A Concise, Enantioselective Synthesis of (-)- and (+)-Hongconin

Optically pure enone 9c, available in three steps from known 6-deoxy D-galactal derivative 7b, reacts with cyanophthalide 6 to directly afford the natural product (-)-hongconin (1), a compound from traditional Chinese medicine recently shown to exhibit antianginal activity. The enantiomer of 1 and its (+)-cis-diastereomer were also synthesized in a parallel fashion from the L-sugar counterpart. The use of C-glycoside Michael acceptors, as opposed to their O-glycoside counterparts, represents a potentially useful simplification of phthalide annulation methodology in synthesizing numerous other such optically pure isochromanquinoids, since it obviates the inconvenience of additional steps late in the synthetic scheme associated with reductive manipulation of a remaining acetal moiety into the desired pyran ring substituent.     

Comparative study of electrochemically directed assembly versus conventional self-assembly of thioacetyl-terminated oligo(phenylene ethynylene)s on gold and platinum surfaces

The assembly of thioacetyl-terminated oligo(phenylene ethynylene)s (OPEs) on Au and Pt surfaces under an electric potential (electrochemical assembly, EA) was compared to assembly at an open circuit (conventional self-assembly, CSA). Cyclic voltammetry and ellipsometry were used to characterize the adsorption kinetics of self-assembled monolayers formed by these two techniques. The adsorption rate of the EA was remarkably faster at positive potentials but slower at negative potentials than that of the CSA, The EA at 400 mV proceeded about 800 times faster than the CSA when exposed to the same solution concentrations. The adsorption rates of both EA and CSA were found to be dependent on the molecular structures of OPEs. OPEs containing electron-donating groups assemble faster than those with electron-withdrawing groups. The amount of time that the thioacetyl-terminated OPE is in the presence of the base, for removal of the acetyl group to generate the thiolate, is called the deprotection time. Deprotection times play a critical role in achieving the maximum difference in adsorption rates between the EA and the CSA. The assembly must be initiated no later than 5 min after the basic deprotection is commenced so that the thiolate concentration remains low. The difference in the adsorption rates between EA and CSA might enable selective deposition of certain OPEs onto specific electrodes.     

Experimental studies of the dynamics of the bond-forming reactions of CF2(2+) with H2O using position-sensitive coincidence spectroscopy

The dynamics of the product channels forming OCF(+)+H(+)+HF and HCF(2) (+)+H(+)+O following the collisions of CF(2) (2+) with H(2)O have been investigated with a new position-sensitive coincidence experiment at a center-of-mass collision energy of 5.6 eV. The results show the formation of OCF(+) occurs via the formation of a doubly charged collision complex [H(2)O-CF(2)](2+) which subsequently undergoes a charge separating dissociation to form H(+) and HOCF(2) (+). The HOCF(2) (+) monocation subsequently fragments to form HF+OCF(+). The lifetimes of the collision complex and the HOCF(2) (+) ion are at least of the order of their rotational period. The kinetic energy release in this reaction indicates that it involves the ground state of CF(2) (2+) and forms the ground electronic states of OCF(+) and HF. The mechanism for forming HCF(2) (+) involves the direct and rapid abstraction of a hydride ion from H(2)O by CF(2) (2+). The resulting OH(+) ion subsequently fragments to H(+)+O, on a time scale at least comparable with its rotational period.     

Interference between the hydrogen bonds to the two rings of nicotine

The biochemical transport and binding of nicotine depends on the hydrogen bonding between water and binding site residues to the pyridine ring and the protonated pyrrolidinium ring. To test the independence of these two moderately separated hydrogen-bonding sites, we have calculated the structures of clusters of protonated nicotine with water and a bicarbonate anion, benzene, indole, or a second water molecule. Unprotonated nicotine-water clusters have also been studied for contrast. The potential energy surfaces are first explored with an intermolecular anisotropic atom-atom model potential. Full geometry optimizations are then carried out using density functional theory to include nonadditive terms in the interaction energies. The presence of the charge on the pyrrolidine nitrogen removes the conventional hydrogen-bonding site on the pyridine ring. The hydrogen-bond ability of this site is nearly recovered when the protonated pyrrolidinium ring is bound to a bicarbonate anion, whereas its interaction with benzene shows a much smaller effect. Indole appears to partially restore the hydrogen-bond ability of the pyridine nitrogen, although indole and benzene both pi-bond to the pyrrolidinium ring. A second hydrogen-bonding water produces a significant conformational distortion of the nicotine. This demonstrates the limitations of the conventional qualitative predictions of hydrogen bonding based on the independence of molecular fragments. It also provides benchmarks for the development of atomistic modeling of biochemical systems.     

