1,738,015 research outputs found
Asymmetric Synthesis of Novel Fluorine-Containing Β-Lactams for Application to Unnatural Β-Amino Acid Synthesis
No full textΒ-Lactams are four-membered cyclic amides which can be found in many biologically active compounds. Β-Lactams may possess two stereocenters. In enantiopure form, Β-lactams can be used as chiral building blocks for the synthesis of various biologically relevant compounds, such as antibacterial drugs, antitumor agents, peptides, peptidomimetics and enzyme inhibitors. In addition, fluorine-containing Β-lactams have drawn substantial interests among bioorganic and medicinal chemists. Studies have shown that the incorporation of fluorine efficiently blocks metabolism of the drug molecules, especially by cytochrome P-450 family of enzymes, providing improved pharmacokinetic properties. Different diastereomers of fluorine-containing Β-lactams with high enantiopurity were synthesized utilizing a base-catalyzed C4-specific epimerization. Highly enantioenriched cis-Β-lactams were obtained using the Staudinger [2+2] ketene-imine cycloaddition reaction, followed by kinetic enzymatic resolution. Ozonolysis of an exocyclic double bond of the enantioenriched cis-Β-lactam afforded the key intermediate for selective epimerization, i.e., a C4-formyl-Β-lactam. The epimerization reaction was optimized for the isolation of trans-Β-lactams. A difluorovinyl group was incorporated through a Wittig reaction, and subsequent modification of the substituents gave the desired 4-difluorovinyl-Β-lactams. Using this protocol, a novel trans-difluorovinyl-Β-lactam, (3S,4S)-1-(tert-butoxycarbonyl)-3-triisopropylsiloxy-4-(2,2-difluoroethenyl)azetidin-2-one with excellent enantiopurity, was obtained. These novel cis- and trans-fluorine-containing Β-lactams can be used for the synthesis of fluorine-containing Α-hydroxy-Β-amino acids and their congeners or for the incorporation of fluorines into biologically active compounds via the Β-lactam synthon method to investigate the effects of the fluorine moieties on their biological activities.Advisor(s): Iwao Ojima. Frank W. Fowler. Committee Member(s): Kathlyn A. Parker. .Stony Brook University Libraries. SBU Graduate School in Department of Chemistry. Lawrence Martin (Dean of Graduate School)
Working train engine 914
No full textWork train heading out, Engine #914. From Peachland Memories, Volume 2
Steam engine 914
No full textA photograph postcard showing steam engine 914, 2-8-0, with 5 cars, passing grain silos
Napierville Junction (NJR) 914
No full textA photograph print showing the Napierville Junction (NJR) 914, 2-8-0, Delson Junction, QC
Inscriptions 914 à 920
No full textInscriptions 914 à 920. In: Revue épigraphique du Midi de la France, tome 3, N°67, 1892. pp. 179-184
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
