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921.
Atom‐Economical Dimerization Strategy by the Rhodium‐Catalyzed Addition of Carboxylic Acids to Allenes: Protecting‐Group‐Free Synthesis of Clavosolide A and Late‐Stage Modification 下载免费PDF全文
Alexander M. Haydl Prof. Dr. Bernhard Breit 《Angewandte Chemie (International ed. in English)》2015,54(51):15530-15534
Natural products of polyketide origin with a high level of symmetry, in particular C2‐symmetric diolides as a special macrolactone‐based product class, often possess a broad spectrum of biological activity. An efficient route to this important structural motif was developed as part of a concise and highly convergent synthesis of clavosolide A. This strategy features an atom‐economic “head‐to‐tail” dimerization by the stereoselective rhodium‐catalyzed addition of carboxylic acids to terminal allenes with the simultaneous construction of two new stereocenters. The excellent efficiency and selectivity with which the C2‐symmetric core structures were obtained are remarkable considering the outcome under classical dimerization conditions. Furthermore, this approach facilitates late‐stage modification and provides ready access to potential new lead structures. 相似文献
922.
Bacterial Synthesis of Unusual Sulfonamide and Sulfone Antibiotics by Flavoenzyme‐Mediated Sulfur Dioxide Capture 下载免费PDF全文
Martin Baunach Dr. Ling Ding Karsten Willing Prof. Dr. Christian Hertweck 《Angewandte Chemie (International ed. in English)》2015,54(45):13279-13283
Sulfa drugs, such as sulfonilamide and dapsone, are classical antibiotics that have been in clinical use worldwide. Despite the relatively simple architectures, practically no natural products are known to feature such aromatic sulfonamide or diarylsulfone substructures. We report the unexpected discovery of three fully unprecedented, sulfonyl‐bridged alkaloid dimers (sulfadixiamycins A–C) from recombinant Streptomyces species harboring the entire xiamycin biosynthesis gene cluster. Sulfadixiamycins exhibit moderate antimycobacterial activities and potent antibiotic activities even against multidrug‐resistant bacteria. Gene inactivation, complementation, and biotransformation experiments revealed that a flavin‐dependent enzyme (XiaH) plays a key role in sulfadixiamycin biosynthesis. XiaH mediates a radical‐based, three‐component reaction involving two equivalents of xiamycin and sulfur dioxide, which is reminiscent of radical styrene/SO2 copolymerization. 相似文献
923.
Highly Regio‐ and Enantioselective Synthesis of N‐Substituted 2‐Pyridones: Iridium‐Catalyzed Intermolecular Asymmetric Allylic Amination 下载免费PDF全文
Xiao Zhang Ze‐Peng Yang Lin Huang Prof. Dr. Shu‐Li You 《Angewandte Chemie (International ed. in English)》2015,54(6):1873-1876
The first iridium‐catalyzed intermolecular asymmetric allylic amination reaction with 2‐hydroxypyridines has been developed, thus providing a highly efficient synthesis of enantioenriched N‐substituted 2‐pyridone derivatives from readily available starting materials. This protocol features a good tolerance of functional groups in both the allylic carbonates and 2‐hydroxypyridines, thereby delivering multifunctionalized heterocyclic products with up to 98 % yield and 99 % ee. 相似文献
924.
The effect of hydrogen-fluorine defects on the conformational energy of polytrifluoroethylene chains
Conformational energy calculations were employed to determine the effect of hydrogen-fluorine defects on the conformation of the polytrifluoroethylene chain. It was determined that the 31 helix was energetically favored for all but the highly syndiotactic polymers, which favored the all-trans conformation. The incorporation of head-to-head linkages into the polymer chain increased the Conformational energy of the 31 helix while the Conformational energy of the all-trans form decreased. The energies of these two conformations did not equalize until the head-to-head concentration reached 20 to 30%, depending on the tacticity of the polymer. The trans, gauche, trans, gauche' conformation was also determined to be energetically favorable for the syndiotactic polymer, but both tacticity defects and head-to-head linkages caused a rapid increase in the Conformational energy. This was, therefore, concluded to be unlikely structure when chemical defects are incorporated into the polymer chain. 相似文献
925.
