The Complete Mechanism of an Aldol Condensation

Automated derivatization with 6-aminoquinolyl-N-hydroxysccinimidyl carbamate for the enantioselective amino acid analysis of neurotensin synthesized by liquid phase peptide synthesis

This study presents an automated derivatization protocol utilizing 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) for enantioselective amino acid analysis of peptides synthesized via liquid phase peptide synthesis, as exemplified by neurotensin. The primary aim was to enhance operational efficiency to manage the derivatization of large sample sets and reduce human error in routine enantioselective amino acid analysis of peptide therapeutics. The chromatographic method based on Chiralpak QN-AX demonstrated enantio- and chemoselectivity for all proteinogenic amino acids (except D-Leu/D-Ile and Glu/pGlu), with quantitative analysis achieved by HPLC-ESI-MS/MS with MRM acquisition through external calibration using stable isotope-labeled internal standards. The goal was to test for racemization of amino acids during peptide synthesis and process optimization, respectively. The results confirmed varying susceptibility to racemization among amino acids during peptide synthesis and cleavage of protection groups, with specific amino acids exhibiting higher levels of D-enantiomer formation. The developed protocol effectively assessed the amino acid composition and stereointegrity of the liquid phase synthesized neurotensin. This research and application highlights the critical role of automation in optimizing peptide analysis workflows and sets the foundation for future improvements in peptide synthesis and chromatographic conditions to enhance specificity, particularly for challenging amino acid pairs. Ultimately, the findings contribute to advancing laboratory practices in peptide chemistry, ensuring the quality and efficacy of peptide-based therapeutics.

Calcium Impregnated Silica Gel in the Domino Reaction Involving Irreversible Aldol Addition, Dehydration, and Michael Addition

An innovative method was developed for the performance of aldol additions in an irreversible fashion by the use of calcium metal impregnated silica gel (Ca@SiO2) as a remarkable reducing reagent. In this approach, Ca@SiO2 drove the reaction forward, prevented reversibility, and ensured the formation of the desired products. Thus, in the presence of Ca@SiO2 (3.0 equiv), aldehydes (1.0 equiv) condensed with ketones (1.0 equiv) in 2-MeTHF to yield α,β-unsaturated enones in 71-90% yields at 25 °C. Additionally, a domino reaction involving successive aldol addition, dehydration, and Michael addition was developed for the preparation of 1,5-diketones. Accordingly, when aldehydes (1.0 equiv) were allowed to react with ketones (2.2 equiv) and Ca@SiO2(4.0 equiv), 1,5-diketones were produced in 67-88% yields. These reactions involved radical processes, where Ca@SiO2 abstracted two α hydrogen atoms from ketones and the oxygen atom from aldehydes to form CaH2@SiO2 and CaO@SiO2, respectively. These species were confirmed by powder X-ray diffraction analysis. The resultant impregnated silica gel species were solid and insoluble in the reaction mixtures, which made the addition reactions irreversible. This method represents a significant advancement in aldol condensation reactions and offers the advantages of both atom economy and atom efficiency.

Atom-Efficient Synthesis of Trimethine Cyanines Using Formaldehyde as a Single-Carbon Source

Herein, we present an innovative and atom-efficient synthesis of trimethine cyanines (Cy3) using formaldehyde (FA) as a single-carbon reagent. The widespread application of Cy3 dyes in bioimaging and genomics/proteomics is often limited by synthetic routes plagued by low atom economy and substantial side-product formation. Through systematic investigation, we have developed a practical and efficient synthetic pathway for both symmetrical and unsymmetrical Cy3 derivatives, significantly minimizing resource utilization. Notably, this approach yields water as the by-product, in alignment with sustainable chemistry principles. Moreover, the efficient one-pot synthesis facilitates the detection of intracellular FA levels, utilizing the fluorescence signal of Cy3 in live cells. It is also possible to detect endogenous FA in the intestinal tissues. We observed a significant decrease in FA in the small intestine of inflammatory bowel disease (IBD) mice as compared to healthy mice. This methodological advancement not only enhances the scope of fluorescent dye synthesis but also contributes to sustainable practices within chemical manufacturing, offering a significant leap forward in the development of environmentally friendly synthetic strategies.

A recombinant L-threonine aldolase with high catalytic efficiency for the asymmetric synthesis of L-threo-phenylserine and L-threo-4-fluorophenylserine

OBJECTIVES

To develop robust variants of L-threonine aldolases (L-TAs), potent catalysts for synthesizing asymmetric β-hydroxy-α-amino acids, it is necessary to identify critical residues beyond the known active site residues.

