Meldrum's acids and 5-alkylidene Meldrum's acids in catalytic carbon-carbon bond-forming processes

Vinylogous and stereoselective domino synthesis of pyrano[2,3-c]pyrroles from alkylidene meldrum's acids

A domino Vinylogous aza-Michael-Aldol-Cyclocondensation (aza-VMAC) reaction was achieved with a series of alkylidene Meldrum's acid derivatives under simple operational conditions paving the way to novel pyrano[2,3-c]pyrroles in an excellent diasteroselectivity (>96 : 4), encompassing the relative control of three contiguous stereocenters.

Formation of bis-spiropyrrolidines from isatin, secondary amines, and alkylidene Meldrum's acids

A catalyst-free synthesis of stable bis-spiropyrrolidines from isatin, secondary amines, and alkylidene Meldrum's acids in MeCN in 75-95% yield is described. The antioxidant and antimicrobial properties of the synthesized compounds are investigated. For this purpose, the radical scavenging activities of four derivatives were studied by radical trapping of diphenylpicrylhydrazine and ferric reduction power experiments. Disk diffusion test on Gram-positive and Gram-negative bacteria was employed to investigate antibacterial activities of five derivatives.

Meldrum's acid assisted formation of tetrahydroquinolin-2-one derivatives a short synthetic pathway to the biologically useful scaffold

A new method for the preparation of tetrahydroquinolin-2-one derivatives is presented. This approach involves a two-step reaction between enaminones and acylating agents, immediately followed by electrophilic cyclization, all within a single synthesis procedure, eliminating the need to isolate intermediates. The entire process is facilitated by the use of acyl Meldrum's acids which not only shortens the preparation time of the substrates but also easily extends the range of substituents That can be used. The method's scope and limitations were evaluated with various reagent combinations thus demonstrating its general applicability to the synthesis of tetrahydroquinolin-2-one core. Interestingly, some exceptions to the regular reaction pathway were observed when a strong EDG (electron donating group) was introduced via acyl Meldrum's acids. The underlying mechanism of this phenomenon was elucidated during the investigation.

Ruthenium-Catalyzed Chemo-Selective Carbene Insertion into C-H Bond of Styrene over Cyclopropanation: C-C Bond Formation

The C-C bond formation reactions are important in organic synthesis. Heck reaction is known to arylate the terminal carbon of olefins; however, direct alkylation of the terminal carbon of olefin is limited. Herein, we report a novel ruthenium-catalyzed selective cross-coupling reaction of styrene and α-diazoesters to form a new C-C bond over cyclopropanation via the C-H insertion process for the first time. Using this novel methodology, a wide variety of substrates have been utilized and a variety of α-vinylated benzylic esters and densely functionalized olefins have been synthesized with good stereoselectivity under mild reaction conditions. The overall reaction process proceeds through the carbene insertion into styrene to form the desired products in good to excellent yields with proper stereoselectivity. The selective C-H inserted product, wide substrate scope, and excellent functional group tolerance are the best features of this work.

Meldrum-Based-1H-1,2,3-Triazoles as Antidiabetic Agents: Synthesis, In Vitro α-Glucosidase Inhibition Activity, Molecular Docking Studies, and In Silico Approach

A series of novel alkyl derivatives (2-5a,b) and 1H-1,2,3-triazole analogues (7a-k) of Meldrum's acid were synthesized in a highly effective way by using "click" chemistry and screened for in vitro α-glucosidase inhibitory activity to examine their antidiabetic potential. ¹H NMR, ¹³C-NMR, and high-resolution electrospray ionization mass spectra (HR-ESI-MS) were used to analyze each of the newly synthesized compounds. Interestingly, these compounds demonstrated high to moderate α-glucosidase inhibitory potency having an IC₅₀ range of 4.63-80.21 μM. Among these derivatives, compound 7i showed extraordinary inhibitory activity and was discovered to be several times more potent than the parent compound Meldrum (1) and the standard drug acarbose. Later, molecular docking was performed to understand the binding mode and the binding strength of all the compounds with the target enzyme, which revealed that all compounds are well fitted in the active site of α-glucosidase. To further ascertain the structure of compounds, suitable X-ray single crystals of compounds 5a, 7a, and 7h were developed and studied. The current investigation has shown that combining 1H-1,2,3-triazole with the Meldrum moiety is beneficial. Furthermore, this is the first time that the aforementioned activity of these compounds has been reported.

