Natural and synthetic chromenes, fused chromenes, and versatility of dihydrobenzo[h]chromenes in organic synthesis

Phenolate-induced intramolecular ring-opening cyclization of N-tosylaziridines: access to functionalized benzoxacycles

Phenolate-induced, diastereo- and regioselective intramolecular exo-tet ring-opening cyclization of N-tosylaziridines has been achieved for the first time.

Photochemical synthesis of benzo[f]chromene

The one-pot photocatalytic synthesis of benzo[f]chromene was performed at room temperature by the addition of benzyl alcohol, malononitrile and β-naphthol in contact with an illuminated nano-SnO₂/TiO₂ composite. This study illustrated the potential of photochemical synthesis of benzo[f]chromene. A simple procedure was proposed for the synthesis of a nano-SnO₂/TiO₂ composite using the sol-gel method by hydrolysis of tin(iv) chloride and titanium(iv) chloride at room temperature in the presence of polyethylene glycol 200, diethanolamine, and distilled water. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and FT-IR were employed to characterize the proposed photocatalyst.

A novel dihydrocoumarin under experimental and theoretical characterization

Coumarins are natural and synthetic active ingredients widely applied in diverse types of medicinal treatments, such as cancer, inflammation, infection, and enzyme inhibition (monoamine oxidase B). Dihydrocoumarin compounds are of great interest in organic chemistry due to their structural versatilities and, as part of our investigations concerning the structural characterization of small molecules, this work focuses on crystal structure and spectroscopic characterization of the synthesized and crystallized compound 4-(4-methoxyphenyl)-3,4-dihydro-chromen-2-one (C₁₆H₁₄O₃). Additionally, a theoretical calculation was performed using density functional theory to analyze the sites where nucleophilic or electrophilic attack took place and to examine the molecular electrostatic potential surface. Throughout all of these calculations, both density functional theory and Car-Parrinello molecular dynamics were performed by fully optimized geometry. The spectroscopic analysis indicated the presence of aromatic carbons and hydrogen atoms, and also the carbonyl and methoxy groups that were confirmed by the crystallographic structure. The C₁₆H₁₄O₃ compound has a non-classical intermolecular interaction of type C-H⋅⋅⋅O that drives the molecular arrangement and the crystal packing. Moreover, the main absorbent groups were characterized throughout calculated harmonic vibrational frequencies. Also, natural bond orbital analysis successfully locates the molecular orbital with π-bonding symmetry and the molecular orbital with π* antibonding symmetry. Finally, the gap between highest occupied and lowest unoccupied molecular orbitals implies in a high kinetic stability and low chemical reactivity of title molecule.

Diversifying Complexity by Domino Benzannulation of Polycyclic Natural Products

Herein we describe a salicylaldehyde-annulation reaction as a plug and play toolkit to diversify the complexity of naturally occurring ketones. The protocol entails the transformation of the polycyclic natural ketone into its propargyl vinyl ether derivative (two synthetic steps) and its microwave-assisted imidazole-catalyzed domino rearrangement to generate the salicylaldehyde ring. This annexed unit allows further synthetic transformations: e.g., the installation of a pharmacophore module to generate natural product-pharmacophore hybrids endowed with unknown biological (pharmaceutical) annotations.

Fused Multifunctionalized Chromenes from Tetraynes and α,β-Unsaturated Aldehydes

The synthesis of fused, highly substituted chromenes in a one-pot-three-step cascade hexadehydro-Diels-Alder reaction of tetraynes followed by an intermolecular α,β-unsaturated aldehyde rearrangement is reported. The target compounds were prepared by the condensation of malonates with 3-bromo-1-propyne, and the resulting 2,2-di(1-propyn-3-yl)malonates underwent cross-coupling with phenylethynyl bromides to afford 2,2-di(5-phenyl-2,4-pentadiynyl)malonates, which underwent intramolecular cyclization to produce tricyclic chromenes. The overall transformation involves the formation of four new C-C bonds and one new C-aryl-O-C-aryl bond by both intramolecular cyclization and intermolecular rearrangement reactions. These constitute a key strategy for the construction of a highly substituted natural chromene core and a robust method for producing other chromenes.

