Synthesis and Conformational Analysis of the Multidrug Resistance-Reversing Agent Hapalosin and Its Non-N-methyl Analog

Hapalosin was initially synthesized by macrolactonization, and a second synthesis was achieved by cycloamidation. In both syntheses, three of the five stereocenters in hapalosin were established by two Brown allylboration reactions. The synthesis of the non-N-Me analog of hapalosin involved chelation-controlled reduction of a gamma-amino-beta-keto ester and cycloamidation. In CDCl(3) at 25 degrees C, synthetic hapalosin exists as a 2.3:1 mixture of conformers, while its non-N-Me analog exists only as a single conformer. (1)H,(1)H-NOESY and computation reveal that the configuration of the amide bond is responsible for the conformations of the two compounds. The major conformer of hapalosin is found to be an s-cis amide, the minor conformer an s-trans amide, and the non-N-Me analog an s-trans amide. Applying distance constraints to protons that exhibit NOESY correlations, computation shows that the major conformer of hapalosin and the non-N-Me analog have very different conformations. By contrast, the minor conformer of hapalosin and the non-N-Me analog have very similar conformations.     

Adsorption of 1,10-phenanthroline within a dodecanethiol monolayer: an approach to a switchable electrode surface

A simple method was used to prepare a "switchable" electrode surface by using self-assembled monolayers of dodecanethiol on a gold electrode. The dodecane-modified electrode was electrochemically inactive until the monolayer was soaked in solutions of 1,10-phenanthroline or 2,2'-bipyridine. The electroactive form of the electrode could be reverted back to the nonelectroactive form by rinsing the electrode. Surface IR results showed that both dodecanethiol and 1,10-phenanthroline exist in the mixed monolayer.     

Investigation of electronic structure and associated nuclear quadrupole interaction of adsorbed fluorine atoms on silicon surface

The electronic energy levels and wave functions of fluorine and chlorine atoms adsorbed at the <111> surface of silicon are obtained by the Hartree-Fock procedure applied to clusters simulating the surface structure. Minimising the total energy with respect to the positions of the halogen atoms, the Si-Cl distance in the adsorbed system was found to be in good agreement with that from a recent SEXAFS measurement, there being no corresponding data presently available for fluorine on silicon surface. The calculated¹⁹ f ^* nuclear quadrupole coupling constants using the electronic wave functions for SiH₃F and the larger cluster Si₄H₉F, simulating the surface, differed substantially in contrast to the situation for the Si-F distance, indicating that the quadrupole coupling is a more sensitive indicator of the adequacy of the cluster chosen to represent the surface system. Experimental results for the coupling constants are awaited for comparison with theory.     

A search for a thermal spike effect in sputtering

The temperature dependence of the sputtering yield was measured for lOOkeV per atom bombardment of silver with molecular and atomic antimony ions. No exponential increase of the yield was found in the temperature range from 25° to 775 °C, although a pronounced nonlinear effect is observed when the yield of these projectiles is compared,Y(Sb ₂ ⁺ )≈ 1.5×[2Y(Sb⁺)]. We conclude that there is no influence of the lattice temperature on collision spikes.     

Resonance-enhanced multiphoton ionization (2+1) of the\tilde B and\tilde C' states of ammonia

Resonance-enhanced multiphoton ionization (REMPI) of the \(\tilde B\) and \(\tilde C'\) states of NH₃ in a pulsed supersonic molecular beam was observed at 304–340 nm, using the multiphoton-ionization mass-spectrometer system described herein. The observed features are the result of a 2+1 MPI process and correspond quite well with the reported 3+1 MPI of NH₃ in the literature from the ground state of opposite parity.     

