozymes (designated Ce6/Pt@PDA-HA, PPCH).

Results

The cascade reactions of PPCH are evidenced by the results that HA exhibits reversing immunosuppressive that converts M2 macrophages into M1 macrophages in situ, while producing H2O2, and then platinum nanozymes further catalyze the H2O2 to produce O2, and O2 produces abundant singlet oxygen (1O2) under the action of Ce6 and low-intensity focused ultrasound (LIFU), resulting in a domino effect and further amplifying the efficacy of SDT. Due to its pH responsiveness and mitochondrial targeting, PPCH effectively accumulates in tumor cells. Under LIFU irradiation, PPCH effectively reverses immunosuppression, alleviates hypoxia in the TME, enhances reactive oxygen species (ROS) generation, and enhances SDT efficacy for eliminating tumor cells in vivo and in vitro. Meanwhile, an in vivo dual-modal imaging including fluorescence and photoacoustic imaging achieves precise tumor diagnosis.

Conclusion

This cascade nanoplatform will provide a promising strategy for enhancing SDT eradication against tumors by modulating immunosuppression and relieving hypoxia.

Diverse Synthesis of 2H-Isoindole-Based Polycyclic Aromatic Compounds

1,3-Disubstituted N-aryl-2H-isoindoles have been synthesized by a cascade reaction of divinyl ethers, which are derived from easily available 4-bromoisocoumarins, with substituted anilines in HFIP. This cascade reaction consists of a ring-opening step through addition-elimination mechanism and the following 5-exo-tet type ring-closing step via the intramolecular nucleophilic substitution reaction. Thus obtained 2H-isoindoles have been derivatized to high-order nitrogen-containing polycycles including less accessible benzo[a]ullazines.

[Development of enzyme immobilization systems for CO2 bioconversion: advances and challenges]

Enzyme-catalyzed CO2 reduction to value-added commodities is important for alleviating the global environmental issues and energy crises due to high selectivity and mild conditions. Owing to high energy density, formic acid or methanol produced from CO2 using formate dehydrogenase (FDH) or multi-enzyme cascades are promising target chemicals for CO2 utilization. However, the low activity, poor stability and low reusability of key enzymes involved in such process hampered its large-scale application. Enzyme immobilization provides an effective solution to these problems and significant progress have been made in immobilization carriers. Moreover, integration of enzyme immobilization with other catalysis techniques have been explored extensively. This review summarized the recent advances in the immobilization of enzymes using membranes, inorganic materials, metal-organic frameworks, covalent organic frameworks and other carriers, and illustrated the characteristics and advantages of different immobilization materials and immobilization methods. The synergistic effects and applications of immobilized enzymes and electrocatalytic or photocatalytic coupling reaction systems for CO2 reduction were further summarized. Finally, the current challenges of enzyme immobilization technology and coupling reaction systems were pointed out and their development prospects were presented.

Enhancing Multistep Reactions: Biomimetic Design of Substrate Channeling Using P22 Virus-Like Particles

Many biocatalytic processes inside cells employ substrate channeling to control the diffusion of intermediates for improved efficiency of enzymatic cascade reactions. This inspirational mechanism offers a strategy for increasing efficiency of multistep biocatalysis, especially where the intermediates are expensive cofactors that require continuous regeneration. However, it is challenging to achieve substrate channeling artificially in vitro due to fast diffusion of small molecules. By mimicking some naturally occurring metabolons, nanoreactors are developed using P22 virus-like particles (VLPs), which enhance the efficiency of nicotinamide adenine dinucleotide (NAD)-dependent multistep biocatalysis by substrate channeling. In this design, NAD-dependent enzyme partners are coencapsulated inside the VLPs, while the cofactor is covalently tethered to the capsid interior through swing arms. The crowded environment inside the VLPs induces colocalization of the enzymes and the immobilized NAD, which shuttles between the enzymes for in situ regeneration without diffusing into the bulk solution. The modularity of the nanoreactors allows to tune their composition and consequently their overall activity, and also remodel them for different reactions by altering enzyme partners. Given the plasticity and versatility, P22 VLPs possess great potential for developing functional materials capable of multistep biotransformations with advantageous properties, including enhanced efficiency and economical usage of enzyme cofactors.