Synthesis, Characterization, and Biological Evaluation of Meldrum's Acid Derivatives: Dual Activity and Molecular Docking Study

In the presented study, eight novel Meldrum's acid derivatives containing various vanillic groups were synthesized. Vanillidene Meldrum's acid compounds were tested against different cancer cell lines and microbes. Out of nine, three showed very good biological activity against E. coli, and HeLa and A549 cell lines. It is shown that the O-alkyl substituted derivatives possessed better antimicrobial and anticancer activities in comparison with the O-acyl ones. The decyl substituted molecule (3i) has the highest activity against E. coli (MIC = 12.4 μM) and cancer cell lines (HeLa, A549, and LS174 = 15.7, 21.8, and 30.5 μM, respectively). The selectivity index of 3i is 4.8 (HeLa). The molecular docking study indicates that compound 3i showed good binding affinity to DNA, E. coli Gyrase B, and topoisomerase II beta. The covalent docking showed that 3i was a Michael acceptor for the nucleophiles Lys and Ser. The best Eb was noted for the topoisomerase II beta-LYS482-3i cluster.

Rigidified Bis(sulfonyl)ethylenes as Effective Michael Acceptors for Asymmetric Catalysis: Application to the Enantioselective Synthesis of Quaternary Hydantoins

The catalytic, enantio- and diastereoselective addition of hydantoin surrogates II to "rigidified" vinylidene bis(sulfone) reagents is developed, thus overcoming the inability of commonly employed β-substituted vinylic sulfones to react. Adducts are transformed in enantioenriched 5,5-disubstituted hydantoins through hydrolysis and reductive desulfonylation processes providing new structures for eventual bioassays. Density functional theory studies that rationalize the observed reactivity and stereoselectivity trends are also provided.

Donor-Acceptor Cyclopropanes: Activation Enabled by a Single, Vinylogous Acceptor

A novel class of highly activated donor-acceptor cyclopropanes bearing only a single, vinylogous acceptor is presented. These strained moieties readily undergo cycloadditions with aldehydes, ketones, thioketones, nitriles, naphth-2-ols and various other substrates to yield the corresponding carbo- and heterocycles. Diastereocontrol can be achieved through the choice of catalyst (Brønsted or Lewis acid). The formation of tetrahydrofurans was shown to be highly enantiospecific when chiral cyclopropanes are employed. A series of mechanistic and kinetic experiments was conducted to elucidate a plausible catalytic cycle and to rationalize the stereochemical outcome.

Practical Synthesis of 7-Azaserotonin and 7-Azamelatonin

A practical method for synthesizing 7-azaserotonin and 7-azamelatonin was developed by using 3-bromo-5-methoxy-1-tosyl-2,3-dihydro-1H-pyrrolo[2,3-b]pyridin-2-ol as a starting material. This compound is a useful reactant for the formal C3-electrophilic reaction. The lactone derivative obtained by the reaction with Meldrum’s acid was used as a key intermediate, in which the C2 unit was introduced into the 7-azaindole skeleton.

Synthesis of fluorenes and their related compounds from biaryls and Meldrum's acid derivatives

A new synthetic method for preparing fluorenes from amino group-containing biaryls and Meldrum's acid derivatives was developed. The reaction proceeded without a catalyst and loss of functional groups. The corresponding six- and seven-membered cyclic products were obtained using biaryl ether and ortho-terphenyl as substrates, respectively.

