Biomimetic cycloaddition approach to tropolone natural products via a tropolone ortho-quinone methide

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.

Harnessing ortho-Quinone Methides in Natural Product Biosynthesis and Biocatalysis

The implementation of ortho-quinone methide (o-QM) intermediates in complex molecule assembly represents a remarkably efficient strategy designed by Nature and utilized by synthetic chemists. o-QMs have been taken advantage of in biomimetic syntheses for decades, yet relatively few examples of o-QM-generating enzymes in natural product biosynthetic pathways have been reported. The biosynthetic enzymes that have been discovered thus far exhibit tremendous potential for biocatalytic applications, enabling the selective production of desirable compounds that are otherwise intractable or inherently difficult to achieve by traditional synthetic methods. Characterization of this biosynthetic machinery has the potential to shine a light on new enzymes capable of similar chemistry on diverse substrates, thus expanding our knowledge of Nature's catalytic repertoire. The presently known o-QM-generating enzymes include flavin-dependent oxidases, hetero-Diels-Alderases, S-adenosyl-l-methionine-dependent pericyclases, and α-ketoglutarate-dependent nonheme iron enzymes. In this review, we discuss their diverse enzymatic mechanisms and potential as biocatalysts in constructing natural product molecules such as cannabinoids.

Construction of bridged polycycles through dearomatization strategies

Bridged polycycles are privileged molecular skeletons with wide occurrence in bioactive natural products and pharmaceuticals. Therefore, they have been the pursing target molecules of numerous chemists. The rapid and convenient generation of sp3-rich complex three-dimensional molecular skeletons from simple and easily available aromatics has made dearomatization a highly valuable synthetic tool for the construction of rigid and challenging bridged rings. This review summarizes the-state-of-the-art advances of dearomatization strategies in the application of bridged ring formation, discusses their advantages and limitations and the in-depth mechanism, and highlights their synthetic value in the total synthesis of natural products. We wish this review will provide an important reference for medicinal and synthetic chemists and will inspire further development in this intriguing research area.

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.

Investigations into a Stoichiometrically Equivalent Intermolecular Oxidopyrylium [5 + 2] Cycloaddition Reaction Leveraging 3-Hydroxy-4-pyrone-Based Oxidopyrylium Dimers

Oxidopyrylium [5 + 2] cycloaddition reactions are powerful strategies for constructing complex bicyclic architectures. However, intermolecular cycloadditions of oxidopyrylium ylides are limited due to competing dimerization processes; consequently, high equivalents of dipolarophiles are often used to help intercept the ylide prior to dimerization. Recent studies by our lab have revealed that oxidopyrylium dimers derived from 3-hydroxy-4-pyrones are capable of reverting back to ylides in situ and as a result can be used as clean oxidopyrylium ylide sources. The following manuscript investigates intermolecular cycloaddition reactions between 3-hydroxy-4-pyrone-derived oxidopyrylium dimers and stoichiometrically equivalent ratios of alkyne dipolarophiles under thermal conditions. With certain reactive alkynes, pure cycloadducts can be obtained following a simple evaporation of the solvent, which is a benefit of the completely atom-economical reaction conditions. However, when less reactive alkynes are used the yields suffer due to a competing dimer rearrangement. Finally, when reactive-yet-volatile alkynes are used, such as methyl propiolate, competing 2:1 ylide/alkyne cycloadducts are observed. Intriguingly, these complex cycloadducts, which can be obtained in good yields from the pure cycloadducts, form with high regio- and stereoselectivities; however, both the regio-and stereoselectivities differ remarkably based on the source of the oxidopyrylium ylide.

Total Mycosynthesis: Rational Bioconstruction and Bioengineering of Fungal Natural Products

Total biosynthesis in fungi is beginning to compete with traditional chemical total synthesis campaigns. Herein, the advantages, disadvantages and future opportunities are discussed within the scope of several recent examples.

