Antiplasmodial Chromanes and Chromenes from the Monotypic Plant Species Koeberlinia spinosa

Tandem Sulfonylative Annulation/Halogenation of 1,7-Enynes with Sodium Sulfinate and TBAX for the Assembly of 4-Methylenechromanes

An effective and stereoselective synthesis of halogenated (E)-4-methylenechromanes with a sulfonyl group was developed via the copper-catalyzed sulfonylative annulation/halogenation of 1,7-enynes, in which sodium sulfinates were used as the sulfonyl reagents and tetrabutylammonium halide provided the halogen sources. The formed alkenyl C-X bonds were valuable and can efficiently undergo the subsequent hydrolysis, alkenylation, alkynylation, arylation, alkylthiolation, and alkoxylation to furnish a series of highly functionalized 4-methylenechromanes.

Stereochemistry of Chiral 2-Substituted Chromanes: Twist of the Dihydropyran Ring and Specific Optical Rotation

Chiral 2-substituted chromanes are important substructures in organic synthesis and appear in numerous natural products. Herein, the correlation between specific optical rotations (SORs) and the stereochemistry at C2 of chiral 2-substituted chromanes was investigated through data mining, quantum-chemical calculations using density functional theory (DFT), and mechanistic analyses. For 2-aliphatic (including acyloxy and alkenyl) chromanes, the P-helicity of the dihydropyran ring usually corresponds to a positive SOR; however, 2-aryl chromanes with P-helicity tend to exhibit negative SORs. 2-Carboxyl (including alkoxycarbonyl and carbonyl) chromanes often display small experimental SORs, and theoretical calculations for them are prone to error because of the fluctuating conformational distribution with computational parameters. Several typical compounds were discussed, including detailed descriptions of the asymmetric synthesis, absolute configuration (AC) assignment methods, and systematic conformational analysis. We hope this work will enrich the knowledge of the stereochemistry of chiral 2-substituted chromanes.

Dual High-Resolution α-Glucosidase and PTP1B Inhibition Profiling Combined with HPLC-PDA-HRMS-SPE-NMR Analysis for the Identification of Potentially Antidiabetic Chromene Meroterpenoids from Rhododendron capitatum

Thirteen previously undescribed chromene meroterpenoids, capitachromenic acids A-M (3-6, 7a, 7b, 8a, 8b, 9a, 9b, 10a, 10b, and 11b), were identified from an ethyl acetate extract of Rhododendron capitatum, using dual high-resolution α-glucosidase and PTP1B inhibition profiling in combination with HPLC-PDA-HRMS-SPE-NMR. In addition, one known chromene meroterpenoid, daurichromenic acid (15), and its biosynthetic precursor, grifolic acid (12), two C-methylated flavanones, (2S)-5,7,4'-trihydroxy-8-methylflavanone (1) and farrerol (2), and two triterpenoids, oleanolic acid (14a) and ursolic acid (14b), were identified. New structures were elucidated by extensive 1D and 2D NMR analysis, and absolute configurations of new chromene meroterpenoids were assigned by analysis of their ECD spectra on the basis of the empirical chromane helicity rule and from Rh₂(OCOCF₃)₄-induced ECD spectra by applying the bulkiness rule. Compounds 5, 9a, 9b, 12, and 15 showed α-glucosidase inhibitory activity with IC₅₀ values ranging from 8.0 to 93.5 μM, while compounds 3, 5, 8b, 9a, 9b, 10b, 11b, 12, and 15 showed PTP1B inhibitory activity with IC₅₀ values ranging from 2.5 to 68.1 μM.

Synthesis and Structural Characterization of 1-(E-1-Arylpropenon-3-yl)-3,4,6-tri-O-benzyl-d-glucals and Their Transformation into Pentasubstituted (2R,3S,4R)-Chromanes via Pd-Catalyzed Cross Dehydrogenative Coupling Reaction

We have developed an efficient methodology for the synthesis of (2R,3S,4R)-2-hydroxymethyl-3,4-dihydroxy-6-aryl-7-aroylchromanes in which the chirality at the C-2, C-3, and C-4 positions is being drawn from C-glucopyranosyl aldehyde, which in turn can be efficiently synthesized from d-glucose. Thus, the synthesis starts with the transformation of sugar aldehyde into 1-(E-1-arylpropenon-3-yl)-3,4,6-tri-O-benzyl-d-glucals using Claisen-Schmidt type condensation reaction with different acetophenones and then to 1,2-disubstituted glucals via Pd(II)-catalyzed cross dehydrogenative coupling reaction, which in turn has been efficiently converted into (2R,3S,4R)-chromanes via 6π-electrocyclization and in situ dehydrogenative aromatization.

