Labdane diterpenes from Haplopappus illinitus

A chemical study of aerial parts of Haplopappus illinitus afforded five labdane diterpenoids whose structures were elucidated by high field NMR spectroscopy and 2D techniques. Four of them are dicarboxylic acids previously reported as the corresponding dimethyl ester derivatives and the fifth is a new compound.     

Theoretical investigations on the solvation process

The conformation energies for monohydrated associates M · H₂O, where M stands for oxirane, aziridine, oxaziridine and cyclopropene, have been obtained by using an electrostatic method, tested in a preceding paper, which relies on SCF molecular potentials calculated exactly. Stable associates have been found in the heterogroup region as well as near the bent bonds (single or double).A discussion is made on the errors in calculating thermodynamic properties for such associates in gas phase by using a priori calculations. As a numerical example the free energy change in the association process is compared for two monohydrated associates of aziridine.

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Zusammenfassung Die Konformationsenergien für Anlagerungsverbindung M · H₂O, wobei M für Oxiran, Aziridin, Oxaziridin und Cyclopropen steht, werden mit einer elektrostatischen Methode berechnet. Dieses Verfahren wurde in einer vorhergehenden Veröffentlichung getestet und beruht auf exakt berechneten SCF-Molekülpotentialen. Stabile Anlagerungsverbindungen wurden für Konformationen gefunden, bei denen das Wassermolekül in der Nähe der Heterogruppe oder der gezogenen Einfach- oder Doppelbindung liegt. Die Fehler bei der Berechnung thermodynamischer Eigenschaften für derartige Anlagerungsverbindungen in der Gasphase werden abgeschätzt. Als ein numerisches Beispiel wird die Differenz der freien Energie bei der Anlagerung für zwei Anlagerungsverbindungen von Aziridin und Molekül Wasser verglichen.

     

Phototactic orientation mechanism in the ciliate Fabrea salina, as inferred from numerical simulations

The marine ciliate Fabrea salina shows a clear positive phototaxis, but the mechanism by which a single cell is able to detect the direction of light and orient its swimming accordingly is still unknown. A simple model of phototaxis is that of a biased random walk, where the bias due to light can affect one or more of the parameters that characterize a random walk, i.e., the mean speed, the frequency distribution of the angles of directional changes and the frequency of directional changes. Since experimental evidence has shown no effect of light on the mean speed of Fabrea salina, we have excluded models depending on this parameter. We have, therefore, investigated the phototactic orientation of Fabrea salina by computer simulation of two simple models, the first where light affects the frequency distribution of the angles of directional changes (model M1) and the second where the light bias modifies the frequency of directional changes (model M2). Simulated M1 cells directly orient their swimming towards the direction of light, regardless of their current swimming orientation; simulated M2 cells, on the contrary, are unable to actively orient their motion, but remain locked along the light direction once they find it by chance. The simulations show that these two orientation models lead to different macroscopic behaviours of the simulated cell populations. By comparing the results of the simulations with the experimental ones, we have found that the phototactic behaviour of real cells is more similar to that of the M2 model.     

Enzymatic resolution of (RS)-β-hydroxy selenides in organic media

Kinetic resolutions of a number of β-hydroxy selenides promoted by enzymes were performed using PPL (free Porcine pancreatic lipase), PSL (Amano PS—free Pseudomonas sp. lipase) and CALB (NOVOZYM 435^®—immobilized Candida Antarctica lipase type B) with (RS)-1-phenylselanyl-propan-2-ol. CALB gave the best results and provided both (R)- and (S)-enantiomers in high enantiomeric purity. A comparative study of the effect of temperature, solvent, enzyme immobilization and the structure of the substrates on the resolution is presented.     