Microneedle-Assisted Transdermal Delivery of 2D Bimetallic Metal-Organic Framework Nanosheet-Based Cascade Biocatalysts for Enhanced Catalytic Therapy of Melanoma

Current conventional treatments for malignant melanoma still face limitations, especially low therapeutic efficacy and serious side effects, and more effective strategies are urgently needed to develop them. Delivering biocatalysts into tumors to efficiently trigger in situ cascade reactions has shown huge potential in producing more therapeutic species or generating stronger tumoricidal effects for augmented tumor therapy. Recently, ultrathin 2D metal-organic framework (MOF) nanosheets have acquired great interest in biocatalysis owing to their large surface areas and abundant accessible active catalytic sites. Herein, an enhanced catalytic therapeutic strategy against melanoma is developed by biocompatible microneedle (MN)-assisted transdermal delivery of a 2D bimetallic MOF nanosheet-based cascade biocatalyst (Cu-TCPP(Fe)@GOD). Profiting from the constructed dissolving MN system, the loaded Cu-TCPP(Fe)@GOD hybrid nanosheets can be accurately delivered into the melanoma sites through skin barriers, and subsequently, trigger the specific cascade catalytic reactions in response to the acidic tumor microenvironment to effectively generate highly toxic hydroxyl radical (^• OH) and deplete glucose nutrient for inducing the death of melanoma cells. The ultimate results prove the high melanoma inhibition effect and biosafety of such therapeutic modality, exhibiting a new and promising strategy to conquer malignant melanoma.

Efficient enzymatic synthesis of d-allulose using a novel d-allulose-3-epimerase from Caballeronia insecticola

BACKGROUND

Rare sugars have become promising 'sugar alternatives' because of their low calories and unique physiological functions. Among the family of rare sugars, d-allulose is one of the sugars attracting interest. Ketose 3-epimerases (KEase), including d-tagatose 3-epimerase (DTEase) and d-allulose 3-epimerase (DAEase), are mainly used for d-allulose production.

RESULTS

In this study, a putative xylose isomerase from Caballeronia insecticola was characterized and identified as a novel DAEase. Caballeronia insecticola DAEase displayed prominent enzymatic properties, and 150 g L^-1 d-allulose was produced from 500 g L^-1 d-fructose in 45 min with a conversion rate of 30% and high productivity of 200 g L^-1  h^-1 . Furthermore, DAEase was employed in a phosphorylation-dephosphorylation cascade reaction, which significantly increased the conversion rate of d-allulose. Under optimized conditions, the conversion rate of d-allulose was approximately 100% when the concentration of d-fructose was 50 mmol L^-1 .

CONCLUSION

This research described a very beneficial and facile approach for d-allulose production based on C. insecticola DAEase. © 2022 Society of Chemical Industry.

Brønsted-Acid-Catalyzed One-Pot Synthesis of β,β-Diaryl Esters: Direct Regioselective Approach to Diverse Arrays of 3-Aryl-1-indanone Cores

A three-component, solvent-dependent, Brønsted-acid-catalyzed reaction of benzaldehydes, silyl enolates and arene nucleophiles has been developed for the synthesis of potential drug candidate 3-aryl-1-indanones. This reaction features the formation of three C-C bonds, high regioselectivity in a one-pot strategy, broad substrate generality, facile scalability (1.04g), high functional group tolerance and viable substrates. The β-O-silyl ethers generated in-situ from the Mukaiyama aldol reaction were subjected to acid-catalyzed benzylic arylation with strong as well as weak nucleophiles, and the resultant β,β-diaryl esters can undergo a third C-C bond formation with excellent regioselectivity through intramolecular cyclization to afford the indanone products in the same pot. Detailed mechanistic insight leads to a feasible reaction pathway. This transformation opens up a practical and adaptable approach to producing a variety of synthetically valuable transformations and enable the synthesis of medicinally valuable (R)-tolterodine and (+)-indatraline.

[Research progress on application of multi-enzyme-catalyzed cascade reactions in enzymatic synthesis of natural products]

As a biocatalyst, enzyme has the advantages of high catalytic efficiency, strong reaction selectivity, specific target products, mild reaction conditions, and environmental friendliness, and serves as an important tool for the synthesis of complex organic molecules. With the continuous development of gene sequencing technology, molecular biology, genetic manipulation, and other technologies, the diversity of enzymes increases steadily and the reactions that can be catalyzed are also gradually diversified. In the process of enzyme-catalyzed synthesis, the majority of common enzymatic reactions can be achieved by single enzyme catalysis, while many complex reactions often require the participation of two or more enzymes. Therefore, the combination of multiple enzymes together to construct the multi-enzyme cascade reactions has become a research hotspot in the field of biochemistry. Nowadays, the biosynthetic pathways of more natural products with complex structures have been clarified, and secondary metabolic enzymes with novel catalytic activities have been identified, discovered, and combined in enzymatic synthesis of natural/unnatural molecules with diverse structures. This study summarized a series of examples of multi-enzyme-catalyzed cascades and highlighted the application of cascade catalysis methods in the synthesis of carbohydrates, nucleosides, flavonoids, terpenes, alkaloids, and chiral molecules. Furthermore, the existing problems and solutions of multi-enzyme-catalyzed cascade method were discussed, and the future development direction was prospected.

