Dehydrative Cyclization Research Articles

Concise Total Synthesis of Crambescin B Methyl Ester

Abstract In this study, the concise total synthesis of crambescin B methyl ester, a cyclic guanidine alkaloid, has been achieved. The key aspects of this new approach include: 1) A Mannich reaction between α-amidosulfone and β-ketoester to construct the main scaffold and 2) Acid-catalyzed dehydrative cyclization leading to enol ether in a highly stereoselective manner. The synthetic approach reported herein is nine steps shorter than our previously published method.

Synthesis of 1,3,4-oxadiazole from 1,2-diacylhydrazines using SO2F2 as a simple and practical cyclization reagent.

The synthesis of 1,3,4-oxadiazoles from 1,2-diacylhydrazines has been achieved using SO₂F₂ as a straightforward and effective reagent for dehydrative cyclisation, both in the absence of metal catalysts and under milder conditions. The reaction exhibited good functional group tolerance and yielded high levels of product.

6-Polyfluoroalkyl-1-arylhexane-1,3,5-triones: Syntheses, ring-chain tautomerism and dehydrative cyclization to 6-polyfluoroalkyl-1-arylpyran-4H-ones

In this work, we present a synthetic protocol for the synthesis of non-symmetrical fluoroalkylated triketones. The synthesis is based on the reaction of benzoyl acetones with esters in the presence of excess lithium hydride. The tautomeric equilibrium of triketones in CDCl3 and DMSO‑d6 solutions was studied. The synthetic application of 1,3,5-triketones has been demonstrated. A method has been developed for the efficient synthesis of 2-aryl-6-(polyfluoroalkyl)-4H-pyran-4-ones by dehydrative cyclization.

Regio‐ and Chemoselectivity of Tandem N‐Ketoalkylation/Cyclization of 2‐Thioxoimidazoline‐4‐One

AbstractThe reaction of 2‐thioxoimidazolidin‐4‐one (thiohydantoin) with aliphatic, aromatic and heteroaromatic α‐iodomethylketones has been investigated in the absence of bases or catalysts. The reaction proceeds as N‐alkylation with subsequent intramolecular dehydrative cyclization. A series of new mono‐ and fused bicyclic iodide and triiodide salts was obtained based on the 5‐oxo‐2‐thioxoimidazolidin‐1‐ium and imidazo[2,1‐b]thiazolium cations. The observed regioselectivity has been rationalized and the mechanism was proposed.

Formation, Identification, and Occurrence of the Furan-Containing β-Carboline Flazin Derived from l-Tryptophan and Carbohydrates.

β-Carbolines (βCs) are bioactive indole alkaloids found in foods and in vivo. This work describes the identification, formation, and occurrence in foods of the βC with a furan moiety flazin (1-[5-(hydroxymethyl)furan-2-yl]-9H-pyrido[3,4-b]indole-3-carboxylic acid). Flazin was formed by the reaction of l-tryptophan with 3-deoxyglucosone but not with 5-hydroxymethylfurfural. Its formation was favored in acidic conditions and heating (70-110 °C). The proposed mechanism of formation occurs through the formation of intermediates 3,4-dihydro-β-carboline-3-carboxylic acid (imines), followed by the oxidation to C═O in the carbohydrate chain and aromatization to βC ring with subsequent dehydration steps and cyclization to afford the furan moiety. Flazin is generated in the reactions of tryptophan with carbohydrates. Its formation from fructose was higher than from glucose, whereas sucrose gave flazin under acidic conditions and heating owing to hydrolysis. Flazin was identified in foods by HPLC-MS, and its content was determined by HPLC-fluorescence. It occurred in numerous processed foods, such as tomato products, including crushed tomato puree, fried tomato, ketchup, tomato juices, and jams, but also in soy sauce, beer, balsamic vinegar, fruit juices, dried fruits, fried onions, and honey. Their concentrations ranged from not detected to 22.3 μg/mL, with the highest mean levels found in tomato concentrate (13.9 μg/g) and soy sauce (9.4 μg/mL). Flazin was formed during the heating process, as shown in fresh tomato juice and crushed tomatoes. These results indicate that flazin is widely present in foods and is daily uptaken in the diet.

