Competing Domino Knoevenagel-Cyclization Sequences with N-Arylcinnamylamines

Domino Knoevenagel-cyclization reactions of N-arylcinnamylamines were carried out with active methylene reagents, which took place with five competing cyclization mechanisms: intramolecular hetero Diels-Alder reaction, stepwise polar [2 + 2] cycloaddition, styryl or aza-Diels-Alder reactions followed by rearomatization, and [1,5]-hydride shift-6-endo cyclization. In the stepwise aza-Diels-Alder reaction, the N-vinylpyridinium moiety acted as an azadiene, producing a condensed heterocycle with tetrahydroquinolizinium and tetrahydroquiniline subunits. Antiproliferative activity with low micromolar IC50 values was identified for some of the novel scaffolds.

Discovery and bioinspired total syntheses of unprecedented sesquiterpenoid dimers unveiled bifurcating [4 + 2] cycloaddition and target differentiation of enantiomers

[4 + 2] cycloaddition has led to diverse polycyclic chiral architectures, serving as novel sources for organic synthesis and biological exploration. Here, an unprecedented class of cadinane sesquiterpene [4 + 2] dimers, henryinins A-E (1-5), with a unique 6/6/6/6/6-fused pentacyclic system, were isolated from Schisandra henryi. The divergent total syntheses of compounds 1-5 and their enantiomers (6-10) were concisely accomplished in eight linear steps using a protection-free approach. Mechanistic studies illustrated the origin of selectivity in the key [4 + 2] cycloaddition as well as the inhibition of reaction pathway bifurcation via desymmetrization. The chemical proteomics results showed that a pair of enantiomers shared common targets (PRDX5 C100 and BLMH C73) and had unique targets (USP45 C588 for 4 and COG7 C419 for 9). This work provides experimental evidence for the discovery of unprecedented cadinane dimers from selective Diels-Alder reaction and a powerful strategy to explore the biological properties of natural products.

Enantioselective Desymmetrization Triggered by Iminium-Enamine Activation: Access to Complex Cyclohepta[b]indoles

The expeditious construction of complex molecules having multiple stereocentres is highly desirable in organic chemistry. In the present communication, we report the development of an organocatalytic asymmetric desymmetrization of prochiral enal-tethered cyclohexadienones via the C3-selective Friedel-Crafts alkylation of indoles triggered by LUMO-lowering iminium activation/HOMO-raising enamine activation. The reaction provides access to bicyclic enones, which further undergo acid-mediated intramolecular annulation from C2-position to afford highly strained cyclohepta[b]indoles with five contiguous stereocentres and three new C-C bonds in excellent enantioselectivity and diastereoselectivity.

Asymmetric Organocatalyzed Cascade Reactions─Merging the pseudo-Halogen and Halogen Effect with Dienamine Catalysis

The combination of asymmetric organocatalysis with the (pseudo)-halogen effect enables the formation of chiral norcarane scaffolds in high yields and selectivities (up to 92% yield, >99% ee, and >95:5 d.r.). This was achieved by reacting (pseudo)-halogenated 3-vinyl chromones with in situ generated chiral dienamines in an inverse electron demand [4 + 2] cycloaddition followed by an intramolecular S_N2 reaction. These scaffolds could easily undergo photoinduced rearrangements or lactonization to form intricate chiral ring structures.

Phosphine-Catalyzed Intramolecular Vinylogous Aldol Reaction of α-Substituted Enones

We demonstrate the first phosphine-catalyzed intramolecular vinylogous aldol reaction (IVAR) of α-substituted enones. This strategy provides access to various pentannulated (hetero)arenes and dibenzocycloheptanones incorporated with two contiguous stereocenters, one of which is an all-carbon quaternary center. The scope of this work is further broadened through elaborations of the IVAR adducts to (i) benzannulated nine-membered carbocyclic systems, (ii) interesting classes of 1,3-dienes, 1,3,5-trienes, and 1-yn-3,5-dienes, and (iii) the analogs of echinolactone D and russujaponol F.

Catalytic Asymmetric Inverse-Electron-Demand Diels-Alder Reactions of 2-Pyrones with Indenes: Total Syntheses of Cephanolides A and B

An inverse-electron-demand Diels-Alder (IEDDA) reaction could complement the conventional normal-electron-demand Diels-Alder reaction in the synthesis of six-membered carbocycles. However, catalytic asymmetric all-carbon-based IEDDA reactions are underdeveloped. Herein, we disclosed a copper-catalyzed asymmetric IEDDA reaction using electron-deficient 3-carboalkoxyl-2-pyrones and electronically unbiased indenes as reactants. This method enables the rapid and enantioselective construction of a wide range of hexahydrofluorenyl bridged-lactone scaffolds. Using this method, asymmetric total syntheses of cephanolides A and B were accomplished.

Asymmetric Michael reaction of 3-homoacyl coumarins with chromone-fused dienes toward enantioenriched coumarin chromone skeletons

Asymmetric Michael reaction of 3-homoacyl coumarins and chromone-fused dienes was developed by employing a chiral squaramide, and a series of coumarin chromone skeletons were furnished in moderate to high yields (up to 99%) and stereoselectivities (up to 98 : 2 dr, 99% ee).

