Enantioselective synthesis of fused cycloheptadienes by a tandem intramolecular cyclopropanation/cope rearrangement sequence

The asymmetric induction in the intramolecular cyclopropanations of allyl v inyldiazoacetates catalyzed by tetrakis[N-[4-dodecylphenyl)sulfonyl]-(S)-pr olinato] dirhodium [Rh-2(S-DOSP)(4)] is very dependent on the allyl substit ution pattern. The reactions of cis-alkenes result in much higher asymmetri c induction than trans-alkenes while the highest enantioselectivity was obt ained with a disubstituted terminal alkene. The intramolecular cyclopropana tion of dienylmethyl vinyldiazoacetates results in the synthesis of fused c ycloheptadiene ring systems with full control of relative stereochemistry a nd variable enantioselectivity. The synthetic utility of this process was d emonstrated by a short synthesis of 5-epi-tremulenolide in 93% ee.