Impact of Na at the low temperature Fe catalysis on high quality cellulose-based graphitic carbon

Biomass-based graphitic carbon is a promising material, but the effect mechanism of inherent alkali metals on the catalytic pyrolysis of biomass is still unclear. In this study, cellulose was selected as the model substance, and the influence mechanism of Na on the graphitization of cellulose catalytic pyrolysis was investigated through a combination of experiments and theoretical calculations. The results indicate that Na can interact with the carbon skeleton through van der Waals forces, thereby altering the spatial structure of graphite crystals and increasing the interlayer spacing of graphite by 27.7%. In addition, Na can coexist with Fe(III) in the form of oxides, while also increasing the reaction energy barrier of reverse hydroxylation and dehydration cyclization, decreasing the generation of reducing gases mainly composed of CO. This impedes the reduction of iron-based catalysts, thereby affecting the formation of the graphite microcrystalline structure. The research indicates that maintaining the ratio of Na content to iron-based catalysts within a 1:1 range is optimal for producing graphite carbon from cellulose.

Kinetic Resolution of Sulfoximines via Asymmetric Organocatalyzed Formation of Benzothiadiazine-1-oxides.

A catalytic kinetic resolution of sulfoximines has been developed through chiral phosphoric acid-catalyzed intramolecular dehydrative cyclizations. A variety of racemic sulfoximines bearing an ortho-amidophenyl moiety underwent asymmetric dehydrative cyclizations using this method, yielding both the recovered sulfoximines and benzothiadiazine-1-oxide products with good to high enantioselectivities (with s-factor up to 61). The diverse derivatizations of the chiral products into a wide range of S-stereogenic center-containing S,N-heterocycles have demonstrated the value of this method.

Brønsted Acid-Catalyzed Dehydrative Nazarov-type Cyclization of CF3-Substituted 3-Indolylallyl Alcohols: Divergent Synthesis of 1-Trifluoromethylated Cyclopenta[b]indoles.

An expedient and efficient synthetic method for the divergent synthesis of 1-trifluoromethylated cyclopenta[b]indoles that relies on Brønsted acid-catalyzed dehydrative Nazarov-type cyclization of CF3-substituted 3-indolylallyl alcohols is described. Two classes of 1-trifluoromethylated cyclopenta[b]indoles can be easily accessed simply by changing the NH-protecting group of indoles. With arylsulfonyl protected 3-indolylallyl alcohols as starting materials, the reaction provided the arylsulfonyl protected 1-trifluoromethylated cyclopenta[b]indoles in good to excellent yields, whereas pivaloyl (Piv) protected substrates led to the formation of NH-free 1-trifluoromethylated cyclolopenta[b]indoles with another alkenyl isomer. This protocol features tunable chemoselectivity, operational simplicity, excellent functional group compatibility, and mild metal-free conditions.

FeCl3-catalyzed regioselective ring-opening of aryl oxirane with 4-hydroxycoumarin for the synthesis of furo[3,2-c]coumarins.

The regioselective ring-opening of aryl oxiranes was investigated with various 4-hydroxycoumarins in dimethyl sulfoxide in the presence of 20 mol% FeCl3 as a catalyst at 110 °C. This approach provided a short and concise synthetic route for the regioselective synthesis of 2-aryl-4H-furo[3,2-c] coumarin derivatives. Product formation occurred through regioselective ring-opening of the aryl oxirane at a less hindered site, followed by dehydration and concomitant cyclization. The salient features of our protocol were: cost-effectiveness; short reaction time; step- and atom economy; easy handling; broad scope of substrates; regioselectivity; good-to-excellent yields; non-requirement of dry solvents, co-catalysts, ligands, or any other additives; inert atmospheric conditions.

Total Synthesis of Cyanobactin Natural Product Balgacyclamide B.

Balgacyclamide A-C are a family of cyanobactin natural products isolated from freshwater cyanobacteria Microcystis aeruginosa. These macrocyclic peptides are characterized by their oxazoline-thiazole core, their 7 or 8 stereocenters, and their antiparasitic activities. Balgacyclamide B is known for its activity towards Plasmodium falciparum chloroquine-resistant strain K1, Trypanosoma brucei rhodesiense, and Leishmania donovani. In this report, the first total synthesis of Balgacyclamide B is described in a 17-steps pathway and a 2 % overall yield. The synthetic pathway toward balgacyclamide B can be adapted for the future syntheses of balgacyclamide A and C. In addition, a brief history background of oxazolines syntheses is shown to emphasize the importance of the cyclization conditions used to interconvert or retain configuration of β-hydroxy amides via dehydrative cyclization.