Regio- and Stereoselective Addition of Brønsted Acids to Yndiamides: Synthesis of N,O,N- and N,S,N-Trisubstituted Ketene Acetals

Yndiamides, N,N-disubstituted alkynes, are versatile building blocks for the synthesis of nitrogen-containing organic molecules. Unlike ynamides, relatives that are inherently polarized by a single nitrogen substituent, their pseudo-symmetric nature renders regioselective reactions challenging. Here we report investigations into the regio­selective addition of Brønsted acids to non-symmetric yndiamides, a reaction that delivers N,O,N- and N,S,N-trisubstituted ketene acetals with excellent regio- and stereoselectivity.

Development of Novel Quinoline-Based Sulfonamides as Selective Cancer-Associated Carbonic Anhydrase Isoform IX Inhibitors

A new series of quinoline-based benzenesulfonamides (QBS) were developed as potential carbonic anhydrase inhibitors (CAIs). The target QBS CAIs is based on the 4-anilinoquinoline scaffold where the primary sulphonamide functionality was grafted at C4 of the anilino moiety as a zinc anchoring group (QBS 13a-c); thereafter, the sulphonamide group was switched to ortho- and meta-positions to afford regioisomers 9a-d and 11a-g. Moreover, a linker elongation approach was adopted where the amino linker was replaced by a hydrazide one to afford QBS 16. All the described QBS have been synthesized and investigated for their CA inhibitory action against hCA I, II, IX and XII. In general, para-sulphonamide derivatives 13a-c displayed the best inhibitory activity against both cancer-related isoforms hCA IX (KIs = 25.8, 5.5 and 18.6 nM, respectively) and hCA XII (KIs = 9.8, 13.2 and 8.7 nM, respectively), beside the excellent hCA IX inhibitory activity exerted by meta-sulphonamide derivative 11c (KI = 8.4 nM). The most promising QBS were further evaluated for their anticancer and pro-apoptotic activities on two cancer cell lines (MDA-MB-231 and MCF-7). In addition, molecular docking simulation studies were applied to justify the acquired CA inhibitory action of the target QBS.

The Catalytic Regio- and Stereoselective Synthesis of 1,6-Diazabicyclo[4.3.0]nonane-2,7-diones

A straightforward synthesis of original 1,6-diazabicyclo[4.3.0]nonane-2,7-diones was achieved through a DBU-organocatalyzed multicomponent Knoevenagel-aza-Michael-Cyclocondensation reaction which takes advantage of an unprecedented highly regio- and diastereoselective conjugate addition of pyridazinones to alkylidene Meldrum's acid intermediates. The key reactive intermediates of this complex process were analyzed by means of electrospray ionization mass spectrometry coupled to ion mobility spectrometry, allowing us to validate the proposed mechanism.

Selected applications of Meldrum's acid - a tutorial

Due to its unique structure and the vast array of substituents that can be attached to its core, Meldrum's acid is a molecule with exceptional chemical properties. In water, it has a remarkably low pKa value of about 4.9. Its C5 position is readily involved in electrophilic substitution reactions whereas the C4 and C6 positions are easily attacked by nucleophiles. At elevated temperatures Meldrum's acid undergoes distinctive decomposition pathways, which can be used in cycloaddition and acylation reactions. In this Tutorial Review, the authors intend to introduce the principles of the synthetic chemistry of Meldrum's acid and provide the essential knowledge for the design and preparation of compounds with desired properties. As there are many reviews focusing on a specific detail of Meldrum's acid chemistry, we would like to give a broader picture of this diverse molecule for undergraduate and graduate students as well as experienced lab leaders. For achieving this goal, some recent advances in using Meldrum's acid derivatives in synthetic scenarios are presented with the hope to further stimulate and promote research leading to additional innovative applications of this synthetically highly relevant molecule.

