Enzymatic formation of pyrrole-containing novel cyclic polyprenoids by bacterial squalene:hopene cyclase

A convergent synthesis provided two pyrrole-containing squalene analogues, in which a C₂₀ isoprene unit is connected to pyrrole, 2-(geranylgeranyl)pyrrole and 2-(farnesyldimethylallyl)pyrrole. When incubated with recombinant squalene:hopene cyclase from Alicyclobacillus acidocaldarius, 2-(farnesyldimethylallyl)pyrrole was enzymatically converted to a 10:1 mixture of a tricyclic and a bicyclic unnatural novel polyprenoids, whereas 2-(geranylgeranyl)pyrrole was not a substrate for the enzyme.     

Enzymatic formation of an unnatural novel tetracyclic sesterterpene by β-amyrin synthase

A convergent synthesis provided a C₂₅ and a C₃₅ oxidopolyprene in which a farnesyl C₁₅ unit is connected in a head-to-head fashion to a geranyl C₁₀ or a geranylgeranyl C₂₀ unit. When incubated with recombinant β-amyrin synthase from Pisum sativum the C₂₅ oxidopolyprene was enzymatically converted to an unnatural novel tetracyclic sesterterpene, while the C₃₅ analogue did not afford any cyclization product.     

Enzymatic formation of indole-containing unnatural cyclic polyprenoids by bacterial squalene:hopene cyclase

[reaction: see text] Two indole-containing substrate analogues, in which a C20 isoprene unit is connected to indole (3-(geranylgeranyl)indole and 3-(farnesyldimethylallyl)indole), were synthesized and tested for enzymatic cyclization by squalene:hopene cyclase from Alicyclobacillus acidocaldarius. Interestingly, 3-(geranylgeranyl)indole was not a substrate for the bacterial squalene cyclase, while 3-(farnesyldimethylallyl)indole was efficiently converted to a 2:1 mixture of unnatural novel products.     

Structure and absolute stereochemistry of novel C15-halogenated acetogenins from the anaspidean mollusc Aplysia dactylomela

Three novel halogenated C₁₅-acetogenins, compounds 1-3, have been isolated, together with known metabolites, from a South China Sea collection of the anaspidean mollusc Aplysia dactylomela. The structures have been suggested by both NMR analysis and comparison with literature data. The structure of 1 was confirmed by a single crystal X-ray study, which also allowed the establishment of its absolute stereochemistry.     

Comparison studies of fullerenes C72, C74, C76 and C78 by tight-binding Monte Carlo and quantum chemical methods

From C₇₂ to C₇₈, the top 20 low-energy isomers screened out from all isomers of each fullerene are optimized and computed by tight-binding Monte Carlo (TBMC), semi-empirical PM3, and ab initio B3LYP/6-31G*//HF/3-21G methods. The comparison results show that the TBMC method can efficiently optimize the structures and correctly predicate the low energy isomers. The relative energies computed by TBMC are in good agreement with the high-lever B3LYP calculation results. Our TBMC and B3LYP results show that the most energetically favorable structure of C₇₂ is not an isomer satisfying the isolated pentagon rule (IPR), which is different with the result by PM3. The symmetry of the most stable IPR isomer tends to low as the fullerene becomes large and several non-isolated-pentagon structures are found to have low symmetries and low energies close to the most stable isomer.     

Semiempirical molecular dynamics studies of C60/C70 fullerene oxides: C60O, C60O2 and C70O

The spectral analysis indicates that all isomers of C₆₀O, C₇₀O and C₆₀O₂ have an epoxide-like structure (an oxygen atom bridging across a C–C bond). According to the geometrical structure analysis, there are two isomers of fullerene monoxide C₆₀O (the 5,6 bond and the 6,6 bond), eight isomers of fullerene monoxide C₇₀O and eight isomers of fullerene dioxide C₆₀O₂. In order to simulate the real reaction conditions at 300 K, the calculation of the different isomers of C₆₀O, C₆₀O₂ and C₇₀O fullerene oxides was carried out using the semiempirical molecular dynamics method with two different approaches: (a) consideration of the geometries and thermodynamic stabilities, and (b) consideration of the ozonolysis mechanism. According to the semiempirical molecular dynamic calculation analysis, the probable product of this ozonolysis reaction is C₆₀O with oxygen bridging over the 6–6 bond (C_2v). The most probable product in this reaction contains oxygen bridging across in the upper part of C₇₀ (6–6 bond in C₇₀O-2 or C₇₀O-4) an epoxide-like structure. C₆₀O₂-1, C₆₀O₂-3 and C₆₀O₂-5 are the most probable products for the fullerene dioxides. All of these reaction products are consistent with the experimental results. It is confirmed that the calculation results with the semiempirical molecular dynamics method are close to the experimental work. The semiempirical molecular dynamics method can offer both the reaction temperature effect by molecular dynamics and electronic structure, dipole moment by quantum chemistry calculation.     