RESULTS

Through virtual screening, a neglected residue Asn305, was identified as critical for catalytic efficiency. Subsequent site-saturation mutagenesis led to a potent variant N305R which exhibited excellent conversions of 88%conv (87%de) and 80%conv (94%de) for the synthesis of L-threo-phenylserine and L-threo-4-fluorophenylserine respectively. This variant not only outperformed the template enzyme, but also represented a promising L-TA for synthesizing the two β-hydroxy-α-amino acids. It was suggested that Arg305 of the variant N305R generated strong cation-arene interaction and electrostatic force with the intermediates, leading to strengthened binding, enhanced L-threo favored orientation and wider entrance.

CONCLUSIONS

Our work not only provided an excellent variant N305R, but also suggested the crucial function of a neglected residue Asn305, which offered valuable experiences for other L-TA studies.

Complete Computational Reaction Mechanism for Foldamer-Catalyzed Aldol Condensation

Foldamers, small synthetic peptides made of α and β-amino acids, have been found to be efficient catalysts for carbon-carbon bond-forming aldol reactions; of particular interest is their ability to catalyze macrocycle ring closure reactions. These catalysts feature a pair of amine groups that are aligned by the helical conformation and act in concert. Kinetic measurements show that the rate of the reaction depends on the identity of the amine side chains present. However, such kinetic analyses and other characterization techniques (e.g. mass spectrometry) can provide only limited information regarding the overall mechanism and rate-determining step of foldamer catalysis. We use semi-empirical density functional tight binding quantum mechanics molecular mechanics metadynamics simulations to determine the free energy and barrier for all elementary steps involved in the ring closure aldol reactions. We have performed calculations for 44 elementary reaction steps to identify key trends regarding amine identity, and provide insight into the intermediates and rate-limiting step of the catalytic cycle. From our results and other known aldol catalysts, we propose foldamer mutants which simulations predict to be better catalysts.

Aldol Condensation for the Construction of Organic Functional Materials

Aldol condensation is a cost-effective and sustainable synthetic method, offering the advantages of low complexity, substrate universality, and high efficiency. Over the past decade, it has become popular for creating next-generation organic functional materials, particularly rigid-rod conjugated (semi)conductors. This review focuses on conjugated small molecules, oligomers, and polymeric (semi)conductors synthesized through aldol condensation, with emphasis on their remarkable features in advancing n-type organic field-effect transistors (OFETs), organic electrochemical transistors (OECTs), organic photovoltaics (OPVs), and organic thermoelectrics (OTEs) as well as NIR-II photothermal conversion. Coherence character, optical properties, microstructure, and chain conformation are investigated to understand material-property relationships. Future applications and challenges in this area are also discussed.

Evaluation of the Char Formation During the Hydrothermal Treatment of Wooden Balls

With wooden balls, a visualization of the hydrothermal carbonization to show the progress of the conversion to char is presented. In the present study, the balls represent the particles of biomass to investigate the differences in conversion outside and inside of biomass particles, during hydrothermal carbonization. A special focus is on hydrochar and pyrochar formation. The wooden balls are treated in subcritical water at 220 °C for holding times between 0 and 960 min. Even after 960 min, hydrolysis of the original biomass is incomplete as cellulose and hemicellulose are linked by lignin, inhibiting the reaction with water. Moreover, two different pathways of char production can be observed. Inside of the wooden ball pyrochar is formed as any water got that deep in, on the surface hydrochar is fixed, originated from the surrounding liquid. On the ground of the HTC reactor, a thin, brittle precipitate of likely hydrochar or humins can be found either from the precipitation of loosely attached compounds on the surface of the biomass or direct precipitation from the liquid.

