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731.
Barluenga J Barrio P Riesgo L López LA Tomás M 《Journal of the American Chemical Society》2007,129(46):14422-14426
A broad range of substituted 2-cyclopentenone derivatives 3-6 are synthesized by the nickel(0)-mediated [3 + 2] cyclization reaction of chromium alkenyl(methoxy)carbene complexes 1 and internal alkynes 2. The reaction takes place with complete regioselectivity with both unactivated alkynes and activated alkynes (electron-withdrawing and electron-donating substituted alkynes). Representative cycloadducts containing boron and tin substituents are further demonstrated to be active partners in classical Pd-catalyzed C-C coupling processes to allow the production of 2-aryl- and 2-alkynyl-substituted cyclopentenones 9-13. 相似文献
732.
Corma A Diaz-Cabanas MJ Rey F Nicolopoulus S Boulahya K 《Chemical communications (Cambridge, England)》2004,(12):1356-1357
The pore topology of ITQ-15 zeolite consists of an ultra-large 14-ring channel that is intersected perpendicularly by a 12-ring pore; acid sites have been introduced in its framework and this unique structure shows advantages over unidirectional ultralarge pore zeolites for diffusing and reacting large molecules. 相似文献
733.
In this article, local optimality in multiobjective combinatorial optimization is used as a baseline for the design and analysis
of two iterative improvement algorithms. Both algorithms search in a neighborhood that is defined on a collection of sets
of feasible solutions and their acceptance criterion is based on outperformance relations. Proofs of the soundness and completeness
of these algorithms are given. 相似文献
734.
H. Schmid E. Burkhardt E. Walker W. Brixel M. Clin J. -P. Rivera J. -L. Jorda M. François K. Yvon 《Zeitschrift für Physik B Condensed Matter》1988,72(3):305-322
The ferroelastic domains of the orthorhombic phase of YBa2Cu3O7- have been observed in polarized light on ceramics and single crystals. By combining polarized light microscopy with the X-ray precession technique, the correlation of the orientation of the orthorhombica- andb-axes with that of thea-b-plane bireflectance, reflection dichroism, transmission dichroism (at a thickness of about 1m), reflection tints generated with compensators and upon uncrossing of polars, as well as the orientation of etch pits has been realized on ferroelastic single domains, bi-domains and more complicated domain patterns. Four ferroelastic orthorhombic domain states have been identified, at variance with former group theoretical considerations, predicting only two states. Ensembles of lamellar domains beyond optical resolution generate strong bireflectance with principal axes rotated by 45° relative to the truea, b-directions. 相似文献
735.
Shi Nian-Qing Prahl Kristine Hendrick Jim Cruz Jose Lu Ping Cho Jae-Yong Jones Sharon Jeffries Thomas 《Applied biochemistry and biotechnology》2000,84(1-9):201-216
Pichia stipitis CBS 6054 will grow on d-xylose, d-arabinose, and l-arabinose. d-Xylose and l-arabinose are abundant in seed hulls of maize, and their utilization is important in processing grain residues. To elucidate
the degradation pathway for l-arabinose, we obtained a mutant, FPL-MY30, that was unable to grow on d-xylose and l-arabinose but that could grow on d-arabinitol. Activity assays of oxidoreductase and pentulokinase enzymes involved in d-xylose, d-arabinose, and l-arabinose pathways indicated that FPL-MY30 is deficient in d-xylitol dehydrogenase (D-XDH), d- and l-arabinitol dehydrogenases, and d-ribitol dehydrogenase. Transforming FPL-MY30 with a gene for xylitol dehydrogenase (PsXYL2), which was cloned from CBS 6054 (Gen Bank AF127801), restored the D-XDH activity and the capacity for FPL-MY30 to grow on
l-arabinose. This suggested that FPL-MY30 is critically deficient in XYL2 and that the d-xylose and l-arabinose metabolic pathways have xylitolas a common intermediate. The capacity for FPL-MY30 to grow on d-arabinitol could proceed through d-ribulose. 相似文献
736.