Synthesis of unnatural pentahydroxylated pyrrolizidines: 5-epi- and 5,7a-di-epi-hyacinthacine C1

Stereocontrolled synthesis of 5-epi- and 5,7a-di-epi-hyacinthacine C₁ (7 and 8), two potential glycosidase inhibitors are described using α,β-unsaturated ketone 9 as homochiral starting material. The key step in the synthesis is the highly diastereoselective dihydroxylation reaction of 9, that allows the obtention of a single bis-hydroxylated ketone (10). Further derivatization into two epimeric mesylate esters followed by internal cyclization form the pyrrolizidinic compounds 7 and 8. This type of compounds can be useful in glycobiology due to their ability to inhibit carbohydrate-processing enzymes.     

Solvothermal synthesis and characterisation of new one-dimensional indium and gallium sulphides: [C10N4H26]0.5[InS2] and [C10N4H26]0.5[GaS2]

Two new main group metal sulphides, [C₁₀N₄H₂₆]_0.5[InS₂] (1) and [C₁₀N₄H₂₆]_0.5[GaS₂] (2) have been prepared solvothermally in the presence of 1,4-bis(3-aminopropyl)piperazine and their crystal structures determined by single-crystal X-ray diffraction. Both compounds are isostructural and crystallise in the monoclinic space group P2₁/n (Z=4), with a=6.5628(5), b=11.2008(9), c=12.6611(9) Å and β=94.410(4)° (wR=0.035) for compound (1) and a=6.1094(5), b=11.2469(9), c=12.7064(10) Å and β=94.313(4)° (wR=0.021) for compound (2). The structure of [C₁₀N₄H₂₆]_0.5[MS₂] (M=In,Ga) consists of one-dimensional [MS₂]⁻ chains which run parallel to the crystallographic a axis and are separated by diprotonated amine molecules. These materials represent the first example of solvothermally prepared one-dimensional gallium and indium sulphides.     

Synthesis of topsentolides A2 and C2, and non-enzymatic conversion of the former to the latter

The first total synthesis of the marine-derived cytotoxin topsentolide A₂, which eventually culminated in its stereochemical determination, was accomplished in 17 steps from a known chiral alcohol. An improved synthesis of its congener, topsentolide C₂, from a synthetic intermediate of topsentolide A₂ was also performed by utilizing the Yamaguchi lactonization to construct its nine-membered lactone ring. Treatment of epoxide ring-containing topsentolide A₂ with HCl/MeOH brought about its quantitative conversion into topsentolide C₂.     

The geometry, vibrational frequencies, thermochemistry, quadrupole moments and electronic structure of C2Na2: Comparison with C2Li2, C2H2, C2F2 and C2Cl2

The electronic structure of Na₂C₂ is studied using ab initio electronic structure methods and is compared to the companion molecule Li₂C₂. Both the linear D_∞h and planar structures are minima on the ground state potential surface with the planar D_2h conformation being the lowest energy form, similar to Li₂C₂. At the CCSD(t) level the planar form is more stable that the linear by 11.2 kcal/mol as compared with 7.34 kcal/mol for Li₂C₂. Both molecules are significantly ionic. The vibrational frequencies, atomization energy at 0 K, D₀, and the standard enthalpy of formation, are calculated and compared to those of Li₂C₂ as well as HCCH, FCCF and ClCCCl. We find D₀ and to be 331.1 and 84.92 kcal/mol for Li₂C₂ and 298.3 and 93.25 kcal/mol for Na₂C₂. We calibrate these by calculating the same quantities for HCCH, FCCF and ClCCCl.     

离子色谱法测定合成气制烯烃产物中的C_1~C_6有机酸

采用离子色谱建立了合成气制烯烃(SGTO)水相产物和油相产物中C1~C6有机酸的测定方法。对分离条件进行了优化,使用标准样品测定了线性范围和工作曲线,考察了方法的精密度和准确度,确定了SGTO油相产物样品的碱洗条件,并对SGTO水相产物和油相产物样品进行了测定。结果表明:C1~C6有机酸的质量浓度在各自配制的质量浓度范围内呈现良好的线性关系,相关系数均大于0.99。标准溶液的回收率测定表明回收率在95.6%~104.3%之间,5次重复测定的相对标准偏差(RSD)在0.4%~3.6%之间,表明该方法具有良好的准确性和精密度。SGTO油相产物中的加标回收率在91.1%~96.8%之间,5次重复测定的RSD为0.7%~2.3%,准确性可以满足实际分析的需要。实际SGTO水相产物和油相产物中C1~C6有机酸的分析结果表明,SGTO水相产物中C2~C4有机酸含量较高,而SGTO油相产物中C4~C6有机酸含量较高。本研究对SGTO反应研究、催化剂制备、工艺优化以及设备材料的选择具有重要意义。     

Synthesis and molecular structures of heptafluoropropylated fullerenes: C70(n-C3F7)8, C70(n-C3F7)6O, and C70(C3F7)4

Ampoule reactions of C₇₀ with n- and i-C₃F₇I were carried out at 250-310 °C. Two step HPLC separations allowed the isolation of several C₇₀(n-C₃F₇)_4-8 and C₇₀(i-C₃F₇)₄ compounds. Crystal and molecular structures of C₇₀(n-C₃F₇)₈-V, C₇₀(n-C₃F₇)₆O, C₇₀(n-C₃F₇)₄, and three isomers of C₇₀(i-C₃F₇)₄ have been determined by X-ray crystallography using synchrotron radiation. Molecular structures of the new compounds were compared with the known examples and discussed in terms of addition patterns and relative energies of their formation.     

