Stereoselective synthesis of 3a,7a-dihydro-3H,4H-furo[3,4-c]pyran-1-ones via intramolecular hetero-Diels-Alder reaction

The synthesis of 3a,7a-dihydro-3H,4H-furo[3,4-c]pyran-1-ones via an intramolecular hetero-Diels-Alder reaction of easily accessible α,β-unsaturated γ-ketoesters was investigated. The reaction was found to proceed in a highly stereoselective way leading to single, cis-configured product isomers. The same diastereomer is formed, independently of the configuration of the enone double bond of the precursor. The respective E- and Z-isomers react either through an endo-E-syn or an exo-Z-syn transition state.     

Neutron scattering from liquid crystal mesophases: Form of the elastic incoherent structure factor for “monocrystal” aligned nematogen with specific Q to n orientation

The elastic incoherent structure function (ElSF), A_o(Q), is derived for specific experimental orientations of the momentum transfer vector, Q, with respect to the nematic director, n, of an aligned “monocrystal” mesophase. The particular uniaxial model first considered is that in which the initial probability component of the orientational correlation function departs from uniformity to become a peaked function. The analytical EISF for Q||n is derived on the basis of the microscopic scattering symmetry and an effectively static principal axis. Brief explicit consideration is given to the EISF for the more general “dynamic principal axis” model with specific Q to an experimental configuration. An alternative closed form is given for S_k (δ′), the dynamical equivalent of an order parameter.     

Time-dependent perturbation: a recursive generation of the transition amplitudes between bound states

We present a new expansion of the solution to the time-dependent Schrödinger equation it ∂U/∂t = {H₀ + V(t)}U. A complete set of eigenvectors of H₀ spanning the Hilbert space in which H₀ and V operate is partitioned in two subsets. Transition amplitudes from the first subspace to the second one are calculated by building an adequate series of intermediate representations with respect to the couplings which produce the transition from the model space into its orthogonal complement. These expansions yield a coupling matrix series V⁽ⁿ⁾ for which an iterative solution V⁽ⁿ⁾ → V⁽ⁿ⁺¹⁾ is derived. This solution leads to a recursive numerical treatment of the calculation of transition amplitudes. A simple example, concerning a harmonic oscillator under an intense laser field, is considered using a Fourier analysis of the perturbation.     

Asymmetric synthesis of (4S,5S)-2-oxo-4-phenyloxazolidine-5-carboxylic acid using a 1,2-addition of PhMgBr to an N-sulfinimine derived from (R)-glyceraldehyde acetonide and (S)-t-BSA

We report an asymmetric synthesis of (4S,5S)-2-oxo-4-phenyloxazolidine-5-carboxylic acid via stereoselective addition of phenylmagnesium bromide (PhMgBr) to an N-sulfinimine derived from (R)-glyceraldehyde acetonide. (S)- and (R)-Glyceraldehyde acetonides, important chiral synthons in synthetic organic chemistry, are prepared from the corresponding epichlorohydrin using an identical synthetic methodology.     

Solvent influence on the viscosity-temperature relationship for dilute polybutadiene solutions

The temperature dependence of the viscosity of dilute polybutadiene solutions has been described by an Arrhenius expression according to Moore's treatment. The activation energy of flow was found to depend on both concentration and molecular weight through an empirical parameter, K_e. This parameter was positive in toluene, zero in decalin and negative in dioxane. This behaviour is discussed in connection with the temperature dependence of the expansion coefficient (x_η) and the excluded volume parameter (z). A recent Yamakawa-Tanaka expression relating x_η and z was used in the theoretical calculation of K_e.     

Two-photon ionization of an inner shell electron of the Cl atom

We have calculated, using second order perturbation theory, the two photon ionization cross section of a K-shell electron of chlorine forE=1.6 keV incident photons. Two classes of intermediate states must be considered, those in which a 1s electron moves to an emptyp-orbital, and those in which an electron from an occupiedp-orbital moves into the continuum. The first class of intermediate states is followed by the ejection of ap-electron into the continuum. The second class of intermediate states is followed by the transfer of a 1s electron into an emptyp-state. The largest contribution comes from 3p →d-continuum followed by 1s → 3p transition. Our result is σ⁽²⁾/I=2.06×10⁴¹ cm⁴/W where σ⁽²⁾ is the two photon ionization cross-section andI is the light intensity.     

