Regioselective Synthesis of Procyanidin B6, A 4-6-Condensed (+)-Catechin Dimer, by Intramolecular Condensation

Lone Pair-π Interactions in Organic Reactions

Noncovalent interactions between a lone pair of electrons and π systems can be categorized into two types based on the nature of π systems. Lone pair-π(C═O) interactions with π systems of unsaturated, polarized bonds are primarily attributed to orbital interactions, whereas lone pair-π(Ar) interactions with π systems of aromatic functional groups result from electrostatic attractions (for electron-deficient aryls) or dispersion attractions and Pauli repulsions (for electron-rich/neutral aryls). Unlike well-established noncovalent interactions, lone pair-π interactions have been comparatively underappreciated or less used to influence reaction outcomes. This review emphasizes experimental and computational studies aimed at integrating lone pair-π interactions into the design of catalytic systems and utilizing these interactions to regulate the reactivity and selectivity of chemical transformations. The role of lone pair-π interactions is highlighted in the stabilization or destabilization of transition states and ground-state binding. Examples influenced by lone pair-π interactions with both unsaturated, polarized bonds and aromatic rings as π systems are included. At variance with previous reviews, the present review is not structured according to the physical origin of particular classes of lone pair-π interactions but is divided into chapters according to ways in which lone pair-π interactions affect kinetics and/or selectivity of reactions.

Optimization parameters for the production of dimer of epicatechin from an endophytic fungus Curvularia australiensis FC2AP using response surface methodology (RSM)

The search for natural therapeutic agents has intensified due to their potential to treat various diseases. Bioactive secondary metabolites from endophytes offer high therapeutic profiles and can be mass-produced after optimizing medium parameters and purification. This investigation aimed to maximize crude pigmented secondary metabolite (CPSM) production from Curvularia australiensis FC2AP by optimizing fermentation conditions statistically. The endophytic fungus produced a maximum yield of 8.81 UL/g from biomass using Sabouraud's Dextrose Broth. After screening essential factors, the Plackett-Burman design was used for factorial optimization, and the Box Behnken design was employed to investigate three significant factors. The final CPSM yield was 12.3 UL/g, approximately 4-fold higher than the preliminary growth medium. Chromatographic purification using a gradient solvent system resulted in six fractions, with the fourth fraction demonstrating the highest bioactivity profile. Structural characterization confirmed this fraction to be a dimer of epicatechin, which has anti-cancer properties, as confirmed through in vivo studies on Sprague Dawley rats. This is the first report of a epicatechin dimer produced from C. australiensis.

Procyanidins and Phlobatannins from the Twigs of Rosa multiflora and Their Neuroprotective Activity

Three new procyanidins (1-3), two new phlobatannins (6 and 7), a new flavan-3,4-diol glycoside (9), and a new neolignan glycoside (10), along with three previously reported compounds (4, 5, and 8) were isolated from the twigs of Rosa multiflora. The chemical structures of the new compounds (1-3, 6, 7, 9, and 10) were characterized by spectroscopic data interpretation, including NMR (¹H and ¹³C NMR, ¹H-¹H COSY, HSQC, HMBC, and NOESY) and HRESIMS analysis. Experimental ECD data analysis was conducted to assign the absolute configurations of the new compounds (1-3, 6, 7, 9, and 10). The absolute configuration of the sugar moieties was verified through a chiral derivatization method and LC-MS analysis. All the isolated compounds (1-10) were evaluated for their anti-neuroinflammatory activity based on inhibitory effects on nitric oxide production using a lipopolysaccharide-stimulated murine microglia BV-2 cell line and for their neurotrophic effects on nerve growth factor induction in C6 glioma.

