Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Donor-acceptor-substituted vinylcyclopropanes such as 1a, 3a, 5a, 5b, 7a, 9a, 9b, and 12a smoothly undergo thermal ring enlargement to functionalized cyclopentene derivatives 2, 4, 6, 8, 10, and 13 at relatively low temperatures (150-190°C) in good yields. This vinylcyclopropane-cyclopentene rearrangement proceeds with low (1a, 3a), moderate (9a, b) or high (7a, 12a) stereoselectivity. In the case of 7a and 12a almost complete retention of configuration with respect to the migrating (methoxy) (phenyl)-substituted carbon atom was observed. The definite mechanistic interpretation of these results is complicated by the stereoisomerization of the starting donor-acceptor-substituted vinylcyclopropanes which proceeds as competing reaction during ring enlargement. Nevertheless, all observations together, in particular the recorded solvent effects, support a stepwise mechanism of the rearrangement with a highly stabilized 1,3-zwitterionic species as common intermediate for cyclopropane stereoisomerization and ring enlargement.
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