Photochemical Rearrangement of a Steroidal α,β-Epoxylactone. Photochemical reactions,XI [1]

The UV. irradiation of 17β-acetoxy-4α, 5α-epoxy-2-oxaandrostan-3-one ( 7 ) yields 17β-acetoxy-2-oxa-10(5 → 4)abeo-4ζ (H)-androsta-3,5-dione ( 11 ). A non-photochemical synthesis of 11 , proceeding in lower yield, is also described.     

Mikrobestimmung von Quecksilber in organischen Verbindungen

Zusammenfassung Eine Methode zur Mikrobestimmung von Quecksilber in organischen Verbindungen wurde beschrieben. 3–5 mg Substanz werden in einem Sauerstoffkolben verbrannt und die Produkte in 4 ml konz. Salpetersäure absorbiert. Die Absorptionslösung wird dann für 6 min gekocht, wobei die vollständige Oxydation zu Quecksilber(II) abläuft. Dieses wird mit 0,005 M Kaliumjodidlösung potentiometrisch titriert. Zur Indikation des Endpunktes wird eine Meßkette aus einer ionenselektiven Elektrode für Jodid und einer Referenzelektrode mit KNO₃-Zwischenelektrolyt verwendet. Die Resultate liegen innerhalb der üblichen Fehlergrenze von ± 0,30%.

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Microassay of mercury in organic substances

Summary A method for microassay of mercury in organic compounds was described. 3–5 mg substance are combusted in an oxygen flask and the products absorbed in 4 ml conc. nitric acid. The absorption solution is then boiled for 6 minutes, during which complete oxidation to mercury(II) occurs. This is titrated potentiometrically with 0.005M potassium iodide solution. A measuring chain consisting of an ion-selective electrode for iodide and a reference electrode with KNO₃ intermediate electrode was used to indicate the endpoint. The results are within the usual ±0.30% limit of error.

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Herrn Prof. Dr. Hans Lieb zum 90. Geburtstag gewidmet.

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Frl. Mirella Piccinini hat an dem experimentellen Teil dieser Arbeit mitgeholfen, wofür ich bestens danke.     

Paired electrosynthesis of cyanoacetic acid

Cyanoacetic acid is formed by cathodic reduction of CO(2) and anodic oxidation of the tetraalkylammonium salt anion; the process is conduced in acetonitrile using a divided cell with a medium porosity glass-frit diaphragm. A mechanism for this paired electrochemical reaction is proposed.     

Synthesis and physicochemical study of the laminin active sequence: SIKVAV

The synthesis, physicochemical characterization, and interaction with membrane model systems of a peptide derived from the PA22-2 region of laminin are described. Surface activity studies indicate that this peptide is able to spread at the air-water interface being the maximal spreading pressure 20 mN/m at subphase concentrations around 10 micro M. Besides, these peptide molecules are also able to form stable monolayers. Physicochemical studies concerning the interaction of this peptide with lipids, organized in mono and bilayers, were carried out using Langmuir balance experiments and polarization fluorescence techniques. The peptide penetrates better in monolayers of DPPC than in those of PC and forms condensed mixed monolayers with DPPC. Energies of mixing are small thus indicating that deviations from ideality were almost negligible. Interactions with bilayers were studied through microviscosity changes (DPH and TMA-DPH probes), membrane permeability alterations (CF, NBD-PE/dithionite), and fusion promotion (NBD-PE/Rh-PE, resonance energy transfer). Results indicate that this sequence interacts very softly with bilayers without promoting changes in their organization. These data as well as the lack of interaction with erythrocytes suggest that coating liposomes with this peptide through chemical amide bonds can render stable inmunoliposomes for further biological applications.     

