Total synthesis of (±)-cyclooroidin

The first total synthesis of (±)-cyclooroidin, a member of the pyrrole-imidazole alkaloid family recently isolated from the sponge Agelas oroides in optically pure form, is described. The synthesis was achieved in nine linear steps, with an overall yield of 10%. Key step was a Wolff bromoketone synthesis performed on the intermediate longamide B.     

Thermal decomposition mechanisms of sequential bipolyesters based on propeneglycol and hydroxybenzoic/phthalic diacid derivatives

Some bipolyesters consisting of 1,2-propene glycol residue (P) and symmetric aromatic diacid residue constituted by two p-oxybenzoyl groups coupled by a terephthaloyl (HTH), phthaloyl (HOH) and isophthaloyl (HIH) group, and a nonsymmetric diacid residue constituted by a p-oxybenzoyl and a terephthaloyl group (HT), have been investigated by direct pyrolysis–mass spectrometry. The results show that intramolecular exchange reactions occurring at the pyrolysis stage cause a reorganization of the copolymer sequences. Sequential bipolyesters I–IV are ideal materials for the investigation of the extent of thermal reorganization induced in the bipolymer sequences by the pyrolytic process.     

Electron-transfer reaction of cinnamic acids and their methyl esters with the DPPH(*) radical in alcoholic solutions

The kinetic behavior of cinnamic acids, their methyl esters, and two catechols 1-10 (ArOH) in the reaction with DPPH(*) in methanol and ethanol is not compatible with a reaction mechanism that involves hydrogen atom abstraction from the hydroxyl group of 1-10 by DPPH(*). The rate of this reaction at 25 degrees C is, in fact, comparatively fast despite that the phenolic OH group of ArOH is hydrogen bonded to solvent molecules. The observed rate constants (k(1)) relative to DPPH(*) + ArOH are 3-5 times larger for the methyl esters than for the corresponding free acids and, for the latter, decrease as their concentration is increased according to the relation k(1) = B/[ArOH](0)(m), where k(1) is given in units of M(-1) s(-1), m is ca. 0.5, and B ranges from 0.02 (p-coumaric acid) to ca. 3.48 (caffeic acid) in methanol and from 0.04 (p-coumaric acid) to ca. 13 (sinapic acid) in ethanol. Apparently, the reaction mechanism of DPPH(*) + ArOH involves a fast electron-transfer process from the phenoxide anion of 1-10 to DPPH(*). Kinetic analysis of the reaction sequence for the free acids leads to an expression for the observed rate constant, k(1), proportional to [ArOH](0)(-1/2) in excellent agreement with the experimental behavior of these phenols. The experimental results are also interpreted in terms of the influence that adventitious acids or bases present in the solvent may have. These impurities dramatically influence the ionization equilibrium of phenols and cause a reduction or an enhancement, respectively, of the measured rate constants.     

Peptide backbone folding induced by the C(alpha)-tetrasubstituted cyclic alpha-amino acids 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) and 1-aminocyclopentane-1-carboxylic acid (Ac5c). A joint computational and experimental study

The conformational study of a new group of synthetic peptides containing 4-amino-1,2-dithiolane-4-carboxylic acid (Adt), a cysteine-related achiral residue, has been carried out through a joint application of computational and experimental methodologies. Molecular Dynamics simulations clearly suggest the tendency of this molecule to adopt a gamma-turn conformation in vacuum and help in analyzing the complex and crucial conformational behaviour of the dithiolane ring which appears to preferentially adopt a C(S)-like structure. Electronic structure calculations carried out in solution using the Density Functional Theory also indicate the preservation of the gamma-like folding in apolar solvents and the helix-like one in more polar solvents. A comparison with the achiral 1-aminocycloalkane-1-carboxylic acid (Ac5c) has been carried out using the same computational tools. NMR and IR data on dipeptide derivatives containing the Adt or Ac5c residue show that in chloroform solution all the models prefer a gamma-turn structure, centered at the cyclic residue, stabilized by an intramolecular H-bond, whereas in a more polar solvent, i.e. dimethyl sulfoxide, this folding is not maintained. The experimental conformational studies, extended to N-Boc protected tripeptides, clearly indicate the remarkable tendency of both the five-membered C(alpha)-tetrasubstituted cyclic amino acids Adt and Ac5c to induce the gamma-turn structure also in models able to adopt the beta-bend conformation.     

Selective crystallization of calcium salts by poly(acrylate)-grafted chitosan

The biopolymer chitosan was chemically modified by grafting polyacrylamide or polyacrylic acid in a homogeneous aqueous phase using potassium persulfate (KPS) as redox initiator system in the presence of N,N-methylene-bis-acrylamide as a crosslinking agent. The influence of the grafted chitosan on calcium salts crystallization in vitro was studied using the sitting-drop method. By using polyacrylamide grafted chitosan as substrate, rosette-like CaSO4 crystals were observed. This was originated by the presence of sulfate coming from the initiator KPS. By comparing crystallization on pure chitosan and on grafted chitosan, a dramatic influence of the grafted polymer on the crystalline habit of both salts was observed. Substrates prepared by combining sulfate with chitosan or sulfate with polyacrylamide did not produce similar CaSO4 morphologies. Moreover, small spheres or donut-shaped CaCO3 crystals on polyacrylic acid grafted chitosan were generated. The particular morphology of CaCO3 crystals depends also on other synthetic parameters such as the molecular weight of the chitosan sample and the KPS concentration.     