Reverse orientation in the ultrasound-assisted [3 + 2]-cycloaddition reaction of nitrile imines with 3-formylchromone-Meldrum's acid adducts

The [3 + 2]-cycloaddition reaction of nitrile imines with 2,2-dimethyl-5-[(4-oxo-4H-chromen-3-yl)methylene]-1,3-dioxane-4,6-dione tends to form the reverse-orientation products under ultrasound irradiation in EtOH in the presence of Et₃N. Evidence for the structure of product 5b was obtained from single-crystal X-ray analysis.

Overcoming Kinetic and Thermodynamic Challenges of Classic Cope Rearrangements

Systematic evaluation of 1,5-dienes bearing 3,3-electron-withdrawing groups and 4-methylation results in the discovery of a Cope rearrangement for Meldrum's acid-containing substrates that have unexpectedly favorable kinetic and thermodynamic profiles. The protocol is quite general due to a concise and convergent synthesis from abundant starting materials. Furthermore, products with an embedded Meldrum's acid moiety are prepared, which, in turn, can yield complex amides under neutral conditions. We have now expanded the scope of the reductive Cope rearrangement, which, via chemoselective reduction, can promote thermodynamically unfavorable [3,3] sigmatropic rearrangements of 3,3-dicyano-1,5-dienes to form reduced Cope rearrangement products. The Cope rearrangement is found to be stereospecific and can yield enantioenriched building blocks when chiral, nonracemic 1,3-disubstituted allylic electrophiles are utilized. We expand further the use of Cope rearrangements for the synthesis of highly valuable building blocks for complex- and drug-like molecular synthesis.

Synthesis of Cyclopentadienes for Cyclopentadienyl Ligands via Cp*RhIII-Catalyzed Formal sp3 C-H Activation/Spiroannulations

An efficient Cp*Rh^III-catalyzed formal C(sp³)-H activation/spiroannulation of alkylidene Meldrum's acids with alkynes has been developed using catalytical Cu(OAc)₂ and air as the oxidant. This reaction demonstrates a new and straightforward approach to spirocyclopentadienes with Meldrum's acid moieties in good to excellent yields under mild reaction conditions with a broad substrate scope. Notably, this protocol provides a novel and straightforward approach to cyclopentadienes with various substitution patterns and the corresponding cyclopentadienyl-type ligands from simple substrates.

Dissociations of free radicals to generate protons, electrophiles or nucleophiles: role in DNA strand breaks

The concept behind the research described in this article was that of marrying the 'soft' methods of radical generation with the effectiveness and flexibility of nucleophile/electrophile synthetic procedures. Classic studies with pulse radiolysis and laser flash photolysis had shown that free radicals could be more acidic than their closed shell counterparts. QM computations harmonised with this and helped to define which radical centres and which structural types were most effective. Radicals based on the sulfonic acid moiety and on the Meldrum's acid moiety (2,2-dimethyl-1,3-dioxane-4,6-dione) were found to be extreme examples in the superacid class. The ethyne unit could be used as a very effective spacer between the radical centre and the site of proton donation. The key factor in promoting acidity was understood to be the thermodynamic stabilisation of the conjugate anion-radicals released on deprotonation. Solvation played a key part in promoting this and theoretical microhydration studies provided notable support. A corollary was that heterolytic dissociations of free radicals to yield either electrophiles or nucleophiles were also enhanced relative to non-radical models. The most effective radical types for spontaneous releases of both these types of reagents were identified. Ethyne units were again effective as spacers. The enhancement of release of phosphate anions by adjacent radical centres was an important special case. Reactive oxygen species and also diradicals from endiyne antibiotics generate C4'-deoxyribose radicals from nucleotides. Radicals of these types spontaneously release phosphate and triphosphate and this is a contributor to DNA and RNA strand breaks.

