The asymmetric catalytic synthesis of beta-substituted tryptophan derivatives was realized in high diastereo- and enantioselectivity by the reaction of glycine derivatives with sulfonylindoles in the presence of catalyst derived from AgCl and a commercially available chiral monodentate phosphoramidite ligand. The resulting adduct was readily converted to beta-substituted tryptophan in 95% overall yield for two steps, which presented a highly efficient route to chiral beta-substituted tryptophan.
[Show abstract][Hide abstract] ABSTRACT: A Brønsted base-catalyzed reaction of nitroalkanes with alkyl electrophiles provides indole heterocycles substituted at C3 bearing a sec-alkyl group with good enantioselectivity (up to 90% ee). Denitration by hydrogenolysis provides a product with equally high ee. An indolenine intermediate is implicated in the addition step, and surprisingly, water cosolvent was found to have a beneficial effect in this step, leading to a one-pot protocol for elimination/enantioselective addition using PBAM, a bis(amidine) chiral nonracemic base.
[Show abstract][Hide abstract] ABSTRACT: The title addition of glycine imines (I) to malonates (II) provides the Michael adducts (III) in high yields and moderate to good diastereo- and enantioselectivities.
[Show abstract][Hide abstract] ABSTRACT: Reaction of sulfonylindoles with protected β-nitro ketones affords the corresponding 3-(2-nitroalkyl)indoles that, under acidic conditions, undergo a sequence of cascade processes finally leading to unsymmetrical 1,4-disubstituted carbazoles.
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