Reactions of Donor-Acceptor Cyclopropanes with Naphthoquinones : Redox and Lewis Acid Catalysis Working in Concert
Reactions of 2-aryl cyclopropane dicarboxylates with naphthoquinones are reported. The key feature was the use of catalytic amounts of SnCl2, which acts as both electron donor and Lewis acid. By an in-situ umpolung of naphthoquinone the formerly electrophilic species is converted into a nucleophile that is able to trigger the ring-opening of the three-membered ring with formation of a new C-C bond. Treatment of these products with base under oxidative conditions resulted - by losing methyl formate - in cyclopentannulated products with fully conjugated π systems exhibiting intensive absorptions in the visible range.