Shimalactone Biosynthesis Involves Spontaneous Double Bicyclo-Ring Formation with 8π-6π Electrocyclization

Shimalactones A and B are neuritogenic polyketides possessing characteristic oxabicyclo[2.2.1]heptane and bicyclo[4.2.0]octadiene ring systems that are produced by the marine fungus Emericella variecolor GF10. We identified a candidate biosynthetic gene cluster and conducted heterologous expression analysis. Expression of ShmA polyketide synthase in Aspergillus oryzae resulted in the production of preshimalactone. Aspergillus oryzae and Saccharomyces cerevisiae transformants expressing ShmA and ShmB produced shimalactones A and B, thus suggesting that the double bicyclo-ring formation reactions proceed non-enzymatically from preshimalactone epoxide. DFT calculations strongly support the idea that oxabicyclo-ring formation and 8π-6π electrocyclization proceed spontaneously after opening of the preshimalactone epoxide ring through protonation. We confirmed the formation of preshimalactone epoxide in vitro, followed by its non-enzymatic conversion to shimalactones in the dark.     

Synthesis and Systematic Structural Analysis of Cationic Half-Sandwich Ruthenium Chalcogenocarbonyl Complexes

Although the chemistry of transition-metal complexes with carbonyl (CO) and thiocarbonyl (CS) ligands has been well developed, their heavier analogues, namely selenocarbonyl (CSe) and tellurocarbonyl (CTe) complexes remain scarce. The limited availability of such CSe and CTe complexes has so far hampered our understanding of the differences between such chalcogenocarbonyl (CE: E=O, S, Se, Te) ligands. Herein, we report the synthesis and properties of a series of cationic half-sandwich ruthenium CE complexes of the type [CpRu(CE)(H₂IMes)(CNCH₂Ts)][BAr^F₄] (Cp=η⁵-C₅H₅⁻; H₂IMes=1,3-dimesitylimidazolin-2-ylidene; Ar^F=3,5-(CF₃)₂C₆H₃). A combination of X-ray diffraction analyses, NMR spectroscopic analyses, and DFT calculations revealed an increasing π-accepting ability of the CE ligands in the order O<S<Se<Te. A variable-temperature NMR analysis of the thus obtained chiral-at-metal CE complexes indicated high stereochemical stability.     

采用X射线吸收光谱表征单原子催化剂:至关重要与持久挑战

X射线吸收光谱(XAS)可为负载型单中心(单原子或单核金属络合物)催化剂的结构和电子特性提供重要信息.虽然XAS技术可表征真实反应条件下、无需长程有序结构的催化剂,并且可提供对于负载型单中心催化剂非常重要的金属-载体界面信息;但是它给出的信息是包括与催化有关或无关的所有负载型金属物种的平均信息.负载型催化剂的准确表征具有长期挑战性,也限制了我们准确地理解催化剂的构效关系.为了更好地利用XAS表征技术,深入研究催化剂的构效关系,并最终用其指导设计开发出高效的催化剂,制备具有均一结构活性位的负载型单中心催化剂,并采用XAS及相关技术对其表征至关重要.本文列举了一些实例以说明XAS在表征具有均一结构活性位的负载型单中心催化剂方面的能力,以及XAS如何与其他技术(如扫描透射电子显微镜和红外光谱)互补,为以分子筛和金属-有机骨架材料为载体而制得的具有均一结构活性位的负载型单中心催化剂提供原子尺度的信息.     

Dominant Li+ ion conduction in Li+/Na+ mixed crystal, 15NaI∙LiBH4

Journal of Solid State Electrochemistry - In the present work, the conduction ions in 15NaI∙LiBH4 are investigated by secondary ion mass spectroscopy (SIMS) and galvanostatic measurements. An...     

An efficient method for 3,4-dihydroisoquinolinium ion formation,leading to a facile introduction of nucleophiles

The ketene silyl acetal derived from ethyl 2-benzyltetrahydroisoquinoline-1-carboxylate undergoes a rapid oxidation reaction with N-bromosuccinimide to form 3,4-dihydroisoquinolinium ion, which reacts with Grignard reagents to give 1,1-disubstituted tetrahydroisoquinolines in good yields.     

Synthesis and Properties of Phosphorus-Containing Aromatic Polyethers

The phosphorus-containing aromatic polyethers were prepared from bis(p-chlorophenyl)phenylphosphine oxide (BCPO) with the sodium salt of several bisphenols by high temperature solution polycondensation. The best result (yield 84%, n _sp /c = 0.15) was obtained from BCPO with bisphenol A (BPA) in dimethyl sulfoxide. However, the polymerization in the solvents such as N, N′-dimethyl-2-pyrrolidone, hexamethylphosphoramide, and N,N′-dimethylformamide, and the polymerization with the other bisphenols HO-C₆H₄-X-C₆H₄?OH, where X = CO, S0₂, CH₃P(O), C₆H₅P(O) in place of BPA gave gumlike polymers. The polymer prepared from BCPO and BPA did not decompose up to ca. 300°C under air or nitrogen atmosphere, but it decomposed slowly at 300–520°C, and decomposed rapidly at 520–540°C. The activation energy (δE) for the maximum rate of weight loss was 47.8 kcal/mole.     

