cas 7743-31-9, <3-(4-Nitrophenyl)-2-propenyl>triphenylphosphoniumbromid

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6- and 8?-Ringschlussreaktionen von 2-Oxaheptatrienyl-Dipolen: Synthese von 3-Vinyl-2,3-dihydrofuranen und 2,3(6,7)-Dihydrooxepinen
The 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 fivemembered ring products (15/16) and, additionally, dihyrooxepins (17 and 29, resp.).As mechanism 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.
Eberbach, Wolfgang,Trostmann, Uwe
p. 2979 - 3003
(2007/10/02)