Biomedical subjects

Pascal Richomme

Publications and source records attributed to Pascal Richomme.

24 records · Page 2Linked to original sources

PubMed2003

Crown-annelated oligothiophenes as model compounds for molecular actuation.

Crown-annelated quater- (4T) and sexithiophenes (6T) with oligooxyethylene chains of various lengths attached at the 3-positions of the terminal thiophene rings have been synthesized. Analysis of the cation-binding properties of the macrocycles by (1)H NMR and UV-vis spectroscopy reveals the formation of a 1:1 complex with Ba(2+), Sr(2+), or Pb(2+) and shows that cation complexation results in a conformational transition in the pi-conjugated system. Theoretical analysis of this process by density functional methods predicts that this conformational transition results in a narrowing of the highest occupied-lowest unoccupied molecular orbital gap of the conjugated system with a decrease of the redox potentials (E(0)(1) and E(0)(2)) associated with the formation of the 4T cation radical and dication. Cyclic voltammetry shows that, depending on the binding constant, the presence of metal cation produces a negative or a positive shift of E(0)(1) while E(0)(2) always shifts negatively. This unusual behavior is discussed in terms of interplay between electrostatic interactions and conformational changes associated with cation binding.

Journal Article↗

PubMed2003

First 2-hydroxy-3-methylbut-3-enyl substituted xanthones isolated from plants: structure elucidation, synthesis and antifungal activity.

Two new 2-hydroxy-3-methylbut-3-enyl substituted xanthones, (+/-)-caledol 1 and (+/-)-dicaledol 2 were isolated from a dichloromethane extract of the leaves of Calophyllum caledonicum (Clusiaceae). Compounds 1 and 2 are the first 2-hydroxy-3-methylbut-3-enyl substituted xanthones isolated from natural source. Their structures were elucidated by means of combined analytical methods including HRFABMS, 1D and 2D NMR spectroscopies and also confirmed by total synthesis using biomimetic ortho-prenylphenols photooxygenation (1O2) as a key step. The antifungal activity against Aspergillus fumigatus is reported.

Aspergillus fumigatus↗

PubMed2002

Synthesis of oligothiophene-bridged bisporphyrins and study of the linkage dependence of the electronic coupling.

A set of twelve porphyrin dimers has been prepared to give information on how the type of connectivity between a porphyrin core and a bridge can influence the interporphyrin electronic interaction. The new porphyrin systems are substituted directly at the meso position with an oligothiophene chain tethered either with a single C-C sigma bond, a trans ethylenyl group, or a acetylenyl group. The compounds are easily obtained by palladium-catalyzed cross-coupling reactions (Stille, Heck, and Sonogashira) between 5-iodo-10,15,20-(3,5-ditert-butylphenyl)porphyrin and the appropriate oligothiophene derivative. This synthetic approach is straightforward and very effective for preparing oligothiophene-based prophyrin systems. The absorption spectra and the fluorescence properties of the dimers demonstrated the crucial importance of the characteristics of the chemical bond used to connect the bridge to the porphyrin unit. The magnitude of the electronic communication can thus be significantly modulated by altering the type of bond connectivity used to link the chromophore to the bridge. The present work shows that an oligothiophene spacer is a viable class of linker for connecting porphyrins, and that a quaterthiophene appended with ethynyl linkages affords a high electronic interaction over a distance as large as 28 A. A detailed computational study of these dimers has clarified the conditions needed for a conjugated system to behave as a molecular wire. These conditions are full planarity of the molecule and proper energy matching between the frontier orbitals of the bridge and the porphyrin. Intermolecular energy transfer in asymmetrical dyads composed of a zinc porphyrin and a freebase porphyrin has been studied by fluorescence spectroscopy. In all systems, this process is more than 98% efficient, and its rate constant decreases steadily in the order 4ZH > 1ZH > 3ZH approximately 2ZH. Thus, the largest rate (kEnT = 1.2 x 10(11) s-1) was found in the dyad linked with bisethynyl quaterthiophene, which represents the longest bridge within the series. These results clearly demonstrate that strong communication and also efficient photoinduced processes can be promoted over a large distance if the electronic structure of the molecular connector is appropriately chosen.

Journal Article↗

PubMed2002

New and antifungal xanthones from Calophyllum caledonicum.

Two new xanthones, namely caledonixanthones E (1) and F (2), were isolated from the stem bark of Calophyllum caledonicum (Guttiferae). The structural elucidation of these compounds was mainly established on the basis of 1D, 2D NMR and HRMS spectroscopic analysis. Among the isolated compounds, eight other known xanthones were also identified in the course of this phytochemical study. In addition to this report, a preliminary evaluation of the antifungal properties of these polyphenolic compounds against Aspergillus fumigatus and Candida albicans is presented.

Antifungal Agents↗

PubMed2002

Synthesis and antimalarial activity of some new 1,2-dioxolane derivatives.

The synthesis of 1,2-dioxolane derivatives in two different acetophenone series, as simplified models of natural coumarins is described. 2-Acetyl-3-acetoxy-4-(3-hydroperoxy-3-methylbut-1-enyl)phenyl acetate and 2-acetyl-5-acetoxy4-(3-hydroperoxy-3-methylbut-1-enyl) phenyl acetate synthons are used as precursors to these structures. In vitro antimalarial activity of the 1,2-dioxolane derivatives has been investigated.

Acetophenones↗

PubMed1999

Electroregulated Metal-Binding with a Crown Ether Tetrathiafulvalene Derivative: Toward Electrochemically Addressed Metal Cation Sponges.

A redox responsive ligand incorporating the tetrathiafulvalene unit has been synthesized. The crystal structure of the free ligand (Z)-1 (C(20)H(30)O(5)S(8), triclinic P&onemacr;, Z = 2, a = 9.087(6) Å, b = 11.637(7) Å, c = 14.370(8) Å, alpha = 65.54(3) degrees, beta = 82.32(5) degrees, gamma = 84.18(6) degrees, V = 1368 Å(3)) shows the redox-active tetrathiafulvalene core to be essentially planar, which allows observation of two reversible one-electron processes upon electrochemical oxidation. The efficiency of this system in the control of the reversible complexation/expulsion sequence of a metallic cation (i.e., Ba(2+)) has been made possible thanks to a combination of (a) an unprecedented high coordination ability among tetrathiafulvalene-based macrocycles as determined by LSI mass spectrometry (log K degrees = 3.5, NBA-matrix) as well as by solution investigations ((1)H NMR and cyclic voltammetry titration studies), which remarkably converge to similar binding constant values (i.e., log K degrees = 4.2-4.3), and (b) reversible metal cation expulsion upon electrochemical oxidation to the dicationic state. A channel-like solid-state structure is observed for the Ba(2+) complex (C(20)H(30)O(5)S(8), Ba(2+)(CF(3)SO(3))(2)(2-), (H(2)O)(2), CD(3)CN, monoclinic C2/c, Z = 8, a = 45.66(1) Å, b = 8.897(5) Å, c = 23.124(8) Å, beta = 105.54(4) degrees, V = 9050 Å(3)), which results from the segregated stacking mode of the crown ether and the redox-active tetrathiafulvalene subunits, respectively.

Journal Article↗