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• W2006291271 abstract "Am Beispiel der aus den Oxiranvorstufen 4–10 durch Erhitzen zugänglichen Carbonyl-Ylide werden erstmals intramolekulare Cycloadditionsreaktionen dieser 1,3-Dipol-Klasse systematisch untersucht. Obwohl mit Ausnahme von 4e und 6a in allen Fällen die Bildung der entsprechenden anellierten und/oder überbrückten Tetrahydrofuran-Derivate (32 – 34, 44 – 46, 50 – 63) nachgewiesen wird, hängen die Bildungsgeschwindigkeit und die Ausbeute der Cycloaddukte ganz entscheidend von der dipolarophilen Qualität der Olefin-Komponente ab: Während nicht-aktivierte C  C-Bindungen diese Cycloadditionen nur langsam und bei relativ geringer Entfernung zu der Dipol-Einheit eingehen (maximale Größe des durch „Einschnürung”︁ entstandenen zweiten Ringes: 9gliedrig), reagieren En-Oxirane mit elektronenarmen Doppelbindungen deutlich schneller und – im Gegensatz zu bisher bekannten Cycloadditionen anderer 1,3-Dipole – auch noch bei großem Abstand der π-Zentren. Auf diese Weise ist es gelungen, Makrocyclen mit bis zu 34 Ringgliedern herzustellen. Die zumeist hohe Regio- und Stereoselektivität bei den intramolekularen Cycloadditionen wird vor allem auf sterische Faktoren zurückgeführt. Intramolecular Cycloadditions with Carbonyl Ylides: From Oxiranes to Large Ring Systems Within the classes of 1,3-dipoles intramolecular cycloaddition reactions with carbonyl ylides, accessible by heating of the oxirane precursors 4 – 10, are studied for the first time in a systematic manner. Although the formation of the corresponding fused and/or bridged tetrahydrofuran derivatives (32 – 34, 44 – 46, 50 – 63) has been established in all cases with the exception of 4e and 6a, the rate of formation as well as the yield of the cycloadducts depend strongly on the dipolarophilicity of the olefinic component: Whereas non-activated C  C bonds undergo the cycloaddition step slowly and only when being arranged in a relatively close position to the dipole unit (the maximum size of the second ring, formed by “tieing”: 9-membered), ene-oxiranes with an electron deficient double bond are reacting much faster and (in contrast to cycloadditions so far known for other 1,3-dipoles) also at a large distance between the π-centers. In this way macro-cycles with up to 34 ring members have been obtained. The high regio- and stereoselectivity observed in most cases of the cycloadditions is explained mainly by steric factors." @default.
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• W2006291271 date "1984-06-01" @default.
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• W2006291271 title "Intramolekulare Cycloadditionen mit Carbonyl-Yliden: Von Oxiranen zu großen Ringen" @default.
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• W2006291271 doi "https://doi.org/10.1002/cber.19841170612" @default.
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