A general equation correlating intramolecular rates with ‘attack’ parameters: distance and angle

Using density functional theory (DFT) at the B3LYP level with the 6-31G(d,p) basis set, a general equation is derived relating activation energy with the distance between the two reactive centers (r_GM), and the hydrogen-bonding angle (α_GM) in an intramolecular proton transfer process. The strong correlation between the values of r_GM and α_GM with the activation energy, ΔG^‡, which reflects the experimental reaction rate, provides an excellent tool to predict reaction rate based on calculated geometrical parameters for a certain system (ΔH^‡, ΔG^‡ vs r_GM and α_GM). The slope of the equation can be used as an indicator of the mode by which the two reacting centers orchestrate in an intramolecular process.     

Geometric parameterization of the YBaCo4O7 structure type: Implications for stability of the hexagonal form and oxygen uptake

We explore the stability of the hexagonal form of MBaCo₄O₇ cobaltites in terms of geometric characteristics of the crystal structure and Global Instability Index (GII) based on the bond-valence considerations. Mismatch between an M^3+/2+ and the three-dimensional network of CoO₄ tetrahedra, whether expressed using an M ionic radii or GII, is shown to essentially determine both the temperature of structural transition to an orthorhombic modification and oxygen storage properties. A number of M cations not reported in the literature are identified to be suitable for the octahedral sites in an MBaCo₄O₇ structure.     

A generalised friction model for the evaluation of angular momentum auto correlation functions

A theory of brownian motion where the friction coefficient is a finite correlation function of molecular torque is used to derive an expression for the angular velocity autocorrelation function, ψ(r). This is related to the orientational a.c.f., φ(t), using an analysis of bandshapes due to Kubo and extended by Shimizu. Both functions φ(t) and ψ(t) contain inherent weaknesses since ψ(t) has a MacLaurin expansion even up to t⁴ only, and π(t) does not reduce to the Kummer function for an ensemble of free rotors in the so-called limit of vanishing friction. Reasons for this behaviour are discussed.     

A direct route for the preparation of Fmoc/OtBu protected iodotyrosine

Herein the synthesis of an Fmoc/O^tBu orthogonally protected iodotyrosine derivative is reported. This has been achieved via a simple two-step process in an overall 58% yield from commercially available Fmoc-Tyr(^tBu)-OH. The Fmoc/O^tBu orthogonally protected iodotyrosine was also shown to be amenable to Suzuki-Miyaura cross-coupling to deliver a novel bi-aryl tyrosine derivative.     

The formation of medium-sized rings by the intramolecular cyclization of arylnitrenium ions

The synthesis of an aminodibenzo[a,c]octadiene and an aminodibenzo[a,c]octatriene, guided by molecular mechanics calculations, was effected by the intramolecular cyclization of the corresponding open chain nitrenium ions. Also reported is the predicted failure to synthesize another aminodibenzo[a,c]octadiene using this method.     

Synthesis of an O-acyl isopeptide by using native chemical ligation to efficiently construct a hydrophobic polypeptide

By using dimethylformamide to suppress the O-to-N acyl migration, we efficiently synthesized an O-acyl isopeptide by native chemical ligation of a peptide-thioester and a Cys-O-acyl isopeptide. The reaction mixture was then loaded onto an octadecylsilane reverse-phase HPLC column, and the isopeptide was purified by using a linear gradient of CH₃CN in 0.1% aqueous trifluoroacetic acid. The recovery rate of the O-acyl isopeptide was considerably higher than that of the corresponding native polypeptide. Synthesis of O-acyl isopeptides via native chemical ligation, with O-to-N acyl migration as the final step to give the native form, has potential as an efficient method of constructing hydrophobic polypeptides.     

Conformational effects on lipase-mediated acylations of 2-substituted cyclohexanols

Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1R)-cyclohexyl acetates and (1S)-cyclohexanols in high yields and ee, but c-4-tert-butyl-c-2-ethenyl-r-1-cyclohexanol was unreactive owing to the steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent acetylations than an alkyl group.     

Stereospecific enzymatic hydrolysis of racemic epoxide: a process for making chiral epoxide

Among various microbial cultures evaluated, Rhodotorula glutinis SC 16293 and Aspergillus niger SC 16311 catalyzed the stereospecific hydrolysis of the racemic epoxide, RS-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-1,2-oxirane, 1 to the corresponding R-diol, R-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-ethane-1,2-diol, 3. The S-epoxide, S-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-1,2-oxirane, 2 remained unreacted in the reaction mixture. A reaction yield of 45–50% (theoretical maximum yield is 50%) and an enantiomeric excess (ee) of >95% were obtained for unreacted S-epoxide 2 using each culture. Addition of 10% methyl tert-butyl ether to an aqueous reaction mixture during hydrolysis by R. glutinis improved the ee of the unreacted S-epoxide 2 to >99% (yield 48%) and that of the R-diol 3 to 79%. Unlike R. glutinis, hydrolysis of racemic epoxide 1 in the presence of 10% methyl tert-butyl ether by A. niger showed an adverse effect and gave S-epoxide 2 in 54% yield and 49% ee.     

