Synthesis of Spiro-Pyrrolidinyloxindoles by Oxidative Rearrangement of N-Acyltetrahydro-β-carbolines Using an Oxone/Aqueous Acetone Mixture
Spiro-pyrrolidinyl-2-oxindoles were prepared by the oxidative rearrangement of Na-acetyl-1,2,3,4-tetrahydro-β-carbolines (THBC) using dimethyldioxirane generated in situ. The Na-acetyl THBC substrates were obtained by Pictet-Spengler and acyl-Pictet-Spengler reactions of L-tryptophan methyl ester, followed by Na-acetylation. The stereoselectivity of the oxidative rearrangement was evaluated and 2D nuclear magnetic resonance (NMR) was used to determine the stereochemistry of the oxindole products relative to L-tryptophan. Density functional theory calculations were consistent with a face selective, substrate controlled, epoxidation of the indolic double bond. The calculations indicated that the resulting epoxide would readily rearrange at room temperature via a concerted ring opening/ring contraction process to give the 3-substituted-2-oxindole.