Development of a Multi-Enzymatic Biocatalytic System through Immobilization on High Quality Few-Layer bio-Graphene

In this work, we report the green production of few-layer bio-Graphene (bG) through liquid exfoliation of graphite in the presence of bovine serum albumin. Microscopic characterization evaluated the quality of the produced nanomaterial, showing the presence of 3-4-layer graphene. Moreover, spectroscopic techniques also confirmed the quality of the resulted bG, as well as the presence of bovine serum albumin on the graphene sheets. Next, for the first time, bG was used as support for the simultaneous covalent co-immobilization of three enzymes, namely β-glucosidase, glucose oxidase, and horseradish peroxidase. The three enzymes were efficiently co-immobilized on bG, demonstrating high immobilization yields and activity recoveries (up to 98.5 and 90%, respectively). Co-immobilization on bG led to an increase of apparent K_M values and a decrease of apparent V_max values, while the stability of the nanobiocatalysts prevailed compared to the free forms of the enzymes. Co-immobilized enzymes exhibited high reusability, preserving a significant part of their activity (up to 72%) after four successive catalytic cycles at 30 °C. Finally, the tri-enzymatic nanobiocatalytic system was applied in three-step cascade reactions, involving, as the first step, the hydrolysis of p-Nitrophenyl-β-D-Glucopyranoside and cellobiose.

Stereoselective Syntheses of Masked β-Amino Acid Containing Phthalides

We herein report a protocol for the asymmetric aldol-initiated cascade addition of isoxazolidin-5-ones to ortho-cyanobenzaldehydes by using Takemoto's bifunctional organocatalyst. This approach allows for the synthesis of various novel β^2,2-amino acid-phthalide conjugates with good enantio- and diastereoselectivities in reasonable yields and the further ring-opening of these compounds to acyclic carboxylic acid derivatives was demonstrated too.

Novel Split Intein-Mediated Enzymatic Channeling Accelerates the Multimeric Bioconversion Pathway of Ginsenoside

Cascade reaction systems, such as protein fusion and synthetic protein scaffold systems, can both spatially control the metabolic flux and boost the productivity of multistep enzymatic reactions. Despite many efforts to generate fusion proteins, this task remains challenging due to the limited expression of complex enzymes. Therefore, we developed a novel fusion system that bypasses the limited expression of complex enzymes via a post-translational linkage. Here, we report a split intein-mediated cascade system wherein orthogonal split inteins serve as adapters for protein ligation. A genetically programmable, self-assembled, and traceless split intein was utilized to generate a biocatalytic cascade to produce the ginsenoside compound K (CK) with various pharmacological activities, including anticarcinogenic, anti-inflammatory, and antidiabetic effects. We used two types of split inteins, consensus atypical (Cat) and Rma DnaB, to form a covalent scaffold with the three enzymes involved in the CK conversion pathway. The multienzymatic complex with a size greater than 240 kDa was successfully assembled in a soluble form and exhibited specific activity toward ginsenoside conversion. Furthermore, our split intein cascade system significantly increased the CK conversion rate and reduced the production time by more than 2-fold. Our multienzymatic cascade system that uses split inteins can be utilized as a platform for regulating multimeric bioconversion pathways and boosting the production of various high-value substances.

Exogenously Triggered Nanozyme for Real-Time Magnetic Resonance Imaging-Guided Synergistic Cascade Tumor Therapy

The uncontrolled treatment process and high concentration of intracellular glutathione compromise the therapeutic efficacies of chemodynamic therapy (CDT). Here, iron oxide nanocrystals embedded in N-doped carbon nanosheets (IONCNs) are designed as a near-infrared light-triggered nanozyme for synergistic cascade tumor therapy. The IONCNs can absorb and convert 980 nm light to local heat, which induces the dissolution of iron oxide for generating Fe²⁺/Fe³⁺ in a weak acid environment, apart from thermal ablation of cancer cells. The formed Fe²⁺ takes on the active site for the Fenton reaction. The formed Fe³⁺ acts as glutathione peroxidase to magnify oxidative stress, improving the antitumor performance. The IONCNs can be used to visually track the treatment process via magnetic resonance imaging. Such IONCNs demonstrate great potential as an exogenously triggered nanozyme via an integrated cascade reaction for imaging-guided synergistic cancer therapy.

Advanced Pyrrolidine-Carbamate Self-Immolative Spacer with Tertiary Amine Handle Induces Superfast Cyclative Drug Release

Amine-carbamate self-immolative (SI) spacers represent practical and versatile tools in targeted prodrugs, but their slow degradation mechanism limits drug activation at the site of disease. We engineered a pyrrolidine-carbamate SI spacer with a tertiary amine handle which strongly accelerates the spacer cyclization to give a bicyclic urea and the free hydroxy groups of either cytotoxic (Camptothecin) or immunostimulatory (Resiquimod) drugs. In silico conformational analysis and p K a calculations suggest a plausible mechanism for the superior efficacy of the advanced SI spacer compared to state-of-art analogues.

