Access to 2-alkyl chromanones via a conjugate addition approach

Orthogonal magnetic orbitals in high spin Cu-VO units: structure, magnetism and EPR study of anisotropic heterometallic complexes

Molecular-based magnetic materials are expected to serve as building blocks for quantum bits. To realize high-dimensional Hilbert space and addressability, we constructed anisotropic multi-level systems based on CuII and VIV with orthogonal magnetic orbitals. The crystal structures and intramolecular magnetic couplings of four CuIIVOII complexes [{CuVO(appen)2}2], [{CuVO(fhma)2EDA}2], [{CuVO(hfca)2EDA}2] and [CuVO(hfca)2DPEDA]n are characterized. Due to the orthogonal magnetic orbitals of CuII and VIV, the Cu-V pairs in the four complexes have strong ferromagnetic couplings, and the coupling strength is linearly related to the dihedral angle between the two equatorial planes of the two coordination polyhedra. Because of the triplet ground state, the system can be described by an effective Hamiltonian model consisting of two S = 1 spins coupled together. The anisotropy parameters of [{CuVO(hfca)2EDA}2] and [CuVO(hfca)2DPEDA]n were obtained by the simulation of X-band continuous wave electron paramagnetic resonance (cw-EPR) spectra, confirming that both complexes have zero-field splitting addressable on the relative energy scale. The results indicate that constructing multi-centre complexes based on orthogonal magnetic orbitals is a promising strategy for designing multidimensional quantum bits.

Total syntheses of (-)-macrocalyxoformins A and B and (-)-ludongnin C

The complex and diverse molecular architectures along with broad biological activities of ent-kauranoids natural products make them an excellent testing ground for the invention of synthetic methods and strategies. Recent efforts notwithstanding, synthetic access to the highly oxidized enmein-type ent-kauranoids still presents considerable challenges to synthetic chemists. Here, we report the enantioselective total syntheses of C-19 oxygenated enmein-type ent-kauranoids, including (-)-macrocalyxoformins A and B and (-)-ludongnin C, along with discussion and study of synthetic strategies. The enabling feature in our synthesis is a devised Ni-catalyzed decarboxylative cyclization/radical-polar crossover/C-acylation cascade that forges a THF ring concomitantly with the β-keto ester group. Mechanistic studies reveal that the C-acylation process in this cascade reaction is achieved through a carboxylation followed by an in situ esterification. Biological evaluation of these synthetic natural products reveals the indispensable role of the ketone on the D ring in their anti-tumor efficacy.

Discovery of 5,7-Dimethoxy-2-(3,4,5-trimethoxyphenoxy)-chromen-4-one with Lipid Lowering Effects in Hepatocytes

The population with nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) is increasing. However, no medicine is indicated for treating these diseases clinically nowadays. Therefore, there is an urgent need to develop a new drug to overcome NAFLD and NASH. Capillarisin, a 2-phenoxychromone originating from Artemisia capillaris Thunb., is well-known for its liver-protective effects. As a result, a series of 2-phenoxychromones was prepared and evaluated for its protective activity against lipid droplet formation in oleic acid (OA)-treated Huh7 cells by means of high-content screening. In the light of the results, the compounds with trimethoxy groups on the phenyl ring possessed better inhibitory properties against lipid accumulation in Huh7 cells, compared to other functional groups on the same ring. Nonetheless, the compounds with a hydroxy group at the C-5 position of the chromone exhibited apparent cytotoxicity. Finally, the active compound, 5,7-dimethoxy-2-(3,4,5-trimethoxyphenoxy)-chromen-4-one (7e), with an IC₅₀ value of 32.2 ± 2.1 μM against lipid accumulation and no significant cytotoxicity, reduced the accumulation of lipid droplets by up-regulating peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1α) to facilitate the catabolism of fat, which shows promise for further optimization to manage NAFLD and NASH.

Asymmetric synthesis of chromanone lactones via vinylogous conjugate addition of butenolide to 2-ester chromones

Chiral chromanone lactones are a class of natural products with important biological activity. We report a direct diastereo- and enantioselective vinylogous conjugate addition of butenolide to 2-ester substituted chromones. The transformation proceeded well in the presence of as low as 1 mol% of a chiral N,N′-dioxide/Sc^III complex, 3 Å MS and a catalytic amount of hexafluoroisopropanol (HFIP). The scope of Michael acceptors includes a variety of substituted chromones at different positions, and the desired chromanone lactones upon reduction are afforded in good yield and diastereoselectivity, and excellent enantioselectivity (up to 99% ee). The strategy could be used in the concise synthesis of blennolide C and gonytolide A, C and G.

