Current Organic Chemistry - Volume 16, Issue 6, 2012

... Naturally occurring (poly)phenols are secondary metabolites that constitute an ubiquitous family of natural products, encountered in both plant1 and marine sources2. For illustration, the number of (poly)phenols isolated from the plant kingdom alone already amounts to several thousands (!) and this number still increases in a continuous and impressive manner. Fortunately, Mother Nature does not accomplish the combinatorial synthesis of this family of natural products by accident, these secondary metabolites being demonstrated to be essential actors in chemical ecology.3,4 And what about (poly)phenols effects on human health ? Several epidemiological studies actually suggest that diets rich in (poly)phenols5 have a positive impact on human health, such diets being associated with reduced risk of a number of chronic diseases including cancers, cardiovascular diseases and age-related disorders.6 Hence, this particularly favorable context leads to the recent emergence of diverse individual (poly)phenols as serious candidates for the prevention and treatment of several human diseases.7 As suggested by the title of this issue, (POLY)PHENOL CHEMISTRY: Part II - Physicochemical Properties & Biological Implications aims at giving state-of-the-art details on the main physicochemical properties of (poly)phenols and/or their subsequent biological benefits. For these purposes, this issue is composed of 4 in-depth reviews written by Olivier DANGLES (University of Avignon, Avignon, France), Maite- Teresa ESCRIBANO-BAILON and Celestino SANTOS-BUELGA (Universidad de Salamanca, Salamanca, Spain), Victor de FREITAS and Nuno MATEUS (Universidade do Porto, Porto, Portugal), and Michel BALTAS et al. (Université de Toulouse, Toulouse, France). The first review of this collection of four essays, contributed by O. DANGLES, points out the remarkable (and popular) antioxidant activity of (poly)phenols by focusing on the corresponding chemical mechanisms and the resulting biological effects.

Polyphenols having catechol (1,2-dihydroxybenzene) nuclei are strong in vitro antioxidants owing to their ability to rapidly reduce reactive oxygen species (ROS), bind transition metal ions as inert complexes and regenerate the potent chain-breaking antioxidant α -tocopherol. Polyphenols provide antioxidant protection to plants by acting as electron donors to peroxidases that reduce hydrogen peroxide into water. Polyphenols are also likely to improve the oxidative stability of food emulsions by binding metal ions and/or reducing ROS at phospholipid or protein interfaces. This protection may extend to the gastric compartment where millimolar concentrations of polyphenols can accumulate after consumption of a meal rich in plant products. Beyond the digestive tract, direct antioxidant effects become quite hypothetical as polyphenols are only poorly absorbed and mostly converted into weakly reducing metabolites. However, despite the low circulating concentrations of polyphenol metabolites, ROS-scavenging and/or inhibition of ROS-producing enzymes could still operate provided that polyphenol metabolites accumulate on the very sites of oxidative stress and inflammation. Finally, oxidation of polyphenols in cells can lead to electrophilic/oxidizing metabolites with a capacity to up-regulate genes coding for antioxidant enzymes. In conclusion, the ability of polyphenols to act as antioxidants in humans must be severely qualified.

Copigmentation is the main colour-stabilizing mechanism in plants and in food products of vegetable origin. It is a spontaneous and exothermic process that consists of the stacking of an organic molecule, called copigment, on the planar polarizable moieties of the anthocyanin coloured forms. Although this phenomenon has long been described, there are some aspects that are still not well understood or controversial like the nature of the interaction pigment to copigment, the way to quantify the extent of the process, its effect on other anthocyanin properties like astringency or reactivity. In this article a review of the most significant advances achieved in the last years in the field of intramolecular and intermolecular copigmentation is presented. Also, the most recent findings regarding wine copigments and their effects on the colour of red wines are revised.

The interactions between polyphenols and proteins have been long described from both nutritional and pharmaceutical perspectives, being relevant for some sensorial and biological properties. Several variables known to drive these interactions have been extensively assayed and are still being presently studied by some research groups. The different physical-chemical features influencing protein- polyphenol interactions make it difficult to clearly establish the mechanisms involved. In a food context, those interactions will be decisive in modulating some organoleptic properties such as astringency of foodstuffs and beverages. This matter has been of a growing interest for the Food Industry. The present review aims to provide the most relevant data gathered so far on the interactions between proteins and polyphenols, essentially from a food context.

Cinnamic acid and its phenolic analogues are natural substances. Chemically, cinnamic acids or the 3-phenyl acrylic acids, offer three main reactive sites: substitution on the phenyl ring, addition on the α ,β -unsaturation and reactions of the carboxylic acid. Owing to these chemical aspects, cinnamic acid derivatives received much attention in medicinal research as traditional as well as valuable scaffolds in recent synthetic bioactive agents. In the last two decades, there has been huge attention towards various cinnamoyl derivatives and their biological efficacy. This review provides a comprehensive literature compilation concerning the synthesis of various cinnamoyl acids, esters, amides, hydrazides and related derivatives and their biological activity evaluations against diseases such as tuberculosis and malaria, which are frequent in developing countries and cardiovascular diseases, which cause a high mortality rate worldwide. We envisage that our effort in this review contributes a much needed and timely addition to the literature of medicinal research.

For long manned space missions, regenerative life support systems must provide carbohydrates. In addition to the necessity to develop such fully closed systems, the formose reaction (FR), the alkaline polymerisation of formaldehyde using Ca(OH)2, seems to be the only current single-step method for the production of synthetic carbohydrates. The FR received much attention with regard to edible carbohydrate production during the 1960s, 70s and 80s. In addition, its importance is also related to the prebiotic and interstellar synthesis of carbohydrates. This review gives an overview about the research attempts that have already been reported on the FR in the literature. The reaction system, mechanism, as well as basic physicochemical parameters are discussed. Many different research groups investigated the fundamentals of the FR as well as the side reaction (cannizzaro reaction (CR)) and downstream reaction (cross cannizzaro reaction (CCR)). In addition, a number of investigations focused on the selective synthesis of carbohydrates. Full conversion of formaldehyde leads to a very complex mixture of compounds, which represents an unfavourable downside of the FR-system. In order to minimize the product distribution, a partial formaldehyde conversion and variation of physicochemical parameters may be helpful. Finally, the FR-system is still not fully unravelled and represents an opportunity for exciting research.

The present review provides a comprehensive summary of microwave-assisted preparation of five-membered azaheterocyclic systems, such as pyrrole, pyrazole, imidazole, triazole, thiazole, isothiazole, oxazole, isoxazole, oxadiazole, thiadiazole and tetrazole.