Synthesis of 1-(Difluoromethyl)alk-1-enes via Palladium-Catalyzed SN2'-Type Substitution Reaction of Difluoromethylated Allylic Phosphates with 1,3-Dicarbonyl Compounds and Imides

Herein, we report the synthesis of 1-(difluoromethyl)alkenes via a palladium-catalyzed reaction of difluoromethyl-substituted allylic phosphates with 1,3-dicarbonyl compounds using PdCl₂(PPh₃)₂ as a precatalyst. 1,3-Dicarbonyl compounds attacked the γ-carbon with respect to the difluoromethyl group to afford their corresponding S_N2'-type substitution products irrespective of the substitution pattern in the allylic phosphates. This regioselectivity has been ascribed to the electronic environment of the unsymmetrical π-allylpalladium intermediate using density functional theory (DFT) calculations. The reaction of difluoromethyl-substituted allylic phosphates with imides was also carried out using a different catalyst system composed of [PdCl(η³-allyl)]₂ and di(diphenylphosphino)butane (dppb).

5-Carbonyl-1,3-oxazine-2,4-diones from N-Cyanosulfoximines and Meldrum's Acid Derivatives

At elevated temperatures, N-cyanosulfoximines react with Meldrum's acid derivatives to give sulfoximines with N-bound 5-carbonyl-1,3-oxazine-2,4-dione groups. A representative product was characterized by single-crystal X-ray structure analysis. The product formation involves an unexpected molecular reorientation requiring several sequential bond-forming and -cleaving processes.

Organocatalytic Multicomponent Synthesis of α/β-Dipeptide Derivatives

A straightforward multicomponent Knoevenagel-aza-Michael-cyclocondensation reaction involving readily available hydroxamic acid-derived from naturally occurring α-amino acids allows a diversity-oriented synthesis of novel isoxazolidin-5-ones possessing an N-protected α-amino acid pendant with good to high diastereoselectivities thanks to a match effect with a chiral organocatalyst. These diversely substituted heterocycles, easily isolated as a single diastereoisomer, proved to be versatile platforms for the formation of an array of α/β-dipeptide fragments.

Copper-catalysed alkylation of heterocyclic acceptors with organometallic reagents

Copper-catalysed asymmetric C–C bond-forming reactions using organometallic reagents have developed into a powerful tool for the synthesis of complex molecules with single or multiple stereogenic centres over the past decades. Among the various acceptors employed in such reactions, those with a heterocyclic core are of particular importance because of the frequent occurrence of heterocyclic scaffolds in the structures of chiral natural products and bioactive molecules. Hence, this review focuses on the progress made over the past 20 years for heterocyclic acceptors.

Stereoselective Sequential Spirocyclopropanation/Cloke-Wilson Rearrangement Reactions for Synthesis of trans-β,γ-Disubstituted γ-Butyrolactones Using Alkylidene Meldrum's Acid and Benzyl Halides

The stereoselective sequential spirocyclopropanation/Cloke-Wilson rearrangement reactions have been developed to synthesize γ-butyrolactones using alkylidene Meldrum's acids and benzyl halides. The DBU-promoted spirocyclopropanation was carried out efficiently at room temperature to generate trans-isomeric spirocyclopropyl Meldrum's acid, and the following stereospecific thermal decarboxylative Cloke-Wilson rearrangement afforded trans-γ-butyrolactones. A variety of aromatic and aliphatic Meldrum's acid derived olefins and benzyl halides were tolerated. Various trans-β,γ-disubstituted γ-butyrolactones were produced with moderate to good overall yields from 46 to 96% and excellent diastereoselectivities.

A thermal decarboxylative Cloke–Wilson rearrangement of dispirocyclopropanes derived from para-quinone methides and bromo-Meldrum's acids: an approach to synthesize spirobutyrolactone para-dienones

An unprecedented approach to synthesize spirobutyrolactone para-dienones from para-quinone methides and bromo-Meldrum's acids has been developed.