We report a direct, diastero- and enantioselective vinylogous 1,4-addition of butanolide to 2-ester chromones. A chiral Sc^III complex enabled the reaction to proceed smoothly to give a variety of chraomanone lactones.

A bifunctional fluorescent probe for simultaneous detection of GSH and H2Sn (n > 1) from different channels with long-wavelength emission

In this work, we presented a long-wavelength emission fluorescent probe DCM-Cou-SePh that can discriminatively detect glutathione (GSH) and hydrogen polysulfides (H₂S_n, n > 1) from green and red emission channels, respectively. With the addition of GSH, probe DCM-Cou-SePh displayed green fluorescence emission (λ_ex/em = 430/530 nm). In the presence of H₂S_n, the probe exhibited a significant fluorescence enhancement in red channel (λ_ex/em = 560/680 nm). We also demonstrated that this probe was suitable to quantitatively detect GSH and H₂S_n with low detection limits (0.12 μM for GSH, 0.19 μM for H₂S_n). Furthermore, DCM-Cou-SePh can be used for sensing endogenous GSH and H₂S_n in living cells by dual-color fluorescence imaging.

Robust, Enantioselective Construction of Challenging, Biologically Relevant Tertiary Ether Stereocenters

A robust, catalytic enantioselective method to construct challenging, biologically relevant, tertiary ether stereocenters has been developed. The process capitalizes on readily accessible bis(oxazoline) ligands to control the facial selectivity of the addition of copper acetylides to benzopyrylium triflates, reactive species generated in situ. Up to 99% enantiomeric excesses are achieved with a broad substrate scope. Using density functional theory (DFT) calculations, the origin of the experimentally observed enantiocontrol was attributed to additional non-covalent interactions observed in the transition state leading to the major enantiomer, such as π-stacking. The resultant substrates have direct applications in the synthesis of naturally occurring bioactive chromanones and tetrahydroxanthones.

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.

Copper Bis(oxazoline)-Catalyzed Enantioselective Alkynylation of Benzopyrylium Ions

The stereocontrolled construction of biologically relevant chromanones and tetrahydroxanthones has been achieved through the addition of alkynes to benzopyrylium trilfates under the influence of copper bis(oxazoline) catalysis. Excellent levels of enantiocontrol (63-98 % ee) are achieved in the addition of a variety of alkynes to an array of chromenones with a hydrogen in the 2-position. Promising levels of enantiocontrol (54-67 % ee) are achieved in the alkynylation of chromenones with esters in the 2-position, generating tertiary ether stereocenters resembling those frequently found in naturally occurring metabolites.

Tandem synthesis of 4-aminoxanthones is controlled by a water-assisted tautomerization: a general straightforward reaction

Iminolactone-aminofuran tautomerization is the rate-determining step in the multicomponent synthesis of xanthones from 3-carbonylchromones, isocyanides and dienophiles.

Silanediol-Catalyzed Chromenone Functionalization

Promising levels of enantiocontrol are observed in the silanediol-catalyzed addition of silyl ketene acetals to benzopyrylium triflates. This rare example of enantioselective, intermolecular chromenone functionalization with carbonyl-containing nucleophiles has potential applications in the synthesis of bioactive chromanones and tetrahydroxanthones.

Total synthesis of (R,R,R)-α-tocopherol through asymmetric Cu-catalyzed 1,4-addition

By introducing a disposable activating substituent at C-3, the asymmetric 1,4-addition to a notoriously unreactive 2-substituted chromenone was achieved with high levels of (2R)-stereoselectivity in the presence of a chiral Cu(I)-phosphoramidite complex as a catalyst. This paved the way for an efficient and conceptually novel synthesis of (R,R,R)-α-tocopherol from readily available starting materials.

Facile synthesis of azaarene-2-substituted chromanone derivatives via tandem sp3 C–H functionalization/decarboxylation of azaarenes with 4-oxo-4H-chromene-3-carboxylic acid

A facile catalyst-free tandem sp³ C–H functionalization/decarboxylation of 4-oxo-4H-chromene-3-carboxylic acid with 2-alkylazaarenes was developed, which can construct the biologically significant azaarene 2-substituted chromanones in a single step.