Discovery of cyclopropyl chromane-derived pyridopyrazine-1,6-dione γ-secretase modulators with robust central efficacy

Herein we describe the discovery of a novel series of cyclopropyl chromane-derived pyridopyrazine-1,6-dione γ-secretase modulators for the treatment of Alzheimer's disease (AD). Using ligand-based design tactics such as conformational analysis and molecular modeling, a cyclopropyl chromane unit was identified as a suitable heterocyclic replacement for a naphthyl moiety that was present in the preliminary lead 4. The optimized lead molecule 44 achieved good central exposure resulting in robust and sustained reduction of brain amyloid-β42 (Aβ42) when dosed orally at 10 mg kg-1 in a rat time-course study. Application of the unpaced isolated heart Langendorff model enabled efficient differentiation of compounds with respect to cardiovascular safety, highlighting how minor structural changes can greatly impact the safety profile within a series of compounds.

Recent developments on ultrasound-assisted one-pot multicomponent synthesis of biologically relevant heterocycles

Heterocycles are the backbone of organic compounds. Specially, N- &O-containing heterocycles represent privileged structural subunits well distributed in naturally occurring compounds with immense biological activities. Multicomponent reactions (MCRs) are becoming valuable tool for synthesizing structurally diverse molecular entities. On the other hand, the last decade has seen a tremendous outburst in modifying chemical processes to make them sustainable for the betterment of our environment. The application of ultrasound in organic synthesis is fulfilling some of the goals of 'green and sustainable chemistry' as it has some advantages over the traditional thermal methods in terms of reaction rates, yields, purity of the products, product selectivity, etc. Therefore the synthesis of biologically relevant heterocycles using one-pot multi-component technique coupled with the application of ultrasound is one of the thrusting areas in the 21st Century among the organic chemists. The present review deals with the "up to date" developments on ultrasound assisted one-pot multi-component synthesis of biologically relevant heterocycles reported so far.

6-exo-trig Michael addition-lactonizations for catalytic enantioselective chromenone synthesis

The catalytic enantioselective 6-exo-trig Michael addition-lactonization of enone-acid substrates to form cis-chromenones with high diastereo- and enantiocontrol was developed using the commercially available isothiourea tetramisole. An acidic workup proved necessary to minimize product epimerization and maximize product er, providing cis-chromenones in excellent yield, and with excellent diastereo- and enantioselectivity.

Enantioselective Synthesis of Chiral Oxygen-Containing Heterocycles Using Copper-Catalyzed Aryl C-O Coupling Reactions via Asymmetric Desymmetrization

An enantioselective desymmetric aryl C-O coupling reaction was demonstrated under the catalysis of CuI and a chiral cyclic diamine ligand. A series of chiral oxygen-containing heterocyclic units such as 2,3-dihydrobenzofurans, chromans, and 1,4-benzodioxanes with tertiary or quaternary stereocarbon centers were synthesized with this method. DFT calculations were also carried out for a better understanding of the model for enantiocontrol.

Total Syntheses of (±)-Rhodonoids A and B and C12-epi-Rhodonoid B

Total syntheses of (±)-rhodonoids A and B and C12-epi-rhodonoid B are described here. A unified strategy employed in these syntheses is an intramolecular oxa-[3 + 3] annulation for accessing the chromene unit. A Fe(OTf)₃-promoted diastereoselective cationic [2 + 2] cycloaddition and a photochemical [2 + 2] cycloaddition were featured to construct the cyclobutane core of (±)-rhodonoids A and B and C12-epi-rhodonoid B, respectively. Fe(OTf)₃ also leads to an interesting bridged tetracycle, which was unambiguously confirmed by single crystal X-ray analysis.

Synthesis and characterization of molybdic acid immobilized on modified magnetic nanoparticles as a new and recyclable catalyst for the synthesis of chromeno[4,3-b]chromenes

Synthesis and characterization of molybdic acid immobilized on modified magnetic nanoparticles as a new catalyst is reported.

Diastereoselective synthesis of benzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles via cycloaddition reaction of benzothiazolium salts with 3-nitrochromenes

Diastereospecific synthesis of benzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles via cycloaddition reaction of benzothiazolium salts with 3-nitrochromenes.

A cascade Claisen rearrangement/o-quinone methide formation/electrocyclization approach to 2H-chromenes

A new approach to 8-substituted 2H-chromenes is developed, featuring a novel cascade aromatic Claisen rearrangement/o-quinone methide formation/6π-electrocyclization.