An Improved Method for the Preparation of Tetra-Coordinate Platinum(II) Chloro-Complexes Containing Bidentate or Terdentate Ligands Starting from cis/trans-[PtCl2(SMe2)2]

An improved method for the preparation of differently charged chelate Pt(II) chloro-complexes is reported. All the complexes have been obtained rapidly and in high yield, by simply reacting equimolar amounts of cis/trans- dichlorobis(dimethylsulphide)platinum(II) with the chelate ligand in an appropriate solvent (CH₂Cl₂, MeOH or H₂O). The ligands chosen were: 1,2-bis(diphenylphosphino)ethane (P—P), pyridine–2-carboxylate (N—O⁻), 2-[(methylthio)methyl]pyridine (N—S), 1,10-phenanthroline (N—N), bis(2-pyridylmethyl)sulphide (N—S—N), di-(2-picolyl)amine (N—N—N), pyridine-2,6-dicarboxylate (O—N—O²⁻) and 2,6-bis(methylthiomethyl)pyridine (S—N—S).     

ELDONET--European Light Dosimeter Network. Structure and functions of the ELDONET server

The European Light Dosimeter Network (ELDONET) project has been designed with the purpose of establishing an efficient system to monitor solar radiation in Europe, in as many as possible locations. This paper describes the structure of the server that collects and processes the data acquired by the different stations belonging to the network, and makes them freely available on the Internet to the scientific community. The server is able to receive data either via FTP from the Internet or via modem and to process them looking for errors or inconsistencies. Moreover, it automatically generates graphs, Web-pages and FTP archives. The server has been active for some years in testing mode and is now fully operative.     

Regiochemical Trends in Intramolecular [2 + 2] Photocycloadditions of 6-(prop-2-enyl)cyclohex-2-enones and 5-(prop-2-enyl)cyclopent-2-enones

The effect of substituents (X ? H, Me, or F at C(6), R ? H or Me at C(2′) of the allyl side chain) on the photoisomerization (λ = 350 nm) of 6-allylcyclohex-2-enones 1 in MeCN is studied. Substituents X control the overall efficiency of intramolecular [2 + 2] photocycloadduct formation (Φ: Me > F > H) but do not exercise an influence on the orientation of addition of the exocyclic double bond to the enone C?C bond. In contrast, replacement of the prop-2-enyl (R ? H) by a 2-methylprop-2-enyl (R ? Me) side chain causes a change in the tricyclo[3.3.1.0^2,7]nonan-6-one 4 vs. tricyclo[4.2,1.0^3,8]nonan-7-one ( 5 ) product ratio from 100:0 (R ? H) to roughly 2:1 (R ? Me) but has almost no bearing on the relative rates of conversion of 1 to products. For C(6)-unsubstituted enones 1aa and 1ba (X ? H), the efficiency of cyclization becomes low enough so that lumiketone rearrangement to bicyclohexanones 6 and 3-isopropylcyclopent-2-enones 9 becomes competitive. Enones 9 undergo consecutive intramolecular [2 + 2] photocycloaddition to tricyclo[3.2.1.0^3,6]octan-2-ones 7 and to tricyclo[3.2.1.0^3,6]octan-7-ones 8 , compounds 8 only being formed when R ? Me.     

Novel rhenium(V) oxo complexes containing bis(pyrazol-1-yl)acetate and bis(pyrazol-1-yl) sulfonate as tripodal N,N,O-heteroscorpionate ligands

