Stereoselective synthesis of monomeric flavonoids

Stereoselective Synthesis of Flavonoids: A Brief Overview

Stereoselective synthesis has been emerging as a resourceful tool because it enables the obtaining of compounds with biological interest and high enantiomeric purity. Flavonoids are natural products with several biological activities. Owing to their biological potential and aiming to achieve enantiomerically pure forms, several methodologies of stereoselective synthesis have been implemented. Those approaches encompass stereoselective chalcone epoxidation, Sharpless asymmetric dihydroxylation, Mitsunobu reaction, and the cycloaddition of 1,4-benzoquinone. Chiral auxiliaries, organo-, organometallic, and biocatalysis, as well as the chiral pool approach were also employed with the goal of obtaining chiral bioactive flavonoids with a high enantiomeric ratio. Additionally, the employment of the Diels-Alder reaction based on the stereodivergent reaction on a racemic mixture strategy or using catalyst complexes to synthesise pure enantiomers of flavonoids was reported. Furthermore, biomimetic pathways displayed another approach as illustrated by the asymmetric coupling of 2-hydroxychalcones driven by visible light. Recently, an asymmetric transfer hydrogen-dynamic kinetic resolution was also applied to synthesise (R,R)-cis-alcohols which, in turn, would be used as building blocks for the stereoselective synthesis of flavonoids.

Synthesis and biological evaluation of chalcone-polyamine conjugates as novel vectorized agents in colorectal and prostate cancer chemotherapy

The aim of this study was to synthesize chalcone-polyamine conjugates in order to enhance bioavailability and selectivity of chalcone core towards cancer cells, using polyamine-based vectors. Indeed, it is well-known that polyamine transport system is upregulated in tumor cells. 3',4,4',5'-tetramethoxychalcone was selected as parent chalcone since it was found to be an efficient anti-proliferative agent on various cancer cells. A series of five chalcone-polyamine conjugates was obtained using the 4-bromopropyloxy-3',4',5'-trimethoxychalcone as a key intermediate. Chalcone core and polyamine tails were fused through an amine bond. These conjugates were found to possess a marked in vitro antiproliferative effect against colorectal (HT-29 and HCT-116) and prostate cancer (PC-3 and DU-145) cell lines. The most active conjugate (compound 8b) was then chosen for further biological evaluations to elucidate mechanisms responsible for its antiproliferative activity. Investigations on cell cycle distribution revealed that this conjugate can prevent the proliferation of human colorectal and prostate cancer cells by blocking the cell cycle at the G1 and G2 phase, respectively. Flow cytometry analysis revealed a sub-G1 peak, characteristic of apoptotic cell population and our inquiries highlighted apoptosis induction at early and later stages through several pro-apoptotic markers. Therefore, this chalcone-N1-spermidine conjugate could be considered as a promising agent for colon and prostatic cancer adjuvant therapy.

(E)-1-(4-Fluoro-2-hydroxyphenyl)-3-(2,3,4-trimethoxyphenyl)prop-2-en-1-one

The crystal structure of a biologically important pharmacophore containing chalcone is reported.

In the title mol­ecule, C₁₈H₁₇FO₅, the conformation about the C=C bond of the central enone group is trans. The dihedral angle between the benzene rings is 13.08 (3)°. The hy­droxy group attached to the benzene ring is involved in an intra­molecular O—H⋯O hydrogen bond. In the crystal, weak C—H⋯O hydrogen bonds link the mol­ecules into chains along [001].

Synthesis of chiral chromanols via a RuPHOX-Ru catalyzed asymmetric hydrogenation of chromones

Chiral chromanols and their derivatives have been synthesized via a RuPHOX-Ru catalyzed asymmetric hydrogenation of chromones in high yields, >20 : 1 drs and with up to 99.9% ee. Control experiments show that the reaction undergoes two sequential asymmetric hydrogenation steps of the C[double bond, length as m-dash]C and C[double bond, length as m-dash]O double bonds. The reaction could be performed on a gram-scale with a relatively low catalyst loading (up to 1000 S/C), and the resulting products can be transformed to several biologically active compounds.

