The efficient synthesis of (3R,4R,5R)-3-amino-4,5-dimethyl-octanoic acid, a chiral β-amino acid with potent affinity for the α2δ protein

A chiral β-amino acid containing three contiguous chiral centers was synthesized efficiently in 11 steps, employing enantio-enriched β-ketoester as a key intermediate, via stereoselective catalytic hydrogenation of the corresponding enamide. Stereoselective 1,4-addition of a methyl group and protonation were key to the preparation of the desired acid 12. Mild and efficient reaction conditions were applied to the enamine formation and protection to avoid epimerization at C-4 of compounds 13 and 14. The final compound was found to display potent affinity for the α₂δ-protein that is a recognized drug target for the treatment of a variety of diseases.     

Reemergent metal-insulator transitions in manganites exposed with spatial confinement

The metal-insulator transition is characterized as a single peak in the temperature-dependent resistivity measurements; exceptions to this have never been seen in any single crystal material system. We show that by reducing a single crystal manganite thin film to a wire with a width comparable to the mesoscopic phase-separated domains inherent in the material, a second and robust metal-insulator transition peak appears in the resistivity versus temperature measurement. This new observation suggests that spatial confinement is a promising route for the discovery of emergent physical phenomena in complex oxides.     

Observation of time dependent relaxation effects in the photoelectron spectra of some metal carbonyls

Photoelectron measurements on transition metal carbonyls in the gas phase were made at various photon energies. Satellite peaks on the low kinetic energy side of the valence orbitals are observed and are attributed to shake up of the CO derived levels. The variation of the intensity of the shake up, relative to the main peak with the kinetic energy at which the shake up peaks are observed, provides evidence for time dependent relaxation of the final state. This data indicates that the shake up intensity is constant for kinetic energies greater than ≈20 eV.     

Extending matchings in planar graphs V

A graph G on at least 2n + 2 vertices in n-extendable if every set of n independent edges extends to (i.e., is a subset of) a perfect matching in G. It is known that no planar graph is 3-extendable. In the present paper we continue to study 2-extendability in the plane. Suppose independent edges e₁ and e₂ are such that the removal of their endvertices leaves at least one odd component C_o. The subgraph G[V(C_o) V(e₁) V(e₂)] is called a generalized butterfly (or gbutterfly). Clearly, a 2-extendable graph can contain no gbutterfly. The converse, however, is false.

We improve upon a previous result by proving that if G is 4-connected, locally connected and planar with an even number of vertices and has no gbutterfly, it is 2-extendable. Sharpness with respect to the various hypotheses of this result is discussed.     

Efficient finite difference derivatives for algebraic modelling systems

A procedure is described which determines Jacobian incidence structure and the constant/nonconstant nature of each Jacobian element via examination of the text of function expression strings. This procedure may be used to minimize the effort required to evaluate by finite differences the Jacobian of a set of functions. Target applications involve algebraic modelling systems and other systems with interpreted functions which require evaluation of first derivatives. Computational experience is presented and discussed.     

Quantum mechanical study of the structure and stability of molecular van der Waals' clusters I: hydrates of SO2

The structure and stability of the mixed clusters SO₂(H₂O)_n, n ranging from 1 to 4, have been examined using semi-empirical quantum mechanical Hartree Fock methods with the improved PM3 parameter set as implemented in MOPAC [1]. Limited molecular dynamics studies together with investigation of the potential surface reveal potential barriers of 1 kcal/mol or less between many of the stable structures for a cluster of a given size. For n=1,2 the results are compared with those ofab initio calculations using a double zeta basis at the MP4 level.Supported in part by NSF Research Experience for Undergraduates     

Total synthesis of (-)-ratjadone

[reaction in text] A convergent asymmetric synthesis of (-)-ratjadone (1) has been achieved, confirming the assignments of stereochemistry for the naturally occurring antifungal metabolite.     

Anomalous fluorescence and low-temperature absorption spectra of naphtho[1,8-cd][1,2,6] thiadiazine and its derivatives: an example of second excited state emission (S2 → S0)

An anomalous fluorescence near 380 nm is detected from heptane solutions on naphtho[1,8-cd][1,2,6] thiadiazine (1), 6,7-dihydroacenaphtho[5,6-cd][1,2,6]thiadiazone (2), its unsaturated derivative (4), and the tetrachloro derivative of 1 (3), when these substances are excited into the S₀→S₂ absorption band near 350 nm. Excitation spectra verify that S₂→S₀ fluorescence is occurring. The measured lifetimes of each fluorescence are 2.2, 3.3, 0.3, 0.4 ns respectively for 1,2,3, and 4. Radiative and non-radiative rate constants are presented. Compounds 1 and 2 exhibit unusual low-temperature absorption spectra at 77 K in 3-methylpetane matrix. The disappearance of the long-wavelength transition at 650 nm is accompanied by a new transition near 450 nm at 77 K. It is theorized that a ground-state dimer can explain these observations.     

Photoelectron spectra of the decomposition of ethylene on (110) tungsten

Photoelectron spectra, LEED patterns, and work function changes were obtained for ethylene adsorbed on (110) tungsten at room temperature, and with subsequent heat treatment. For saturated adsorption of C₂H₄ on (110) W at room temperature, features in the photoelectron spectrum were observed which are believed to be due to the C, HCC, and Cmetal bonds in an adsorbed species of the form C₂H₂. The work function decreased by 1.2 eV at saturation, but LEED showed no change from the clean surface pattern. Upon heating to ≈ 500 K, where hydrogen is known to desorb, the CH bond was broken, whereas the CC and Cmetal bonds remained. The work function increased, from saturation, by ≈ 0.6 eV and the LEED pattern exhibited a large diffuse background with no new spots. Upon heating to ≈ 1100 K the CC bond broke and the LEED pattern ordered into the characteristics carbon contamination pattern.