Asymmetric construction of six vicinal stereogenic centers on hexahydroxanthones via organocatalytic one-pot reactions

Inspired by the chemistry and biology of hexahydroxanthones, herein we report an organocatalytic Michael-Michael-Aldol-decarboxylation reaction that provides efficient access to biologically interesting fully substituted hexahydroxanthones bearing six contiguous stereogenic centers from readily accessible materials in acceptable yields (up to 63%) and excellent stereoselectivities (up to 10 : 1 dr and >99% ee). In other words, the reaction efficiently produces three chemical bonds and up to six vicinal stereogenic centers in a one-pot operation. In particular, to our knowledge, this is an asymmetric organocatalytic strategy enabling the first construction of six vicinal stereogenic centers on non-spirocyclic hexahydroxanthone frameworks.

Recent advances of chromone-based reactants in the catalytic asymmetric domino annulation reaction

Chiral polycyclic chromanones are important heterocyclic frameworks that constitute the core structures of many natural products and bioactive molecules.

Enantioselective Synthesis of cis-Decalin Derivatives by the Inverse-Electron-Demand Diels-Alder Reaction of 2-Pyrones

A novel strategy for the synthesis of cis-decalins by an ytterbium-catalyzed asymmetric inverse-electron-demand Diels-Alder reaction of 2-pyrones and silyl cyclohexadienol ethers is reported here. A broad range of synthetically important cis-decalin derivatives with multiple contiguous stereogenic centers and functionalities are obtained in good yields and stereoselectivities. A full set of diastereomeric substituted cis-decalin motifs are readily accessible by tuning the absolute configurations of substituted silyl cyclohexadienol ethers (R or S) as well as the ligands (R or S). The synthetic potential is showcased by the enantioselective total synthesis of 4-amorphen-11-ol, and further demonstrated by the first total synthesis of cis-crotonin.

NHC-Catalyzed Cascade Reaction between β-Methyl Enals and Dienones for Quick Construction of Complex Multicyclic Lactones

A NHC-promoted cascade reaction between β-methyl enal and dienone is developed for quick access to multicyclic lactone molecules bearing quaternary chiral carbon centers. Our study constitutes the first 1,6-addition of the acylazolium vinyl enolate γ-carbon via NHC catalysis and provides rapid access to complex lactone molecules that are otherwise difficult to prepare. The structurally sophisticated lactone products bearing up to four fused ring structures are afforded in up to quantitative yields with good to excellent enantioselectivities.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Asymmetric pyrone Diels-Alder reactions enabled by dienamine catalysis

Despite the proven value in utilizing pyrone dienes to create molecular complexity via Diels-Alder reactions with varied dienophiles, few examples of effective catalytic, asymmetric variants of this process have been developed. Herein, we show that the use of Jørgensen-Hayashi-type catalysts can convert an array of α,β-unsaturated aldehydes into chiral dienamines that can formally add in a Diels-Alder fashion to a number of electron-deficient pyrones of the coumalate-type to generate optically active [2.2.2]-bicyclic lactones. In most cases, the reactions proceed with good to excellent diastereo- and enantiocontrol (up to 99% ee). Models to explain that stereoselectivity, as well as several additional transformations of the resultant products, are also presented.

A New Macrodiolide and Two New Polycyclic Chromones from the Fungus Penicillium sp. SCSIO041218

A new macrodiolide, mangrovlide A (1) and two new polycyclic chromones, penixanthones C (2) and D (3), as well as four other known compounds (4-7), have been isolated from the mangrove sediment derived fungus Penicillium sp. SCSIO041218, cultured in the 0.25% NaCl rice substrate. The structures of the new compounds were determined by analysis of the NMR and MS spectroscopic data. Compound 1 possesses a 10-membered macrodiolide unit, while 2 and 3 are chromones with an unprecedented 6/6/6/5 polycyclic skeleton. Compounds 1-7 were evaluated for their cytotoxicities, while all the compounds displayed weak or no activity.

A bifunctional pyrazolone–chromone synthon directed organocatalytic double Michael cascade reaction: forging five stereocenters in structurally diverse hexahydroxanthones

The merging of two or more known natural product-based scaffolds is a powerful and routine strategy to develop bioactive small molecules.

Bifunctional Brønsted base catalyzed inverse-electron-demand aza-Diels–Alder reactions of saccharin-derived 1-azadienes with azlactones

An enantioselective inverse-electron-demand aza-Diels–Alder reaction of saccharin-derived 1-azadienes and azlactones was developed using a phenylalanine-derived bifunctional Brønsted base-squaramide catalyst.

Stereocontrolled construction of six vicinal stereogenic centers on a hexahydroxanthone framework through a formal [2+1+3] annulation

The first example of a bifunctional donor–donor 3C synthon formed in situ from an activated methine with nitromethane through a [2+1] Michael addition, further directing a one-pot organocascade Michael/Henry cycloaddition was developed.

