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Stereoselective double lewis acid/organo-catalyzed dimerization of donor-acceptor cyclopropanes into substituted 2-oxabicyclo[3.3.0]octanes.

A new approach for the dimerization of donor-acceptor cyclopropanes (2-arylcyclopropane-1,1-dicarboxylates) under double-catalysis conditions by treatment with 20 mol % of GaCl(3) and dimethyl 3,5-dimethyl-1-pyrazoline-3,5-dicarboxylate as a specific organocatalyst has been found. Under these conditions, the starting compounds are regio- and stereospecifically converted into polysubstituted 2-oxabicyclo[3.3.0]octanes. Two new rings, one C-O bond, and two C-C bonds are formed in this process, and four stereocenters are thus created. The reaction mechanism was thoroughly studied by NMR spectroscopy, and a number of intermediates were detected.