Under pressure: predicting pressurized metered dose inhaler interactions using the atomic force microscope

Drug particulate interactions in pressurized metered dose inhalers (pMDI) may lead to a decrease in aerosolization efficiency and subsequent efficacy in patient treatment. The interactions between salbutamol sulfate (commonly used in Ventolin pMDIs) and a series of pMDI canister materials were investigated using the atomic force microscope (AFM) colloid probe technique. Approximately 4000 individual force-distance curves were determined for a drug probe and three surfaces (10 x 10 mum areas) in situ, in a model propellant. The area under each force-distance curve was integrated to obtain separation energy values. Median separation energy values followed the rank order borosilicate glass > aluminum > PTFE, suggesting PTFE to be the most suitable canister coating.     

Use of the hydroxyl radical and gel electrophoresis to study DNA structure

The hydroxyl radical has been used as a chemical probe to study in solution the structure of DNA and DNA-protein complexes. The hydroxyl radical abstracts a deoxyribose hydrogen atom, cleaving one strand of the DNA. The cutting pattern, visualized by separating the cleavage products using gel electrophoresis, shows the reactivity of each backbone position toward the radical. This method has been applied to studies of DNA bending and helical twist. Phased runs of adenines (adenine tracts) cause sequence-directed DNA bending. The hydroxyl radical cleavage of a bent DNA fragment containing short adenine tracts phased with the helix screw gives rise to an unusual cutting pattern. The hydroxyl radical cleavage rate decreases in the 5' to 3' direction along each adenine tract, with a minimum at the 3' end of each adenine tract. The cleavage of the matching thymine tract is similar, but the minimum in the pattern is offset in the 3' direction. This pattern on the autoradiograph of the gel is interpreted to indicate that bending is accompanied by a narrow minor groove in the DNA molecule. Furthermore, hydroxyl radical cleavage results in different cutting patterns for two similar sequences, (CGA4T4)5 and (CGT4A4)5, which have been shown to be bent and relatively straight, respectively. The hydroxyl radical method has also been used to determine the helical repeat of the metallothionein IIA gene to be about 10.5 base pairs per turn. Methods of optimizing the hydroxyl radical reaction for DNA-protein footprinting are discussed. Because individual gel bands give information about cutting frequency at particular positions in the backbone, gel resolution and clear autoradiographs are important to this work.(ABSTRACT TRUNCATED AT 250 WORDS)     

Steroids and steroidases—XIX : Comparison of diazomethane and tiffeneau-demjanov homologations of 5α-3-oxosteroids. Evidence for predominant equatorial approach of the C-3 carbonyl group by diazomethane

Quantitative comparisons of the product ratios of the mechanistically similar diazomethane and Tiffeneau-Demjanov homologations of 17β-hydroxy-5α-androstan-3-one and 5α-cholestan-3-one have shown that equatorial approach of diazomethane to the C-3 CO group predominates to the extent of 70–79%. The data for both the C-17β-OH and -C₈H₁₇ substituted steroids are in close agreement thereby confirming that the C-17 substituents do not exert any significant long range effect on the reactions studied.     

The matching of electrostatic extrema: A useful method in drug design? A study of phosphodiesterase III inhibitors

Summary Ligands which bind to a specific protein binding site are often expected to have a similar electrostatic environment which complements that of the binding site. One method of assessing molecular electrostatic similarity is to examine the possible overlay of the maxima and minima in the electrostatic potential outside the molecules and thereby match the regions where strong electrostatic interactions, including hydrogen bonds, with the residues of the binding site may be possible. This approach is validated with accurate calculations of the electrostatic potential, derived from a distributed multipole analysis of an ab initio charge density of the molecule, so that the effects of lone pair and -electron density are correctly included. We have applied this method to the phosphodiesterase (PDE) III substrate adenosine-3,5-cyclic monophosphate (cAMP) and a range of nonspecific and specific PDE III inhibitors. Despite the structural variation between cAMP and the inhibitors, it is possible to match three or four extrema to produce relative orientations in which the inhibitors are sufficiently sterically and electrostatically similar to the natural substrate to account for their affinity for PDE III. This matching of extrema is more apparent using the accurate electrostatic models than it was when this approach was first applied, using semiempirical point charge models. These results reinforce the hypothesis of electrostatic similarity and give weight to the technique of extrema matching as a useful tool in drug design.