1 Introduction

2 Synthetic Examples

2.1 2-Pyridones

2.2 Cytochalasans

2.3 Sorbicillinoids

2.4 Decalins: Solanapyrone

2.5 α-Pyrone Polyenes: Citreoviridin and Aurovertin

2.6 Anditomin and Related Meroterpenoids

2.7 Tropolone Sesquiterpenoids

3 Conclusion

Synthetic Biology Driven Biosynthesis of Unnatural Tropolone Sesquiterpenoids

Tropolone sesquiterpenoids (TS) are an intriguing family of biologically active fungal meroterpenoids that arise through a unique intermolecular hetero Diels-Alder (hDA) reaction between humulene and tropolones. Here, we report on the combinatorial biosynthesis of a series of unprecedented analogs of the TS pycnidione 1 and xenovulene A 2. In a systematic synthetic biology driven approach, we recombined genes from three TS biosynthetic gene clusters (pycnidione 1, xenovulene A 2 and eupenifeldin 3) in the fungal host Aspergillus oryzae NSAR1. Rational design of the reconstituted pathways granted control over the number of hDA reactions taking place, the chemical nature of the fused polyketide moiety (tropolono- vs. monobenzo-pyranyl) and the degree of hydroxylation. Formation of unexpected monobenzopyranyl sesquiterpenoids was investigated using isotope-feeding studies to reveal a new and highly unusual oxidative ring contraction rearrangement.

Phomeroids A and B: two novel cytotoxic meroterpenoids from the deep-sea-derived fungus Phomopsis tersa FS441

Phomeroids A (1) and B (2), two novel meroterpenoids representing two types of skeletons, together with one known homologous analogue (3) were isolated from the deep-sea-derived fungus Phomopsis tersa FS441.

Maltol- and Allomaltol-Derived Oxidopyrylium Ylides: Methyl Substitution Pattern Kinetically Influences [5 + 3] Dimerization versus [5 + 2] Cycloaddition Reactions

Oxidopyrylium ylides are useful intermediates in synthetic organic chemistry because of their capability of forming structurally complex cycloadducts. They can also self-dimerize via [5 + 3] cycloaddition, which is an oft-reported side reaction that can negatively impact [5 + 2] cycloadduct yields and efficiency. In select instances, these dimers can be synthesized and used as the source of oxidopyrylium ylide, although the generality of this process remains unclear. Thus, how the substitution pattern governs both dimerization and cycloaddition reactions is of fundamental interest to probe factors to regulate them. The following manuscript details our findings that maltol-derived oxidopyrylium ylides (i.e., with ortho methyl substitution relative to oxide) can be trapped prior to dimerization more efficiently than the regioisomeric allomaltol-derived ylide (i.e., with a para methyl substitution relative to oxide). Density functional theory studies provide evidence in support of a sterically (kinetically) controlled mechanism, whereby gauche interactions between appendages of the approaching maltol-derived ylides are privileged by higher barriers for dimerization and thus are readily intercepted by dipolarophiles via [5 + 2] cycloadditions.

Enzymatic Intermolecular Hetero-Diels-Alder Reaction in the Biosynthesis of Tropolonic Sesquiterpenes

Diels-Alder reactions are among the most powerful synthetic transformations to construct complex natural products. Despite that increasing of enzymatic intramolecular Diels-Alder reactions have been discovered, natural intermolecular Diels-Alderases are rarely described. Here, we report an intermolecular hetero-Diels-Alder reaction in the biosynthesis of tropolonic sesquiterpenes and functionally characterize EupfF as the first fungal intermolecular hetero-Diels-Alderase. We demonstrate that EupfF catalyzed the dehydration of a hydroxymethyl-containing tropolone (5) to generate a reactive tropolone o-quinone methide (6) and might further stereoselectively control the subsequent intermolecular hetero-Diels-Alder reaction with (1E,4E,8Z)-humulenol (8) to produce enantiomerically pure neosetophomone B (1). Our results reveal the biosynthetic pathway of 1 and expand the repertoire of activities of Diels-Alder cyclases.

Phomanolides C-F from a Phoma sp.: Meroterpenoids Generated via Hetero-Diels-Alder Reactions

Phomanolides C-F (1-4), four new meroterpenoids, were isolated from a Phoma sp., together with the known phomanolides A (5) and B (6); their structures were elucidated primarily by NMR experiments. The absolute configurations of 1-3 were assigned by electronic circular dichroism calculations, and that of 4 was established by X-ray diffraction analysis using Cu Kα radiation. Compounds 1-3 incorporate an unprecedented trioxa[4.4.3]propellane subunit in their skeletons. Compounds 2 and 4 were weakly cytotoxic.