Solvent-free cross aldol condensation of aldehydes and ketones over SrMo1-xNixO3-δ perovskite nanocrystals as heterogeneous catalysts

Aldol condensation is arguably one of the most fascinating reactions that leads to the formation of C-C bonds. Its use in the pharmaceutical industry to synthesis complex drugs from simple aldehydes and ketones has become of paramount importance. Although this is one reaction that has lured a lot of attention, not enough has been explored in heterogeneous catalysis. In this work we have successfully synthesized multicationic perovskites via the soft-template method and characterized them thoroughly. The synthesized perovskite nanocrystals were found to have small SBET however their catalytic application in the conversion of benzaldehyde (BAL) in the aldol condensation with diethyl ketone (DEK) was found to be astonishing. The synthesis was confirmed using many techniques, from determining the oxidation states of the materials using XPS. This gave access to determine the coordination of the metals in the perovskite lattice and also qualitatively assess the oxygen environments that exist. The oxygen vacancies and SBET were used to assess the activity of the perovskite catalysts in the cross-aldol condensation reaction. The optimal conditions for this aldol condensation were found to be 120 °C after 25 h with no solvent using SrMo0.5Ni0.5O3-δ inorganic perovskite which had the highest amount of oxygen vacant sites which gave a conversion of 88 % and an 82 % selectivity towards the desired cross-aldol condensation product. The use of dimethylformamide (DMF) for this reaction is discouraged as it reacts with BAL to produce a higher amide.

Synthesis of Novel 1,4-Diketone Derivatives and Their Further Cyclization

One of the important reactions to obtain a new carbon-carbon bond is the Stetter reaction, which is generally via a nucleophilic catalyst like cyanide or thiazolium-NHC catalysts. In particular, 1,4-diketones with very functional properties are obtained by the Stetter reaction with the intermolecular reaction of an aldehyde and an α,β-unsaturated ketone. In this study, we synthesized new derivatives (substituted arenoxy) of 1,4-diketone compounds (2a-2n) with useful features by a new version of the Stetter reaction method. In our work, arenoxy benzaldehyde derivatives with different structures as the Michael donor and methyl vinyl ketone as the Michael acceptor were used for the intermolecular Stetter reaction. The reaction was catalyzed by 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride (3b), using triethylamine for the basic medium and dimethyl sulfoxide as the solvent. As a result, some novel arenoxy-substituted 1,4-diketones were gained with good yields at room temperature within 24 h through an intermolecular Stetter reaction. In addition, new furan and pyrrole derivatives were prepared by performing the cyclization reaction with one of the obtained new diketone compounds.

Facile Access to Fabricate Carbon Dots and Perspective of Large-Scale Applications

Carbon dots (CDs), fluorescent carbon nanoparticles with particle sizes < 10 nm, are constantly being developed for potential large-scale applications. Recently, methods allow CD synthesis to be carried out on large-scale preparation in a controlled fashion are potentially important for multiple disciplines, including bottom-up strategy, top-down method. In this review, the recent progresses in the research of the methods for large-scale production of CDs and their functionalization are summarized. Especially, the methods of CD synthesis, such as large-scale preparation, hydrothermal/solvothermal, microwave-assisted, magnetic hyperthermia microfluidic and other methods, along with functionalization of CDs, are summarized in detail. By promising applications of CDs, there are three aspects have been already reported, such as enhancing mechanical properties, flame retardancy, and energy storage. Also, future development of CDs is prospected.

The complete mechanism of an aldol condensation in water

The base-catalyzed aldol condensation between benzaldehyde and p-acetylbenzoic acid in water shows an inverse solvent kinetic isotope effect, k_3,D2O/k_3,H2O, of 1.33 ± 0.03. The reaction is definitely faster in D₂O. This is interpreted to mean that the rate-limiting step in a five-step mechanism is Step 5, the final elimination of hydroxide from the enolate intermediate, not the formation of that intermediate. This is the same result and the same conclusion as from earlier studies in aqueous acetonitrile and refutes a suggestion, based on computations, that the rate-limiting step would change in water. Those computations are criticized as implying impossibly large isotope effects.

Transition metal-free functionalization of 2-oxindoles via sequential aldol and reductive aldol reactions using rongalite as a C1 reagent

A sequential one-pot classical aldol, transition-metal and hydride-free reductive aldol reaction is reported here for C(sp3)- H functionalization of 2-oxindoles using the multifaceted reagent rongalite. Here, rongalite functions as a hydride-free reducing agent and double C1 unit donor. This protocol enables the synthesis of a wide range of 3-methylindoline-2-ones and 3-(hydroxymethyl)-3-methylindolin-2-ones from 2-oxindoles (65-95% yields), which are the synthetic precursors for many natural products. Some of the important aspects of this synthetic approach include one-pot methylation and hydroxymethylation, low-cost rongalite (ca. $0.03 per 1 g), mild reaction conditions and applicability to gram-scale synthesis.