Dynamics of a dissipative two level system. II

The problem of quantum coherence in a symmetric double well potential with a dimensionless damping coefficient α for classical motion is studied within a spin 1/2-boson model. The experimentally measured probabilityp(t) of refinding a definite initial state after timet is approximately expressed by the transverse spin relaxation function ?(t), which is determined from a three-pole approximation, that incorporates both the correct long and short time behaviour. For a bare tunnelsplitting δ small compared to the heat bath cutoffω _c we find, that the oscillating component of ?(t) is negligible compared to the relaxational one unless α is of orderΔ/ω _c . Thusp(t)?(1+exp(?νt))/2 with a mean tunneling rateν proportional to \(\tilde \Delta = \Delta (\Delta /\omega _c )^{\frac{\alpha }{{2 - \alpha }}} \) for α < 2 andT ? \(\tilde \Delta \) and proportional toΔ ²/ω _c ·(T/ω _c ) ^α?1 otherwise. The results apply directly to recent measurements of the dynamics of flux states in a SQUID.     

Perfluormethyl-element-liganden XIII [1]: ein neuer weg zu zweikernkomplexen des typs M2(CO)8E(CF3)2X (M = Mn,Re)

The reactions of M(CO)₅X ( M = Mn, Re; X = Cl, Br, I) with E₂(CF₃)₄ (E = P, As) between 50 and 90°C yield binuclear complexes of the type M₂(CO)₈E(CF₃)₂X with two different bridging ligands, the formation of which is influenced by M (Mn > Re), E (P > As) , and X(I > Br > Cl). The main by-product is the symmetrical system M₂(CO)₈[E(CF₃)₂]₂, which is however not formed by the partial replacement of X by E(CF₃)₂ since this reaction requires temperatures above 120°C. The observed products can be explained by a three-step reaction path starting with the cleavage of E₂(CF₃)₄ followed by the subtitution of a cis-CO group in the M(CO)₅X component by M(CO)₅E(CF₃)₂ and the ring closure.     

Haloaldehyde polymers. IX. Polybromodichloroacetaldehyde

Bromodichloroacetaldehyde was synthesized by two methods. The first synthesis started from chloral, which was allowed to react with Ph₃P and the resultant compound brominated and hydrolyzed to give bromodichloroacetaldehyde in an overall yield of 60%. Purification by repeated distillation from P₂O₅ gave polymerization grade bromodichloroacetabldehyde. Bromodichloroacetaldehyde could also be synthesized by bromination of dichloroacetaldehyde diethyl acetal. The yields of this synthesis were only 20–30%, and the aldehyde could not be purified readily to give polymerization grade monomer. Bromodichloroacetaldehyde could be homopolymerized at ?30°C with anionic and also some cationic initiators to a polymer which was insoluble and did not melt but degraded to monomer above 200°C. The ceiling temperature of the polymerization was ?15°C in 1M solution. Bromodichloroacetaldehyde could also be copolymerized with isocyanates, primarily aryl isocyanates, and also with chloral.     

Anomalous solvent shift effect and observation of phosphorescence in bilirubin

Bilirubin IX-α has a large extinction coefficient but shows a weak blue-shift in solvents of increasing dielectric constant. A blue-shift is typical of an n → π^* transition, and is here interpreted in terms of the amide group present in the terminal pyrrole rings. Compounds undergoing n → π^* transitions usually form triplet states. With bilirubin, an emission is observed at 77 K; evidence is presented that this may be phosphorescence from an excited triplet state. The energy of this triplet level is 230 kJ mole^?1, thus bilirubin should be capable of sensitizing the formation of ¹Δ_g O₂.     

Electronic transitions and magnetic circular dichroism of the naphthalene monogenegative ion

The optical absorption and magnetic circular dichroism (MCD) spectra of the mononegative ion of naphthalene in a solution of 2-methyltetrahydrofuran have been measured at room temperature over the 25000–41000 cm^?1 spectral region. Experimental values of the MCD parameter B/D are compared with theoretical data obtained by means of an LCAO SCF CI calculation according to Pariser, Parr and Pople. The agreement between theory and experiment is rather good.