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 “one-pot” 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.