Synthesis of procyanidins by stepwise- and self-condensation using 3,4-cis-4-acetoxy-3-O-acetyl-4-dehydro-5,7,3′,4′-tetra-O-benzyl-(+)-catechin and (−)-epicatechin as a key building monomer

Computational Study on the Conformational Flexibility-Mediated Intramolecular Oxidative Spirocyclization of Procyanidin B4

Procyanidins, found widely in foods and beverages, are prone to oxidation, yet the chemical structures of their oxidation products and the mechanisms involved remain unclear. Herein, we report that the conformation of procyanidin B4 influences its oxidation products and their stereochemistry. Eight spirocyclized oxidation products were obtained from procyanidin B4 and classified as S- or R-forms based on the configuration of the spiro carbons. The ratios of S- and R-forms derived from the compact and extended rotamers of procyanidin B4, respectively, varied with the solvent. DFT calculations suggested that the four lowest-energy conformers of procyanidin B4 are diverged by interflavan bond rotation and heterocyclic ring inversion. Conformations with an axial-oriented B-ring were estimated as reactive conformations showing proximity between reaction sites on the B- and D-rings. Moreover, the extended rotamer bearing the axially oriented B-ring showed greater stabilization by noncovalent interactions (NCIs), such as OH-π interactions, compared to the counterpart of the compact rotamer. This NCI-based stabilization accounts for a higher production of the R-form despite the predominant presence of the compact rotamer in H2O. These findings highlight the conformational effects that bias the stereoselectivity of oxidative spirocyclization in procyanidin B4, advancing our understanding of procyanidin oxidation mechanisms and product stereochemistry.

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.

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.

Conversion of flavonol glycoside to anthocyanin: an interpretation of the oxidation–reduction relationship of biosynthetic flavonoid-intermediates

An efficient conversion of rutin to the corresponding anthocyanin, cyanidin 3-O-rutinoside, was established. Clemmensen-type reduction of rutin gave a mixture of flav-2-en-3-ol and two flav-3-en-3-ols, which were easily oxidised by air to give the anthocyanin. The interconversion reactions of these flavonoids provide insight into their biosynthetic pathway.

An efficient conversion of rutin to the corresponding anthocyanin, cyanidin 3-O-rutinoside by Clemmensen-type reduction followed by air oxidation was established.

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

Proanthocyanidins, also known as condensed tannins or oligomeric flavonoids, are found in many edible plants and exhibit interesting biological activities. Herein, we report a new, simple method for the stereoselective synthesis of procyanidin B6, a (+)-catechin-(4-6)-(+)-catechin dimer, by Lewis acid-catalyzed intramolecular condensation. The 5-O-t-butyldimethylsilyl (TBDMS) group of 5,7,3′4′-tetra-O-TBDMS-(+)-catechin was regioselectively removed using trifluoroacetic acid, leading to the “regio-controlled” synthesis of procyanidin B6. The 5-hydroxyl group of the 7,3′,4′-tri-O-TBDMS-(+)-catechin nucleophile and the 3-hydroxyl group of 5,7,3′,4′-tetra-O-benzylated-(+)-catechin electrophile were connected with an azelaic acid. The subsequent SnCl₄-catalyzed intramolecular condensation proceeded smoothly to give the 4-6-condensed catechin dimer. This is the first report on the complete regioselective synthesis of a 4-6-connected oligomer without modifying the 8-position.

Structure-Activity Relationship of Oligomeric Flavan-3-ols: Importance of the Upper-Unit B-ring Hydroxyl Groups in the Dimeric Structure for Strong Activities

Proanthocyanidins, which are composed of oligomeric flavan-3-ol units, are contained in various foodstuffs (e.g., fruits, vegetables, and drinks) and are strongly biologically active compounds. We investigated which element of the proanthocyanidin structure is primarily responsible for this functionality. In this study, we elucidate the importance of the upper-unit of 4-8 condensed dimeric flavan-3-ols for antimicrobial activity against Saccharomyces cerevisiae (S. cerevisiae) and cervical epithelioid carcinoma cell line HeLa S3 proliferation inhibitory activity. To clarify the important constituent unit of proanthocyanidin, we synthesized four dimeric compounds, (-)-epigallocatechin-[4,8]-(+)-catechin, (-)-epigallocatechin-[4,8]-(-)-epigallocatechin, (-)-epigallocatechin-[4,8]-(-)-epigallocatechin-3-O-gallate, and (+)-catechin-[4,8]-(-)-epigallocatechin and performed structure-activity relationship (SAR) studies. In addition to antimicrobial activity against S. cerevisiae and proliferation inhibitory activity on HeLa S3 cells, the correlation of 2,2-diphenyl-l-picrylhydrazyl radical scavenging activity with the number of phenolic hydroxyl groups was low. On the basis of the results of our SAR studies, we concluded that B-ring hydroxyl groups of the upper-unit of the dimer are crucially important for strong and effective activity.

Synthesis of Arecatannin A1 from Dimeric Epicatechin Electrophile

Synthesis of arrecatannin A1 (1) was accomplished from dimeric epicatechin electrophile, which was prepared by Zn(OTf)2 mediated self-condensation, and monomeric catechin nucleophile. The condensation was successfully worked using Yb(OTf)3 as a Lewis acid in good yield.

Inhibitory activity of synthesized acetylated Procyanidin B1 analogs against HeLa S3 cells proliferation

Proanthocyanidins, also known as condensed tannins and/or oligomeric flavonoids, occur in many edible plants and have various interesting biological activities. Previously, we reported a synthetic method for the preparation of various procyanidins in pure form and described their biological activities. Here, we describe the synthesis of procyanidin B1 acetylated analogs and discuss their inhibition activities against HeLa S3 cell proliferation. Surprisingly, the lower-unit acetylated procyanidin B1 strongly inhibited the proliferation of HeLa S3 cells. This molecule showed much stronger inhibitory activity than did epigallocatechin-3-O-gallate (EGCG), green tea polyphenol, and dimeric compounds that included EGCG as a unit. This result suggests that the phenolic hydroxyl groups of the upper-units in flavan-3-ols are important for their inhibitory activity against cancer cell proliferation and that a hydrophobic lower unit dimer enhances this activity.

Unified approach to catechin hetero-oligomers: first total synthesis of trimer EZ-EG-CA isolated from Ziziphus jujuba

A catechin hetero-trimer isolated from Ziziphus jujuba has been synthesized. Among three constituent monomers, (-)-epiafzelechin and (-)-epigallocatechin were prepared by de novo synthesis. Trimer formation relied on the unified approach to oligomers based on the bromo-capping and the orthogonal activation, reaching the reported structure of the natural product.

Plant polyphenols: chemical properties, biological activities, and synthesis

Eating five servings of fruits and vegetables per day! This is what is highly recommended and heavily advertised nowadays to the general public to stay fit and healthy! Drinking green tea on a regular basis, eating chocolate from time to time, as well as savoring a couple of glasses of red wine per day have been claimed to increase life expectancy even further! Why? The answer is in fact still under scientific scrutiny, but a particular class of compounds naturally occurring in fruits and vegetables is considered to be crucial for the expression of such human health benefits: the polyphenols! What are these plant products really? What are their physicochemical properties? How do they express their biological activity? Are they really valuable for disease prevention? Can they be used to develop new pharmaceutical drugs? What recent progress has been made toward their preparation by organic synthesis? This Review gives answers from a chemical perspective, summarizes the state of the art, and highlights the most significant advances in the field of polyphenol research.