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- W2171088081 abstract "Es werden thermisch induzierte Ringerweiterungsreaktionen von Butadienyloxiranen beschrieben. Wahrend die 3(E)-konfigurierten Epoxytriene 11 ganz uberwiegend zu cis-3-Vinyl-2,3-dihydrofuranen 15 reagieren und nur die Derivate a, c, h auch geringe Anteile der trans-Isomeren 16 bilden, entsteht beim Erhitzen der 3(Z)-Butadienyloxirane 12 stets ein Dreikomponentengemisch, welches neben den Funfringprodukten 15/16 noch zusatzlich die Dihydrooxepine 17 (bzw. 29) enthalt. Als Mechanismus der Ringerweiterungsreaktionen wird eine mehrstufige Sequenz uber 2-Oxaheptatrienyl-Dipole diskutiert, welche unter Beteiligung von sechs Elektronen zu Dihydrofuranen oder von acht Elektronen zu Dihydrooxepinen elektrocyclisieren. Im Zusammenhang mit 1,7-Cyclisierungsreaktionen von anderen erweiterten Dipol-Systemen wird auf die entscheidende Bedeutung der Stereochemie in den Ausgangsprodukten hingewiesen.6- and 8π-Cyclisation Reactions of 2-Oxaheptatrienyl Dipoles: Synthesis of 3-Vinyl-2,3-dihydrofurans and 2,3(6,7)-DihydrooxepinsThe thermally induced ring expansion reactions of several butadienyloxiranes are described. The 3(E)-configurated epoxytrienes 11 are transformed predominantly into cis-3-vinyl-2,3-dihydrofurans 15, the trans-isomers 16 being formed as a minor component only with the derivatives a, c, and h. In contrast, thermolysis of the 3(Z) -butadienyloxiranes 12 leads always to a mixture of five-membered ring products (15/16) and, additionally, dihydrooxepins (17 and 29. resp.). As mechanisms of the ring expansion reactions a multistep sequence is proposed with 2-oxaheptatrienyl dipoles as intermediates, which undergo electrocyclisation with participation of either six or eight electrons leading to dihydrofurans and dihydrooxepins, resp. With relation to 1,7-cyclisation reactions of other extended dipole systems the importance of the stereochemistry in the starting materials is emphasised." @default.
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- W2171088081 date "1981-09-01" @default.
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- W2171088081 title "6- und 8π-Ringschlußreaktionen von 2-Oxaheptatrienyl-Dipolen: Synthese von 3-Vinyl-2,3-dihydrofuranen und 2,3(6,7)-Dihydrooxepinen" @default.
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- W2171088081 doi "https://doi.org/10.1002/cber.19811140907" @default.
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