Chemoenzymatic Oxosulfonylation-Bioreduction Sequence for the Stereoselective Synthesis of β-Hydroxy Sulfones

A series of optically active β-hydroxy sulfones has been obtained through an oxosulfonylation-stereoselective reduction sequence in aqueous medium. Firstly, β-keto sulfones were synthesized from arylacetylenes and sodium sulfinates to subsequently develop the carbonyl reduction in a highly selective fashion using alcohol dehydrogenases as biocatalysts. Optimization of the chemical oxosulfonylation reaction was investigated, finding inexpensive iron(III) chloride hexahydrate (FeCl₃  ⋅ 6H₂ O) as the catalyst of choice. The selection of isopropanol in the alcohol-water media resulted in high compatibility with the enzymatic process for enzyme cofactor recycling purposes, providing a straightforward access to both (R)- and (S)-β-hydroxy sulfones. The practical usefulness of this transformation was illustrated by describing the synthesis of a chiral intermediate of Apremilast. Interestingly, the development of a chemoenzymatic cascade approach avoided the isolation of β-keto sulfone intermediates, which allowed the preparation of chiral β-hydroxy sulfones in high conversion values (83-94 %) and excellent optical purities (94 to >99 % ee).

Bioproduction of Natural Phenethyl Acetate, Phenylacetic Acid, Ethyl Phenylacetate, and Phenethyl Phenylacetate from Renewable Feedstock

Natural phenethyl acetate (PEA), phenylacetic acid (PAA), ethyl phenylacetate (Et-PA), and phenethyl phenylacetate (PE-PA) are highly desirable aroma chemicals, but with limited availability and high price. Here, green, sustainable, and efficient bioproduction of these chemicals as natural products from renewable feedstocks was developed. PEA and PAA were synthesized from l-phenylalanine (l-Phe) via novel six- and five-enzyme cascades, respectively. Whole-cell-based cascade biotransformation of 100 mm l-Phe in a two-phase system (aqueous/organic: 1 : 0.5 v/v) containing ethyl oleate or biodiesel as green solvent gave 13.6 g L^-1 PEA (83.1 % conv.) and 11.6 g L^-1 PAA (87.1 % conv.), respectively. Coupled fermentation and biotransformation approach produced 10.4 g L^-1 PEA and 9.2 g L^-1 PAA from glucose or glycerol, respectively. The biosynthesized PAA was converted to natural Et-PA and PE-PA by esterification using lipases with ethanol or 2-phenylethanol derived from sugar, affording 2.7 g L^-1 Et-PA (83.1 % conv.) and 4.6 g L^-1 PE-PA (96.3 % conv.), respectively.

Injectable Hydrogel Based on Defect-Rich Multi-Nanozymes for Diabetic Wound Healing via an Oxygen Self-Supplying Cascade Reaction

Diabetic wound healing remains challenging owing to the risk for bacterial infection, hypoxia, excessive glucose levels, and oxidative stress. Glucose-activated cascade reactions can consume glucose and eradicate bacteria, avoiding the direct use of hydrogen peroxide (H₂ O₂ ) and wound pH restriction on peroxidase-like activity. However, the anoxic microenvironment in diabetic wounds impedes the cascade reaction due to the oxygen (O₂ ) dependence of glucose oxidation. Herein, defect-rich molybdenum disulfide nanosheets loaded with bovine serum albumin-modified gold nanoparticle (MoS₂ @Au@BSA NSs) heterostructures are designed and anchored onto injectable hydrogels to promote diabetic wound healing through an O₂ self-supplying cascade reaction. BSA decoration decreases the particle size of Au, increasing the activity of multiple enzymes. Glucose oxidase-like Au catalyzes the oxidation of glucose into gluconic acid and H₂ O₂ , which is transformed into a hydroxyl radical (•OH) catalyzed by peroxidase-like MoS₂ @Au@BSA to eradicate bacteria. When the wound pH reaches an alkalescent condition, MoS₂ @Au@BSA mimicks superoxide dismutase to transform superoxide anions into O₂ and H₂ O₂ , and decomposes endogenous and exogenous H₂ O₂ into O₂ via catalase-like mechanisms, reducing oxidative stress, alleviating hypoxia, and facilitating glucose oxidation. The MoS₂ @Au@BSA nanozyme-anchored injectable hydrogel, composed of oxidized dextran and glycol chitosan crosslinked through a Schiff base, significantly accelerates diabetic wound healing.

Hollow Cuprous Oxide@Nitrogen-Doped Carbon Nanocapsules for Cascade Chemodynamic Therapy

Cuprous-based nanozymes have demonstrated great potential for cascade chemodynamic therapy (CDT) due to their higher catalytic efficiency and simple reaction conditions. Here, hollow cuprous oxide@nitrogen-doped carbon (HCONC) dual-shell structures are designed as nanozymes for CDT oncotherapy. This HCONC with a size distribution of 130 nm is synthesized by a one-step hydrothermal method using cupric nitrate and dimethyl formamide as precursors. The thin-layer carbon (1.88 nm) of HCONC enhances the water-stability and reduces the systemic toxicity of cuprous oxide nanocrystals. The dissolved Cu⁺ of HCONC in acid solution induces a Fenton-like reaction and exhibits a fast reaction rate for catalyzing H₂ O₂ into highly toxic hydroxyl radicals (·OH). Meanwhile, the formed Cu⁺ consumes oversaturated glutathione (GSH) to avoid its destruction of ROS at the intracellular level. In general, both cellular and animal experiments show that HCONC demonstrates excellent antitumor ability without causing significant systemic toxicity, which may present tremendous potential for clinical cancer therapy.

