Total synthesis of (-)-bitungolide F and determination of its absolute stereochemistry

A highly convergent total synthesis of bitungolide F leading to the assignment of its absolute stereochemistry is described. The key steps include a Horner-Wadsworth-Emmons olefination to construct the C7-C8 bond, a Wittig reaction to introduce the conjugate E,E-olefinic moiety in the molecule, and finally a ring-closing metathesis reaction to construct the six-membered alpha,beta-unsaturated delta-lactone of the molecule. Modified Evans's syn-aldol reaction, using Crimmins's protocol, was used to install the stereochemistries at the C4 and C5 centers. The stereochemistry at C9 was introduced by means of hydroxy-directed reduction of the C9 keto using Evans's protocol.     

Intramolecular Cyclization of Carbonate and Thiocarbonate Derivatives of myo‐Inositol in the Solid State: Implications for Acyl Group Transfer Reactions in Molecular Crystals

Racemic 4‐O‐phenoxycarbonyl and 4‐O‐phenoxythiocarbonyl derivatives of myo‐inositol orthoformate undergo thermal intramolecular cyclization in the solid state to yield the corresponding 4,6‐bridged carbonates and thiocarbonates, respectively. The thermal cyclization also occurs in the solution and molten states, but less efficiently, suggesting that these cyclization reactions are aided by molecular pre‐organization, although not strictly topochemically controlled. Crystal structures of two carbonates and a thiocarbonate clearly revealed that the relative orientation of the electrophile and the nucleophile in the crystal lattice facilitates the intramolecular cyclization reaction and forbids the intermolecular reaction. The correlation observed between the chemical reactivity and the non‐covalent interactions in the crystal of the reactants provides a way to estimate the chemical stability of analogous molecules in the solid state.     

Comparison of racemic epi‐inosose and (−)‐epi‐inosose

The conversion of myo‐inositol to epi‐inositol can be achieved by the hydride reduction of an intermediate epi‐inosose derived from myo‐inositol. (−)‐epi‐Inosose, (I), crystallized in the monoclinic space group P2₁, with two independent molecules in the asymmetric unit [Hosomi et al. (2000). Acta Cryst. C 56 , e584–e585]. On the other hand, (2RS,3SR,5SR,6SR)‐epi‐inosose, C₆H₁₀O₆, (II), crystallized in the orthorhombic space group Pca2₁. Interestingly, the conformation of the molecules in the two structures is nearly the same, the only difference being the orientation of the C‐3 and C‐4 hydroxy H atoms. As a result, the molecular organization achieved mainly through strong O—H...O hydrogen bonding in the racemic and homochiral lattices is similar. The compound also follows Wallach's rule, in that the racemic crystals are denser than the optically active form.     

Near Ultraviolet Absorption Spectrum of Ethylpyrazine

Abstract

Vibrational analysis of the electronic absorption spectrum of ethylpyrazine observed around 320 nm is carried out. The bands are ascribed to the longest wavelength one of the four possible π? ← n systems. A “vibronic” interaction between the excited state of this system and another excited state of the molecule is observed. A π? ← π system with practically no vibrational structure is also observed and discussed.     

High Pressure Studies on Partially Bilayer and Monolayer Smectics

We have carried out pressure studies on the eighth and ninth members of the homologous series 4-alkoxybenzoyloxy-4′-cyanoazobenzene (nOBCAB). It is found that the pressure behavior of the monolayer smectic A (A₁) phase is entirely different from that of the partially bilayer smectic A (A₁) phase. The A₁ -nematic phase boundary is a straight line at all pressures and the range of the A_d phase increases with increasing pressure. On the other hand, the A_d -nematic phase boundary is elliptic in shape and the A_d phase gets bounded. These results emphasize the fact that the pressure behavior of the A phase is strongly related to the extent of overlap of the molecules in the layer.     

Total synthesis of (+)-epiquinamide from d-mannitol

The total synthesis of (+)-epiquinamide, a novel quinolizidine alkaloid isolated from the Ecuadoran poison frog, Epipedobates tricolor is described. The key step includes a ring-closing metathesis reaction to construct both the six member rings. d-Mannitol was used as a chiral pool material.     

Thermal analysis of binding of iodinated contrast agents to TiO2

Iodinated contrast agents such as iohexol and diatrizoate are pharmaceutical compounds of emerging concern in sewage and drinking water. These are resistant to removal through conventional water treatment processes. Photocatalytic degradation (PD) of these compounds over titanium dioxide has been suggested as a possible technique for the removal of iohexol and diatrizoate from sewage water. Several studies have evaluated these compounds, finding that the two do not respond in the same way to the presence of photocatalyst, which could imply that direct oxidation by surface electron holes is a major route for one compound but not the other. It is necessary that analytes adsorb to the photocatalyst for this oxidation to occur. We employ thermogravimetric analysis to characterize desorption of these analytes from two different forms of titanium dioxide. Compared to pure analyte and pure photocatalyst, additional peaks are seen near 700 °C when the contrast agents are adsorbed to titanium dioxide. This implies that direct oxidation by electron holes is not a primary factor in the PD of iodinated contrast agents. We evaluate the effects of analyte concentration and titanium dioxide crystal structure on these results to address the general applicability of our approach.     

Improving database enrichment through ensemble docking

While it may seem intuitive that using an ensemble of multiple conformations of a receptor in structure-based virtual screening experiments would necessarily yield improved enrichment of actives relative to using just a single receptor, it turns out that at least in the p38 MAP kinase model system studied here, a very large majority of all possible ensembles do not yield improved enrichment of actives. However, there are combinations of receptor structures that do lead to improved enrichment results. We present here a method to select the ensembles that produce the best enrichments that does not rely on knowledge of active compounds or sophisticated analyses of the 3D receptor structures. In the system studied here, the small fraction of ensembles of up to 3 receptors that do yield good enrichments of actives were identified by selecting ensembles that have the best mean GlideScore for the top 1% of the docked ligands in a database screen of actives and drug-like "decoy" ligands. Ensembles of two receptors identified using this mean GlideScore metric generally outperform single receptors, while ensembles of three receptors identified using this metric consistently give optimal enrichment factors in which, for example, 40% of the known actives outrank all the other ligands in the database.