An alternative multipolar expansion for intermolecular potential functions

We have derived a new multipolar expansion for intermolecular potential-energy functions with applications in molecular physics, theoretical chemistry, and mathematical physics. The new formulation employs a separation of radial and angular terms with a simple index structure that leads to computational efficiency and ease of physical interpretation. For the case of the Coulomb interaction, we compare the present formulation with two conventional multipole expansions: the Cartesian tensor and the irreducible spherical tensor expansions. The new formalism leads to efficient numerical algorithms that are useful for general applications beyond intermolecular potentials. In addition to the electrostatic Coulomb interaction, we illustrate the formalism with applications to special function theory and a bipolar expansion involved in potential theory.     

Terbium-sensitized luminescence determination of levofloxacin in tablets and human urine and serum

A selective and sensitive luminescence method for the determination of levofloxacin is described. The method is based in the luminescence signal from a terbium(III)-levofloxacin complex, in a micellar solution of sodium dodecyl sulfate (SDS), using a chemical deoxygenation agent (Na2SO3). The method allows the determination of 8-600 ng mL-1 of levofloxacin in 10 mM SDS solution containing 0.04 M acetic acid-sodium acetate buffer (pH 6) and 7.5 mM Na2SO3 with lambda exc = 292 nm and lambda em = 546 nm. The luminescence method was applied to the determination of the levofloxacin in a Spanish commercialized pharmaceutical formulation Tavanic (Hoechst Marion Roussel). Good concordance was found between the nominal and experimental values (500 and 488 mg, respectively), with a relative standard deviation (RSD) of 0.6%. The proposed method was shown to be 100-fold more sensitive than the spectrophotometric method, and nearly 2-fold more sensitive than the fluorescence method. The method was also applied to levofloxacin determination in human serum (by external calibration method) and urine (by standard additions method), spiked at levels found after drug administration at normal clinical doses. Average recoveries found were 90.1 (RSD 1%) and 102 (RSD 1.9%), respectively.     

Stericaliy Congested Molecules: 2,2′-[(Biaryldiyl)bis(oxy)]bis[1,3,2-oxazaphospholidines]

The original suggestion that a through-space mechanism was operative in the seven-bond J(P, P) coupling constant of 30.3 Hz observed for 3.3′-bis(1,1-dimethylethyl)-2,2′-[3,3′,5,5′-tetrakis(1,1-dimethylethyl)-1,1′-biphenyl-2,2′-diyl]bis(oxy)}bis[1,3,2-oxazaphospholidine] ( 1a )) was investigated. In the solid-state CP-MAS ³¹PNMR spectrum of 1a , two nonequivalent P -atoms were observed; sufficient resolution could not be obtained to determine whether P, P coupling was present. The preparation and spectral data of the N-methyl analogue 1b and of the acyclic N-isopropyl analogue 6 (Scheme 1) provided evidence that a) the essentially exclusive formation (R*, R*,S*)- 1a in the reaction of the disodium biphenyldiolate 3a with the phosphorochloridite 4a is the result of significant differences in the free energy of activation (ΔG*) for the formation of the various diastereoisomers due to the steric congestion within the molecule and that b) the magnitude of the observed P,P coupling is dependent upon the degree of conformational freedom within the molecule. In the ³¹P-NMR spectrum of the P-sulfide 7 , which was prepared by the reaction of la with sulfur, 2s resonances were observed that strongly suggested that the lone electrons pair on P are involved in the mechanism for the transmission of coupling data. The (4S,5R) -12 and (4R, 5S) -12 of la were prepared in a three-step reaction sequence starting from the corresponding enantiomerically pure norephredine 8 (Scheme 2). Both (4S, 5R)- and (4R, 5S) -12 were obtained as a diastereoisomer mixture that differ by the configuration of the axis of chirality, i.e., (R*R*,R*)- and (R*,S*,R*) -12 were obtained. The major diastereoisomer was obtained upon recrystallization, and the atropisomers were observed to equilibrate in solution by monitoring the H? C(5) resonance in the ¹H-NMR with time (ΔG° = 0.4 kcal/mol; Fig. 2). The process observed corresponds to the restricted rotation about the central single bond of the biphenyl system. The isolation of an atropisomer with only a single ortho substituent on each aryl ring is quite rare. In the ¹³C-NMR spectrum of both (R*,R*,R*)- and (R*,S*,R*) -12 , C(5) is two-bond-coupled to the oxazaphospholidine P-atom (²J(C(5),P((2)) = 8.5 Hz) that is further virtually coupled to the P-atom of the other oxazaphospholidine ring (7J(P(2),P(2′)) = 30 Hz; ⁹J(C(5),P(2′)) = 0 Hz; δ(P(2)) = δ(P(2′)) = 136 ppm. In the ³¹P-NMR spectrum of (R*,R*,S*) -12 , which was prepared from the racemic chloridite (mixture of three diastereoisomers was obtained), a ⁷J(P(2),P(2′) of 36 Hz was observed. These observations provide strong evidence that seven-bond P,P coupling occurs in all three diastereoisomers of 12 . The observed P,P coupling is both independent of the configuration of the chiral axis and the configuration of the asymmetric P-centers. This independence of P,P coupling upon the configuration on P implies also the independence of the observed coupling upon the orientation of the lone-pair of electrons on P provided that the conformations of the diastereoisomers are similar in solution. The X-ray crystal structure of the complex formed from 1a and dichloro(cycloocta-1,5-diene)platinum(II) was obtained and the solid-state structure discussed. The major diastereoisomer of (4S,5R) -12 was used as a chiral ligand in asymmetric hydrosilylation and hydrogenation reactions (Scheme 3).     