Cucurbit[7]uril-Modulated H/D Exchange of α-Carbonyl Hydrogen: Deceleration in Alkali and Acceleration in Acid Conditions

Supramolecular catalysis based on host-guest interactions has aroused much attention in past decades. Among the various strategies, modulation of the reactivity of key intermediates is an effective strategy to achieve high-efficiency supramolecular catalysis. Here, we report that by utilizing the host-guest interaction of cucurbit[7]uril (CB[7]), the reactivity of anionic enolate and cationic oxonium, the intermediates of H/D exchange of the α-carbonyl hydrogen in alkali and acid conditions, respectively, could be modulated effectively. On one hand, in alkaline conditions, both the electrostatic effect and the steric hindrance effect of CB[7] disfavored formation of the enolate anion intermediate. On the other hand, in acidic conditions, the oxonium was stabilized and the solvent effect was weakened by the electrostatic effect of CB[7]. As a result, the H/D exchange of 1-(4-acetylphenyl)-N,N,N-trimethylmethanaminium bromide is decelerated in alkaline and accelerated in acidic conditions. It is promising that the highly polar portals of CB[n] molecules together with their well-defined host-guest chemistry may be applied to modulate the reactivity of other kinds of ionic intermediates in an effective and convenient way, thus enriching the toolkit of supramolecular catalysis.

BF3 -Catalyzed Mukaiyama aldol reaction of acetaldehyde with 2-siloxy-1-propene

The Mukaiyama aldol reaction is a powerful tool for the construction of the carbon-carbon bond and the formation of β-hydroxycarbonyl compounds. In this work, the mechanism of acetaldehyde and 2-siloxy-1-propene both in the absence and presence of the catalyst BF₃ was investigated based on density functional theory. The mechanism includes two major steps: the formation of the carbon-carbon bond and the removal of SiH₃ /BF₂ by water. The energy barrier of the carbon-carbon bond formation process in the presence of BF₃ is obviously lower, indicating that BF₃ is a good catalyst for this reaction. In terms of molecular configuration, the different tensions between the five-membered-ring and six-membered-ring can be considered as the possible reason for the catalytic effect of BF₃ . In terms of charge transfer, the charges of natural population analysis in the carbon atom of the carbonyl group in acetaldehyde becomes more positive, which is easier to attack by nucleophiles and promote the nucleophilic process.

Comment on "Topography of the free energy landscape of Claisen-Schmidt condensation: solvent and temperature effects on the rate-controlling step" by N. D. Coutinho, H. G. Machado, V. H. Carvalho-Silva and W. A. da Silva, Phys. Chem. Chem. Phys., 2021, , 6738

The referenced article in PCCP presents calculations of solvent kinetic isotope effects that indicate that the rate-limiting step in base-catalyzed chalcone formation in aqueous solution becomes the second enolization. This disputes our previous conclusion, based on experimental isotope effects in aqueous acetonitrile, that the rate-limiting step is the final loss of hydroxide and formation of the C-C double bond. That conclusion is here affirmed as general for any protic solvent, and it is further concluded that those calculations are flawed.

Reply to the 'Comment on "Topography of the Free Energy Landscape on the Claisen-Schmidt Condensation: Solvent and Temperature Effect in the Rate-Controlling Step"' by N. D. Coutinho, H. G. Machado, V. H. Carvalho-Silva and W. A. da Silva, Phys. Chem. Chem. Phys., 2021, , 6738

In the Comment on our paper on the description of the Gibbs Free energy profile of Claisen-Schmidt condensation, it is claimed that our calculations are flawed due to inconsistencies with experimental isotope effects in aqueous acetonitrile. In this Reply, we presented rigorous arguments, ambiguities in the Comment and new calculations confirming the consistency of our results: (i) small differences in the relative energetic barriers in both experimental and theoretical curves make the assignment of the rate-limiting step debatable, making the concept of RCS questionable; (ii) it is shown how the misinterpretation of the elementary steps and of the overall processes rate constants led the Comment to incorrect conclusions about the behavior of the inverse isotopic effect; (iii) neglect in the Comment of the inverse kinetic isotope effect in step R2 due to the hybridization conversion, and of the inverse equilibrium isotopic effect for step R1 to describe an overall iKIE > 1, (iv) an erroneous suggestion in the Comment that the disagreement between experimental kinetic parameters is due to the fact that acetonitrile is not used in previous experimental works, when contradictorily the literature recommends it as being indispensable to allow kinetic accuracy; and (v) new calculations improved by explicit-implicit hybrid treatment again ensure that step R4, and not step R5, can assume the role of RCS in protic solvents. Recognizing that questioning is an excellent path for promoting understanding, we hope that the answers provided here will help to clarify and expand the pertinent topics under discussion.

