Stabilization and structure elucidation of unstable Corynoxine-type stereoisomers using tartaric acid

In oxindole alkaloids, stereoisomer formation occurs via bond cleavage at the spiro carbon center, followed by an intramolecular Mannich reaction. Since the possible common transition state possesses two prochiral faces, four stereoisomers at the C-3 and C-7 positions are potentially generated; however, few have been reported in nature. In this study, the four corynoxine stereoisomers were isolated, and their structures were elucidated. Two of these stereoisomers were found to be structurally unstable. Addtion of tartaric acid as a chemical trap reagent stabilized these isomers named corynoxines C and D and revealed their stereochemistries as 3R, 7S and 3R, 7R, respectively.