Photophysics and redox behavior of chiral transition metal polymers

The absorption and emission spectra, excited-state lifetimes, quantum yields, and electrochemical measurements have been obtained for a new series of chiral complexes based on three different chiral 2,2':6',2' '-terpyridine ligands, (-)-ctpy, (-)-[ctpy-x-ctpy], and (-)-[ctpy-b-ctpy], with one, two, or multiple Ru metal centers. The room-temperature absorption and emission maxima of [[((-)-ctpy)Ru]-(-)-[ctpy-b-ctpy]-[Ru((-)-ctpy)]](PF(6))(4) and ((-)-[ctpy-b-ctpy])-[[Ru((-)-[ctpy-b-ctpy])](PF(6))(2)](n) were shifted to lower energies and also exhibited significantly longer luminescence lifetimes when compared to [Ru((-)-ctpy)(2)](PF(6))(2), [[((-)-ctpy)Ru]-(-)-[ctpy-x-ctpy]-[Ru((-)-ctpy)]](PF(6))(4), and ((-)-[ctpy-x-ctpy])-[[Ru((-)-[ctpy-x-ctpy])](PF(6))(2)](n). In terms of their electrochemical behavior, all of the complexes studied exhibited one Ru-centered and two ligand-centered redox waves and the [[((-)-ctpy)Ru]-(-)-[ctpy-x-ctpy]-[Ru((-)-ctpy)]](PF(6))(4), ((-)-[ctpy-x-ctpy])-[[Ru((-)-[ctpy-x-ctpy])](PF(6))(2)](n), and ((-)-[ctpy-b-ctpy])-[[Ru((-)-[ctpy-b-ctpy])](PF(6))(2)](n)() complexes were found to electrodeposit upon ligand-based reduction. The difference between the formal potentials of the Ru-centered and the first ligand-centered (least negative) waves corresponded linearly with the changes in the observed emission energies. The shifts in energy are discussed using a particle-in-a-box model, and the luminescence lifetimes are discussed in terms of the structure of the excited-state manifold.     

Direct versus resonances mediated F+OH collisions on a new 3A" potential energy surface

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.     

Stepwise retro 1,3-dipolar cycloaddition induced by electron. Impact on 3,5-diphenyl-1,2,4-oxadiazole

The retro 1,3-dipolar cycloaddition induced by electron impact on 3,5-diphenyl-1,2,4-oxadiazole is interpreted as a two-step process on the basis of the energetics and kinetics of the fragments [C₇H₅NO]⁺., [C₆H₅CN]⁺. and [C₆H₅CO]⁺.