(E)-5-Benzyl-7-(3,4-dimethoxybenzylidene)-3-(3,4-dimethoxyphenyl)-2-phenyl-3,3a,4,5,6,7-hexahydro-2H-pyrazolo[4,3c] Pyridine

A new pyrazolo-pyridine analogue (title compound) was synthesized in two steps. The first stage was synthesis of monoketone curcumin analogue through Claisen–Schmidt reaction. The second stage was synthesis of the title compound through intermolecular cyclization under reflux condition. The structure of the title compound has been confirmed by spectroscopic analysis including UV, FT-IR, HRMS, 1D NMR (1H-NMR, 13C-NMR, 1D-TOCSY), and 2D NMR (COSY, HSQC, HMBC). Based on the DPPH assay, the compound has moderate antioxidant activity, with an IC50 value of 194.06 ± 7.88 µg/mL (0.337 mM).

Aldol Condensation Versus Superbase-Catalyzed Addition of Ketones to Acetylenes: A Quantum-Chemical and Experimental Study

The mechanism of aldol condensation of ketones in KOH/DMSO superbasic media has been investigated using the B2PLYP(D2)/6-311+G**//B3LYP/6-31+G* quantum-chemical approach. It is found that the interaction of three ketone molecules resulting in the formation of the cyclohex-2-enone structure [isophorone or 3,5-dicyclohexyl-5-methylspiro(5.5)undec-2-en-1-one] is thermodynamically more favorable than the interaction of two, three, or four molecules of ketone, resulting in the formation of linear products of the condensation. The formation of the condensation products with the isophorone skeleton can significantly hinder the cascade reactions of ketones with acetylenes [to afford 6,8-dioxabicyclo(3.2.1)octanes or acylcyclopentenols] promoted by superbases. In particular, the kinetically more preferable reactions of autovinylation of 2-methyl-3-butyn-2-ol and autocondensation of acetone are the reasons why interaction of acetone with acetylene does not lead to the products of the cascade assemblies. The predominant formation of the products of these side reactions is confirmed experimentally.

Kilogram-Scale Synthesis and Functionalization of Carbon Dots for Superior Electrochemical Potassium Storage

Carbon dot is a type of carbon material with an ultrasmall size of less than 10 nm for all three dimensions, which has attracted more and more attention due to its useful merits. Unfortunately, the complicated synthesis method and low yield largely limit its wide large-scale application. Herein, an inexpensive and high-efficiency aldol condensation method under ambient temperature and pressure was proposed for the large-scale synthesis of CDs, which can obtain products with 1.083 kg in 2 h and realize the functionalization of carbon dots doped with nitrogen (NCDs) and sulfur/nitrogen doubly (NSCDs), and then the mechanism and structure of CDs formation were explained. Moreover, utilizing the feature of controllable assembly of carbon dots, and combined with theoretical calculations, we have designed functionalized 1D carbon fibers (CF) to construct high-performance potassium storage anode materials through the assembly of carbon dots induced by a Zn compound. Benefitting from the microstructure and surface functional groups derived from CDs, the N-doped CF (NCF700) exhibits superior electrochemical energy storage performance for potassium ion batteries (PIBs). This study provides a low-cost and high-yield method to produce CDs and promotes the practical application of CDs in electrochemical energy storage.

Topography of the free energy landscape of Claisen-Schmidt condensation: solvent and temperature effects on the rate-controlling step

Recent studies have found that hydroxide elimination and the C[double bond, length as m-dash]C bond formation step in base-promoted aldol condensation have a strong influence on the overall rate of the reaction, in contrast to the well-accepted first enolization or C-C bond formation step. Here, applying theoretical models to the prototypical reaction of chalcone formation, the complete free energy profile of Claisen-Schmidt condensation is assessed, revealing how a protic solvent and a slight increase in temperature can induce the second enolization as the rate-controlling step (RCS). It is also observed: i) the nonexistence of a step with a much higher energetic barrier than the others, making the concept of RCS debatable; and ii) that the overall inverse kinetic isotopic effect does not exclude second enolization as a RCS in protic continuum medium. We expect that these results can expand the understanding of the decisive role of physicochemical factors on the choose of the RCS in the aldol condensation.