Total Synthesis of (+)-Medicarpin

(+)-Medicarpin has been synthesized asymmetrically for the first time in a linear scalable process with an overall yield of 11%. The two chiral centers were constructed in one step via condensation using a chiral oxazolidinone auxiliary. This method will likely accelerate research on medicarpin as an erythropoietin inducer for erythropoietin-deficient diseases.

Two polymorphs of trans-[3-(3-nitro-phen-yl)oxiran-2-yl](phen-yl)methanone

The title compound, C15H11NO4, crystallizes in two polymorphic forms, centrosymmetric monoclinic and chiral ortho-rhom-bic. The geometry of the mol-ecules in the two polymorphs is slightly different, possibly due to inter-molecular inter-actions. There are no classical hydrogen bonding in these two structures. However, a number of C-H⋯O inter-molecular inter-actions, involving both O atoms of the nitro as well the benzoyl groups, stabilize the crystal structures.

(E)-3-(4-Hy-droxy-3-meth-oxy-phen-yl)-1-(4-hy-droxy-phen-yl)prop-2-en-1-one

In the title compound, C₁₆H₁₄O₄, there is an intra­molecular O—H⋯O hydrogen bond. The benzene rings are inclined to one another by 13.89 (9)°. The prop-2-en-1-one group is twisted slightly, the O=C—C_ar—C_ar (ar = aromatic) and C=C—C=O torsion angles being −10.4 (3) and −7.4 (3)°, respectively. In the crystal, mol­ecules are linked by O—H⋯O hydrogen bonds, forming chains along [100]. These chains are further linked by O—H⋯O hydrogen bonds, forming corrugated sheets lying parallel to (010). There are C—H⋯π inter­actions present within the sheets.

Chalcones with electron-withdrawing and electron-donating substituents: anticancer activity against TRAIL resistant cancer cells, structure-activity relationship analysis and regulation of apoptotic proteins

In the present study, a series of 46 chalcones were synthesised and evaluated for antiproliferative activities against the human TRAIL-resistant breast (MCF-7, MDA-MB-231), cervical (HeLa), ovarian (Caov-3), lung (A549), liver (HepG2), colorectal (HT-29), nasopharyngeal (CNE-1), erythromyeloblastoid (K-562) and T-lymphoblastoid (CEM-SS) cancer cells. The chalcone 38 containing an amino (-NH2) group on ring A was the most potent and selective against cancer cells. The effects of the chalcone 38 on regulation of 43 apoptosis-related markers in HT-29 cells were determined. The results showed that 20 apoptotic markers (Bad, Bax, Bcl-2, Bcl-w, Bid, Bim, CD40, Fas, HSP27, IGF-1, IGFBP-4, IGFBP-5, Livin, p21, Survivin, sTNF-R2, TRAIL-R2, XIAP, caspase-3 and caspase-8) were either up regulated or down regulated.

[3-Benzoyl-2,4-bis(3-nitrophenyl)cyclobutyl](phenyl)methanone

The asymmetric unit of the title compound, C₃₀H₂₂N₂O₆, comprises a half-mol­ecule of the cyclo­butane derivative. The least-squares planes defined by the respective C atoms of the aromatic substituents inter­sect at angles of 76.81 (7) and 89.22 (8)° with the least-squares plane defined by the C atoms of the cyclo­butane ring. In the crystal, C—H⋯O contacts connect the mol­ecules into a three-dimensional network. The shortest centroid–centroid distance between the two different aromatic rings is 3.9601 (8) Å.

Boesenbergia rotunda: From Ethnomedicine to Drug Discovery

Boesenbergia rotunda is a herb from the Boesenbergia genera under the Zingiberaceae family. B. rotunda is widely found in Asian countries where it is commonly used as a food ingredient and in ethnomedicinal preparations. The popularity of its ethnomedicinal usage has drawn the attention of scientists worldwide to further investigate its medicinal properties. Advancement in drug design and discovery research has led to the development of synthetic drugs from B. rotunda metabolites via bioinformatics and medicinal chemistry studies. Furthermore, with the advent of genomics, transcriptomics, proteomics, and metabolomics, new insights on the biosynthetic pathways of B. rotunda metabolites can be elucidated, enabling researchers to predict the potential bioactive compounds responsible for the medicinal properties of the plant. The vast biological activities exhibited by the compounds obtained from B. rotunda warrant further investigation through studies such as drug discovery, polypharmacology, and drug delivery using nanotechnology.