Jing Liu Gang Zhang Shengru Long Xiaojun Wang Jie Yang 《Journal of Macromolecular Science: Physics》2013,52(2):219-230
A useful annealing treatment above glass transition temperature of high molecular weight amorphous m-poly(arylene sulfide sulfone amide) (m-PASSA) was employed in this work. No crystalline or cross-link structures were formed according to X-ray diffraction, differential scanning calorimetry, and nuclear magnetic resonance results; nevertheless, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis results showed that the number of terminal carboxyl groups of annealed m-PASSA decreased after the annealing treatment. The volatiles formed in the decomposition of m-PASSA were investigated by combined thermogravimetric analysis and online FTIR. The initial volatile products in the decomposition experiments supported a process that involved the cleavage of an aromatic carboxyl group, followed by the emission of major volatile products—carbon dioxide, sulfur dioxide, and benzenthiol—released from the amide, sulfone, and diphenyl sulphide groups in m-PASSA. Annealed m-PASSA exhibited different thermal properties, as evidenced by thermogravimetric analysis, compared to the untreated polymer. The degradation activation energy increased from 154.4 kJ/mol to 173.6 kJ/mol after annealing treatment, and the lifetime was extended by more than 1000 times. 相似文献
926.
Célia BillaudJean-Philippe Goddard Thierry Le Gall Charles Mioskowski 《Tetrahedron letters》2003,44(24):4451-4454
The reaction of sulfone anions with trialkylboranes followed by thermal isomerization of the obtained boron compounds in the presence of excess borane-methyl sulfide complex and by alkaline hydroperoxide oxidation yields primary alcohols. 相似文献
927.
《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(1):361-368
A synthetic route to enantiomerically pure (1R,2S)‐1‐phenylphospholane‐2‐carboxylic acid ( 1 ), which is a phosphorus analogue of proline, has been established. A key step is the deprotonation–carboxylation of the 1‐phenylphospholane borane complex 3 by using sBuLi/1,2‐dipiperidinoethane (DPE). Configurational stability of the key intermediate, the amine‐coordinated α‐phosphinoalkyllithium borane complex 4 , was investigated by employing lithiodestannylation–carboxylation of both diastereomers of the 1‐phenyl‐2‐trimethylstannylphospholane borane complex 7 in the presence of several kinds of amines, and as a result, 4 was found to be configurationally labile even at ?100 °C. The key intermediate, the DPE‐coordinated trans‐1‐phenyl‐2‐phospholanyllithium borane complex 9 , was isolated, and the structure was identified by X‐ray crystal structure analysis. This is the first X‐ray crystal structure determined for an α‐monophosphinoalkyllithium borane complex. Remarkably, the alkyllithium complex is monomeric and tricoordinate at the lithium center with a slightly pyramidalized environment, and the existence of a Li? C bond (2.170 Å) has been confirmed. Moreover, 1H–7Li HOESY and 6Li NMR analyses suggested the structure of 9 in solution as well as the existence of an equilibrium between 9 , its cis isomer, and the ion pair 8 at room temperature, which was extremely biased towards 9 at ?100 °C. Finally, 1 was used as a chiral ligand in a palladium‐catalyzed allylic substitution, and the desired product was obtained in high yield with good enantioselectivity. 相似文献
928.
929.
Dae Sik Kim Ho Bum Park Jae Young Jang Young Moo Lee 《Journal of polymer science. Part A, Polymer chemistry》2005,43(22):5620-5631
New sulfonated poly(imidoaryl ether sulfone) copolymers derived from sulfonated 4,4′‐dichlorodiphenyl sulfone, 4,4′‐dichlorodiphenyl sulfone, and imidoaryl biphenol were evaluated as polymer electrolyte membranes for direct methanol fuel cells. The sulfonated membranes were characterized with Fourier transform infrared spectroscopy, thermogravimetric analysis, and proton nuclear magnetic resonance spectra. The state of water in the membranes was measured with differential scanning calorimetry, and the existence of free water and bound water was discussed in terms of the sulfonation level. The 10 wt % weight loss temperatures of these copolymers were above 470 °C, indicating excellent thermooxidative stability to meet the severe criteria of harsh fuel‐cell conditions. The proton conductivities of the membranes ranged from 3.8 × 10?2 to 5 × 10?2 S/cm at 90 °C, depending on the degree of sulfonation. The sulfonated membranes maintained the original proton conductivity even after a boiling water test, and this indicated the excellent hydrolytic stability of the membranes. The methanol permeabilities ranged from 1.65 × 10?8 to 5.14 × 10?8 cm2/s and were lower than those of other conventional sulfonated ionomer membranes, particularly commercial perfluorinated sulfonated ionomer (Nafion). The properties of proton and methanol transport were discussed with respect to the state of water in the membranes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5620–5631, 2005 相似文献
930.