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The Heteroaromatic Cope Rearrangement of 1-Pyridyl-, 1-Furyl-, and 1-Thienyl-2-vinylcyclopropanes
Maas, Gerhard,Hummel, Christine
, p. 3679 - 3696 (2007/10/02)
The 1-pyridyl-2-vinylcyclopropanes 3a and 5a rearrange to the cycloheptapyridines 10a and b on heating in an aromatic solvent.The reaction may be considered as an aromatic Cope rearrangement to 9, followed by a rearomatization step.N-Methylation of the starting vinylcyclopropane lowers the necessary reaction temperature markedly.The primary products 19 and 20 of the Cope rearrangement of the 1-furyl- or 1-thienyl-2-vinylcyclopropanes 3, 5c, d are thermally stable under the reaction conditions.The rearomatization of 19 and 20 is catalyzed by electrophiles (H+, CH3I) and, to a lower extent, by bases (pyridine).On heating to about 200 deg C in an inert aromatic solvent, 19a undergoes Retro-Cope rearrangement to 3c, aromatization to 21 as well as rearrangement to the cyclopentene 23.In all cases, the (Z)-vinylcyclopropanes undergo the Cope rearrangement faster than their (E)-isomers.
