One-pot synthesis of conformationally restricted spirooxindoles

Diastereomeric three-, five- and six-membered spirocycloalkyloxindoles were successfully synthesized in a rapid and convenient manner from readily accessible starting materials in moderate to high yields using 1-methyl-3-acetonitriloxindole after a one-pot base-mediated double-alkylation strategy. It was found that the diastereoselection is dependent on the reaction conditions and the spirocycloalkyl ring size, with the 3R^∗,8R^∗ diastereomers being thermodynamically favored under the basic reaction conditions for three- and five-membered rings, and the 3R^∗,8S^∗ diastereomer in the case of six-membered rings, as predicted by DFT calculations. The relative stereochemistry was supported by 2D NMR spectra and X-ray crystal structural analysis. The conformational rigidity of the spirocycloalkyloxindoles in solution was established based on NMR experimental and theoretical DFT approaches.