Stereoretentive Catalytic [3+2]-Cycloaddition/Rearrangement/Decarboxylation Reactions of Indoles with Non-Racemic Donor-Acceptor Cyclopropanes

A highly enantioselective synthesis of chiral dihydro-3H-carbazole-2-carboxylate derivatives is reported via a "onepot" cyclopentannulation-rearrangement cascade reaction that is sequentially catalyzed by nickel(II) perchlorate hexahydrate and scandium(III) trifluoromethanesulfonate with 3-methylindole and non-racemic donor-acceptor cyclopropanes in high yields and enantioretention under mild reaction conditions. Highly diastereoselective [3+2]-cycloaddition is dependent on 3-methylindole substituents. In addition, a further transformation of these dihydro-3H-carbazole-2-carboxylates via hydrolysis and decarboxylation under unexpectedly mild reaction conditions provides straightforward access to the decarboxylated compounds in moderate yields with high retention of enantiomeric purity.