Enhancing selectivity in oxidation catalysis with sol-gel nanocomposites

Valuable organic compounds such as alpha-hydroxy acids are easily synthesised with relevant selectivity enhancement using a sol-gel hydrophobized nanostructured silica matrix doped with the organocatalyst TEMPO: A materials science based synthetic route which cannot be achieved via classical homogeneous synthesis.     

Mass spectrometric analysis of androstan-17β-ol-3-one and androstadiene-17β-ol-3-one isomers

Mass spectrometric identification and characterization of steroids using electrospray ionization and tandem mass spectrometry has advantages in drug testing and doping control analysis attributable to limitations of gas chromatography followed by electron ionization mass spectrometry. Steroids with an androstadiene-17beta-ol-3-one nucleus and double bonds located either at C-1 and C-4, C-4 and C-9, or C-4 and C-6 were used to determine characteristic fragmentation pathways. Diagnostic dissociation routes are proposed using deuterium labeling, MS3 experiments, and analyses of structurally closely related compounds. Steroids such as boldenone (androst-1,4-diene-17beta-ol-3-one) produced characteristic product ions at m/z 121, 135, and 147. Compounds with double bonds at C-4 and C-9 generated abundant product ions at m/z 145 and 147. Conjugated double bonds at C-4 and C-6 gave rise to an intense and characteristic signal at m/z 133. Stereochemical differentiation between 5alpha- and 5beta-isomers of androstan-17beta-ol-3-ones was possible because of significant differences in relative abundance of product ions generated by elimination of acetone from alpha,beta-saturated 3-keto steroids.     

Evaluation of the antioxidant activity by flow injection analysis method with electrochemically generated ABTS radical cation

A Trolox equivalent antioxidant capacity (TEAC) decolourisation assay was adapted to a flow injection analysis (FIA) system and a simple and rapid method for antioxidant activity evaluation was developed. To avoid the time consuming step of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS) radical cation preparation by chemical oxidation of ABTS, as in the original TAEC assay, and hence, to shorten the analysis time, the ABTS radical cation was generated on-line by electrochemical oxidation of ABTS in the flow-through electrolysis cell forming a part of the FIA system. The proposed method was optimised with respect to a flow rate, injection volume and ABTS radical cation/carrier ratio. Under the optimised conditions linear calibration graphs for Trolox were obtained over the range 10-100 microM, with a limit of detection 1.6 microM. Good reproducibility (relative standard deviation 1.95%) and sample throughput (32 samples per hour) were achieved. The developed method was applied to the evaluation of the antioxidant activity of pure compounds and samples of some common beverages. In both cases a good correlation between the results obtained by the proposed method and TEAC values evaluated by the classic TAEC decolourisation assay was obtained (r(2)= 0.996 for pure compounds and r(2)= 0.957 for beverage samples).     

Characterisation of fresh and aged terpenic resins by micro-FTIR and GC-MS analyses of varnishes in XVI-XVII centuries paintings

Resinous materials, generally added to drying oils, were often employed as final protective films in paintings, both in tempera and oil techniques. Most of the resins used in fine arts come from exudates of plants. Natural resins are mainly composed by terpenoids. Among these resins, dammar, rosin, mastic, elemi gum, Venice turpentine were frequently employed in easel paintings. The aim of this work is the identification of natural resins in samples collected from works of art. In this paper we propose to use micro-FTIR spectrometry as a preliminary and non destructive technique to detect the presence of resinous materials in microsamples collected from paintings, followed by GC-MS analyses to obtain a precise identification of the resin. To reach this aim commercial resins, employed as standard, were analysed previously by micro-FTIR spectrometry and then analysed by GC-MS chromatography, searching characteristic chemical compounds used as "markers" of resins. The results obtained on standards allowed us to successfully analyse samples collected from two works of art: the "Portrait of a Young Gentleman", by Lorenzo Lotto (XVI century), and the "Madonna with the Infant Jesus and Saints": by Carlo Maratta (XVII secolo).     

Synthetic Applications of 2-(1,3-Dithian-2-yl)indoles. 7. Synthesis of Aspidospermidine

A new method of synthesizing the alkaloid aspidospermidine (1), based on building ring E on the pyridocarbazole [ABCD] ring structure, is reported. The preparation of the pyridocarbazole framework of Aspidosperma alkaloids is a new three-step synthetic application of 2-(1,3-dithian-2-yl)indoles. A tandem conjugate addition-alkylation reaction starting from indolyldithiane (4), 3-methylenelactam 6, and EtI yields the adduct 17. Treatment of lactam 17 with DIBALH leads to formation of the naphthyridoindole 18. Compound 18 isomerizes in aqueous AcOH to yield pyridocarbazole 3. Finally, closure of ring E and subsequent reduction of the dithiane ring produces aspidospermidine. Pyridocarbazoles 2 and 10 were prepared as models.