Bifunctional oxindole-chromone 4C building block directed asymmetric synthesis of bispirocyclic hexahydroxanthones featuring five contiguous stereocenters and two side-by-side oxindoles

Optically active small molecules based on privileged natural product frameworks and rich in three-dimensional complexity are in high demand.

An asymmetric iminium ion catalysis-enabled cascade cycloaddition reaction of chromone-oxindole synthons with enals: construction of a spirooxindole–hexahydroxanthone framework

We report the first asymmetric iminium ion catalysis-enabled cascade cycloaddition reaction of bifunctional chromone-oxindole synthons and α,β-unsaturated aldehydes.

Organocatalyzed Synthesis of [3.2.1] Bicyclooctanes

Organocatalysis constitutes one of the main research areas in organic chemistry from the last two decades. This chemistry has been applied to the synthesis of many natural products and structures in a manner that reduces the residues and so the ecological impact. In this review, we consider the work that has been done for the synthesis of bicyclo[3.2.1]octane framework. This structure is present in many natural products with very important biological activities.

Highly enantioselective synthesis of fused bicyclic dihydropyranones via low-loading N-heterocyclic carbene organocatalysis

Highly diastereo and enantioselective [4+2] cycloadditions have been achieved between pyrrolidone-derived cyclic enones and α-haloaldehydes under mild conditions. Relying on extremely reactive in-situ generated chiral N-heterocyclic carbenes, this stereoselective annulation proceeds efficiently even on the gram scale with the catalyst loading as low as 0.025 mol% (250 ppm). A variety of cis-substituted bicyclic dihydropyranones can be produced in up to 96% yield with up to >99% ee. In addition, simple, inexpensive linear aldehydes such as n-propanal can be used directly in asymmetric cycloadditions via oxidative N-heterocyclic carbene organocatalysis with low catalyst loading. This method may provide an economical and practical approach for the asymmetric synthesis of medicinally relevant molecules.

Organocatalytic asymmetric synthesis of tetrahydrocarbazoles via an inverse-electron-demand Diels–Alder reaction of 2,3-indole-dienes with enals

A novel 2,3-indole-diene synthon was designed and first applied to an inverse-electron-demand Diels–Alder reaction to construct tetrahydrocarbazoles with excellent stereoselectivities.

Unprecedented formation of 3-(tetrahydrofuran-2-yl)-4H-chromen-4-one in a reaction between 3,3a-dihydro-9H-furo[3,4-b]chromen-9-one and malononitrile

Chromone skeletons are widespread among natural products as well as bioactive molecules. Here, we describe an unprecedented reaction of furo[3,4-b]chromen-9-one with malononitrile to afford 3-(tetrahydrofuran-2-yl)-4H-chromen-4-ones. Experimental data suggest that a sequence of Michael/retro-Michael/nucleophilic addition is involved in this unprecedented transformation.

Asymmetric cycloaddition reactions catalysed by diarylprolinol silyl ethers

Organocatalysis as a paint palette for asymmetric cycloaddition reactions.

Facile synthesis of novel spiroheterocycles via diastereoselective aziridination of cyclic enones

A highly diastereoselective aziridination of cyclic enones has been developed under mild conditions, giving an efficient access to a class of novel spiroaziridines which show promising antibacterial activity.

Old tricks, new dogs: organocatalytic dienamine activation of α,β-unsaturated aldehydes

Chiral secondary amines are some of the most commonly used kinds of catalysts. They have become a reliable tool for the α- and β-activation of carbonyl compounds, via HOMO, SOMO or LUMO activation pathways. Recently, chemists have turned their attention to the development of novel organocatalytic strategies for remote functionalisation, targeting stereocentres even more distant from the catalyst-activation site, through dienamine, trienamine, and vinylogous iminium ion pathways (γ-, ε- and δ-positions, respectively). Here we outline and discuss the state-of-the-art in dienamine activation, classifying examples according to the different reactive activation pathways followed by the formed dienamine intermediate (1,3-, 1,5-, 2,5- and 4,5-functionalisation) and the reaction type developed, as determined by the structure and the nature of electrophiles and nucleophiles.

Enantioselective Vinylogous Organocascade Reactions

Cascade reactions are powerful tools for rapidly assembling complex molecular architectures from readily available starting materials in a single synthetic operation. Their marriage with asymmetric organocatalysis has led to the development of novel techniques, which are now recognized as reliable strategies for the one-pot enantioselective synthesis of stereochemically dense molecules. In recent years, even more complex synthetic challenges have been addressed by applying the principle of vinylogy to the realm of organocascade catalysis. The key to the success of vinylogous organocascade reactions is the unique ability of the chiral organocatalyst to transfer reactivity to a distal position without losing control on the stereo-determining events. This approach has greatly expanded the synthetic horizons of the field by providing the possibility of forging multiple stereocenters in remote positions from the catalyst's point of action with high selectivity, while simultaneously constructing multiple new bonds. This article critically describes the developments achieved in the field of enantioselective vinylogous organocascade reactions, charting the ideas, the conceptual advances, and the milestone reactions that have been essential for reaching highly practical levels of synthetic efficiency.