Design and synthesis of quinoline-pyrimidine inspired hybrids as potential plasmodial inhibitors

Presently, artemisinin-based combination therapy (ACT) is the first-line therapy of Plasmodium falciparum malaria. With the emergence of malaria parasites that are resistant to ACT, alternative antimalarial therapies are urgently needed. In line with this, we designed and synthesised a series of novel N-(7-chloroquinolin-4-yl)-N'-(4,6-diphenylpyrimidin-2-yl)alkanediamine hybrids (6a-7c) and evaluated their inhibitory activity against the NF54 chloroquine-susceptible strain as a promising class of antimalarial compounds. The antiplasmodial screening revealed that seven analogues showed promising to good activity with half-maximal inhibitory concentration (IC₅₀) = 0.32 μM-4.30 μM. Compound 7a with 1,4-diamine butyl linker and 4-hydroxyl phenyl on fourth and sixth position of pyrimidine core showed the most prominent activity with an IC₅₀ value of 0.32 ± 0.06 μM, with a favourable safety profile of 9.79 to human kidney epithelial (HEK293) cells. The remaining six analogues showed moderate activity with IC₅₀ values ranging from 7.50 μM to 83.01 μM. We further investigated the binding affinities of the molecules to two essential cytosolic P. falciparum heat shock protein 70 homologues; PfHsp70-1 and PfHsp70-z. Compound 7a exhibited the highest binding affinity for both PfHsp70s with K_D in a lower nanomolar range (4.4-11.4 nM). Furthermore, molecular docking revealed that compounds 6, 6k, 7b and 7a exhibited better fitness in PfHsp70-1 with compound 7a showing the highest and lowest binding scores of -9.8 kcal/mol. Therefore, we speculate that PfHsp70-1 is one of the targets of these inhibitors. The bioisoteric replacement of the groups at phenyl ring at the fourth and sixth position of the pyrimidine core had a constructive association with antiplasmodial activity. The promising antiplasmodial activity of the synthesised analogues illustrates how crucial molecular hybridisation is as a strategy in the development of quinoline-pyrimidine hybrids as prospective antiprotozoal agents.

Low-Temperature Aldol Condensation of Aldehydes on R-TiO2(100)-(1 × 1): Exceptional Selectivity for α,β-Unsaturated Enal Production

Selective C-C coupling of oxygenates via aldol condensation has the potential to produce useful chemicals from aldehydes and ketones. Here we report a combined experimental and theoretical study on the aldol condensation of unbranched aldehydes (C_nH_2n+1-CHO, n = 1-4) on rutile (R)-TiO₂(100)-(1 × 1). Experimental results show that the R-TiO₂(100)-(1 × 1) surface has a very high reactivity and selectivity for aldol product formation from tested aldehydes at room temperature. Theoretical calculations indicate that the CH₃CHO enolization and the aldol dehydration occur with low energy barriers, and the 3-butanolal intermediate adsorbs on R-TiO₂(100)-(1 × 1) stably, suggesting that the surface has a "modest" acid-base strength for efficient crotonaldehyde formation. The adsorption configuration of CH₃CHO and surface structure of R-TiO₂(100)-(1 × 1) may contribute to the exclusive selectivity of (E)-crotonaldehyde formation, which provides us a deep insight into the high selectivity of aldol condensation of aldehydes on the TiO₂ catalyst.

On-line attenuated total reflection infrared spectroscopy (ATR-IR): a powerful tool for investigating the methyl cyclopentenone synthesis process

On-line attenuated total reflection infrared spectroscopy (ATR-IR) was used to gain a good understanding of the kinetics and mechanism for methyl cyclopentenone (MCP) synthesis from 2-methylfuran and formaldehyde in a four-step reaction. Combining in situ IR monitoring and a quantitative univariate model, the mechanisms for the main side reactions were discussed in depth. The presence and forming mechanism of the side product generated in step 1 (Mannich reaction) were reported for the first time. Off-line ¹H NMR and GC-MS were used as reference tools to further clarify the structure of the side product. Results also show that an undesirable side reaction will take place if the reaction time for step 2 is longer than 3 h. Possible mechanisms for side reactions and optimized experimental conditions were suggested for the purpose of improving the selectivity of the main reaction to efficiently facilitate the yield of MCP. The present study demonstrates that on-line ATR-IR can be a powerful tool to gain insight into the process understanding of various chemical reactions, providing a solid theoretical foundation for highly efficient, large-scale synthesis of MCP.

