Synthesis of the dysiherbaine tetrahydropyran core employing a tethered aminohydroxylation reaction.

A stereocontrolled and scalable synthesis of an advanced intermediate of the dysiherbaine tetrahydropyran core has been achieved in 11 steps and 27% overall yield. The key feature of this synthetic approach is the application of the Donohoe tethered aminohydroxylation reaction to install the amino diol and establish the four contiguous syn stereocenters on the tetrahydropyran ring.

Synthesis, Characterization, and Photochemistry of a Dinuclear Cyanide-Bridged Iron(II)-Platinum(IV) Mixed-Valence Compound and Its Implications for the Corresponding Iron(II)-Platinum(IV)-Iron(II) Complex.

The mixed-valence compound [(NH(3))(5)Pt(IV)(&mgr;-NC)Fe(II)(CN)(5)].6H(2)O was synthesized by the substitution reaction of [Pt(IV)(NH(3))(5)OSO(2)CF(3)](OSO(2)CF(3))(3) and [Fe(II)(CN)(6)](4)(-) in aqueous solution and was characterized by UV/vis, IR, and resonance Raman spectroscopies, cyclic voltammetry, and single-crystal X-ray diffractometry. The monoclinic crystal (space group P2(1)/m (No. 11)) consists of a dinuclear, cyanide-bridged Fe(II)-Pt(IV) moiety with unit cell dimensions of a = 9.3241(5) Å, b = 14.0466(7) Å, c = 9.6938(4) Å, beta = 111.467(2) degrees, and Z = 2. There are also an average of six waters of hydration per unit cell. The R-factors for this structure are R = 3.66% and R(w) = 7.90%. The electronic spectrum reveals a broad intervalent (IT) charge-transfer absorption at approximately 420 nm (epsilon = 540 M(-)(1) cm(-)(1)). Both the ground-state spectroscopy and the electrochemistry of this compound are very similar to those of the corresponding trinuclear adduct [(NC)(5)Fe(II)(&mgr;-CN)Pt(IV)(NH(3))(4)(&mgr;-NC)Fe(II)(CN)(5)](4)(-), which has been reported previously. Classical Marcus-Hush theory has been applied in the analysis of the IT band of the dinuclear compound in an effort to elucidate a fuller understanding of the photophysics of the trinuclear complex. The data suggest that this latter, centrosymmetric species can be treated theoretically as two back-to-back dinuclear donor-acceptor (D-A) compounds of the form D-A/A-D, where the Pt(IV) inversion center acts as the acceptor for both halves of the molecule. The photochemistry of the dinuclear complex was also investigated.

Diastereoselective synthesis of glutamate-appended oxolane rings: synthesis of (s)-(+)-lycoperdic acid.

The stereocontrolled synthesis of the glutamate-containing natural product (S)-(+)-lycoperdic acid is described. The key transformation in the synthetic route was an efficient diastereoselective annulation of an oxolane ring onto a pyroglutamate scaffold to construct either a gamma,gamma-disubstituted glutamate-appended tetrahydrofuran or a gamma-lactone. The reaction sequence also featured an improved method for the halogenation of pyroglutamate derivatives in high yield with enhanced stereoselection.

Electrochemical determination of activation energies for methanol oxidation on polycrystalline platinum in acidic and alkaline electrolytes.

The oxidation pathways of methanol (MeOH) have been the subject of intense research due to its possible application as a liquid fuel in polyelectrolyte membrane (PEM) fuel cells. The design of improved catalysts for MeOH oxidation requires a deep understanding of these complex oxidation pathways. This paper will provide a discussion of the literature concerning the extensive research carried out in acidic and alkaline electrolytes. It will highlight techniques that have proven useful in the determination of product ratios, analysis of surface poisoning, anion adsorption, and oxide formation processes, in addition to the effects of temperature on the MeOH oxidation pathways at bulk polycrystalline platinum (Pt(poly)) electrodes. This discussion will provide a framework with which to begin the analysis of activation energy (E(a)) values. This kinetic parameter may prove useful in characterizing the rate-limiting step of the MeOH oxidation at an electrode surface. This paper will present a procedure for the determination of E(a) values for MeOH oxidation at a Pt(poly) electrode in acidic and alkaline media. Values from 24-76 kJ mol(-1) in acidic media and from 36-86 kJ mol(-1) in alkaline media were calculated and found to be a function of applied potential and direction of the potential sweep in a voltammetric experiment. Factors that influence the magnitude of the calculated E(a) include surface poisoning from MeOH oxidation intermediates, anion adsorption from the electrolyte, pH effects, and oxide formation processes. These factors are all potential, and temperature, dependent and must clearly be addressed when citing E(a) values in the literature. Comparison of E(a) values must be between systems of comparable electrochemical environment and at the same potential. E(a) values obtained on bulk Pt(poly), compared with other catalysts, may give insight into the superiority of other Pt-based catalysts for MeOH oxidation and lead to the development of new catalysts which lower the E(a) barrier at a given potential, thus driving MeOH oxidation to completion.

