DFT 1H-1H coupling constants in the conformational analysis and stereoisomeric differentiation of 6-heptenyl-2H-pyran-2-ones: configurational reassignment of synargentolide A

Dihydro-furanones from Hyptis species: Chemical correlations and DFT-NMR/ECD calculations for stereochemical assignments

Dihydro-furanones are bioactive compounds isolated from various plants, marine fungi, and sponges. The present investigation describes the isolation by recycling HPLC and structural characterization by NMR of four previously undescribed 2(5H)-furanones, monticofuranolide A and pectinolides N-P, one phenylpropanoid, rosmarinic acid, and five known flavonoids, in addition to the undescribed natural flavonoid, 2R,3R-dihydrogossipetin or 5,7,8,3',4'-pentahydroxy flavanonol, from collections of H. monticola Mart. ex Benth and Hyptis pectinata (L.) Poit. Chemical correlations, resembling the biogenetic relationship of the isolated 2(5H)-furanones with their 5,6-dihydro-2H-pyran-2-one precursors, were accomplished to confirm their absolute configuration. Density functional theory-NMR/ECD calculations have been used to solve the absolute configuration for this type of compounds.

Recent Advances in the Stereoselective Total Synthesis of Natural Pyranones Having Long Side Chains

Pyranone natural products have attracted great attention in recent years from chemists and biologists due to their fascinating stereoisomeric structural features and impressive bioactivities. A large number of stereoselective total syntheses of these compounds have been described in the literature. The natural pyranones with long side chains have recently received significant importance in the synthetic field. In the present article, we aim to review the modern progress of the stereoselective total syntheses of these natural pyranones containing long-chain substituents.

Recent trends in the structural revision of natural products

Covering: 2012 to 2017 This article reviews recent reports on the structural revision of natural products. Through a critical assessment of the original and revised published structures, the article addresses why each structure was targeted for revision, discusses the techniques and key discrepancies that led to the proposal of the revised structure, and offers measures that may have been taken during the original structure determination to prevent error. With the revised structures in hand, weaknesses of original proposals are assessed, providing a better understanding on the logic behind structure determination.

Antifungal Phenylpropanoid Glycosides from Lippia rubella

Lippia species share various pharmacological activities and are used in traditional cooking and medicine worldwide. Combined chromatographic techniques such as column chromatography, high-performance liquid chromatography, and countercurrent chromatography led to the purification of two new antifungal phenylpropanoid glycosides, lippiarubelloside A (1) and lippiarubelloside B (2), by bioactivity-directed fractionation of an ethanol-soluble extract from Lippia rubella, in addition to the known active related compounds forsythoside A (3), verbascoside (4), isoverbascoside (5), and poliumoside (6). The structures of compounds 1 and 2 were determined by comparison of their NMR spectroscopic data with the prototype active compound 4. Cryptococcus neoformans, which causes opportunistic lung infections, was sensitive to compounds 1-6 in the concentration range of 15-125 μg/mL. A synergistic effect (FIC_index = 0.5) between 3 and amphotericin B was demonstrated. The glycosylated flavonoids pectolinarin (7), linarin (8), and siparunoside (9) were also isolated.

Structure Elucidation, Conformation, and Configuration of Cytotoxic 6-Heptyl-5,6-dihydro-2 H-pyran-2-ones from Hyptis Species and Their Molecular Docking to α-Tubulin

Cytotoxic 6-heptyl-5,6-dihydro-2 H-pyran-2-ones are chemical markers of Hyptis (Lamiaceae) and are responsible for some of the therapeutic properties of species with relevance to traditional medicine. The present investigation describes the isolation of known pectinolides A-C (1-3), in addition to the new pectinolides I-M (4-8), from two Mexican collections of H. pectinata by HPLC. The novel biosynthetically related monticolides A (9) and B (10) were also isolated by high-speed countercurrent chromatography from H. monticola, an endemic species of the Brazilian southeastern high-altitude regions. A combination of chemical correlations, chiroptical measurements, and Mosher ester NMR analysis was used to confirm their absolute configuration. The utility of DFT-NMR chemical shifts and J_H-H calculations was assessed for epimer differentiation. Molecular docking studies indicated that 6-heptyl-5,6-dihydro-2 H-pyran-2-ones have a high affinity for the pironetin-binding site of α-tubulin, which may be a possible mechanism contributing to the cytotoxic potential of these small and flexible molecules.

Study of E/Z isomerization of (arylamino)methylidenefuran-2(3H)-ones by 1 H, 13 C, 15 N spectroscopy and DFT calculations in different solvents

The structure and configuration of the series of previously unknown arylaminomethylidenefuran-2(3H)-ones have been determined in solution by ¹ H, ¹³ C, ¹⁵ N nuclear magnetic resonance spectroscopy including two-dimensional experiments such as ¹ H─¹ H COSY, dqCOSY, ¹ H─¹³ C HSQC, ¹ H─¹³ C HMBC. It was found that synthesized substances exist as an equilibrium mixture of E- and Z-enamines in solution. It was established on the basis of density functional theory calculations that the exchange between the two push-pull enamines is a simple rotation around an exocyclic partial double bond that depends on the effect of the solvents. Copyright © 2017 John Wiley & Sons, Ltd.

Complementarity of DFT Calculations, NMR Anisotropy, and ECD for the Configurational Analysis of Brevipolides K-O from Hyptis brevipes

Brevipolides K-O (1-5), five new cytotoxic 6-(6'-cinnamoyloxy-2',5'-epoxy-1'-hydroxyheptyl)-5,6-dihydro-2H-pyran-2-ones (IC₅₀ values against six cancer cell lines, 1.7-10 μM), were purified by recycling HPLC from Hyptis brevipes. The structures, containing a distinctive tetrahydrofuran ring, were established by comprehensive quantum mechanical calculations and experimental spectroscopic analysis of their NMR and ECD data. Detailed analysis of the experimental NMR ¹H-¹H vicinal coupling constants in comparison with the corresponding DFT-calculated values at the B3LYP/DGDZVP level confirmed the absolute configuration of 3 and revealed its conformational preferences, which were further strengthened by NOESY correlations. NMR anisotropy experiments by the application of Mosher's ester methodology and chemical correlations were also used to conclude that this novel brevipolide series (1-5) share the same absolute configuration corresponding to C-6(R), C-1'(S), C-2'(R), C-5'(S), and C-6'(S).