Enantioselective Cleavage of Cyclobutanols Through Ir-Catalyzed C-C Bond Activation: Mechanistic and Synthetic Aspects

The Ir-catalyzed conversion of prochiral tert-cyclobutanols to β-methyl-substituted ketones proceeds under comparably mild conditions in toluene (45-110 °C) and is particularly suited for the enantioselective desymmetrization of β-oxy-substituted substrates to give products with a quaternary chirality center with up to 95 % ee using DTBM-SegPhos as a chiral ligand. Deuteration experiments and kinetic isotope effect measurements revealed major mechanistic differences to related RhI -catalyzed transformations. Supported by DFT calculations we propose the initial formation of an IrIII hydride intermediate, which then undergoes a β-C elimination (C-C bond activation) prior to reductive C-H elimination. The computational model also allows the prediction of the stereochemical outcome. The Ir-catalyzed cyclobutanol cleavage is broadly applicable but fails for substrates bearing strongly coordinating groups. The method is of particular value for the stereo-controlled synthesis of substituted chromanes related to the tocopherols and other natural products.

Computational insight into the mechanism and origin of high regioselectivity in the ring-opening cyclization of spirocyclopropanes with stabilized sulfonium ylides by the DFT

Chromanes with high bioactivity play an important role in nature, and cyclization reactions of cyclopropanes with sulfonium ylides to form chromane skeletons have attracted great attention of scientists. The mechanism as well as origins of regioselectivity and stereoselectivity for the ring-opening/cyclization reactions between cyclohexane-1,3-dione-2-spirocyclopropanes and stabilized sulfonium ylides in CH₂Cl₂ were investigated by using the density functional theory (DFT) M06-2X/6-311+G(d,p)//M06-2X/6-31G(d,p) method combined with the SMD model. The calculated results revealed that the reaction process involved two key steps: the ring-opening step and the cyclization step, with the former being the rate-determining and stereoselectivity-determining step. The regioselectivity of the ring-opening step of spirocyclopropane indicated that the tertiary carbon was more preferential than the secondary one when sulfonium ylide attacked spirocyclopropane. The theoretical results confirmed that the stereoselectivity of the reaction to form the trans-isomer product is more favorable than the cis-isomer, and the calculated trans/cis ratio is in accordance with the experiment. Moreover, the conceptual density functional theory reactivity indices suggest that the electronic effect controls the regioselectivity. What is more, the stereoselectivity analyzed by weak non-covalent interaction also shows the importance of electronic effect. Graphical Abstract.

Garcinoic Acids and a Benzophenone Derivative from the Seeds of Garcinia kola and Their Antibacterial Activities against Oral Bacterial Pathogenic Organisms

In this study, three new garcinoic acid dimers, δ,δ-bigarcinoic acid (1), δ,δ-bi-O-garcinoic acid (2), and γ,δ-bi-O-garcinoic acid (3), and a new benzophenone derivative, (8E)-4-geranyl-3,5-dihydroxybenzophenone (4), as well as seven known compounds (5-11) were isolated from the seeds of Garcinia kola. The structures of the new compounds were elucidated using MALDI-TOF-MS and spectroscopic data, including 1D and 2D NMR and electronic circular dichroism spectra. All of the isolated compounds were evaluated for their antimicrobial activity against two oral pathogens, Porphyromonas gingivalis and Streptococcus sobrinus. Among them, 4 and δ-garcinoic acid (6) exhibited antimicrobial activity against both of these microorganisms (MICs of 31.3-62.5 μM for P. gingivalis and 15.6-31.3 μM for S. sobrinus). These results indicate that some chemical constituents in G. kola seeds have potential application in the prevention of oral diseases.

Galtonosides A-E: Antiproliferative and Antiplasmodial Cholestane Glycosides from Galtonia regalis

An extract of Galtonia regalis from the Natural Products Discovery Institute showed moderate antiplasmodial activity, with an IC50 value less than 1.25 μg/mL. The two known cholestane glycosides 1 and 2 and the five new cholestane glycosides galtonosides A-E (3-7) were isolated after bioassay-directed fractionation. The structures of the new compounds were determined by interpretation of their NMR and mass spectra. Among these compounds, galtonoside B (4) displayed the most potent antiplasmodial activity, with an IC50 value of 0.214 μM against the drug-resistant Dd2 strain of Plasmodium falciparum.