1-(3-Benzyl-4,6-dibenzyloxy-2-hydroxyphenyl)ethanone

The title compound, C₂₉H₂₆O₄, is essentially planar in the acetophenone portion that includes both the hy­droxy and a benz­yloxy O atoms, with an r.m.s. deviation of 0.0311 Å. The other two substituents inter­sect the plane at 70.45 (3) and 59.55 (4)°. In the molecule there is an intramolecular O—H⋯O hydrogen bond. In the crystal, mol­ecules are linked by C—H⋯O hydrogen bonds, as well as C—H⋯π and π-stacking inter­actions, with centroid–centroid distances 3.6570 (2) Å.

Asymmetric Methods for the Synthesis of Flavanones, Chromanones, and Azaflavanones

Flavanones, chromanones, and related structures are privileged natural products that display a wide variety of biological activities. Although flavanoids are abundant in nature, there are a limited number of available general and efficient synthetic methods for accessing molecules of this class in a stereoselective manner. Their structurally simple architectures belie the difficulties involved in installation and maintenance of the stereogenic configuration at the C2 position, which can be sensitive and can undergo epimerization under mildly acidic, basic, and thermal reaction conditions. This review presents the methods currently used to access these related structures. The synthetic methods include manipulation of the flavone/flavanone core, carbon-carbon bond formation, and carbon-heteroatom bond formation.

Epiafzelechin from the root bark of Cassia sieberiana: detection by DART mass spectrometry, spectroscopic characterization, and antioxidant properties

The root bark of Cassia sieberiana was analyzed using direct analysis in real time mass spectrometry, and a main flavonoid component with an [M + H](+) mass of 275 was identified. The flavonoid, epiafzelechin, was isolated and fully characterized with the concerted use of NMR spectroscopy, circular dichroism, and optical rotation. Electronic circular dichroism and optical rotation TDDFT calculations were also performed, and their agreement with the experimental results confirmed the enantiomeric identity of the isolated natural product. The antioxidant activity of the compound was also investigated.

(2E)-3-[4-(Dimethyl-amino)-phen-yl]-1-(4-fluoro-phen-yl)prop-2-en-1-one

The mean planes of the two benzene rings in the title compound, C(17)H(16)FNO, are twisted slightly, making a dihedral angle of 7.8 (1)°. The prop-2-en-1-one group is also twisted slightly with a C-C-C-O torsion angle of -11.6 (3)°. In the crystal, weak inter-molecular C-H⋯O inter-actions link pairs of mol-ecules, forming centrosymmetric dimers.

2',3,4,4',5-Penta-meth-oxy-chalcone

In the title chalcone [systemetic name 1-(2,4-dimeth­oxy­phen­yl)-3-(3,4,5-trimeth­oxy­phen­yl)prop-2-en-1-one], C₂₀H₂₂O₆, the dihedral angle between the plane of the two benzene rings is 7.03 (4)° with all but one of the meth­oxy groups essentially co-planar with these rings [C—C—O—C torsion angles = −76.1 (2), −0.7 (3), 1.8 (3), −6.2 (3), 2.0 (3)°]. An intra­molecular C—H⋯O inter­action occurs. The crystal packing is stabilized by weak inter­molecular C—H⋯O hydrogen bonds.

2',3,4,4'-Tetra-meth-oxy-chalcone

In the title compound [systematic name: 1-(2,4-dimethoxyphenyl)-3-(3,4-dimethoxyphenyl)prop-2-en-1-one], C₁₉H₂₀O₅, the dihedral angle between the benzene rings is 26.88 (5)°. One of the meth­oxy groups is twisted slightly away from the plane [C—O—C—C torsion angle = −12.8 (3)°] while the others are almost co-planer [C—O—C—C torsion angles = −3.2 (3), 2.6 (3) and −3.6 (3)°]. The crystal packing is stabilized by inter­molecular C—H⋯O inter­actions. A weak intra­molecular C—H⋯O inter­action occurs.