Mössbauer study of LiFeVPO x as a new cathode material for lithium-ion battery

The Mössbauer spectrum of LiFeVPO _x , LiFeV_0.5PO _x and LiFePO _x glasses prepared by conventional melt-quenching method for cathode active material is composed of a doublet due to distorted Fe^IIIO₄ tetrahedra. The Mössbauer spectrum of LiFePO _x glass has an additional doublet due to distorted Fe^IIO₆ octahedra. Heat treatment of LiFeVPO _x and LiFeV_0.5PO _x glasses at a given temperature close to each crystallization temperature causes a marked decrease in the value of Δ, reflecting a decrease in the distortion or an increase in the local symmetry of distorted Fe^IIIO₄ tetrahedra. Heat treatment of LiFeVPO _x glass causes an increase in the electric conductivity from the order of 10^?7 to 10^?3 S·cm^?1, together with an increase in the specific discharge-and charge-capacity of a coin-type Li-ion cell from 50 to 150 mAh·g^?1. These results prove that structural relaxation of the glass network causes an increase in the electric conductivity and an increase in the energy density of the Li-ion cell.     

The Curl–Divergence equations for the electronic non-adiabatic coupling terms: study of the C2H molecule and the H2+H system

This Letter is part of an effort to use the Curl equations to calculate non-adiabatic coupling terms, subject to ab initio boundary conditions. As examples we consider two-state, planar, systems characterized by two coordinates, θ and q and treat the corresponding non-adiabatic coupling terms, namely, τ_θ(q,θ) and τ_q(q,θ). The theory, which yields τ_q(q,θ) once τ_θ(q,θ) is given, is applied to three cases: an analytical model and two ab initio treatments – one for the C₂H molecule and one for the H+H₂ molecular system. In all three cases encouraging agreements were obtained between the theoretical τ_q(q,θ) values and the ab initio ones.     

Approximate analytic expression for the electrophoretic mobility of a cylindrical colloidal particle. Relaxation effect

An approximate expression is derived for the electrophoretic mobility μ _⊥ of an infinitely long cylindrical particle of radius a and zeta potential ζ oriented perpendicular to an applied electric field in an electrolyte solution. The obtained expression is applicable for cylinders with large radii such that κa≥ca. 30, where κ is the Debye–Hückel parameter. It is shown that the obtained mobility expression μ _⊥, which consists of Smoluchowski’s equation and the correction term of the order of exp(ze∣ζ∣/2kT)/κa (where z is the valence of counterions in the electrolyte solution, e is the elementary electric charge, k is Boltzmann’s constant, and T is the absolute temperature), coincides with the mobility expression for a sphere of radius a and zeta potential ζ correct to the order of exp(ze∣ζ∣/2kT)/κa.     

Synthesis of chiral ortho-thio-substituted phenyl phosphonodiamidates via a P-S to P-C rearrangement

The ortho-lithiation of a phenyl phosphorodiamidothioate derived from an enantiopure C₂-symmetric diamine is studied. It is shown that the migration of the diaminophosphoryl group from sulfur to carbon, leading to an ortho-sulfanylated phenyl phosphonodiamidate, only occurs in the presence of an alkylating agent or a Lewis acid as BF₃·Et₂O. The influence of the chiral diaminophosphoryl group on the stereoselectivity of the oxidation of the ortho-sulfanyl or alkylsulfanyl group is also examined.     

Development of a convenient route for the preparation of the N-Cbz-protected guaninyl synthon required for Boc-mediated PNA synthesis

An efficient, high yielding, chemo- and regioselective, five-step synthetic route to N²-Cbz-guanin-9-yl acetic acid has been developed, which avoids the use of triphosgene. After formation of the N²-Boc protected purine from 2-amino-6-chloropurine, two successive base-controlled alkylations allowed an N⁹-tert-butyl acetate function followed by an N²-Cbz moiety to be installed. The selectivities of these reactions were confirmed through an X-ray crystallographic study of the 6-(2-nitrophenoxy) analogue. Final hydrolytic dechlorination and removal of the Boc and tert-butyl ester protecting groups were accomplished concomitantly under acidic conditions to afford the guanin-9-yl PNA monomer synthon in an overall yield of 53%.