Multicomponent synthesis of pyroglutamic acid derivatives via Knoevenagel-Michael-hydrolysis-lactamization-decarboxylation (KMHL-D) sequence

A novel and practical method for the synthesis of 3-substituted pyroglutamic acid derivatives is described. One pot multicomponent reaction of Meldrum's acid, aldehyde and Schiff's base followed an unprecedented chemoselective Knoevenagel-Michael-hydrolysis-lactamization domino sequence to afford 4-carboxy 3-substituted pyroglutamic acid derivatives under mild conditions. A carboxy intermediate formed appears to accelerate its own formation. The generality of the synthesis is exemplified by the use of a wide variety of aldehydes including enolizable aliphatic aldehydes, while substrates are stable under reaction conditions.

Enantioselective Catalytic Transformations of Barbituric Acid Derivatives

Since the beginning of the 20th century, numerous research efforts made elegant use of barbituric acid derivatives as building blocks for the elaboration of more complex and useful molecules in the field of pharmaceutical chemistry and material sciences. However, the construction of chiral scaffolds by the catalytic enantioselective transformation of barbituric acid and derivatives has only emerged recently. The specific properties of these rather planar scaffolds, which also encompass either a high Brønsted acidity concerning the native barbituric acid or the marked electrophilic character of alkylidene barbituric acids, required specific developments to achieve efficient asymmetric processes. This review covers the enantioselective catalytic reactions developed for barbituric acid platforms using an organocatalytic and metal-based enantioselective sequences. These achievements currently allow several unique addition and annulation reactions towards the construction of high valued chiral heterocycles from barbituric acid derivatives along with innovative enantioselective developments.

Transient [3,3] Cope rearrangement of 3,3-dicyano-1,5-dienes: computational analysis and 2-step synthesis of arylcycloheptanes

A simple and modular route to arylcycloheptene scaffolds is reported. The two-step route from Knoevenagel adducts and allylic electrophiles is made possible through the design of a Cope rearrangement that utilizes a "traceless" activating group to promote an otherwise thermodynamically unfavorable transformation. Experimentally, the [3,3] rearrangement occurrs transiently at room temperature with a computed barrier of 19.5 kcal mol-1, which ultimately allows for three-component bis-allylation. Ring-closing metathesis delivers the arylcycloheptane and removes the activating group. This report describes the design and optimization of the methodology, scope and mechanistic studies, and computational analysis.

Vitrimeric Silicone Elastomers Enabled by Dynamic Meldrum's Acid-Derived Cross-Links

Current vitrimer technology uses only a handful of distinct reactions for cross-linking. New dynamic reactions can diversify vitrimer functionality and properties. In this paper, reversible cross-links formed by conjugate addition-elimination of thiols with a Meldrum's acid derivative enable compression-remolding of silicone elastomers. After 10 remolding cycles, there is no discernible deterioration of mechanical properties (Young's modulus, T_g, rubbery plateau E'), nor is there a change in stress relaxation activation energy. This robust new cross-linker could be implemented in any number of systems that currently use permanent thiol-ene cross-linking, expanding the scope of recyclable materials.

A Unique (3+2) Annulation Reaction between Meldrum's Acid and Nitrones: Mechanistic Insight by ESI-IMS-MS and DFT Studies

The fragile intermediates of the domino process leading to an isoxazolidin-5-one, triggered by unique reactivity between Meldrum's acid and an N-benzyl nitrone in the presence of a Brønsted base, were determined thanks to the softness and accuracy of electrospray ionization mass spectrometry coupled to ion mobility spectrometry (ESI-IMS-MS). The combined DFT study shed light on the overall organocatalytic sequence that starts with a stepwise (3+2) annulation reaction that is followed by a decarboxylative protonation sequence encompassing a stereoselective pathway issue.

Self-crosslinkable and modifiable polysiloxanes possessing Meldrum's acid groups

Meldrum's acid functionalized poly(dimethylsiloxane)s exhibiting self-crosslinking and post-modifiable features.