A General Biomimetic Hetero-Diels-Alder Approach to the Core Skeletons of Xenovulene A and the Sterhirsutins A and B

A biomimetic, regio- and stereoselective approach to the 5,6,11-tricyclic core skeleton of xenovulene A, as well as sterhirsutins A and B, is described. The key steps are a biomimetic inverse-electron-demand hetero-Diels-Alder cycloaddition of α-humulene and a ribose-derived vinyl ketone, followed by acid-catalyzed rearrangement of the 1,3-dioxolane that neighbors the resultant cyclic enol ether.

Efficient generation of an oxidopyrylium ylide using a Pd catalyst and its [5+2] cycloadditions with several dipolarophiles

An efficient method for the generation of an oxidopyrylium ylide from 6-acetoxy-6-acetoxymethyl-2H-pyran-3(6H)-one using a Pd catalyst and [5+2] cycloadditions of the resulting ylide are described. Among substituted styrene derivatives as dipolarophiles, electron-rich styrenes showed higher yield (up to 80%). The [5+2] cycloaddition reactions can also be applied to exo-methylene cyclic compounds, and an improved method for the synthesis of polygalolide intermediate has been demonstrated.

Fluorous-Phase Approach to α-Hydroxytropolone Synthesis

α-Hydroxytropolones (αHTs) are troponoids that demonstrate inhibition against an array of therapeutically significant targets, making them potential drug leads for several human diseases. We have utilized a recently discovered one-pot three-component oxidopyrylium cycloaddition in a solid-supported synthesis of αHTs. Though the procedure is time efficient and generates assay-ready molecules, the system suffers from low yields and an inability to perform reaction modifications on resin-bound intermediates. In order to combat these issues with the solid-phase platform, we incorporated fluorous tags into our synthetic route. Through the implementation of fluorous phase chemistry, we demonstrate a substantial increase in the overall yield of αHTs, as well as an ability to execute metal-catalyzed cross coupling and amide coupling on fluorous tagged intermediates. We also show that tagged molecules can be separated from nonfluorous impurities, and vice versa, by utilizing fluorous liquid-liquid and solid-phase extractions. Hence, these proof-of-principle investigations describe the viability of a fluorous phase approach to αHT synthesis and its potential to serve as a combinatorial technique to produce structurally diverse substrates.

Traceless Solid-Phase α-Hydroxytropolone Synthesis

α-Hydroxytropolones are established inhibitors of several therapeutically relevant binuclear metalloenzymes, and thus lead drug targets for various human diseases. We have leveraged a recently-disclosed three-component oxidopyrylium cycloaddition in the first solid-phase synthesis of α-hydroxytropolones. We also showed that, while minor impurities exist after cleavage and aqueous wash, the semi-crude products display activity in HIV RT-associated RNaseH enzymatic and cell-based assays consistent with pure molecules made in solution phase. These proof-of-principle studies demonstrate the feasibility of solid-phase α-hydroxytropolone synthesis and its potential to serve as a powerful platform for α-hydroxytropolone-based drug discovery and development.

Discovery and Development of a Three-Component Oxidopyrylium [5 + 2] Cycloaddition

α-Hydroxy-γ-pyrone-based oxidopyrylium cycloaddition reactions are useful methods for accessing a highly diverse range of oxabicyclo[3.2.1]octane products. Intermolecular variants of the reaction require the formation of a methyl triflate-based pre-ylide salt that upon treatment with base in the presence of alkenes or alkynes leads to α-methoxyenone-containing bicyclic products. Herein, we describe our discovery that the use of ethanol-stabilized chloroform as solvent leads to the generation of α-ethoxyenone-containing bicyclic byproducts. This three-component process was further optimized by gently heating a mixture of a purified version of the oxidopyrylium dimer in the presence of an alcohol prior to addition of a dipolarophile. Using this convenient procedure, several new oxidopyrylium cycloaddition products can be generated in moderate yields. We also highlight the method in a tandem ring-opening/debenzylation method for the generation of α-hydroxytropolones.