Efficient Concentration of Functional Polyphenols Using Their Interaction with Gelatin

Among polyphenol compounds, the flavan-3-ol structure, which is the basic unit of green tea catechins and the galloyl groups contained in green tea catechins are known to exhibit various functions. In this paper, we discuss how to concentrate highly functional polyphenol compounds by exploiting the interaction between gelatin and the catechol structures. First, we confirmed the interaction between heat-stabilized gelatin and flavan-3-ol derivatives, including synthesized compounds. When green tea leaf extract containing a large amount of flavan-3-ol derivatives was incubated with gelatin, most of the polyphenol compounds it contained were adsorbed. Because the compounds adsorbed on gelatin could not be eluted, DPPH radical and ABTS radical scavenging activity tests were conducted using the as-prepared gelatin-polyphenol complex. Radical scavenging activity was observed when the compounds were adsorbed on gelatin and heating at 90 °C for 5 min did not have a significant effect on their activity. These results suggest that functional polyphenols can be efficiently concentrated using heat-stabilized gelatin and retain their functionality while adsorbed.

Computational Analysis of the Selective Formation of the C4α-C8' Bond in the Intermolecular Coupling of Catechin Derivatives

Procyanidin B3 is a natural flavonoid composed of two catechins connected via a C4α-C8' bond. The couplings of catechin derivatives, promoted by Lewis acids, have been widely applied to the syntheses of procyanidin B3 and related flavonoids because the reactions construct the C4α-C8' bond in a highly stereo- and regioselective manner. However, the structural complexity of the catechin derivatives has complicated the exploration of a detailed mechanism for this selectivity. Here, we report the results of a computational study to provide plausible origins for the selective C4α-C8' bond formation of catechin derivatives 1 and 2 by using models 5 and 7. Although a systematic search did not provide S_N2-like transition states, we successfully identified transition states TS-A, TS-B, and TS-C for the S_N1-type C4α-C8', C4β-C8', and C4α-C6' bond formations, respectively, from a total of 233 transition states to justify the stereo- and regioselectivity of the experimental results. The analysis of these structures by NCIPLOT mapping and the distortion/interaction strain model suggests that the eclipsed interaction at the forming C-C bond between the electrophile and the nucleophile destabilizes TS-B, while the strain of the electrophile destabilizes TS-C. Consequently, the C4α-C8' bond is formed via the lowest energy transition state TS-A.

Flavan-3-ols Content in Red Raspberry Leaves Increases under Blue Led-Light Irradiation

Berry fruits are well known to contain large amounts of polyphenol compounds. Among them, flavan-3-ol derivatives are a group of secondary metabolism compounds currently attracting a great deal of attention owing to their health benefits. Not only the fruits, but also the leaves of raspberry plants, are highly esteemed for tea making around the world and are largely used for food. In this report, we discuss the results of our study on the effect of light and temperature on polyphenol accumulation in raspberry leaves. When raspberry was cultivated in a plant factory unit and light intensity, wavelength, and temperature were varied, the amount of total polyphenol increased under blue light. Quantitative determination of (+)-catechin, (⁻)-epicatechin, procyanidin B4, flavan-3-ol trimer, which are flavan-3-ol derivatives, was carried out using HPLC, whereby we confirmed their increase under blue light. Semi-quantitative RT-PCR showed correlation between chalcone synthase (CHS) gene expression and the amounts of the compounds measured in the leaves.

A Computational Study on the Stereo- and Regioselective Formation of the C4α-C6' Bond of Tethered Catechin Moieties by an Exhaustive Search of the Transition States

We previously reported the total synthesis of procyanidin B6 by using the stereo- and regioselective C-C bond formation of tethered catechin moieties as the key step. The reaction afforded the product bearing a new C4α-C6' bond linkage instead of the inherently preferable C4α-C8' bond. However, the origin of this selectivity remained unclear due to the complex structure of the substrate. Here we report the results of computational exploration of this C-C bond formation to gain mechanistic insights into the selectivity. The computational study of highly flexible compounds was realized by an exhaustive search of transition states. A large library of candidate transition states was generated by a conformational search of constrained models using molecular mechanics simulations and semiempirical molecular orbital calculations. Subsequent DFT-based transition state calculations provided 367 transition states for C4-C6' and C4-C8' bond formations. Comparison of the geometries and energies showed that the C4α-C6' linkage is preferentially formed via two competing transition states, leading to a C6'-diastereomeric mixture. Interactive atomic distances and visualization of the nonbonding interactions suggest the importance of nonclassical hydrogen bonding and CH-π, π-π, and lone pair-π interactions in stabilizing the two transition states. The present study supports preferential C4α-C6' bond formation of the tethered catechins.