不同预处理的氧化亚铜纳米线电极诱导的晶面效应提升电化学CO2还原为多碳产物

近年来,由于化石燃料不断消耗造成的二氧化碳气体过量排放,对人类生活环境造成越来越大的威胁.电催化二氧化碳还原反应是一种很有前景的解决方法,可回收废弃的二氧化碳并通过将其转化为可再生的燃料和化学品来最终实现碳循环.在各种还原产物中,多碳化学产物因其具有高能量密度和高商业价值而备受青睐.然而,由于涉及多个复杂的反应途径,设...     

Synthesis, structural characterization and reactivity of the amino borane 1-(NPh2)-2-[B(C6F5)2]C6H4

The bright red title compound 1 was synthesized from (2-lithiophenyl)diphenylamine and bis(pentafluorophenyl)boron chloride. Its reactions with small acids like H₂O and HCl proceeded easily giving zwitterionic compounds. For 1 and its water adduct 2 the crystal structures were determined, the latter featuring an ammonium borate structure containing a short intramolecular hydrogen bond bridge. Treatment of 1 with Jutzi's acid, [H(OEt₂)₂][B(C₆F₅)₄], did not result in protonation of the nitrogen, but reaction of 1 with LiH in the presence of 12-crown-4, led to the isolation of the aminoborate [1-(Ph₂N)-2-{B(H)(C₆F₅)₂}C₆H₄][Li(12-crown-4)] (3). Borohydride 3 reacted with Jutzi's acid to regenerate 1 and liberate hydrogen.     

The geometry of Goldstein and Kline's C11H11 cation

Semi-empirical computations on the C₁₁H₁₁ cation reported by Goldstein and Kline indicate that a classical structure suggested by models is not an appropriate representation of the system. A system with C_s symmetry differing in detail from the “quasi D_2d” form proposed by the discoverers is found to be the stable form. The reasons for easy rearrangement in this system are discussed.     

Reactions of the dications C7H6, C7H7, and C7H8 with methane: Predominance of doubly charged intermediates

The reactions of methane with the dications C₇H₆²⁺, C₇H₇²⁺, and C₇H₈²⁺ generated by electron ionization of toluene are studied using mass-spectrometry tools. It is shown that the reactivity is dominated by the formation of doubly charged intermediates, which can either eliminate molecular hydrogen to yield doubly charged products or undergo charge-separation reactions leading to the formation of a methyl cation and the corresponding C₇H_n+1⁺ monocation. Typical processes observed for dications, like electron transfer or proton transfer, are largely suppressed. The theoretically derived mechanism of the reaction between C₇H₆²⁺ and CH₄ indicates that the formation of the doubly charged intermediate is kinetically preferred at low internal energies of the reactants. In agreement, the experimental results show a pronounced hydrogen scrambling and dominant formation of the doubly charged products at low collision energies, whereas direct hydride transfer prevails at larger collision energies.     

Cover Picture: Polyketide‐Chain Branching by an Enzymatic Michael Addition (Angew. Chem. Int. Ed. 27/2009)

The fungus Rhizopus microsporus …? ; shown in the micrograph on the cover picture, harbors the endosymbiotic bacteria Burkholderia rhizoxinica that produces the antimitotic polyketide macrolide rhizoxin (see structure). In their Communication on page 5001 ff. , C. Hertweck and co‐workers show that the enzyme rhizoxin polyketide synthase is uniquely capable of introducing a β branch (highlighted by the magnifying glass) by a Michael‐type addition of a malonyl unit to an acryloyl intermediate.

     

Desmethylubiquinone Q2 from the Far-Eastern ascidian Aplidium glabrum: structure and synthesis

Two new diprenylquinones, glabruquinone A (desmethylubiquinone Q₂) having cancer preventive properties and its minor isomer Glabruquinone B were isolated from the ascidian Aplidium glabrum. Their structures have been elucidated by NMR and mass spectra and confirmed by synthesis.     

Synthesis and reactivity of [N(C6H4Br)3][B(C6F5)4]: the X-ray crystal structure of [Fe(C5H5)2][B(C6F5)4]

A new chemical oxidant [N(4-C₆H₄Br)₃][B(C₆F₅)₄], was prepared and used to synthesize [Fe(C₅H₅)₂][B(C₆F₅)₄]. The crystal structure of [Fe(C₅H₅)₂][B(C₆F₅)₄] was determined.