Computational and experimental structure–reactivity relationships: evidence for a side reaction in Alpine-Borane reductions of d-benzaldehydes

Extraordinary stereoselectivity, approaching 100%, has been reported in the reductions of d-benzaldehydes by B-isopinocampheyl-9-borabicyclo[3.3.1]nonane (Alpine-Borane). This is likely because of the extreme size disparity of groups on either side of the carbonyl. Here, we present a structure–reactivity study whereby the reductions of variably substituted d-benzaldehydes are explored using highly sensitive measures for enantiomeric excess and relative reactivity. These results are compared to the relative rates predicted from density functional calculations. The results indicate that 2,6-disubstitution adversely affects the stereoselectivity by means of a non-selective reduction via the dehydroboration product of Alpine-Borane, 9-borabicyclo[3.3.1]nonane.     

Bacterial protein structures reveal phylum dependent divergence

Protein sequence space is vast compared to protein fold space. This raises important questions about how structures adapt to evolutionary changes in protein sequences. A growing trend is to regard protein fold space as a continuum rather than a series of discrete structures. From this perspective, homologous protein structures within the same functional classification should reveal a constant rate of structural drift relative to sequence changes. The clusters of orthologous groups (COG) classification system was used to annotate homologous bacterial protein structures in the Protein Data Bank (PDB). The structures and sequences of proteins within each COG were compared against each other to establish their relatedness. As expected, the analysis demonstrates a sharp structural divergence between the bacterial phyla Firmicutes and Proteobacteria. Additionally, each COG had a distinct sequence/structure relationship, indicating that different evolutionary pressures affect the degree of structural divergence. However, our analysis also shows the relative drift rate between sequence identity and structure divergence remains constant.     

Gas-Phase Fluorescence Excitation and Emission Spectroscopy of Three Xanthene Dyes (Rhodamine 575, Rhodamine 590 and Rhodamine 6G) in a Quadrupole Ion Trap Mass Spectrometer

The gas-phase fluorescence excitation, emission and photodissociation characteristics of three xanthene dyes (rhodamine 575, rhodamine 590, and rhodamine 6G) have been investigated in a quadrupole ion trap mass spectrometer. Measured gas-phase excitation and dispersed emission spectra are compared with solution-phase spectra and computations. The excitation and emission maxima for all three protonated dyes lie at higher energy in the gas phase than in solution. The measured Stokes shifts are significantly smaller for the isolated gaseous ions than the solvated ions. Laser power-dependence measurements indicate that absorption of multiple photons is required for photodissociation. Redshifts and broadening of the dispersed fluorescence spectra at high excitation laser power provide evidence of gradual heating of the ion population, pointing to a mechanism of sequential multiple-photon activation through absorption/emission cycling. The relative brightness in the gas phase follows the order R575(1.00) < R590(1.15) < R6G(1.29). Fluorescence emission from several mass-selected product ions has been measured.     

Gold‐ and Silver‐Catalyzed Reactions of Propargylic Alcohols in the Presence of Protic Additives

A wide range of primary, secondary and tertiary propargylic alcohols undergo a Meyer–Schuster rearrangement to give enones at room temperature in the presence of a gold(I) catalyst and small quantities of MeOH or 4‐methoxyphenylboronic acid. The syntheses of the enone natural products isoegomaketone and daphenone were achieved using this reaction as the key step. The rearrangement of primary propargylic alcohols can readily be combined in a one‐pot procedure with the addition of a nucleophile to the resulting terminal enone, to give β‐aryl, β‐alkoxy, β‐amino or β‐sulfido ketones. Propargylic alcohols bearing an adjacent electron‐rich aryl group can also undergo silver‐catalyzed substitution of the alcohol with oxygen, nitrogen and carbon nucleophiles. This latter reaction was initially observed with a batch of gold catalyst that was probably contaminated with small quantities of silver salt.     

Catalytic Enantioselective Desymmetrisation as a Tool for the Synthesis of Hodgkinsine and Hodgkinsine B

Two palladium‐catalysed amination protocols are deployed in the desymmetrisation of the complex dimeric alkaloid meso‐chimonanthine. The power of these transformations is showcased in an efficient formal and total synthesis of the natural products hodgkinsine and hodgkinsine B, respectively.     