A Guanosine-Quadruplex Hydrogel as Cascade Reaction Container Consuming Endogenous Glucose for Infected Wound Treatment-A Study in Diabetic Mice

Diabetic foot ulcers infected with antibiotic-resistant bacteria form a severe complication of diabetes. Antimicrobial-loaded hydrogels are used as a dressing for infected wounds, but the ongoing rise in the number of antimicrobial-resistant infections necessitates new, nonantibiotic based designs. Here, a guanosine-quadruplex (G₄ )-hydrogel composed of guanosine, 2-formylphenylboronic acid, and putrescine is designed and used as a cascade-reaction container. The G₄ -hydrogel is loaded with glucose-oxidase and hemin. The first cascade-reaction, initiated by glucose-oxidase, transforms glucose and O₂  into gluconic acid and H₂ O₂ . In vitro, this reaction is most influential on killing Staphylococcus aureus or Pseudomonas aeruginosa in suspension, but showed limited killing of bacteria in biofilm-modes of growth. The second cascade-reaction, however, transforming H₂ O₂  into reactive-oxygen-species (ROS), also enhances killing of biofilm bacteria due to hemin penetration into biofilms and interaction with eDNA G-quadruplexes in the biofilm matrix. Therewith, the second cascade-reaction generates ROS close to the target bacteria, facilitating killing despite the short life-time of ROS. Healing of infected wounds in diabetic mice proceeds faster upon coverage by these G₄ -hydrogels than by clinically common ciprofloxacin irrigation. Moreover, local glucose concentrations around infected wounds decrease. Concluding, a G₄ -hydrogel loaded with glucose-oxidase and hemin is a good candidate for infected wound dressings, particularly in diabetic patients.

Redox Replacement of Silver on MOF-Derived Cu/C Nanoparticles on Gas Diffusion Electrodes for Electrocatalytic CO2 Reduction

Bimetallic tandem catalysts have emerged as a promising strategy to locally increase the CO flux during electrochemical CO₂ reduction, so as to maximize the rate of conversion to C-C-coupled products. Considering this, a novel Cu/C-Ag nanostructured catalyst has been prepared by a redox replacement process, in which the ratio of the two metals can be tuned by the replacement time. An optimum Cu/Ag composition with similarly sized particles showed the highest CO₂ conversion to C₂₊ products compared to non-Ag-modified gas-diffusion electrodes. Gas chromatography and in-situ Raman measurements in a CO₂ gas diffusion cell suggest the formation of top-bound linear adsorbed *CO followed by consumption of CO in the successive cascade steps, as evidenced by the increasingνC-H bands. These findings suggest that two mechanisms operate simultaneously towards the production of HCO₂ H and C-C-coupled products on the Cu/Ag bimetallic surface.

Cooperative NHC/Photoredox Catalyzed Ring-Opening of Aryl Cyclopropanes to 1-Aroyloxylated-3-Acylated Alkanes

Cyclopropanes are an important class of building blocks in organic synthesis. Herein, a ring‐opening/arylcarboxylation/acylation cascade reaction for the 1,3‐difunctionalization of aryl cyclopropanes enabled by cooperative NHC and organophotoredox catalysis is reported. The cascade works on monosubstituted cyclopropanes that are in contrast to the heavily investigated donor–acceptor cyclopropanes more challenging to be difunctionalized. The key step is a radical/radical cross coupling of a benzylic radical generated in the photoredox catalysis cycle with a ketyl radical from the NHC catalysis cycle. The transformation features metal‐free reaction conditions and tolerates a diverse range of functionalities.

Synthetic Silica Nano-Organelles for Regulation of Cascade Reactions in Multi-Compartmentalized Systems

In eukaryotic cells, enzymes are compartmentalized into specific organelles so that different reactions and processes can be performed efficiently and with a high degree of control. In this work, we show that these features can be artificially emulated in robust synthetic organelles constructed using an enzyme co-compartmentalization strategy. We describe an in-situ encapsulation approach that allows enzymes to be loaded into silica nanoreactors in well-defined compositions. The nanoreactors can be combined into integrated systems to produce a desired reaction outcome. We used the selective enzyme co-compartmentalization and nanoreactor integration to regulate competitive cascade reactions and to modulate the kinetics of sequential reactions involving multiple nanoreactors. Furthermore, we show that the nanoreactors can be efficiently loaded into giant polymer vesicles, resulting in multi-compartmentalized microreactors.

