Absolute stereochemistry of amphidinolides G and H

[structure: see text] Absolute stereochemistry of amphidinolides G (1) and H (2), potent cytotoxic 27- and 26-membered macrolides, respectively, isolated from a marine dinoflagellate Amphidinium sp., was determined by X-ray diffraction analysis, synthesis of a degradation product (3) of 2, and interconversion between 1 and 2.     

Relative Lewis basicities of six AI(ORF)4- superweak anions and the structures of LiA

The relative Lewis basicities of six Al(ORF)4- ions, Al[OC(CH3)(CF3)2]4-, Al(OC(CF3)3]4-, Al(OCPh(CF3)2]4-, Al[OC[4-C6H4(tBu)](CF3)2]4-, Al(OC(Cy)(CF3)2]4-, and Al(OCPh2(CF3)]4-, have been determined by measuring their relative coordinating abilities towards Li+ in dichloromethane. The relative Li- Lewis basicities of the Al(ORF)4- ions are linearly related to the aqueous pKa values of the corresponding parent HORF fluoroalcohols. The Lewis basicity of Al[OCH(CF3)2]4- could not be measured because two of these anions can coordinate to one Li+ cation. The structures of LiAl[OCH(CF3)2]4 and [1-Et-3-Me-1,3-C3H3N2][Li[Al[OCH(CF3)2)4]2] were determined.     

Guest shape-responsive fitting of porous coordination polymer with shrinkable framework

In situ synchrotron X-ray powder diffraction patterns of porous coordination polymers [[Cu(2)(pzdc)(2)(bpy)].G] have been measured (pzdc = pyrazine-2,3-dicarboxylate, bpy = 4,4'-bipyridine) (where G = H(2)O for CPL-2 superset H(2)()O, G = benzene for CPL-2 superset benzene, and G = void for the apohost). The structures of apohost and CPL-2 superset benzene were determined from Rietveld analysis. Adsorption of benzene in the channels induced a remarkable contraction in the crystal (b axis; 6.8%, volume; 4.9%), although the channels were occupied by the benzene molecules. This crystal transformation provides a new pore structure that is well suited for benzene molecules, and we denote it as a "shape-responsive fitting" transformation. This type of pore gives rise to a new guideline: frameworks can be composed of flexible motifs that are linked via strong bond and/or stiff motifs that are connected via weaker bonds.     

Rapid racemization of chiral non-racemic sec-alcohols catalyzed by (η-C5(CH3)5)Ru complexes bearing tertiary phosphine-primary amine chelate ligands

A ternary catalyst system of Cp*RuCl(cod)-2-diphenylphosphinoethylamine-KOt-Bu (Cp*=η⁵-C₅(CH₃)₅, cod=1,5-cyclooctadiene) causes rapid racemization of chiral non-racemic sec-alcohols, which results from the reversible hydrogen transfer between sec-alcohols and ketones. Both tertiary phosphine and primary amine functionalities in the ligand are responsible for the high rate.     

Total synthesis of (+)-tubelactomicin A. 2. Synthesis of the upper-half segment and completion of the total synthesis

[reaction: see text]. We have completed the total synthesis of natural (+)-tubelactomicin A (1), a 16-membered macrolide antibiotic. This Letter presents a highly efficient synthesis of the upper-half segment (C14-C24) and the completion of the total synthesis featuring a high-yielding Stille coupling for the connection of the upper-half and lower-half segments and Mukaiyama macrolactonization for the construction of the entire structure of 1.     

Measurement of hyperfine structure constants,g values and tensor polarizability of excited states of Sm I

Atomic-beam laser spectroscopy with fluorescence detection is applied to investigate the hyperfine structure (hfs), Stark and Zeeman effects of the electric dipole transitions of Sm i. The measurements are performed for the four transitions of 0 → 15650.55, 292.58 → 15650.55, 292.58 → 15567.32, and 1489.55 → 16890.59 cm^-1. The hfs constants of the upper and lower levels and the isotope shifts of the transitions are determined. The tensor polarizability of the 15650.55 cm^-1 level is found to be α₂ = -556.2 ± 11.8 kHz/(kV/cm)². Thanks to the accurate g values of the lower levels [Childs, W.J., Goodman, L.S.: Phys. Rev. A 6, 2011 (1972)], the g values of the upper levels are obtained from the observed Zeeman splittings. The values are found to be -0.115907(79), 2.22259(20) and 1.599968(51) for the 15650.55 cm^-1 .1, 15567.32 cm^-1 and 16890.59.cm^-1 levels, respectively.     

Distributed computing and NMR constraint-based high-resolution structure determination: applied for bioactive Peptide endothelin-1 to determine C-terminal folding

Distributed computing has been implemented to the solution structure determination of endothelin-1 to evaluate efficiency of the method for NMR constraint-based structure calculations. A key target of the investigation was determination of the C-terminal folding of the peptide, which had been dispersed in previous studies of NMR, despite its pharmacological significances. With use of tens of thousands of random initial structures to explore the conformational space comprehensively, we determined high-resolution structures with good convergences of C-terminal as well as previously defined N-terminal structures. The previous studies had missed the C-terminal convergence because of initial structure dependencies trapped in localized folding of the N-terminal region, which are strongly constricted by two disulfide bonds.     

Thermal and photochemical rearrangements of 1,4-dithiin sulfoxides

Thermolysis of 2,5-diphenyl-1,4-dithiin-1-oxide afforded 2-formyl-2,4-diphenyl-1,3-dithiole, which was obtained also in photolysis along with another rearranged product, 2-benzoyl-4-phenyl-1,3-dithiole.     

DNA-bleomycin interaction: Nucleotide sequence-specific binding and cleavage of DNA by bleomycin

Biosynthetic intermediates and synthetic analogues of bleomycin (BLM) have been investigated for their metal binding, dioxygen activation, and DNA cleavage. Molecular O₂ was activated by the Fe(II) complex of a synthetic model ligand. Nucleotide sequence specificities in DNA cleavage by the BLM-Fe(II) and deglyco-BLM-Fe(II) complexes were almost identical. It has been shown that (1) the β-aminoalanine-pyrimidine-β-hydroxyhistidine portion of BLM is essential for the metal binding and dioxygen activation and (2) the bithiazole moiety contributes to the specific binding to guanine base of DNA.     

Rapid determination of indium,nickel and copper in NBS standard reference material by substoichiometric radioactivation analysis

A method of rapid determination for indium, nickel and copper in Spinach (NBS; SRM-1570) by substoichiometric radioactivation analysis is described. The method is based on the principle that an equal amount of non-irradiated test sample is added to the irradiated standard sample and subsequent substoichiometric extraction for the irradiated test and standard samples is carried out. Indium is extracted as diethyldithiocarbamate into carbon tetrachloride, nickel as dimethylglyoximate into chloroform and copper as dithizonate into carbon tetrachloride. The radioactivities of these extracts are measured by NaI(Tl) or Ge(Li) detector coupled with pulse-height analyser. The analytical results obtained by the method were in good agreement with our published values and certified values by NBS.