In-situ formed aryl acid-triggered intramolecular dehydrative cyclization whilst supported Pd-catalyzed carbonylative synthesis of 2-aryl benzimidazoles

Herein, polystyrene supported palladium (Pd@PS) nanoparticles (NPs) catalyzed carbonylative synthesis of 2-aryl benzimidazoles from aryl iodides and aromatic diamines has been introduced. In-situ formed aryl carboxylic acid was revealed as a catalyst for dehydrative cyclization of secondary (2°) and tertiary (3°) amides formed via aminocarbonylation between aryl iodides and aromatic diamines. Herein, oxalic acid acts as carbon monoxide (CO) and carbon dioxide (CO2) gases source plausibly leading to aminocarbonylation via CO gas as major pathway and carboxylation of aryl iodides via CO2 as minor pathway. The Pd@PS catalyst was found to be easily separable, air/moisture stable and recyclable under the targeted reaction conditions. In addition, the present protocol requires only catalytic amount of inorganic base (K2CO3) to achieve the highest yields of desired products.

Mechanistic and Structural Insights into a Divergent PLP-Dependent l-Enduracididine Cyclase from a Toxic Cyanobacterium.

Cyclic arginine noncanonical amino acids (ncAAs) are found in several actinobacterial peptide natural products with therapeutically useful antibacterial properties. The preparation of ncAAs like enduracididine and capreomycidine currently takes multiple biosynthetic or chemosynthetic steps, thus limiting the commercial availability and applicability of these cyclic guanidine-containing amino acids. We recently discovered and characterized the biosynthetic pathway of guanitoxin, a potent freshwater cyanobacterial neurotoxin, that contains an arginine-derived cyclic guanidine phosphate within its highly polar structure. The ncAA l-enduracididine is an early intermediate in guanitoxin biosynthesis and is produced by GntC, a unique pyridoxal-5'-phosphate (PLP)-dependent enzyme. GntC catalyzes a cyclodehydration from a stereoselectively γ-hydroxylated l-arginine precursor via a reaction that functionally and mechanistically diverges from previously established actinobacterial cyclic arginine ncAA pathways. Herein, we interrogate l-enduracididine biosynthesis from the cyanobacterium Sphaerospermopsis torques-reginae ITEP-024 using spectroscopy, stable isotope labeling techniques, and X-ray crystallography structure-guided site-directed mutagenesis. GntC initially facilitates the reversible deprotonations of the α- and β-positions of its substrate before catalyzing an irreversible diastereoselective dehydration and subsequent intramolecular cyclization. The comparison of holo- and substrate-bound GntC structures and activity assays on site-specific mutants further identified amino acid residues that contribute to the overall catalytic mechanism. These interdisciplinary efforts at structurally and functionally characterizing GntC enable an improved understanding of how nature divergently produces cyclic arginine ncAAs and generate additional tools for their biocatalytic production and downstream biological applications.

Comparative Transcriptomic and Proteomic Analyses Provide New Insights into the Tolerance to Cyclic Dehydration in a Lichen Phycobiont.

Desiccation tolerance (DT) is relatively frequent in non-vascular plants and green algae. However, it is poorly understood how successive dehydration/rehydration (D/R) cycles shape their transcriptomes and proteomes. Here, we report a comprehensive analysis of adjustments on both transcript and protein profiles in response to successive D/R cycles in Coccomyxa simplex (Csol), isolated from the lichen Solorina saccata. A total of 1833 transcripts and 2332 proteins were differentially abundant as a consequence of D/R; however, only 315 of these transcripts/proteins showed similar trends. Variations in both transcriptomes and proteomes along D/R cycles together with functional analyses revealed an extensive decrease in transcript and protein levels during dehydration, most of them involved in gene expression, metabolism, substance transport, signalling and folding catalysis, among other cellular functions. At the same time, a series of protective transcripts/proteins, such as those related to antioxidant defence, polyol metabolism and autophagy, was upregulated during dehydration. Overall, our results show a transient decrease in most cellular functions as a result of drying and a gradual reactivation of specific cell processes to accommodate the hydration status along successive D/R cycles. This study provides new insights into key mechanisms involved in the DT of Csol and probably other dehydration-tolerant microalgae. In addition, functionally characterising the high number of genes/proteins of unknown functions found in this study may lead to the discovery of new DT mechanisms.

2-Amino-1,3-benzothiazole: Endo N-Alkylation with α-Iodo Methyl Ketones Followed by Cyclization.