Metathesis Cascade-Triggered Depolymerization of Enyne Self-Immolative Polymers*

A novel class of enyne self-immolative polymers (SIPs) capable of metathesis cascade-triggered depolymerization is reported. Studies on model compounds established 1,6-enyne structures for efficient metathesis cascade reactions. SIPs incorporating the optimized 1,6-enyne motif were prepared via both polycondensation and iterative exponential growth approaches. These SIPs demonstrated excellent stability in strong acid, base, nucleophiles, or at elevated temperatures, and can undergo efficient and complete depolymerization once triggered by a metathesis catalyst. Further studies revealed that introducing a terminal alkene to the chain end of the enyne SIPs improved the depolymerization efficiency, and established their potential as stimuli-responsive materials.

Stereodivergent Synthesis of Enantioenriched γ-Butyrolactones Bearing Two Vicinal Stereocenters Enabled by Synergistic Copper and Iridium Catalysis

By virtue of a fundamentally new reaction model of azomethine ylide serving as a two-atom synthon, we present the first example of stereodivergent preparation of γ-butyrolactones via synergistic Cu/Ir-catalyzed asymmetric cascade allylation/lactonization, and all four stereoisomers of γ-butyrolactones bearing two vicinal stereocenters are accessible with excellent diastereoselective and enantioselective control. The chiral Ir^III -π-allyl intermediate was separated and characterized to understand the origin of the regio- and stereoselectivity of the initial C-C bond formation process. Control experiments shed some light on the catalyst/substrate and catalyst/catalyst interactions in this dual catalytic system to rationalize the related kinetic/dynamic kinetic resolution process with different catalyst combinations. The enantioenriched γ-butyrolactone products were converted into an array of structurally complex chiral molecules and organocatalysts that were otherwise inaccessible.

An Alternative Route to Complex Allenes or Cyclooctatrienes via a Suzuki Cyclocarbopalladation Cascade

The 4-exo-dig cyclocarbopalladation of vinyl bromides substituted with a triple or double bond resulted in impressive cascade reactions leading to different compounds under Suzuki cross-coupling conditions upon a slight modification of the starting material. When the starting compound carries a triple bond, a single cascade occurs providing a structure containing an allene, a tetrasubstituted cyclopropane, and a cyclobutene with complete stereoselectivity. When the related starting material possessing a double bond is reacted under the same conditions in the presence of various vinyl boronic esters or acids, an efficient 8π-electrocyclization provides tricyclic systems comprised of a cyclobutene unit, as well as a cyclooctatriene. Five carbons of the latter can be selectively decorated with different substituents depending on the choice of the starting material and the boronic coupling partner.

A Dual-Nanozyme-Catalyzed Cascade Reactor for Enhanced Photodynamic Oncotherapy against Tumor Hypoxia

Tumor hypoxia is a typical characteristic of tumor microenvironment (TME), which seriously compromises the therapeutic effect of photodynamic therapy (PDT). The development of nanozymes with oxygen-generation ability is a promising strategy to overcome the oxygen-dependent of PDT but remained a great challenge. Herein, a dual-nanozymes based cascade reactor HAMF is proposed to alleviate tumor hypoxia for enhanced PDT. The hollow mesoporous silica nanoparticles (HMSNs) are constructed as an excellent nanocarrier to load ultra-small gold nanoparticles (Au NPs) and manganese dioxide (MnO₂ ) shell via in situ reduction method, and further coordination with an efficient photosensitizer 4-DCF-MPYM (4-FM), a thermally activated delayed fluorescence (TADF) fluorescein derivative. With the response to TME, MnO₂ can catalyze endogenous H₂ O₂ into O₂ and subsequently accelerating glucose oxidation by Au NPs to produce additional H₂ O₂ , which is reversely used as the substrate for MnO₂ -catalyzed reaction, thereby constantly producing singlet oxygen (¹ O₂ ) for enhanced PDT upon light irradiation. This work proposed a cascade reactor based on dual-nanozyme to relieve tumor hypoxia for effective tumor suppression, which may enrich the application of multi-nanozymes in biomedicine.

Cascade Reaction to Selectively Synthesize Multifunctional Indole Derivatives by IrIII -Catalyzed C-H Activation

An effective and condition-controlled way to synthesize with high selectivity a variety of functionalized indoles with potent biological properties has been developed. Notably, 2,4-dialkynyl indole products were obtained by direct double C-H bond alkynylation, whereas alkynyl at the C4 position could convert to carbonyl to generate 2-alkynyl-3,4-diacetyl indoles fast and effectively. Additionally, a one-pot relay catalytic reaction led to 2,5-di-alkynyl-3,4-diacetyl indoles when using a carbonyl group as the directing group and by controlling the type and quantity of additives. A possible mechanism was proposed based on many studies including deuterium-exchange experiments, the necessary conditions of product conversion, and the effect of water on the reaction.

