Stereoselective synthesis and self-assembling capabilities of heterocyclic cinnamic acids

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Cinnamic acid (E-3-phenylpropenoic acid) is a well-known material, because it can be found in cinnamon, which was mentioned for the first time on ancient Egyptian hieroglyphs (2000 B.C). The oil of leafs and the bark of Cinnamomum Verum (cinnamon plants are native in Sri Lanka) contains cinnamaldehyde as a main component (50−80%) and several percent of esters and terpenes. Cinnamon (the bark of the plant) and the oil of the leafs have been widely used as flavor and food preservative as well for thousands of years, because some of its components, mostly the cinnamic acid derivatives, have antimicrobial effect on pathogenic bacteria and on human diseases like upper respiratory tract infection caused by Pneumococcus. As it was mentioned, cinnamic acid can be found in cinnamon, due to the slow oxidation of cinnamaldehyde to the more stable E-cinnamic acid on contacting with air. Cinnamic acid was synthesized first by W.H. Perkin in 1868 but, its use and those of its derivatives were delayed until world war II., where the first UV-protective sunscreen was prepared from cinnamate esters for the soldiers fighting in the Pacific area. The second “big boom” in the history of cinnamic acid, came at the late 80’s, when they gained importance as sample matrices in the MALDI−MS technique due to its high absorbance in the UV range (the average molar extinction coefficient is around: 19556 M-1cm-1). The substituted derivatives of cinnamic acids and some heteroatom-containing derivatives came into the focus of our research group, since they proved to be good models for studying hydrogen bonding driven assembling in solutions as well as in the solid state. It was found that they were capable short- and long-range ordering in solution and the solid state, respectively. The fundamental units were found to be the dimers, kept together by strong hydrogen bonds, while the multimers of the dimers interacting with weaker (aromatic)C−H…X (where X = O, N, S) close contacts. In most of these studies, the combination of spectroscopic methods and molecular modelling was applied for the characterization of the structure-forming interactions. We developed several methods for the stereoselective synthesis of heterocyclic cinnamic acids; the Knövenagel-Döbner condensation for the E-selective and the Horner-Wadsworth-Emmons olefination for the Z-selective preparation of cinnamic acids. The UV-induced photoisomerisation is found to be useful as well to prepare E-Z isomer mixtures from the more easily accessible E isomer. The structure forming properties were studied intensely with spectroscopyc methods (IR, 1H NMR, COSY, XRD) and many secondary interactions were found among them, including double hydrogen bonds between the carboxylic group, weak hydrogen bonds among the aromatic protons and pi-stacking with the aromatic rings. The self-assembling capabilities were studied over metal surfaces as well with the combination of IRM, SEM and AFM. High-order organization was found on each surface indicating well the presence of various interactions. The experiments were checked by quantum chemical calculations, including conformational analysis and precise energy and geometry calculations for the extended hydrogen-bonded aggregates.

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