Reactions of 2-amino-1,3-benzothiazole with aliphatic, aromatic and heteroaromatic α-iodoketones in the absence of bases or catalysts have been studied. The reaction proceeds by N-alkylation of the endocyclic nitrogen atom followed by intramolecular dehydrative cyclization. The regioselectivity is explained and the mechanism of the reaction is proposed. A number of new linear and cyclic iodide and triiodide benzothiazolium salts have been obtained and their structure proved by NMR and UV spectroscopy.

Electrochemical Synthesis of Hetero[7]helicenes Containing Pyrrole and Furan Rings via an Oxidative Heterocoupling and Dehydrative Cyclization Sequence

AbstractThe electrochemical synthesis of hetero[7]helicenes including pyrrole and furan rings has been established. A single electrochemical operation led to an oxidative heterocoupling and dehydrative cyclization sequence to afford oxaza[7]helicenes in 50–86% yields with 45–77% Faradic efficiencies. Their derivatization and chiroptical properties were also investigated.magnified image

Hydrogenation of levulinic acid to γ-valerolactone over single-atom Pt confined in Sn-modified MIL-101(Fe).

We synthesize a Sn-modified MIL-101(Fe), which can confine Pt to the single-atom scale. This novel Pt@MIL(FeSn) catalyst efficiently hydrogenates levulinic acid to γ-valerolactone (TOF: 1386 h-1, yield: >99%) at only 100 °C and 1 MPa of H2via α-angelica lactone as an intermediate. This could be the first report on switching the reaction path from 4-hydroxypentanoic acid to α-angelica lactone under very mild conditions. Incorporating Sn into MIL-101(Fe) enables the creation of abundant micro-pores less than 1 nm and Lewis acidic sites that stabilize Pt0 atoms. The ensemble of active Pt atoms and a Lewis acid can synergistically enhance adsorption of the CO bond and facilitate dehydrative cyclization of levulinic acid.

Hydrogen bonding-catalyzed synthesis of 1,4-dioxanes from dehydrative cyclization of vicinal diols in ionic liquids

[HO-EtMIm][OTf] efficiently catalyzes the dehydrative cyclization of vicinal diols under metal-free conditions, producing 1,4-dioxanes in 100% selectivity.

Dihydropyrrole-3-thiones: one-pot synthesis from propargylamines, acyl chlorides and sodium sulfide.

An efficient one-pot synthesis of 1,2,5-trisubstituted-1,2-dihydro-3H-pyrrole-3-thiones (up to 91% yield), representatives of essentially new heterocyclic systems, by the successive treatment of available propargylamines with acyl chlorides (PdCl2/CuI/Ph3P/Et3N, toluene, 40-45 °C, 3 h) and sodium sulfide (Na2S·9H2O, EtOH, 20-25 °C, 7 h) has been developed. The synthesis comprises the addition of sulfide anions to the formed aminoacetylenic ketones followed by dehydrative cyclization of the prototropically rearranged adducts.

Two-Step Synthesis, Structure, and Optical Features of a Double Hetero[7]helicene.

A novel double aza-oxa[7]helicene was synthesized from the commercially available N1,N4-di(naphthalen-2-yl)benzene-1,4-diamine and p-benzoquinone in two steps. Combining the acid-mediated annulation with the electrochemical sequential reaction (oxidative coupling and dehydrative cyclization) afforded this double hetero[7]helicene. Moreover, the structural and optical features of this molecule have been studied using X-ray crystallographic analysis, and the absorption and emission behaviors were rationalized based on DFT calculations.

A Facile Synthesis of 2-Oxazolines via Dehydrative Cyclization Promoted by Triflic Acid.

2-oxazolines are common moieties in numerous natural products, pharmaceuticals, and functional copolymers. Current methods for synthesizing 2-oxazolines mainly rely on stoichiometric dehydration agents or catalytic dehydration promoted by specific catalysts. These conditions either generate stoichiometric amounts of waste or require forcing azeotropic reflux conditions. As such, a practical and robust method that promotes dehydrative cyclization while generating no byproducts would be attractive to oxazoline production. Herein, we report a triflic acid (TfOH)-promoted dehydrative cyclization of N-(2-hydroxyethyl)amides for synthesizing 2-oxazolines. This reaction tolerates various functional groups and generates water as the only byproduct. This method affords oxazoline with inversion of α-hydroxyl stereochemistry, suggesting that alcohol is activated as a leaving group under these conditions. Furthermore, the one-pot synthesis protocol of 2-oxazolines directly from carboxylic acids and amino alcohols is also provided.