Preparation of gem-dimethylcyclopropane-fused compounds through sigmatropic rearrangements. On/off-switching of the tautomerization of 3,4-homotropilidene by steric hindrance

Cyclopropanation of 4,8,8-trimethylcycloheptatriene having an ether function at the 3-position by unsubstituted carbenoid addition resulted in a complex mixture mainly due to quick valence tautomerization of the produced 3,4-homotropilidene analogue during the reaction. Dihalocarbene addition to the same substrate proceeded exclusively at the 3,4-position to give an adduct, where the tautomerization process was interrupted by steric hindrance caused by the halogen substituents. Removal of the halogen atoms by reduction promotes the tautomerization to give a gem-dimethylcyclopropane-fused product. Stereospecificity of the tautomerization was also demonstrated by obtaining the product in an optically pure state.