Intramolecular didehydro-Diels-Alder reaction and its impact on the structure-function properties of environmentally sensitive fluorophores

Visible Light-Promoted Regioselective Benzannulation of Vinyl Sulfoxonium Ylides with Ynoates

Herein, we report a highly regioselective [4 + 2]-annulation of vinyl sulfoxonium ylides with ynoates under light-mediated conditions. The reaction proceeds through the new dienyl sulfoxonium ylide, which undergoes photolysis under blue light irradiation to give highly substituted naphthalene scaffolds. The method presented here operates at room temperature and does not require the addition of an external photosensitizer. The in situ-generated dienyl sulfoxonium ylide absorbs light and acts as a photosensitizer for the formation of arenes. The synthetic potential of these benzannulations was further illustrated by various synthetic transformations and a scale-up reaction. Moreover, control experiments and quantum chemical calculations reveal the mechanistic details of the developed reaction.

Intramolecular Diels-Alder Reaction of Cyclopropenyl Vinylarenes: Access to Benzonorcarane Derivatives

Intramolecular Diels-Alder vinylarene reaction (IMDAV) is a [4 + 2] cycloaddition that employs styrene derivatives as conjugated dienes, whose poor reactivity arises from the required loss of aromaticity, which is recovered by a subsequent [1,3]-H shift. Herein, we describe the use of cyclopropene as a dienophile, harnessing its strain energy to drive the IMDAV reaction. Benzonorcarane scaffolds form in good yields, excellent stereoselectivity, and broad functional tolerance. Theoretical calculations and NMR studies have revealed significant mechanistic insights.

Sequential Strategies to Trigger Mild Dearomative Diels-Alder Cyclizations

Dihydronaphthalenes are present in several functional molecules, but their assembly is challenging. However, a sequential strategy can induce the key annullation of an alkyne with a vinylarene under mild conditions. Products form in good yields, with ample functional tolerance via domino nucleophilic substitution and dearomative Diels-Alder and ene reactions. DFT modeling data show that alkali cations are crucial to ensure a smooth dearomative cyclization on the in situ generated intermediates.

Intramolecular Diels-Alder Reactions of α-Bromostyrene-Functionalized Unsaturated Carboxamides

Intramolecular cycloaddition reactions of α-bromostyrene-functionalized amides of monomethyl fumarate were investigated. The reaction of the amides with Et₃N in toluene at 110 °C gave 1,4-dihydronaphthalenes. The 1,4-dihydronaphthalenes may be produced via the intramolecular Diels-Alder reaction, proton transfer, and dehydrobromination by a base, along with C═C bond isomerization by proton transfer. The reaction of amide derivatives with halogen on a benzene ring and alkali metal carbonates in toluene at 110 °C gave naphthalene derivatives directly. Dehydrogenation of various 1,4-dihydronaphthalenes with cesium or rubidium carbonate in toluene at 110 °C gave naphthalene derivatives. The retardation by TEMPO, acceleration by air for some substrates, and density functional theory calculations suggest a radical mechanism caused by intervention of molecular oxygen.

A Removable Acyl Group Promoted the Intramolecular Dehydro-Diels-Alder Reaction of Styrene-Ynes: Highly Chemoselective Synthesis of Aryldihydronaphthalene Derivatives

A removable acyl group promoted the intramolecular didehydro-Diels-Alder reaction of styrene-ynes under mild reaction conditions is proposed. The reaction is free of metals and catalysts, is easy to perform, and exhibits good functional group tolerance, providing a highly chemoselective approach for obtaining the valuable aryldihydronaphthalene derivatives.

Facile synthesis of diverse hetero polyaromatic hydrocarbons (PAHs) via the styryl Diels–Alder reaction of conjugated diynes

The intramolecular styryl Diels–Alder reaction with conjugated diynes under thermally stable triazole-gold (TA–Au) catalytic conditions and the sequential transformation through alkyne activation to access various PAHs with high efficiency was reported for the first time.

Ambient Synthesis of Tricyclic Naphthalenes via Stepwise Styryl-yne Dearomative Diels-Alder Cyclization

A cascade of styrylynols promoted by MnO₂ allows the synthesis of fused tricycles with a naphthalene core. The reaction occurs under ambient conditions, offering a practical synthetic tool because of the inexpensive and abundant manganese species. The method affords products through the sequential oxidation of a propargyl alcohol, stepwise Diels-Alder cyclization, and finally rearomatization. According to density functional theory, the usually unfavorable stepwise Diels-Alder mechanism is instead a general tool for eliciting otherwise challenging dearomative annulation.

Practical access to fluorescent 2,3-naphthalimide derivatives via didehydro-Diels-Alder reaction

A practical and efficient approach for the synthesis of fluorescent 2,3-naphthalimide derivatives has been developed from readily available starting materials via an intramolecular didehydro-Diels-Alder reaction, which proceeded well under room temperature, exhibiting a wide substrate scope and good functional group tolerance. The practicability of this methodology has been verified by one-step synthesis of the environmentally sensitive fluorophore 6-DMN on a gram scale with a shorter time, fewer steps and less waste disposal, and without the utilization of toxic transition metals. The present experimental and computational studies support the crucial role of the propiolimide moiety in the transformation.