Exploring Aldol Reactions on DNA and Applications to Produce Diverse Structures: An Example of Expanding Chemical Space of DNA-Encoded Compounds by Diversity-Oriented Synthesis

A DNA-encoded chemical library (DECL) is built with combinatorial chemistry, which works by bringing chemical fragments together to generate diverse structures. However, chemical diversity of DNA-encoded chemical libraries is often limited by DNA compatible synthetic reactions. This report shows a conceptual strategy to expand chemical space of DNA-encoded chemical libraries by incorporation of diversity-oriented synthesis in DECL synthesis. We developed Aldol reactions on DNA in a combinatorial way. After obtaining DNA-tagged α, β-unsaturated ketones which represent important chemical intermediates, many distinct structures with skeletal diversities are achieved by diversity-oriented synthesis.

Identification of rapid access to polycyclic systems via a base-catalyzed cascade cyclization reaction and their biological evaluation

A base-mediated cascade reaction between malonate esters and acrolein was developed to access complex polycyclic systems. This novel tandem reaction enables the simultaneous generation of up to seven new bonds and at least three new stereogenic centers. Mechanistic studies indicate a series of nucleophilic 1,4 and 1,6 Michael addition reactions occur, followed by an aldol condensation reaction, culminating in the formation of three fused rings. The compounds were characterized by NMR studies and the stereochemistry was confirmed by X-ray analysis. The ability to generate multigram quantities of such complex molecular scaffolds renders the method promising for medicinal chemistry campaigns. Herein, we also demonstrate that the lead compounds display promising anti-proliferative activities against human cancer cell models.

Fluorescence spectroscopy studies of crossed aldol reactions: a reactive Nile red dye reveals catalyst-dependent product formation

A highly fluorescent, aldol-reactive derivative of the dye Nile red is synthesized and evaluated as an in situ probe of crossed aldol reactions.

Chalcones and Flavanones Bearing Hydroxyl and/or Methoxyl Groups: Synthesis and Biological Assessments

Chalcones and flavanones are isomeric structures and also classes of natural products, belonging to the flavonoid family. Moreover, their wide range of biological activities makes them key scaffolds for the synthesis of new and more efficient drugs. In this work, the synthesis of hydroxy and/or methoxychalcones was studied using less common bases, such as sodium hydride (NaH) and lithium bis(trimethylsilyl)amide (LiHMDS), in the aldol condensation. The results show that the use of NaH was more effective for the synthesis of 2′-hydroxychalcone derivatives, while LiHMDS led to the synthesis of polyhydroxylated chalcones in a one-pot process. During this study, it was also possible to establish the conditions that favor their isomerization into flavanones, allowing at the same time the synthesis of hydroxy and/or methoxyflavanones. The chalcones and flavanones obtained were evaluated to disclose their antioxidant, anticholinesterasic, antibacterial and antitumor activities. 2′,4′,4-Trihydroxychalcone was the most active compound in terms of antioxidant, anti-butyrylcholinesterase (IC50 26.55 ± 0.55 μg/mL, similar to control drug donepezil, IC50 28.94 ± 1.76 μg/mL) and antimicrobial activity. 4′,7-Dihydroxyflavanone presented dual inhibition, that is, the ability to inhibit both cholinesterases. 4′-Hydroxy-5,7-dimethoxyflavanone and 2′-hydroxy-4-methoxychalcone were the compounds with the best antitumor activity. The substitution pattern and the biological assay results allowed the establishment of some structure/activity relationships.

Histidine-based copper tetrapeptides as enantioselective catalysts for aldol reactions

Copper(ii)-peptides are widely used as industrial catalysts such as in the aerobic oxidation of organic molecules, formation of new C–H bonds and in the azide–alkyne cycloaddition reaction. The length of peptides and the effect of adding copper metal into peptides were questioned in their field of applications. Five novel histidine-based tetrapeptides with the sequences HAAD (P1), HAFD (P2), HAVD (P3), AGHD (P4) and PGHD (P5) were synthesized using the solid phase peptide scheme and analysed with high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) with percentage purities as high as 99.5%. All the peptides were positively charged (+1) and the molecular weight calculated from m/z values of MS results coincided with the theoretical molecular weight of the peptides. Copper(ii)-peptides derived from these peptides and copper(ii) acetate monohydrate (CuP1–CuP5) in a 1 : 2 ratio was synthesised, purified and characterised by ultraviolet-visible spectroscopy (UV-Vis), ultraviolet-fluorescence spectroscopy (fluorescence) and fourier transform infrared spectroscopy (FTIR), circular dichroism spectroscopy (CD) and optical rotation polarimetry. It provided the necessary information on the secondary structure and the successful binding of copper(ii) to the specific amino acids, hence leading to the putative geometry of copper(ii)-peptides and the difference in the chirality of amino acids, peptides and copper(ii)-peptides. The catalytic activities of the synthesised complexes were evaluated. CuP1 & CuP3 catalysed both the asymmetric aldol reactions with high enantioselectivity of p-nitrobenzaldehyde with cyclohexanone (% ee = 87.3 & 80.3, respectively) and of p-anisaldehyde with cyclohexanone (% ee = 95.5 & 90.9, respectively).