Synthesis and structure of an arylpyran normal, pseudodiacid showing recognition

Abstract  

Oxidation of 1,4-bis(4′-oxo-2′,2′-dimethylpent-2-yl)benzene with hypochlorite produces 1,4-bis(3′-carboxy-2′-methylbut-2-yl)benzene and 3-(4′-carboxyphenyl)-3,3-dimethylpropanoic acid. Cyclization of this mixture forms 3,3,7,7-tetramethyl-1,2,3,5,6,7-hexahydro-s-indacen-1,5-dione, 3,3,7,7-tetramethyl-1,2,3,5,6,7-hexahydro-as-indacen-1,5-dione (5) and 6-carboxy-3,3-dimethyl-1-indanone (6). Ketoacid (6) is converted to the arylpyran pseudoacid 7-carboxy-3-hydroxy-4,4-dimethylisobenzopyran-1-one (7). In the crystal structure of (7), carboxylic acid and the pseudoacid groups each form complementary dimer hydrogen bonds linking the molecules in chains. Contact O···O distances reflect their differing energetics, with pseudoacyl O···O at 2.78(1)Å and carboxylic O···O at 2.62(1)Å.     

Differentiation of 3-O-sulfated heparin disaccharide isomers: Identification of structural aspects of the heparin CCL2 binding motif

The presence of 3-O-sulfated glucosamine residues in heparin or heparan sulfate plays a role in binding to antithrombin III and HSV infection. In this study, tandem mass spectrometry was used to differentiate between two heparin disaccharide isomers containing variable sulfate at C6 in a common disaccharide and C3 in a more rare one. The dissociation patterns shown by MS² and MS³ were clearly distinguishable between the isomers, allowing their differentiation and quantitation. Using this technique, we show that an octasaccharide with 11 sulfate groups with high affinity for inflammatory chemokine CCL2 does not contain 3-O-sulfated disaccharides.     

Characterization and application of SuperLig<Superscript>®</Superscript> 620 solid phase extraction resin for automated process monitoring of <Superscript>90</Superscript>Sr

Characterization of SuperLig^® 620 solid phase extraction resin was performed in order to develop an automated on-line process monitor for ⁹⁰Sr. The main focus was on strontium separation from barium, with the goal of developing an automated separation process for ⁹⁰Sr in high-level wastes. High-level waste contains significant ¹³⁷Cs activity, of which ^137mBa is of great concern as an interference to the quantification of strontium. In addition barium, yttrium and plutonium were studied as potential interferences to strontium uptake and detection. A number of complexants were studied in a series of batch K_d experiments, as SuperLig^® 620 was not previously known to elute strontium in typical mineral acids. The optimal separation was found using a 2 M nitric acid load solution with a strontium elution step of ~0.49 M ammonium citrate and a barium elution step of ~1.8 M ammonium citrate. ⁹⁰Sr quantification of Hanford high-level tank waste was performed on a sequential injection analysis microfluidics system coupled to a flow-cell detector. The results of the on-line procedure are compared to standard radiochemical techniques in this paper.     

Investigation of a catalyst ink dispersion using both ultra-small-angle X-ray scattering and cryogenic TEM

The dispersion of Nafion ionomer particles and Pt/C catalyst aggregates in liquid media was studied using both ultra-small-angle X-ray scattering (USAXS) and cryogenic TEM. A systematic approach was taken to study first the dispersion of each component (i.e., ionomer particles and Pt/C aggregates), then the combination of the components, and last the catalyst ink. Multiple-level curve fitting was used to extract the particle size, size distribution, and geometry of the Pt/C aggregates and the Nafion particles in liquid media from the scattering data. The results suggest that the particle size, size distribution, and geometry are not uniform throughout the systems but rather vary significantly. It was found that the interaction of each component (i.e., the Nafion ionomer particles and the Pt/C aggregates) occurs in the dispersion. Cryogenic TEM was used to observe the size and geometry of the particles in liquid directly and to validate the scattering results. The TEM results showed excellent agreement.