"Golden" Cascade Cyclization to Benzo[c]-Phenanthridines

Herein, we describe a gold-catalyzed cascade cyclization of Boc-protected benzylamines bearing two tethered alkyne moieties in a domino reaction initiated by a 6-endo-dig cyclization. The reaction was screened intensively, and the scope was explored, resulting in nine new Boc-protected dihydrobenzo[c]phenanthridines with yields of up to 98 %; even a π-extension and two bidirectional approaches were successful. Furthermore, thermal cleavage of the Boc group and subsequent oxidation gave substituted benzo[c]phenanthridines in up to quantitative yields. Two bidirectional approaches under the optimized conditions were successful, and the resulting π-extended molecules were tested as organic semiconductors in organic thin-film transistors.

Recent Advances on Photoinduced Cascade Strategies for the Synthesis of N-Heterocycles

Over the last decade, visible-light photocatalysis has proved to be a powerful tool for the construction of N-heterocyclic frameworks, important constituents of natural products, insecticides, pharmacologically relevant therapeutic agents and catalysts. This account highlights recent developments and established methods towards the photocatalytic cascades for preparation of different classes of N-heterocycles, giving emphasis on our contribution to the field.

Asymmetric Total Syntheses of 8,9-Seco-ent-kaurane Diterpenoids Enabled by an Electrochemical ODI-[5+2] Cascade

An electrochemical ODI-[5+2] cascade reaction was developed which enables the rapid assembly of diversely functionalized bicyclo[3.2.1]octadienones from sensitive ethynylphenols. By combining a directed retro-aldol/aldol process, a [2,3]-sigmatropic rearrangement, and an Al(O-iPr)₃ -promoted reductive 1,3-transposition, the asymmetric total syntheses of five 8,9-seco-ent-kauranoids-(-)-shikoccin, (-)-O-methylshikoccin, (-)-epoxyshikoccin, (+)-O-methylepoxyshikoccin, and (+)-rabdo-hakusin-have been achieved in a concise and efficient manner.

Palladium-Catalyzed Cascade to Benzoxepins by Using Vinyl-Substituted Donor-Acceptor Cyclopropanes

A palladium-catalyzed intermolecular cascade (4+3) cyclocondensation of salicylaldehydes and vinylcyclopropanes is reported. A key feature of the reaction is the use of a phosphonate group as an acceptor moiety on the cyclopropane, exploiting its propensity to undergo olefination with aldehydes. Subsequent O-allylation enabled the formation of a range of substituted benzoxepinsWith a novel chiral ligand, the products were obtained in generally good yield and with reasonable enantioselectivity.

Hypervalent-Iodine(III)-Mediated Tandem Oxidative Dearomatization/Aziridination of Phenolic Amines: Synthesis of Functionalized Unactivated Aziridines

A new hypervalent-iodine(III)-mediated tandem reaction involving oxidative dearomatization and in situ aziridination of phenolic amines is described, providing a mild and effective method for the assembly of structurally interesting and synthetically useful aziridines. Importantly, the densely functionalized aziridines resulting from this unprecedented tandem reaction offer a platform for expeditious access to architecturally diverse aza-heterocycles through transformations initiated by selective ring-opening of aziridines.

Total Synthesis of (+)-Alsmaphorazine C and Formal Synthesis of (+)-Strictamine: A Photo-Fries Approach

A bioinspired photo-Fries/imine capture cascade reaction was developed in continuous-flow mode, which facilitated the rapid construction of a series of diversely functionalized 2,7-heterocycle-fused tetrahydrocarbazoles, the ubiquitous core structures embedded in strychnos and akuammiline-type monoterpene indole alkaloids. The synthetic utility of this novel method has been preliminarily explored by the first total synthesis of (+)-alsmaphorazine C and formal synthesis of (+)-strictamine in a concise and efficient manner.

Synthetic Enzyme-Catalyzed CO2 Fixation Reactions

In recent years, (de)carboxylases that catalyze reversible (de)carboxylation have been targeted for application as carboxylation catalysts. This has led to the development of proof-of-concept (bio)synthetic CO2 fixation routes for chemical production. However, further progress towards industrial application has been hampered by the thermodynamic constraint that accompanies fixing CO2 to organic molecules. In this Review, biocatalytic carboxylation methods are discussed with emphases on the diverse strategies devised to alleviate the inherent thermodynamic constraints and their application in synthetic CO2 -fixation cascades.

Catalytic Enantioselective Isocyanide-Based Reactions: Beyond Passerini and Ugi Multicomponent Reactions

The development of catalytic enantioselective isocyanide-based reactions is currently of great interest because the resulting products are valuable in organic synthesis, pharmacological chemistry, and materials science. This review assembles and comprehensively summarizes the recent achievements in this rapidly growing area according to the reaction types. Special attention is paid to the advantages, limitations, possible mechanisms, and synthetic applications of each reaction. In addition, a personal outlook on the opportunities for further exploration is given at the end.