Zhu L Ostash B Rix U Nur-E-Alam M Mayers A Luzhetskyy A Mendez C Salas JA Bechthold A Fedorenko V Rohr J 《The Journal of organic chemistry》2005,70(2):631-638
The angucycline antibiotic family of the landomycins displays potent antitumor activity. To elucidate early post polyketide synthase (PKS) tailoring steps of the landomycin E biosynthetic pathway in Streptomyces globisporus 1912, the mutant S. globisporus M12 was prepared through gene replacement experiment of lndM2. It encodes an enzyme with putative oxygenase and reductase domains, according to sequencing of the gene and its counterpart lanM2 from S. cyanogenus S136 landomycin A biosynthetic gene cluster. The isolation of the novel shunt products 11-hydroxytetrangomycin and 4-hydroxytetrangomycin along with the well-known angucyclines tetrangomycin and tetrangulol from the culture of S. globisporus M12 provides evidence for the involvement of lndM2 in the early biosynthetic pathway of the landomycins, in particular in the formation of the alicyclic 6-hydroxy function of the landomycin aglycon. We therefore propose LndM2 to be responsible for both hydroxylation of the 6-position and its subsequent reduction. These reactions are necessary before the glycosylation reactions can occur. The results are in agreement with the originally published structure of landomycin but do not support the recently suggested revised structure. 相似文献
737.
Juan Alfonso Redondo Enrique Martínez‐Campos Laetitia Plet Mnica Prez‐Perrino Rodrigo Navarro Guillermo Corrales Abhay Pandit Helmut Reinecke Alberto Gallardo Jos Luis Lpez‐Lacomba Alfonso Fernndez‐Mayoralas Carlos Elvira 《Macromolecular rapid communications》2016,37(7):575-583
The incorporation of cyclodextrins (CDs) to nonviral cationic polymer vectors is very attractive due to recent studies that report a clear improvement of their cytocompatibility and transfection efficiency. However, a systematic study on the influence of the CD derivatization is still lacking. In this work, the relevance of β‐CD permethylation has been addressed by preparing and evaluating two series of copolymers of the cationic N‐ethyl pyrrolidine methacrylamide (EPA) and styrenic units bearing pendant hydroxylated and permethylated β‐CDs (HCDSt and MeCDSt, respectively). For both cell lines, CDs permethylation shows a strong influence on plasmid DNA complexation, “in vitro” cytocompatibility and transfection efficiency of the resulting copolymers over two murine cell lines. While the incorporation of the hydroxylated CD moiety increased the cytotoxicity of the copolymers in comparison with their homopolycationic counterpart, the permethylated copolymers have shown full cytocompatibility as well as superior transfection efficiency than the controls. This behavior has been related to the different chemical nature of both units and tentatively to a different distribution of units along the polymeric chains. Cellular internalization analysis with fluorescent copolymers supports this behavior.
738.
Barba C Carmona D García JI Lamata MP Mayoral JA Salvatella L Viguri F 《The Journal of organic chemistry》2006,71(26):9831-9840
A B3LYP/6-31G* study has been carried out for the reactions of methacrolein with cyclopentadiene, parent nitrone, 1-pyrroline-1-oxide, and (Z)-C,N-diphenylnitrone, in which the coordination of a Lewis acid (borane) and the solvent polarity (dichloromethane) have been taken into account. Calculated activation parameters, regioselectivities (for 1,3-dipolar cycloaddition reactions), and endo/exo stereoselectivities show good agreement with available experimental data. Gas-phase calculations show a varied behavior of the s-cis/s-trans TS stability for noncatalyzed reactions (from the systematic s-cis preference for the cyclopentadiene reaction to the systematic s-trans predilection encountered in the diphenylnitrone cycloaddition). BH3 coordination leads to a preferential stabilization of s-trans TSs in the reactions of cyclopentadiene (exo approach) and diphenylnitrone but a larger stabilization of s-cis structures in the processes involving the parent nitrone or 1-pyrroline-1-oxide. Additionally, a rather systematic preferential stabilization of s-trans structures is induced by solvent polarity in most reactions. As a consequence, an s-trans preference is predicted in solution for both thermal and catalyzed types of reactions in most approaches. Such a conclusion is consistent with some experimental results suggesting a preference for a particular conformation of the methacrolein-Lewis acid complexes. 相似文献
739.
Fernández N Carrillo L Vicario JL Badía D Reyes E 《Chemical communications (Cambridge, England)》2011,47(45):12313-12315
We have developed a highly efficient procedure for carrying out the catalytic enantioselective (3+2) cycloaddition between enals and stable azomethine ylides such as isoquinolinium and phthalizinium methylides. Under the optimized reaction conditions highly substituted chiral pyrroloisoquinolines and pyrrolophthalazines have been obtained in high yields and excellent diastereo- and enantioselectivities. 相似文献
740.