Sequential In-catalyzed intramolecular hydroarylation and Pd-catalyzed cross-coupling reactions using bromopropargyl aryl ethers and amines

A sequential one-pot In-catalyzed intramolecular hydroarylation and Pd-catalyzed cross-coupling reaction using bromopropargyl aryl ethers and amines has been developed.

The study of metal-free and palladium-catalysed synthesis of benzochromenes via direct C–H arylation using unactivated aryl benzyl ethers derived from essential oils as raw materials

The synthesis of 6H-benzo[c]chromenes, from phenol-rich essential oils, was studied through two approaches, establishing the Pd-approach as the most efficient protocol over the metal-free process.

Transition-Metal-Free Cross-Coupling of Indium Organometallics with Chromene and Isochroman Acetals Mediated by BF3·OEt2

A transition-metal-free coupling of triorganoindium reagents with benzopyranyl acetals mediated by a Lewis acid has been developed. The reaction of R3In with chromene and isochroman acetals in the presence of BF3·OEt2 afforded 2-substituted chromenes and 1-substituted isochromans, respectively, in good yields. The reactions proceed with a variety of triorganoindium reagents (aryl, heteroaryl, alkynyl, alkenyl, alkyl) using only 50 mol % of the organometallic, thus demonstrating the efficiency of these species. Preliminary mechanistic studies indicate the formation of an oxocarbenium ion intermediate in the presence of the Lewis acid.

An efficient one-pot synthesis of thiochromeno[3,4-d]pyrimidines derivatives: Inducing ROS dependent antibacterial and anti-biofilm activities

An efficient synthesis of thiochromeno[3,4-d]pyrimidine derivatives has been achieved successfully via a one-pot three-component reaction of thiochrome-4-one, aromatic aldehyde and thiourea in the presence of 1-butyl-3-methyl imidazolium hydrogen sulphate [Bmim]HSO4. This new protocol has the advantages of environmental friendliness, high yields, short reaction times, and convenient operation. Furthermore, among all the tested derivatives, compounds 4b and 4c exhibited promising antibacterial, minimum bactericidal concentration and anti-biofilm activities against Staphylococcus aureus MTCC 96, Staphylococcus aureus MLS16 MTCC 2940 and Bacillus subtilis MTCC 121. The compound 4c also showed promising intracellular ROS accumulation in Staphylococcus aureus MLS16 MTCC 2940 comparable to that of ciprofloxacin resulting in apoptotic cell death of the bacterium.

Biological importance of structurally diversified chromenes

Compounds incorporating the chromene scaffold are largely present in natural products and display a wide variety of biological activities. Their low toxicity combined to the broad pharmacological properties have inspired medicinal chemists in the search for new therapeutic agents. This review covers the literature between 1993 and on the biological activity of 2H- and 4H-chromenes, both from natural and synthetic origin. Includes a section that identifies a selection of chromene-based natural products, followed by recent literature on bioactive natural chromenes and the corresponding source, covering plants and fruits. Synthetic chromenes are equally important and a separate section addresses the use of these derivatives as new leads for drug discovery. Different biological targets were identified, namely those associated with anticancer, antimicrobial, anti-inflammatory, antithrombotic and antipsychotic activities.

Aniline-Promoted Cyclization-Replacement Cascade Reactions of 2-Hydroxycinnamaldehydes with Various Carbonic Nucleophiles through In Situ Formed N,O-Acetals

In this study, we report the harnessing of new reactivity of N,O-acetals in an aminocatalytic fashion for organic synthesis. Unlike widely used strategies requiring the use of acids and/or elevated temperatures, direct replacement of the amine component of the N,O-acetals by carbon-centered nucleophiles for C-C bond formation is realized under mild reaction conditions. Furthermore, without necessary preformation of the N,O-acetals, an amine-catalyzed in situ formation of N,O-acetals is developed. Coupling both reactions into a one-pot operation enables the achievement of a catalytic process. We demonstrate the employment of simple anilines as promoters for the cyclization-substitution cascade reactions of trans-2-hydroxycinnamaldehydes with various carbonic nucleophiles including indoles, pyrroles, naphthols, phenols, and silyl enol ethers. The process offers an alternative approach to structurally diverse, "privileged" 2-substituted 2H-chromenes. The synthetic power of the new process is furthermore shown by its application in a 2-step synthesis of the natural product candenatenin E and for the facile installation of 2-substituted 2H-chromene moieties into biologically active indoles.