Design, synthesis, and biological evaluation of a scaffold for iGluR ligands based on the structure of (-)-dysiherbaine.

The design and synthesis of four 2,2-disubstituted dihydrobenzofurans that are structurally related to several glutamate-containing natural products, including (-)-dysiherbaine, is described. Biological evaluation of these analogs shows that one is a KA receptor antagonist and another is an NMDA receptor agonist.

A dual electrolyte H2/O2 planar membraneless microchannel fuel cell system with open circuit potentials in excess of 1.4 V.

A dual electrolyte H2/O2 fuel cell system employing a planar microfluidic membraneless fuel cell has been investigated and compared to single electrolyte H2/O2 systems under analogous conditions. The fuel is H2 dissolved in 0.1 M KOH (pH 13), and the oxidant is O2 dissolved in 0.1 M H2SO4 (pH 0.9), comprising a system with a calculated thermodynamic potential of 1.943 V (when 1 M H2 and O2 concentrations are assumed). This value is well above the calculated thermodynamic maximum of 1.229 V for an acid, or alkaline, single electrolyte H2/O2 fuel cell. Experimentally, open-circuit potentials in excess of 1.4 V have been achieved with the dual electrolyte system. This is a 500 mV increase in the open circuit potentials observed for single electrolyte H2/O2 systems also studied. The dual electrolyte fuel cell system shows power generation of 0.6 mW/cm2 from a single device, which is nearly 0.25 mW/cm2)greater than the values obtained for single electrolyte H2/O2 fuel cell systems studied. Microchannels of varying dimensions have been employed to study both the single and dual electrolyte H2/O2 systems. Channel thickness variation and the flow rate dependences of power generation are also addressed.

Excited-state electronic coupling and photoinduced multiple electron transfer in two related ligand-bridged hexanuclear mixed-valence compounds.

The synthesis, characterization, electrochemical, photophysical, and photochemical properties of two hexanuclear mixed-valence compounds are reported. Each supramolecular species consists of two cyano-bridged [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(3)L-NC-Fe(II)(CN)(5)] triads that are linked to each other through a Pt(IV)-L-Pt(IV) bridge, where L = 4,4'-dipyridyl (bpy) or 3,3'-dimethyl-4,4'-dipyridyl (dmb). The major difference between the two compounds is the electronic nature of the bridging ligand between the two Pt atoms. Both species exhibit a broad Fe(II) --> Pt(IV) intervalent (IT) absorption band at 421 nm with an oscillator strength that is approximately four times that for [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(5)] and twice that for [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(4)-NC-Fe(II)(CN)(5)].(4-) When L = bpy, the resonance Raman spectrum obtained by irradiating the IT band at 488 nm exhibits several dipyridyl ring modes at 1604, 1291, and 1234 cm(-1) which are not present in the spectrum when L = dmb. In addition, femtosecond pump-probe spectroscopy performed at 400 nm yields a transient bleach of the IT absorption band with a single exponential decay of 3.5 ps for L = bpy, compared with only 1.8 ps for L = dmb and 2.1 ps for [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(4)-NC-Fe(II)(CN)(5)].(4-) Last, prolonged irradiation of the complexes at 488 nm leads to the formation of 4 equiv of ferricyanide with a quantum efficiency of 0.0014 for L = bpy and 0.0011 for L = dmb. The transient absorption, resonance Raman, and photochemical data suggest that the degree of excited electronic coupling in these compounds is tunable by changing the electronic nature of the Pt-L-Pt bridging ligand.