Reactions of [NBu4][Re(O)Cl4] with bis(pyrazol-1-yl)methane (bpzm) and bis(pyrazol-1-yl)acetate (Hbpza) and with the lithium salts lithium [bis(3,5-dimethylpyrazol-1-yl)acetate] (Libdmpza) and lithium [bis(3,5-dimethylpyrazol-1-yl)methanesulfonate] (Libdmpzs) produce a series of new compounds containing either a kappa2-N,N bidentate pyrazolyl ligand [Re(O)(bpzm)Cl3 (1), Re(O)(bpzm)(OMe)Cl2 (2), Re(O)(bpzaOMe)(OMe)Cl2 (4)] or a kappa3-N,N,O heteroscorpionate [Re(O)(bpza)Cl2 (3), Re(O)(bdmpza)Cl2 isomers 5 and 6, Re(O)(bdmpza)(OMe)Cl (7), Re(O)(bdmpza)(OEt)Cl (8), Re(O)(bdmpzs)(OMe)Cl (9), Re(O)(bdmpzs)(OEt)Cl (10)]. X-ray analyses of 1 and 3 show in both cases a distorted octahedral environment around the rhenium atom. The nature and the geometry of the products are strongly determined by the reaction solvent and by the heteroscorpionate ligand itself. When scorpionates bear methylated pyrazolyl rings mixed heterocomplexes Re(O)(bdmpza)(glycol) (11) and Re(O)(bdmpzs)(glycol) (12) are obtained (H2glycol = ethylene glycol). Also 11 shows an octahedral geometry as assessed by X-ray study.     

Synthesis,chemical, and electrochemical characterization of the [Ag13{μ3-Fe(CO)4}] n− (n = 3, 4, 5) cluster anions: X-Ray structural determination of [N(PPh3)2]3 [Ag12(μ12-Ag){μ3Fe(CO)4}8]1

The oxidation of the [Fe(CO)₄]^2– dianion with Ag⁺ salts occurs through a particularinner-sphere mechanism, which involves an intermediate cascade of silver clusters stabilized by Fe(CO)₄ ligands. The last detectable Ag-Fe cluster of the sequence is the [Ag₁₃{-Fe(CO)₄}₈]^3– trianion, which has been selectively obtained by using ca. 1.7 equivalents of Ag⁺ per mole of [Fe(CO)₄]^2–. The [Ag₁₃{-Fe(CO)₄}₈]^3–- trianion has been isolated in a crystalline state with several quaternary cations, and has been characterized by X-ray diffraction studies of its bis(triphenylphosphine)iminium salt. [N(PPh₃)₂]₃ [Ag₁₃{ ₃-Fe(CO)₄}₈]·2(CH₃)₂CO, monoclinic, space group P2₁ (No.4),a = 16.284(2) Å,b =18.767(5) Å,c = 25.905(4) Å, = 90.46(1)°,V = 7916(3) ų,Z = 2,R = 0.0324. The molecular structure of the anion consists of a centered cuboctahedron of silver atoms with the triangular faces capped by Fe(CO)₄ units. Chemical reduction of ( Ag₁₃{ ₃-Fe(CO)₄}₈]^3– affords the corresponding [Ag₁₃{ ₃-Fe(CO)₄)₈]^4–, which in turn gives [Ag₁₃{ ₃-Fe(CO)₄)₈]^5– and [Ag₆{ ₃-Fe(CO)₄}₄]^– upon further reduction. Electrochemical investigations confirm the reversibility of the [Ag₁₃{ ₃-Fe(CO)₄}₈]^3–/4– redox change. Furthermore, in spite of some electrode poisoning effects, evidence of the existence of the [Ag₁₃{ ₃-Fe(CO)₄}₈]^5– pentaanion was obtained. The yet structurally uncharacterized [Ag₆{ ₃-Fe(CO)₄)₄]^2– dianion is quantitatively obtained by reaction of [Fe(CO)₄]^2– with ca. 1.5 equivalents of Ag⁺ or by addition of one equivalent of Ag⁺ to solutions of the [Ag₅{Fe(CO)₄}₄]^3– trianion. All attempts to isolate its quaternary salts as crystalline materials failed owing to formation of amorphous insoluble precipitates. The above series of ₃-Fe(CO)₄ octa-capped cuboctahedral Ag₁₃ clusters can be envisioned as the Ag⁺ . Ag and Ag^– cryptates of the [Ag₁₂{}₃-Fe(CO)₄}₈]^4– cryptand. respectively.Dedicated to Prof L. F. Dahl on his 65th birthday.