One hundred years of benzotropone chemistry

One-Pot Synthesis of Functionalized Benzotropones via a Phthalide Ring-Opening/Intramolecular Aldol Condensation Cascade

A one-pot protocol was developed for the synthesis of functionalized benzotropone derivatives via a nucleophilic phthalide ring opening by a 5-lithiated dioxinone derivative, followed by an intramolecular aldol condensation. The method demonstrates exceptional versatility with diverse substrates, yielding a variety of functionalized benzotropones. Subsequent transformations of the obtained benzotropone derivatives were explored for their potential applications.

Synthesis of 2,3-Benzotropones via Palladium(II)-Catalyzed Aerobic Dehydrogenation from 1-Benzosuberones and Sequential Diels-Alder Reaction to Yield Benzobicyclo[3.2.2]nonenones

An approach to 2,3-benzotropone from 1-benzosuberone via palladium(II)-catalyzed aerobic dehydrogenation was developed. This method first provided a catalytic route to various 2,3-benzotropones from their corresponding 1-benzosuberones in good yields. In addition, the reaction could be applied to a one-pot Diels-Alder reaction with maleimide, providing a complex benzobicyclo[3.2.2]nonenone in ≤90% yield. Kinetic analysis supporting our proposed mechanism was also performed, underscoring the robustness of the developed synthetic pathway.

Convenient synthesis of thiolated 2,7-disubstituted tropones via double C-N bond cleavage of tropinone derivatives

A range of 2,4-dialkylidenetropinone-derived quaternary ammonium salts smoothly reacted with thiols in the presence of tributylamine, delivering structurally diverse thiolated 2,7-disubstituted tropones in moderate to excellent yields with high site selectivity. The reaction employs readily available feedstocks and reagents, is free of transition metals, tolerates various functional groups, and can be easily scaled up.

Access to 2-Alkyltropones via Organic Base-Catalyzed Tandem Deamination and Aldol Condensation of Tropinone-Derived Quaternary Ammonium Salts

The tropone skeleton exists in a number of natural products and bioactive substances, and currently, the applications of substituted tropones are significantly limited by their preparative methods. Herein, we report a very convenient method to access 2-alkyltropones via organic base-catalyzed tandem reaction of tropinone-derived quaternary ammonium salts. Tropinone methiodide reacted with a wide variety of aromatic and aliphatic aldehydes in the presence of 1,4-diazabicyclo[2.2.2]octane to afford structurally diverse 2-alkyltropones in moderate to excellent yields with extremely high site selectivity. The reaction employs readily available feedstocks and reagents, is free of transition metals and compatible with water and air, tolerates a variety of functional groups, and can be easily scaled up. Moreover, the products are amenable to various synthetic transformations. Preliminary mechanistic studies revealed that the reaction proceeded via tandem deamination, aldol condensation, and isomerization.

Construction of Unique Spiro[dibenzo[a,f]azulene-6,2'-indenes] via Unprecedented Annulation of ortho-C-H Bond of Benzylidene Group

The domino reaction of alkyl and aryl isocyanides with two molecules of 2-arylidene-1,3-indanediones in acetonitrile at 80 °C resulted in unique functionalized spiro[dibenzo[a,f]azulene-6,2'-indenes] in good yields, in which the two 2-arylidene-1,3-indanediones acted as different building blocks to construct the polycyclic system. More importantly, the unprecedented anticipation of the ortho-position of benzylidene group to form a novel dibenzo[a,f]azulene ring through a formal [5 + 2] cycloaddition process was first observed. On the other hand, DABCO-promoted reaction of the isocyanides with two molecules of 2-arylidene-1,3-indanediones in acetonitrile at 80 °C afforded functionalized spiro[cyclopenta[a]-indene-2,2'-indene] derivatives.

