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.

Plant Polyphenols: Chemical Properties, Biological Activities, and Synthesis

Dietary phenolics: chemistry, bioavailability and effects on health.

Proanthocyanidins (syn condensed tannins) are complex flavonoid polymers naturally present in cereals, legume seeds and particularly abundant in some fruits and fruit juices. They share some common structural features—phenolic nature and high molecular weight—with phenolic polymers found in black tea and red wine (called here tannin-like compounds). The polymeric nature of proanthocyanidins makes their analysis and estimation in food difficult. For this reason, little is known about their consumption, although they likely contribute a large part of the daily polyphenol intake. They also share common physicochemical properties: they form stable complexes with metal ions and with proteins and are, like other polyphenols, good reducing agents. Many of their biological effects of nutritional interest derive from these properties. As metal ion chelators, they influence the bioavailability of several minerals. The nutritional significance of the non-specific complexation of proteins is less clear. As reducing agents, they may participate in the prevention of cancers, both of the digestive tract and inner organs. They may also protect LDLs against oxidation and inhibit platelet aggregation and therefore prevent cardiovascular diseases. In vitro, animal and human studies on the prevention of these chronic diseases are reviewed with particular attention to wine and tea polyphenols. The lack of data on their bioavailability and the paucity of human studies are emphasised.

© 2000 Society of Chemical Industry

Proanthocyanidins and tannin-like compounds – nature, occurrence, dietary intake and effects on nutrition and health

Phenolic secondary metabolites play an important role in plant-derived food quality, as they affect quality characteristics such as appearance, flavour and health-promoting properties. Their content in foods is affected by many factors that influence phenolic stability, biosynthesis and degradation. In terms of their biosynthesis the key enzyme phenylalanine ammonia-lyase (PAL) is especially relevant, as it can be induced by different stress (environmental) conditions. In addition, polyphenol oxidases (PPO) and peroxidases (POD) are the main enzymes responsible for quality loss due to phenolic degradation. The different factors affecting phenolic-related food quality are reviewed. These include internal (genetic) and environmental (agronomic) factors, technological treatments applied during postharvest storage of fruits and vegetables, as well as processing and storage of the processed products. The different strategies that are required to either maintain or enhance the phenolic-related quality of foods are critically reviewed. Genetic modification designed to decrease polyphenol oxidases or peroxidases is not always a feasible method, owing to side problems related to the growth and defence of the plant. Agronomic treatments can be used to enhance the phenolic content and pigmentation of fruits and vegetables, although the information available on this topic is very scarce and even contradictory. Some postharvest treatments (cold storage, controlled or modified atmospheres, etc) can also improve phenolic-related quality, as well as new processing methods such as irradiation (gamma, UV), high-field electric pulses, high hydrostatic pressures and microwaves. Les composes phenoliques, metabolites secondaires des plantes, jouent un role dans la qualite des fruits, au niveau de l'apparence, de la flaveur et des qualites nutritives. L'enzyme cle de leur biosynthese est la phenylalanine ammonium-lyase, regulee par les facteurs environnementaux. La polyphenol oxydase et la peroxydase sont responsables des degradations donc des pertes de qualite. Les differents facteurs internes (genetiques) et externes (agronomiques) affectant la qualite des produits vegetaux sont etudies. Les differentes mesures envisageables (modification genetique, traitements chimiques ou physiques apres recolte) pour modifier le contenu en compose phenoliques et donc la qualite des produits vegetaux sont etudies.

Phenolic compounds and related enzymes as determinants of quality in fruits and vegetables

The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

Complete and unequivocal 1H and 13C NMR assignments for a range of green tea polyphenols [epiafzelechin, catechin, epicatechin, gallocatechin, epigallocatechin, catechin-3-O-gallate, epicatechin-3-O-gallate, gallocatechin-3-O-gallate, epigallocatechin-3-O-gallate and epigallocatechin-3-O-(3′-O-methyl)-gallate] in acetone-d6 solvent have been achieved using the 2D proton–carbon correlation experiments HMQC and HMBC and also 1D NOE difference spectroscopy.

1H and 13C NMR Assignments of Some Green Tea Polyphenols

Amaryllidaceae and Sceletium alkaloids

Amaryllidaceae, Sceletium, imidazole, oxazole, thiazole, peptide and miscellaneous alkaloids (July 1998 to June 1999)

Amaryllidaceae, Sceletium, imidazole, oxazole, thiazole, peptide and miscellaneous alkaloids

Caffeine is found in both coffee and tea whilst polyphenols are present in a wide variety of foods and beverages. Theaflavin and its gallate esters are polyphenolic molecules which can be isolated from black tea infusions. The theaflavin family of polyphenols contribute to the taste and colour of tea, and their complexation with caffeine is thought to be largely responsible for the formation of tea cream, a precipitate that forms as tea cools. The self-association of theaflavin and caffeine was studied using nuclear magnetic resonance methods (chemical shift changes and self-diffusion constants on dilution) and it is shown that caffeine forms stacks of molecules (Ks = 7.9 l mol−1 at 300 K), while theaflavin forms stable dimers (Ks = 230 l mol−1). The theaflavin monomer consists of a planar benzotropolone ring system, with the two flavan rings approximately orthogonal to this plane, and stacked against each other. In the dimer, two benzotropolone rings align with an antiparallel geometry. Two molecules of caffeine bind to one molecule of theaflavin in a strictly sequential manner, with first and second association constants of 11.9 and 16.5 l mol−1, respectively. It is proposed that the first caffeine inserts between the two flavan rings, and the second then binds to the newly liberated flavan surface.

John R. Lewis

Papers

1H and 13C NMR Assignments of Some Green Tea Polyphenols

Amaryllidaceae and Sceletium alkaloids

Amaryllidaceae, Sceletium, imidazole, oxazole, thiazole, peptide and miscellaneous alkaloids (July 1998 to June 1999)

Amaryllidaceae, Sceletium, imidazole, oxazole, thiazole, peptide and miscellaneous alkaloids

The self-association of the black tea polyphenol theaflavin and its complexation with caffeine