P5 with the sequence H₂N-PGHD-CONH.

Mechanism and rate constant of proline-catalysed asymmetric aldol reaction of acetone and p-nitrobenzaldehyde in solution medium: Density-functional theory computation

In search of new ways to improve catalyst design, the current research focused on using quantum mechanical descriptors to investigate the effect of proline as a catalyst for mechanism and rate of asymmetric aldol reaction. A plausible mechanism of reaction between acetone and 4-nitrobenzaldehyde in acetone medium was developed using highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies calculated via density functional theory (DFT) at the 6-31G∗/B3LYP level of theory. New mechanistic steps were proposed and found to follow, with expansion, the previously reported iminium-enamine route of typical class 1 aldolase enzymes. From the elementary steps, the first step which involves a bimolecular collision of acetone and proline was considered as the rate-determining step, having the highest activation energy of 59.07 kJ mol^-1. The mechanism was used to develop the rate law from which the overall rate constant was calculated and found to be 4.04×10-8dm3mol-1s-1 . The new mechanistic insights and the explicit computation of the rate constant further improve the kinetic knowledge of the reaction.

On the asymmetric autocatalysis of aldol reactions: The case of 4-nitrobenzaldehyde and acetone. A critical appraisal with a focus on theory

Under neutral conditions, spontaneous mirror symmetry breaking has been occasionally reported for aldol reactions starting from achiral reagents and conditions. Chiral induction might be interpreted in terms of autocatalysis exerted by chiral mono-aldol or bis-aldol products as source of initial enantiomeric excesses, which may account for such experimental observations. We describe here a thorough Density Functional Theory (DFT) study on this complex and otherwise difficult problem, which provides some insights into this phenomenon. The picture adds further rationale to an in-depth analysis by Moyano et al, who showed the isolation and characterization of bis-aldol adducts and their participation in a complex network of reversible steps. However, the lack of enantiodiscrimination (ees vanish rapidly in solution) suggests, according to the present results, a weak association in complexes formed by the catalysts and substrates. The latter would also be consistent with almost flat transition states having similar heights for competitive catalyst-bound transition structures (actually, we were unable to locate them at the level explored). Overall, neither autocatalysis as once conjectured nor mutual inhibition of enantiomers appears to be operating mechanisms. Asymmetric amplification in early stages harnessing unavoidable enantiomeric imbalances in reaction mixtures of chiral products represents a plausible interpretation.

Contrasting C- and O-Atom Reactivities of Neutral Ketone and Enolate Forms of 3-Sulfonyloxyimino-2-methyl-1-phenyl-1-butanones

The mechanisms of intramolecular cyclization of 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanones (1) under basic (DABCO and t-BuOK) and acidic (AcOH and TFA) conditions were investigated by means of experimental and computational methods. The ketone, enol, and enolate forms of 1 can afford different intramolecular cyclization products (2, 3, 4), depending on the conditions. The results of the reaction of 1 under basic conditions suggest intermediacy of neutral enol (DABCO) and anionic enolate (t-BuOK), while the results under acidic conditions (AcOH and TFA) indicate involvement of neutral ketones, which exhibit reactivities arising from both the oxygen lone-pair electrons (O atom reactivity) and carbon σ-electrons (C atom reactivity). The neutral enol in DABCO afforded 2H-azirine 4. On the other hand, the products (isoxazole 2 and oxazole 3) generated from the ketone form and from the enolate form are the same, but the reaction mechanisms are apparently different. The results demonstrate ambident-like reactivity of neutral ketone in the 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanone system.

A DFT study on the aldol condensation reaction on MgO in the process of ethanol to 1,3-butadiene: understanding the structure–activity relationship

The mechanism of aldol condensation on MgO surfaces with different structures was investigated to illustrate the structure–activity relationship.