Direct Two Carbon Ring Expansion of 1-Indanones with Ynones: An Eco-Friendly, One-Flask Approach to Functionally Enriched 5H-Benzo[7]annulenes

Direct 2C-ring expansion of 1-indanones with ynones to 5H-benzo[7]annulenes has been observed, and its generality has been gauged (19 examples). Overall, this simple and convenient cascade process to 5H-benzo[7]annulenes involves engagement of 1-indanone with two ynone moieties with formation of three new C-C σ-bonds, cleavage of C-C σ-bond, and concurrent functionality amplification. The resulting seven-membered ring, laced with an opportunistic disposition of four proximal functional groups, offers avenues for their further productive interplay. Our new approach embraces many green and eco-friendly features.

Lactam-fused tropolones: a new tunable, environmentally sensitive fluorophore class

Fluorescent small-molecules capable of altering their profiles in response to environmental changes are exceptionally valuable tool compounds throughout the scientific community. The following manuscriipt describes a new class of fluorescent small molecules based on lactam-fused tropolones that are responsive to a dynamic range of environmental changes. These molecules can be easily obtained through a rapid annulation procedure between appropriately functionalized tropolones and primary amines, which is often complete within minutes at room temperature. Molecules generated through this approach have been identified with fluoresence emission across the visible light spectra, and can be tuned based on either the tropolone or amine component. They are also highly responsive to changes in solvent, pH, and certain divalent metal ions. Tropolone-fused lactams thus represent a new class of tunable fluorescent small molecules that could find value throughout the scientific community.

Carbocycloaddition Strategies for Troponoid Synthesis

Tropone is the prototypical aromatic 7-membered ring, and can be found in virtually any undergraduate textbook as a key example of non-benzenoid aromaticity. Aside from this important historical role, tropone is also of high interest as a uniquely reactive synthon in complex chemical synthesis as well as a valuable chemotype in drug design. More recently, there has been growing interest in the utility of tropones for catalysis and material science. Thus, synthetic strategies capable of synthesizing functional tropones are key to fully exploiting the potential of this aromatic ring system. Cycloaddition reactions are particularly powerful methods for constructing carbocycles, and these strategies in turn have proven to be powerful for generating troponoids. The following review article provides an overview of strategies for troponoids wherein the 7-membered carbocycle is generated through a cycloaddition reaction. Representative examples of each strategy are also provided.

Dearomative Ring Expansion of Polycyclic Arenes

Benzocycloheptenes constitute a common structural motif embedded in many natural products and biologically active compounds. Herein, we report their concise preparation from non‐activated polycyclic arenes using a two‐step sequence involving dearomative [4+2]‐cycloaddition with arenophile in combination with palladium‐catalyzed cyclopropanation, followed by cycloreversion‐initiated ring expansion. The described strategy provides a working alternative to the Buchner reaction, which is limited to monocyclic arenes. Overall, this methylene‐insertion molecular editing approach enables rapid and direct conversion of simple (hetero)arenes into a range of substituted (aza)benzocycloheptatrienes, which can undergo a myriad of downstream functionalizations.

Migratory cycloisomerization of 1,3-dien-5-ynes conjugated with pseudopeptides in assembly of benzo[7]annulenes

A novel domino cycloisomerization of 1,3-dien-5-ynes for the synthesis of 7H-benzo[7]annulenes is reported. The noticeable feature of this domino reaction involves the assembly of the fused bicyclic motifs through a transamidation/5-exo-trig cyclization/8π-electrocyclization sequence in a single step. Finally, mechanistic investigations were conducted experimentally and supported by DFT calculations.

On-Surface Thermal Stability of a Graphenic Structure Incorporating a Tropone Moiety

On-surface synthesis, complementary to wet chemistry, has been demonstrated to be a valid approach for the synthesis of tailored graphenic nanostructures with atomic precision. Among the different existing strategies used to tune the optoelectronic and magnetic properties of these nanostructures, the introduction of non-hexagonal rings inducing out-of-plane distortions is a promising pathway that has been scarcely explored on surfaces. Here, we demonstrate that non-hexagonal rings, in the form of tropone (cycloheptatrienone) moieties, are thermally transformed into phenyl or cyclopentadienone moieties upon an unprecedented surface-mediated retro–Buchner-type reaction involving a decarbonylation or an intramolecular rearrangement of the CO unit, respectively.

