Das Substitutionsmuster der Dien‐Teilstruktur bestimmt, wie leicht elektronenreiche Aryldienone eine AlCl3‐katalysierte Nazarov‐Cyclisierung eingehen (siehe Schema). Bei α,γ‐substituierten Systemen ...wird die starke 1,3‐Allylspannung des Reaktanten im Übergangszustand der Elektrocyclisierung verringert, was die Reaktivität erhöht. DFT‐Rechnungen können diese Art der Reaktivität vorhersagen.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Eine neue (wenngleich noch hypothetische) Verbindungsklasse wird beschrieben – die „Sigmatrop‐Verschiebomere.“ Dabei handelt es sich um Polymere mit lokalen „Defekten“ in Form von Einfach‐ oder ...Doppelbindungen, die durch sigmatrope Umlagerungen im Polymergerüst fortschreiten (siehe Schema).
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The structures, obtained by X-ray crystallography, of the binding sites of catalytic antibodies raised to bind different phosphonates are compared. Although the amino acid sequences differ, all ...exhibit a tetrahedral array of hydrogen bond donors (a ‘canonical binding array’) complementary to the tetrahedral anion, which represents a ‘transition state epitope’ for the basic hydrolysis of esters and amides. Antibodies for phosphates, arsonates, and sulfonates are found also to possess the tetrahedral anion canonical binding array.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A library of novel, propeller-shaped dispirotriheterocyclic isoxazolinopiperidinochromanones is reported. Each rigid dispirotriheterocycle was prepared in five linear steps from commercially ...available tert-butyl 4-oxopiperidine-1-carboxylate and various derivatives of 1-(2-hydroxyphenyl)ethanone, benzaldehyde oxime, and carboxylic acids. Computational chemistry was employed to analyze the three-dimensional geometries of these dispirotriheterocycles, as well as to generate chemoinformatic bioavailability data. X-ray crystallographic structure determination verified the regioselectivity of the nitrile oxide 1,3-dipolar cycloaddition reaction. The resulting library of compounds has been added to the National Institutes of Health repository (∼10 mg of each with ≥90% purity) for pilot-scale biomedical studies with bioassay data available at the National Center for Biotechnology Information PubChem database.
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IJS, KILJ, NUK, PNG, UL, UM
A multitude of challenges facing modern synthesis have an enamine component as part of a solution. Transition‐metal‐catalyzed rearrangement of allylamines to enamines has occupied a prominent place ...among catalytic transformations. Similar to more conventional synthetic approaches to enamine intermediates, this chemistry typically leads to the formation of (E) isomers. Recently, a RhI‐catalyzed rearrangement of N‐allylaziridines to (Z)‐N‐alkenylaziridines was reported with kinetic stereoselectivities between 75:25 and 95:5 (Z/E). This exciting result warranted a mechanistic explanation. Herein we describe the results of quantum chemical calculations B3LYP/6‐31+G(d,p)‐LANL2DZ aimed at evaluating competing mechanisms for this isomerization. We have compared and contrasted two mechanisms for the rearrangement: one that proceeds through an azametallocyclopentene intermediate and another through a hydrometalation/β‐hydride elimination. We find that the latter mechanism corresponds to the lower energy pathway, which, counterintuitively, exhibits a kinetic preference for the formation of products with (Z) C=C double bonds. A close correspondence between product ratios determined by simple Boltzmann distribution calculations based on these theoretical results and the previously reported ratios is observed. Lastly, we have examined whether the observed stereocontrol is exclusive to strained amines, and we find that the unique characteristics of the aziridine ring, compared to other amines, prove to be essential. Given the privileged status of the enamine functional group in synthesis, application of the “strain effect” in a range of metal‐catalyzed processes is expected to have broad consequences.
RhI‐catalyzed rearrangement of N‐allylaziridines to (Z)‐N‐alkenylaziridines responsible for stereoselectivities between 75:25 and 95:5 (Z/E) are studied herein with B3LYP/6‐31+G(d,p)‐LANL2DZ. A hydrometalation/β‐hydride elimination mechanism counterintuitively exhibits a kinetic preference for the formation of products with (Z) C=C double bonds.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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