Cyclopropanation with Fischer Acyloxycarbene Complexes: Preparation of Cyclopropane and Cycloheptane-Fused γ-Lactones

摘要

The Cope rearrangement is known as the therrnochernically induced [3,3]-sigmatropic rearrangement of 1,5-hexadienes, one of the pericyclic reactions involving the migration of two iz bonds and breaking and forming of one a bond.1 This reaction has been applied to a vast number of substrates to access not only novel chemical identities, but also molecules of medicinal importance. The Cope rearrangement always suffers from a high degree of reversibility being driven toward the formation of the higher substituted defines in an equilibrium mixture (la and 2a). To shift the equilibrium toward one side, an additional electron-rich functionality [-OH (oxy-Cope),-NR2 (amino-Cope), etc.] may be required as depicted in lb to make the rearrangement to 2b essentially irreversible by the subsequent formation of 3b (Fig. I). The necessity for a directing group becomes even more apparent if one of the double bonds on a 1,5-hexadiene is part of an aromatic system. For example, the homoallylarene 4 fails to undergo the Cope rearrangement to (Fig. 2), since there are no factors that compensate for the loss of the aromatic stabilization. In contrast, the anionic oxy-Cope rearrangement of was found to give rise to the ketone 7, countering the dearomatization by forming an enolate 8.