Complex Natural Products Derived from Pyrogallols

Pyrogallols (1,2,3-trihydroxybenzenes) are abundant in Nature, easily oxidized, and are central precursors to important natural products. The rich chemistry of their oxidized derivatives, the hydroxy-o-quinones, has been studied for over a century and still attracts the interest of the scientific community. Only in the last ten years have critical insights of pyrogallol chemistry from the mid-twentieth century been applied to modern natural product synthesis. Historical studies of pyrogallol chemistry, including [5+2], [4+2], and formal [5+5] cycloadditions are discussed here and reactivity guidelines established. The application and remarkable selectivity of these cycloadditions is then showcased in the recent syntheses of several fungal natural products, including dibefurin, epicolactone, the merocytochalasans, and preuisolactone A. The authors hope that this contribution will spark further interest in the fascinating chemistry of pyrogallols and natural products derived from them.

Construction of Structurally Rigid Azulen-6-ones via Migratory Rearrangement of Spirocycles and Their Photophysical Studies

We report the synthesis of polysubstituted 1,1,2,3-tetraarylazulen-6-ones through the ring opening and scaffold rearrangement of spirocycles in the presence of ferric chloride under ambient conditions. Control experiments shown that this reaction may proceeded through a radical cation mechanism and futher theoretical calculation revealed that the electronic distribution of the radical cation intermediate dominated this oxidative rearrangement instead of dehydrogenation. It is noteworthy that some interesting photophysical properties, including aggregation-induced emission, halochromism, and two-photon fluorescence, were discovered for the azulen-6-one 2a, making them to work as promising functional materials in optical-related fields.

Synthesis and Characterization of 1-Hydroxy-4,5-arene-Fused Tropylium Derivatives

The properties of 1-hydroxy-4,5-arene-fused tropyliums were assessed based on experimental and theoretical investigations. An X-ray crystallographic analysis revealed a decrease of bond alternation in the seven-membered ring of 1-hydroxy-4,5-benzotropylium derivatives compared with that of the parent 4,5-benzotropones, which is indicative of an increase in aromaticity upon protonation. NICS and AICD calculations also supported the increased aromaticity of 1-hydroxy-4,5-arene-fused tropylium. The pK_a values for a series of 1-hydroxy-4,5-arene-fused tropylium derivatives were also determined.

Atom-Economic Synthesis of Highly Functionalized Bridged Ring Systems Initiated by Ring Expansion of Indene-1,3-dione

An atom economic procedure for the regioselective synthesis of bridged seven-membered-ring compounds from simple reactants such as ynones and indene-1,3-dione has been developed. This process was realized through the one-pot reactions of ring-expansion of indene-1,3-dione with alkynyl ketones and successive formal [4+2] cycloaddition. The Michael addition reaction is the key for the regioselectivity of the formal [4+2] cycloaddition.

Synthesis of 4,5-Benzotropone π Complexes of Iron, Rhodium, and Iridium and Their Potential Use in Catalytic Borrowing-Hydrogen Reactions

The syntheses of rhodium, iridium, and iron π complexes bearing 4,5-benzotropone ligands are reported. X-ray crystallographic analyses revealed that a tropone core coordinates to a metal center in a η⁴ manner with a tub-form geometry. Some of the benzotropone π complexes exhibited catalytic activity for N-alkylation of aniline by borrowing hydrogen.

Tropolone natural products

This review on natural products containing a tropolonoid motif highlights analytical methods applied for structural identification and biosynthetic pathway analysis, the ecological context and the pharmacological potential of this compound class.