Photodimerizations of hydroxy- and benzoylated 4-azachalcones and quantum chemical investigation of the reactions

Photodimerization reactions of compounds 4-6 gave four new cyclobutane-containing compounds (7-9) with full control over the stereochemistry at the four stereogenic centers. These new cyclobutane-containing compounds had beta-truxinic (7a), delta-truxinic (7b and 9), and epsilon-truxillic (8) structures. However, o-, m-, and p-hydroxy 4-azachalcones (1-3) did not give photochemical cyclization products under any conditions (in solvent or in their solid or molten states). Experimental data suggested the possibility of frontier orbital control over stereochemical behavior, so some theoretical calculations were performed. Full geometrical optimization of compounds 1-9 was performed via DFT B3LYP/6-31(+)G**, and their electronic structures were also investigated. The geometries of the singlet and triplet states were initially optimized by density functional theory (DFT) and the configuration interaction singles (CIS) B3LYP/3-21(+)G** level. An additional calculation was performed for the triplet state using the ground-state geometry. The possible photochemical dimerization products of compounds 7-9 (a-g) and the intrinsic reaction coordinates (IRCs) of the reactions of compounds 4-6 were calculated theoretically by the DFT/3-21(+)G** method. The configurations (reactant, transition state, product, and reaction pathway) corresponding to the stationary points (minima or saddle points) were determined. The intrinsic reaction coordinates were followed to verify the energy profiles that connect each TS to the appropriate local minimum. The dimeric products expected from the calculations coincided with the dimers produced experimentally.

(2E)-1-(3-Chloro-phen-yl)-3-phenyl-prop-2-en-1-one

In the title compound, C(15)H(11)ClO, the dihedral angle between the mean planes of the benzene ring and the chloro-substituted benzene ring is 48.8 (3)°. The dihedral angles between the mean plane of the prop-2-ene-1-one group and the mean planes of the 3-chloro-phenyl and benzene rings are 27.0 (4) and 27.9 (3)°, respectively. In the crystal, weak inter-molecular C-H⋯π-ring inter-actions occur.

Bioactivity and structure of biophenols as mediators of chronic diseases

Biophenols and their associated activity have generated intense interest. Current topics of debate are their bioavailability and bioactivity. It is generally assumed that their plasma concentrations are insufficient to produce the health benefits previously attributed to their consumption. However, data on localized in vivo concentrations are not available and many questions remain unanswered. Potential mechanisms by which they may exert significant bioactivity are discussed together with structure activity relationships. Biophenols are highly reactive species and they can react with a range of other compounds. Products of their reaction when functioning as antioxidants are examined.

Hypericum in infection: Identification of anti-viral and anti-inflammatory constituents

The Iowa Center for Research on Botanical Dietary Supplements seeks to optimize Echinacea, Hypericum, and Prunella botanical supplements for human-health benefit, emphasizing antiviral, anti-inflammatory and anti-pain activities. This mini-review reports on ongoing studies on Hypericum. The Center uses the genetically diverse, well-documented Hypericum populations collected and maintained at the USDA-ARS North Central Regional Plant Introduction Station (NCRPIS), and the strength of research in synthetic chemistry at Iowa State University to tap natural diversity, to help discover key constituents and interactions among constituents that impact bioactivity and toxicity. The NCRPIS has acquired more than 180 distinct populations of Hypericum, with a focus on Hypericum perforatum L. (Hypericaceae), representing about 13% of currently recognized taxa. Center chemists have developed novel synthetic pathways for key flavones, acyl phloroglucinols, hyperolactones and a tetralin that have been found in Hypericum, and these compounds are used as standards and for bioactivity studies. Both light-dependent and light-independent anti-viral activities have been identified by using bioactivity-guided fractionation of H. perforatum and a HIV-1 infection test system. Our Center has focused on light-independent activity, potentially due to novel chemicals, and polar fractions are undergoing further fractionation. Anti-inflammatory activity has been found to be light-independent, and fractionation of a flavonoid-rich extract revealed four compounds (amentoflavone, chlorogenic acid, pseudohypericin and quercetin) that interacted in the light to inhibit lipopolysaccharide-induced prostaglandin E(2) activity. The Center continues to explore novel populations of H. perforatum and related species to identify constituents and interactions of constituents that contribute to potential health benefits related to infection.