The condensation cyclization between isatins and 5-methoxy tryptamine catalyzed by chiral phosphoric acids provides spirooxindole tetrahydro-β-carboline products in excellent yields (up to 99%) and enantioselectivity (up to 98:2 er). A comparison of catalysts provides insight for the substrate scope and factors responsible for efficient catalytic activity and selectivity in the spirocyclization. Chiral phosphoric acids with different 3,3′-substitution on the binaphthyl system and opposite axial chirality afford the spiroindolone product with the same absolute configuration.

We report the Lewis acid catalyzed additions of allylsilanes to N-Boc-iminooxindoles and the formation of novel silicon-containing spirocarbamates via intramolecular trapping of a β-silyl carbocation by an N-Boc group. Several transformations display the synthetic utility of these spirocarbamate oxindoles, including a reductive cyclization to access new silylated furoindoline derivatives.

Herein, we describe an enantioselective Cu­(II)-catalyzed spiroannulation of N-Boc-iminooxindoles with allylsilanes where a significant positive nonlinear effect (NLE) is observed. EPR spectroscopic studies of the copper­(II) species present under synthetically relevant conditions reveal explicit spectroscopic evidence based on analysis of the metal center for the species responsible for the positive NLE in a metal-catalyzed system. EPR spectroscopy indicates that formation of a heterochiral ML2 species under scalemic conditions enriches the effective enantiopurity of the catalytically active species, leading to the asymmetric amplification observed in the spiroannulation. Mathematical analysis of the positive NLE reveals a high thermodynamic preference toward formation of the heterochiral ML2, which has a low relative reactivity when compared to the homochiral ML2.

A Lewis acid-catalyzed stereoselective [3+2] annulation of crotylsilanes with iminooxindoles is reported to access 2,3'-pyrrolidinyl spirooxindoles with four stereocenters. The addition of NaBArF significantly enhances reactivity, allowing either metal salts or acidic clay to be effective catalysts for the stereoselective reaction.