Abstract
A highly diastereo- and enantioselective Mannich-type reaction of 3-aryloxindoles with N-Boc aldimines was achieved under the catalysis of axially chiral ammonium betaines. This catalytic method provides a new tool for the construction of consecutive quaternary and tertiary stereogenic carbon centers on biologically intriguing molecular frameworks with high fidelity.
Highlights
Chiral indole alkaloids possessing C-3 quaternary indoline frameworks are an important class of biologically relevant molecules, and numerous efforts have been made for the development of reliable synthetic methodologies to enable the installation of the C-3 stereogenic center [1-4]
The reaction of N-Boc 3-phenyloxindole (2a) with benzaldehyde-derived N-Boc imine 3a [36] was conducted in the presence of a catalytic amount of chiral ammonium betaine 1a (5 mol %) in toluene with 4 Å molecular sieves (MS 4 Å) at −60 °C
We have clearly demonstrated that chiral ammonium betaine 1c acts as a uniquely effective catalyst in promoting a Mannich-type reaction between 3-aryloxindoles and N-Boc aldimines with high levels of diastereo- and enantioselectivity under mild conditions
Summary
Chiral indole alkaloids possessing C-3 quaternary indoline frameworks are an important class of biologically relevant molecules, and numerous efforts have been made for the development of reliable synthetic methodologies to enable the installation of the C-3 stereogenic center [1-4]. Successful examples of Mannich-type reactions with imines are surprisingly limited despite allowing efficient construction of vicinal quaternary and tertiary stereocenters [9-22]. Taking advantage of this unique property, we have developed a series of highly stereoselective transformations, and disclose the effectiveness of 1 in solving a challenging problem regarding the rigorous control of the relative and absolute stereochemistry in the asymmetric Mannich-type reaction of 3-aryloxindoles.
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