Palladium-catalyzed asymmetric synthesis of axially chiral (allenylmethyl)silanes and chirality transfer to stereogenic carbon centers in S(E)' reactions

[reaction: see text] Novel stereoselective reactions of 4-substituted-1-trimethylsilyl-2,3-butadienes ((allenylmethyl)silanes) were developed. The axially chiral (allenylmethyl)silanes were prepared from (3-bromopenta-2,4-dienyl)trimethylsilane by a Pd-catalyzed asymmetric reaction with soft nucleophiles with up to 88% enantioselectivity. The (allenylmethyl)silanes reacted with acetals in the presence of a TiCl(4) promoter to give 1,3-diene derivatives via an S(E)' pathway. The 1,3-dienyl products have (E)-geometry exclusively and up to 88%( )()chirality transfer from the axially chiral allenes to the centrally chiral 1,3-dienes was observed in the S(E)' reaction.     

Structure of the water/platinum interface--a first principles simulation under bias potential

Ab initio molecular dynamics simulations have been performed on the water/Pt interface. When the surface is neutral, water is found to form a contact layer directing its O atom toward the surface, i.e., O-down configuration. When the surface is negatively biased, the contact layer shows a significant structural change. The O-down configuration is converted mostly to the H-down configuration. As the surface is biased more strongly, we find that a hydrophobic double layer is formed in the contact layer.     

Simple chiral diene ligands provide high enantioselectivities in transition-metal-catalyzed conjugate addition reactions

Chiral dienes possessing the bicyclo[2.2.2]octadiene framework were prepared readily through the [4 + 2] cycloaddition of ( R)-alpha-phellandrene with methyl propiolate as the key step. Diene 9, substituted with a tertiary alcohol on one of the two double bonds, is prepared in just one step from the cycloadduct and is highly effective as a chiral ligand for rhodium-catalyzed asymmetric conjugate addition reactions, giving the corresponding addition products with higher enantioselectivity than other chiral dienes.     

Rhodium-catalyzed kinetic resolution of tertiary homoallyl alcohols via stereoselective carbon-carbon bond cleavage

A nonenzymatic kinetic resolution of tertiary homoallyl alcohols has been developed through a rhodium-catalyzed retro-allylation reaction under simple conditions. Selectivity factors of up to 12 have been achieved by employing (R)-H8-binap as the ligand, and the reaction can be conducted on a preparative scale.     

Biosensor immunoassay for the screening of dioxin-like polychlorinated biphenyls in retail fish

Dioxin-like polychlorinated biphenyls (DL-PCBs) often make up the majority of the toxic equivalent (TEQ) contribution of dioxins found in fish samples. For the purpose of making risk assessments, it is therefore important to develop screening methods for determining TEQ concentrations of DL-PCBs in retail fish. We have developed a rapid biosensor immunoassay (BIA) for DL-PCBs that uses a surface plasmon resonance sensor (Biacore 3000). The BIA is highly specific for 2,3',4,4',5-pentachlorobiphenyl (PCB 118) that is generally the most abundant DL-PCB isomer found in fish. The fish extracts were first cleaned up on a multilayer silica gel column followed by an alumina column, then subjected to the assay. The quantitative limit of the assay was 1 ng PCB 118 per gram of tested sample. Dilution and recovery tests using purified fish extracts suggested that the matrix effect was minimized in the assay by diluting the analyzed samples. The assay results for retail fish samples (n=7) agreed well with those obtained by an enzyme-linked immunoassay (ELISA) using the same monoclonal antibody: ELISA has been already validated for determining DL-PCBs in fish samples, so BIA performs well in this analysis. Finally, BIA results for the TEQ concentrations of DL-PCBs in retail fish samples (n=10) correlated well with those obtained by high-resolution gas chromatography coupled to high-resolution mass spectrometry (r=0.89). Our method is therefore useful for screening retail fish to determine the TEQ concentrations of DL-PCBs.     

Biosynthesis of octosyl acid A: Incorporation of C-13 labeled glucose

D-[1 and 6-¹³C]Glucose feeding experiments followed by ¹³C-NMR analysis supported that octosyl acid A is a shunt metabolite of the polyoxin biosynthesis, being formed by the condensation of uridine and phosphoenolpyruvate.     

Palladium-phosphine complex catalyzed cross-coupling reaction of 1-methyl-2-pyrrolyl-magnesium bromide and -zinc chloride with organic halides

1-Methyl-2-pyrrolyl-magnesium bromide and -zinc chloride, which were prepared from 1-methyl-2-pyrrolyllithium with MgBr₂ and ZnCl₂ respectively, reacted with aryl- and heteroaromatic halides to give the corresponding 2-substituted pyrroles in good to excellent yields in the presence of palladium-phosphine complexes as catalysts.