Phomanolides A and B from the Fungus Phoma sp.: Meroterpenoids Derived from a Putative Tropolonic Sesquiterpene via Hetero-Diels-Alder Reactions

Phomanolides A (1) and B (2), unique meroterpenoids with new pentacyclic and tetracyclic skeletons, respectively, and phomanoxide (3), the double-epoxidation product of a putative biosynthetic precursor of 1 and 2, were isolated from the solid substrate fermentation cultures of the fungus Phoma sp., along with the known compound eupenifeldin (4). The structures of 1-3 were elucidated based on NMR spectroscopic data and electronic circular dichroism calculations and further secured by X-ray crystallography. Biogenetically, compounds 1 and 2 could be derived from a hypothetical monotropolonic sesquiterpene intermediate via hetero-Diels-Alder reactions. Compound 4 showed potent antiproliferative effects against three human glioma cell lines, with IC50 values of 0.08-0.13 μM.

Synthesis of Naturally Occurring Tropones and Tropolones

Tropones and tropolones are an important class of seven-membered non-benzenoid aromatic compounds. They can be prepared directly by oxidation of seven-membered rings. They can also be derived from cyclization or cycloaddition of appropriate precursors followed by elimination or rearrangement. This review discusses the types of naturally occurring tropones and tropolones and outlines important methods developed for the synthesis of tropone and tropolone natural products.

The biology and synthesis of α-hydroxytropolones

α-Hydroxytropolones are a subclass of the troponoid family of natural products that are of high interest due to their broad biological activity and potential as treatment options for several diseases. Despite this promise, there have been scarce synthetic chemistry-driven optimization studies on the molecules. The following review highlights key developments in the biological studies conducted on α-hydroxytropolones to date, including the few synthetic chemistry-driven optimization studies. In addition, we provide an overview of the methods currently available to access these molecules. This review is intended to serve as a resource for those interested in biological activity of α-hydroxytropolones, and inspire the development of new synthetic methods and strategies that could aid in this pursuit.

Triflic acid-mediated rearrangements of 3-methoxy-8-oxabicyclo[3.2.1]octa-3,6-dien-2-ones: synthesis of methoxytropolones and furans

Methoxytropolones are useful scaffolds for therapeutic development because of their known biological activity and established value in the synthesis of α-hydroxytropolones. Upon treatment with triflic acid, a series of 3-methoxy-8-oxabicyclo[3.2.1]octa-3,6-dien-2-ones rearrange rapidly and cleanly to form methoxytropolones. Interestingly, bicycles that are derived from dimethyl acetylenedicarboxylate (R(2) = R(3) = CO2Me) instead form furans as the major product.

Synthesis of a highly functionalized core of verrillin

An efficient stereoselective synthesis of furanoverrillin (5), a highly functionalized core of verrillin (1), is reported. The synthetic strategy is based on constructing bicyclic lactone 17 prior to the 10-membered ring macrocyclization. The effect of the C4 methyl group on the furan reactivity is also discussed.

An oxidopyrylium cyclization/ring-opening route to polysubstituted α-hydroxytropolones

α-Hydroxytropolones are a class of molecules with therapeutic potential against several human diseases. However, structure-activity relationship studies on these molecules have been limited due to a scarcity of efficient synthetic methods to access them. It is demonstrated herein that α-hydroxytropolones can be generated through a BCl(3)-mediated ring-opening/aromatization/demethylation process on 8-oxabicyclo[3.2.1]octenes. Used in conjunction with an improved method based on established oxidopyrylium dipolar cycloadditions, several polysubstituted α-hydroxytropolones can be accessed in three steps from readily available α-hydroxy-γ-pyrones.

A new and efficient method for o-quinone methide intermediate generation: application to the biomimetic synthesis of the benzopyran derived natural products (±)-lucidene and (±)-alboatrin

Lucidene and alboatrin are complex benzopyran derived natural products. A key step in their biogenesis may involve a hetero Diels-Alder cycloaddition between an o-quinone methide intermediate with a simple, or activated tri-substituted olefin. Experimental evidence is provided to support this hypothesis, with the biomimetic synthesis of both (+/-)-lucidene and (+/-)-alboatrin successfully achieved using a new and efficient method for o-quinone methide generation.