A General Organocatalytic System for Enantioselective Radical Conjugate Additions to Enals

Herein, we report a general iminium ion-based catalytic method for the enantioselective conjugate addition of carbon-centered radicals to aliphatic and aromatic enals. The process uses an organic photoredox catalyst, which absorbs blue light to generate radicals from stable precursors, in combination with a chiral amine catalyst, which secures a consistently high level of stereoselectivity. The generality of this catalytic platform is demonstrated by the stereoselective interception of a wide variety of radicals, including non-stabilized primary ones which are generally difficult to engage in asymmetric processes. The system also served to develop organocatalytic cascade reactions that combine an iminium-ion-based radical trap with an enamine-mediated step, affording stereochemically dense chiral products in one-step.

Urchin-Shaped Metal Organic/Hydrogen-Bonded Framework Nanocomposite as a Multifunctional Nanoreactor for Catalysis-Enhanced Synergetic Therapy

Ultrasound (US)-induced sonodynamic therapy (SDT) is an efficient and precise method against tumor, and the integration of multiple cancer therapies has been proved as a promising strategy for better therapeutic effects. Herein, for the first time, a multifunctional nanoreactor has been fabricated by integrating Fe-MIL-88B-NH₂, PFC-1, and glucose oxidase (GO_x) to form urchin-like Fe-MIL-88B-NH₂@PFC-1-GO_x (MPG) nanoparticles as Fenton's reagent, a sonosensitizer, and a tumor microenvironment (TME) modulator. In detail, MPG can generate ^•OH for chemodynamic therapy (CDT) and deplete glutathione (GSH) to alleviate the antioxidant ability of cancer cells. Moreover, catalase (CAT)-like MPG can react with H₂O₂ to generate O₂ for relieving hypoxia in TME, enhancing GO_x-catalyzed glucose oxidation to produce H₂O₂ and gluconic acid. Then, the regenerated H₂O₂ can promote the Fenton reaction to achieve GO_x catalysis-enhanced CDT. Owing to its large π-electron conjugated system, MPG also serves as an ideal sonosensitizer, realizing a burst generation of ¹O₂ under US irradiation for efficient SDT. Therefore, the tumor treatment will be notably enhanced by MPG-based synergetic CDT/SDT/starvation therapy via a series of cascade reactions. Overall, this work develops a versatile nanoreactor with improved tumor treatment effectiveness and broadens the application prospects of porous materials in the field of biomedical research.

Reductive Arylation of Amides via a Nickel-Catalyzed Suzuki-Miyaura-Coupling and Transfer-Hydrogenation Cascade

We report a means to achieve the addition of two disparate nucleophiles to the amide carbonyl carbon in a single operational step. Our method takes advantage of non-precious-metal catalysis and allows for the facile conversion of amides to chiral alcohols via a one-pot Suzuki-Miyaura cross-coupling/transfer-hydrogenation process. This study is anticipated to promote the development of new transformations that allow for the conversion of carboxylic acid derivatives to functional groups bearing stereogenic centers via cascade processes.

Scalable and Recyclable All-Organic Colloidal Cascade Catalysts

We report on the synthesis of core–shell microparticles (CSMs) with an acid catalyst in the core and a base catalyst in the shell by surfactant‐free emulsion polymerization (SFEP). The organocatalytic monomers were separately copolymerized in three synthetic steps allowing the spatial separation of incompatible acid and base catalysts within the CSMs. Importantly, a protected and thermo‐decomposable sulfonate monomer was used as acid source to circumvent the neutralization of the base catalyst during shell formation, which was key to obtain stable, catalytically active CSMs. The catalysts showed excellent performance in an established one‐pot model cascade reaction in various solvents (including water), which involved an acid‐catalyzed deacetalization followed by a base‐catalyzed Knoevenagel condensation. The CSMs are easily recycled, modified, and their synthesis is scalable, making them promising candidates for organocatalytic applications.

Rational Design of Pomegranate-like Base-Acid Bifunctional β Zeolite by Steam-Assisted Crystallization for the Tandem Deacetalization-Knoevenagel Condensation

A highly crystalline pomegranate-like base-acid bifunctional beta zeolite was successfully synthesized by the steam-assisted crystallization method using a basic nitrided N-beta as the starting material. The secondary crystal growth of a beta zeolite generating acid functionality occurred over the outer surface and intercrystalline void spaces of the N-beta zeolite. The pomegranate-like N-beta@H-beta zeolite had a high surface area and base-acid dual functionality because of the well-connected framework topologies of the H-beta and N-beta crystallites. The N-beta@H-beta zeolite exhibited a superior yield of benzylidenemalononitrile during the tandem deacetalization-Knoevenagel condensation of benzaldehyde dimethyl acetal and malononitrile compared to H-beta, N-beta, and their physical mixture. This is likely due to the isolated and balanced activity of the base- and acid-catalyzed reactions.