An Alternative Molecular View of Evolution: How DNA was Altered over Geological Time

Four natural phenomena are cited for their defiance of conventional neo-Darwinian analysis: human intelligence; cat domesticity; the Cambrian explosion; and convergent evolution. 1. Humans are now far more intelligent than needed in their hunting–gathering days >10,000 years ago. 2. Domestic cats evolved from wildcats via major genetic and physical changes, all occurring in less than 12,000 years. 3. The Cambrian explosion refers to the remarkable expansion of species that mystifies evolutionists, as there is a total lack of fossil evidence for precursors of this abundant new life. 4. Convergent evolution often involves formation of complex, multigene traits in two or more species that have no common ancestor. These four evolutionary riddles are discussed in terms of a proposed “preassembly” mechanism in which genes and gene precursors are collected silently and randomly over extensive time periods within huge non-coding sections of DNA. This is followed by epigenetic release of the genes, when the environment so allows, and by natural selection. In neo-Darwinism, macroevolution of complex traits involves multiple mutation/selections, with each of the resulting intermediates being more favorable to the species than the previous one. Preassembly, in contrast, invokes natural selection only after a partially or fully formed trait is already in place. Preassembly does not supplant neo-Darwinism but, instead, supplements neo-Darwinism in those important instances where the classical theory is wanting.

Fluorescence Quenching of Carbonyl-Twisted 5Acyl-1-dimethylaminonaphthalenes by Alcohols

Derivatives of 1-dimethylamino-5-propionylnaphthalene that constrain the carbonyl group into a five-, six-, and seven-membered ring were prepared, and their fluorescence quenching in protic solvents was studied. Evidence for enhanced quenching due to carbonyl twisting out of the molecular plane is presented, but this effect is heavily masked by the strong quenching by all of the derivatives and by the ring size-dependent deactivation seen in polar, aprotic solvents. Calculations show strong, ring size-dependent vibrational coupling between the carbonyl group and the naphthalene ring in the first excited state.

Synthesis of Multisubstituted 1-Naphthoic Acids via Ru-Catalyzed C-H Activation and Double-Alkyne Annulation under Air

An efficient [2 + 2 + 2] benzannulation of phthalic acids/anhydrides with two alkynes was developed for synthesis of multisubstituted 1-naphthoic acids via Ru-catalyzed C-H activation. The reaction preceded well using atmospheric oxygen as the sole oxidant with high atom/step economies. Facilitated by the free carboxyl group, the products can be easily converted to diverse polycyclic molecules.

1,5-Prodan Emits from a Planar Intramolecular Charge-Transfer Excited State

1-Propionyl-5-dimethylaminonaphthalene (8, 1,5-Prodan) and two derivatives where the amino group is constrained in a seven-membered (9) and five-membered (10) ring are prepared. All three exhibit strong fluorescence and similar degrees of solvatochromism. Their fluorescence is strongly quenched in alcohol solvents. Because the amino group in 9 and especially 10 is forced to be coplanar with the naphthalene ring, the similar photophysical behavior of all three suggests that emission arises from a planar excited state (planar intramolecular charge transfer).

The Cation-π Interaction Enables a Halo-Tag Fluorogenic Probe for Fast No-Wash Live Cell Imaging and Gel-Free Protein Quantification

The design of fluorogenic probes for a Halo tag is highly desirable but challenging. Previous work achieved this goal by controlling the chemical switch of spirolactones upon the covalent conjugation between the Halo tag and probes or by incorporating a “channel dye” into the substrate binding tunnel of the Halo tag. In this work, we have developed a novel class of Halo-tag fluorogenic probes that are derived from solvatochromic fluorophores. The optimal probe, harboring a benzothiadiazole scaffold, exhibits a 1000-fold fluorescence enhancement upon reaction with the Halo tag. Structural, computational, and biochemical studies reveal that the benzene ring of a tryptophan residue engages in a cation−π interaction with the dimethylamino electron-donating group of the benzothiadiazole fluorophore in its excited state. We further demonstrate using noncanonical fluorinated tryptophan that the cation−π interaction directly contributes to the fluorogenicity of the benzothiadiazole fluorophore. Mechanistically, this interaction could contribute to the fluorogenicity by promoting the excited-state charge separation and inhibiting the twisting motion of the dimethylamino group, both leading to an enhanced fluorogenicity. Finally, we demonstrate the utility of the probe in no-wash direct imaging of Halo-tagged proteins in live cells. In addition, the fluorogenic nature of the probe enables a gel-free quantification of fusion proteins expressed in mammalian cells, an application that was not possible with previously nonfluorogenic Halo-tag probes. The unique mechanism revealed by this work suggests that incorporation of an excited-state cation−π interaction could be a feasible strategy for enhancing the optical performance of fluorophores and fluorogenic sensors.

Modern Synthetic Avenues for the Preparation of Functional Fluorophores

Biomedical research relies on the fast and accurate profiling of specific biomolecules and cells in a non‐invasive manner. Functional fluorophores are powerful tools for such studies. As these sophisticated structures are often difficult to access through conventional synthetic strategies, new chemical processes have been developed in the past few years. In this Minireview, we describe the most recent advances in the design, preparation, and fine‐tuning of fluorophores by means of multicomponent reactions, C−H activation processes, cycloadditions, and biomolecule‐based chemical transformations.

Isocyanide based [4+1] cycloaddition reactions: an indispensable tool in multi-component reactions (MCRs)

The advent of cycloaddition reactions in the synthesis of heterocycles and their ever burgeoning applications in the fields of materials chemistry, catalysis and drugs have been a profound scientific development.