Conjugate Addition of Carbon Acids to β,γ-Unsaturated α-Keto Esters: Product Tautomerism and Applications for Asymmetric Synthesis of Benzo[a]phenazin-5-ol Derivatives

A correlation between the equilibrium ratio of tautomeric products generated by the asymmetric Michael reactions of cyclic carbon acids with β,γ-unsaturated α-keto esters and the chemical shift of the α-proton in starting nucleophilic substrates was revealed which makes equilibration predictable. New tetrahydropyran-fused benzo[a]phenazins were enantioselectively (up to 99% ee) synthesized from β,γ-unsaturated α-keto esters and benzo[a]phenazin-5-ol, a powerful anti-cancer agent sAJM589. Facile recyclability of catalyst Ia in the catalytic reactions was demonstrated.

Raistrickiones A-E from a Highly Productive Strain of Penicillium raistrickii Generated through Thermo Change

Three new diastereomers of polyketides (PKs), raistrickiones A−C (1⁻3), together with two new analogues, raistrickiones D and E (4 and 5), were isolated from a highly productive strain of Penicillium raistrickii, which was subjected to an experimental thermo-change strategy to tap its potential of producing new secondary metabolites. Metabolites 1 and 2 existed in a diastereomeric mixture in the crystal packing according to the X-ray data, and were laboriously separated by semi-preparative HPLC on a chiral column. The structures of 1⁻5 were determined on the basis of the detailed analyses of the spectroscopic data (UV, IR, HRESIMS, 1D, and 2D NMR), single-crystal X-ray diffractions, and comparison of the experimental and calculated electronic circular dichroism spectra. Compounds 1⁻5 represented the first case of 3,5-dihydroxy-4-methylbenzoyl derivatives of natural products. Compounds 1⁻5 exhibited moderate radical scavenging activities against 1,1-diphenyl-2-picrylhydrazyl radical 2,2-diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl (DPPH).

Stachybotrychromenes A-C: novel cytotoxic meroterpenoids from Stachybotrys sp

In the course of gaining new insights into the secondary metabolite profile of various Stachybotrys strains, in particular concerning triprenyl phenol-like compounds, so far, unknown metabolites with analogous structural features were discovered. Three novel meroterpenoids containing a chromene ring moiety, namely stachybotrychromenes A–C, were isolated from solid culture of the filamentous fungus Stachybotrys chartarum DSMZ 12880 (chemotype S). Their structures were elucidated by means of comprehensive spectroscopic analysis (1D and 2D NMR, ESI-HRMS, and CD) as well as by comparison with spectroscopic data of structural analogues described in literature. Stachybotrychromenes A and B exhibited moderate cytotoxic effects on HepG2 cells after 24 h with corresponding IC₅₀ values of 73.7 and 28.2 μM, respectively. Stachybotrychromene C showed no significant cytotoxic activity up to 100 μM. Moreover, it is noteworthy that stachybotrychromenes A–C are produced not only by S. chartarum chemotype S but also S. chartarum chemotype A and Stachybotrys chlorohalonata.

Synthesis and Evaluation of Thiochroman-4-One Derivatives as Potential Leishmanicidal Agents

The S-containing heterocyclic compounds benzothiopyrans or thiochromones stand out as having promising biological activities due to their structural relationship with chromones (benzopyrans), which are widely known as privileged scaffolds in medicinal chemistry. In this work, we report the synthesis of 35 thiochromone derivatives and the in vitro antileishmanial and cytotoxic activities. Compounds were tested against intracellular amastigotes of Leishmania panamensis and cytotoxic activity against human monocytes (U-937 ATCC CRL-1593.2). Compounds bearing a vinyl sulfone moiety, 4h, 4i, 4j, 4k, 4l and 4m, displayed the highest antileishmanial activity, with EC₅₀ values lower than 10 μM and an index of selectivity over 100 for compounds 4j and 4l. When the double bond or the sulfone moiety was removed, the activity decreased. Our results show that thiochromones bearing a vinyl sulfone moiety are endowed with high antileishmanial activity and low cytotoxicity.