Stereodivergent Synthesis and Stereochemical Reassignment of the C79–C104 Fragment of Symbiodinolide

We have synthesized eight possible diastereoisomers 3 a – h of the C79–C97 fragment of symbiodinolide ( 1 ) in a stereodivergent manner by utilizing a dithiane addition to the aldehyde as a key step. Comparison of the ¹³C NMR chemical shifts of the natural product 1 and the synthetic products 3 a – h indicated that the relative stereostructure of this fragment in symbiodinolide ( 1 ) is that represented in 3 a or f . We have stereodivergently synthesized eight possible diastereoisomers of the C94–C104 fragment 4 a – h , and we have compared their ¹³C NMR chemical shifts with those of the natural product, which established the relative stereochemistry of this fragment to be that described in diastereoisomers 4 a or e . By combining the stereostructural outcomes of the C79–C97 and C94–C104 fragments, we have proposed four candidate compounds of the C79–C104 fragment 2 a – d . We also synthesized diastereoisomers 2 a and b ( 2 a in the preceding article; Chem. Eur. J. 2015 , DOI: 10.1002/chem.201503880) by a Julia–Kocienski olefination and diastereoisomers 2 c and d by a Wittig reaction. By comparing the ¹³C NMR chemical shifts of natural symbiodinolide ( 1 ) with those of the synthetic products 2 a – d , we have reassigned the stereostructure of the C79–C104 fragment of natural product 1 to be that depicted in diastereoisomer 2 b .     

Stereoselective Synthesis of the Proposed C79–C104 Fragment of Symbiodinolide

Stereoselective and streamlined synthesis of the proposed C79–C104 fragment 2 of symbiodinolide ( 1 ), a polyol marine natural product with a molecular weight of 2860, was achieved. In the synthetic route, the proposed C79–C104 fragment 2 was synthesized by utilizing a Julia–Kocienski olefination and subsequent Sharpless asymmetric dihydroxylation as key transformations in a convergent manner. Detailed comparison of the ¹³C NMR chemical shifts between the natural product and the synthetic C79–C104 fragment 2 revealed that the stereostructure at the C91–C99 carbon chain moiety of symbiodinolide ( 1 ) should be reinvestigated.     

Reactive regioregular oligothiophenes and polythiophenes with Zincke salt structure: Synthesis,reactions, and chemical properties

Polythiophenes with reactive Zincke salt structure, P4ThPy⁺DNP(Cl^?)‐a and P5ThPy⁺DNP(Cl^?)‐a , were synthesized by the oxidation polymerization of oligothiophenes, such as 3'‐(4‐N‐(2,4‐dinitrophenyl)pyridinium chloride)?2,2':5',2'';5'',2'''‐quarterthiophene ( 4ThPy⁺DNP(Cl^?) ) and 4''‐(4‐N‐(2,4‐dinitrophenyl)pyridinium chloride)?2,2';5',2'';5'',2''';5''',2''''‐quinquethiophene ( 5ThPy⁺DNP(Cl^?) ), with iron(III) chloride. The reaction of P5ThPy⁺DNP(Cl^?)‐a with R‐NH₂ [R = n‐hexyl (Hex) and phenyl (Ph)] substituted the 2,4‐dinitrophenyl group into the R group with the elimination of 2,4‐dinitroaniline to yield P5ThPy⁺R(Cl^?) . Similarly, model compounds, 4ThPy⁺R(Cl^?) and 5ThPy⁺R(Cl^?) (R = Hex and Ph), were also synthesized. In contrast to the photoluminescent 4ThPy and 5ThPy , the compounds P4ThPy⁺DNP(Cl^?)‐a , P5ThPy⁺DNP(Cl^?)‐a , and P5ThPy⁺R(Cl^?) showed no photoluminescence because their internal pyridinium rings acted as quenchers. Cyclic voltammetry measurements suggested that P4ThPy⁺DNP(Cl^?)‐a , P5ThPy⁺DNP(Cl^?)‐a , and P5ThPy⁺R(Cl^?) received an electrochemical reduction of the pyridinium and 2,4‐dinitrophenyl groups and oxidation of the polymer backbone. P4ThPy⁺DNP(Cl^?)‐a and P5ThPy⁺DNP(Cl^?)‐a were electrically conductive (ρ = 3.0 × 10 ^{? 6} S cm ^{? 1} and 2.1 × 10 ^{? 6} S cm ^{? 1}, respectively) in the nondoped state. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 481–492     

Azo benzene derivatives

Pressure-volume isotherms for 4-(trans-4-butylcylohexyl)benzonitrile were measured by means of piston-cylinder method. This substance has a monotropic nematic phase at atmospheric pressure. But over a pressure, the nature of isotropic to nematic phase transition changed from monotropic to enantiotropic phase transition. It is the first observation in P-V-T experiments that the monotropic isotropic-nematic transition changes enantiotropic under pressure.