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Novel urea-linked cinchona-calixarene hybrid-type receptors for efficient chromatographic enantiomer separation of carbamate-protected cyclic amino acids.
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Two novel diastereomeric cinchona-calixarene hybrid-type receptors (SOs) were synthesized by inter-linking 9-amino(9-deoxy)-quinine (AQN)/9-amino(9-deoxy)-epiquinine (eAQN) and a calix[4]arene scaffold via an urea functional unit. Silica-supported chiral stationary phases (CSPs) derived from these
Mechanistic investigations into the enantioselective Conia-ene reaction catalyzed by cinchona-derived amino urea pre-catalysts and Cu(I).
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The enantioselective Conia-ene cyclization of alkyne-tethered β-ketoesters is efficiently catalyzed by the combination of cinchona-derived amino-urea pre-catalysts and copper(I) salts. The reaction scope is broad and a series of substrates can be efficiently cyclized with high yields and
A new class of urea-substituted cinchona alkaloids promote highly enantioselective nitroaldol reactions of trifluoromethylketones.
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The first class of bifunctional cinchona-alkaloid catalysts incorporating a urea moiety at C-5' has been developed. These materials catalyze the efficient and highly enantioselective 1,2-addition of nitromethane to trifluoromethylketones to form synthetically pliable products incorporating a
Asymmetric catalysis with bifunctional cinchona alkaloid-based urea and thiourea organocatalysts.
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Cinchona alkaloid derivatives modified to include a (thio)urea component have emerged in the last three years as readily accessible, robust and tunable bifunctional organocatalysts for a range of synthetically useful transformations. The origins of these catalysts and the major developments in their
Cinchona Urea-Catalyzed Asymmetric Sulfa-Michael Reactions: The Brønsted Acid-Hydrogen Bonding Model.
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The cinchona alkaloid-derived urea-catalyzed asymmetric conjugate addition of aromatic thiols to cycloalkenones was studied using density functional theory (DFT). Deprotonation of the thiol gives a protonated amine that activates the electrophile by Brønsted acid catalysis, while the urea group
Urea- and thiourea-substituted cinchona alkaloid derivatives as highly efficient bifunctional organocatalysts for the asymmetric addition of malonate to nitroalkenes: inversion of configuration at C9 dramatically improves catalyst performance.
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Cinchona Alkaloid Catalyzed Sulfa-Michael Addition Reactions Leading to Enantiopure β-Functionalized Cysteines.
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Sulfa-Michael additions to α,β-unsaturated N-acylated oxazolidin-2-ones and related α,β-unsaturated α-amino acid derivatives have been enantioselectively catalyzed by Cinchona alkaloids functionalized with a hydrogen bond donating group at the C6' position. The series of Cinchona alkaloids includes
Cinchona Alkaloid-Squaramide Catalyzed Sulfa-Michael Addition Reaction: Mode of Bifunctional Activation and Origin of Stereoinduction.
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The mechanism of the enantioselective sulfa-Michael addition reaction catalyzed by a cinchona alkaloid-squaramide bifunctional organocatalyst was studied using density functional theory (DFT). Four possible modes of dual activation mechanism via hydrogen bonds were considered. Our study showed that
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