Asymmetric 1,4-Michael Addition in Aqueous Medium Using Hydrophobic Chiral Organocatalysts

Domino Reaction for the Synthesis of Substituted Pyrano[3',4':3,4]cyclopenta[1,2-c]chromenes Involving Double Ring Formation

Herein, we present a green cascade approach for synthesizing a range of chemoselective polysubstituted pyrano[3',4':3,4]cyclopenta[1,2-c]chromenes containing a chiral stereocenter. The strategy involves a metal-free nucleophilic reaction between dialkyl (2E)-2-{2-oxo-3-[(2E)-3-aryl-2-propenoyl]-2H-chromen-4-yl}-2-butenedioates and α,α-dicyanoolefins under reflux conditions in ethanol solvent. Mechanistic studies have shown that the reaction proceeds via a cascade of steps, including Michael addition, intramolecular C- and O-cyclization, intramolecular trans-esterification, [1,2]-H and [1,5]-methoxy- and ethoxy carbonyl shift, and finally aromatization to yield the desired product. In this method, three bonds (2C-C and C-O) form in one pot, simultaneously forming two new cyclopentane and pyrano rings by double cyclization reactions. Other advantages of this method are that it has a simple operation, readily available starting materials, a chemoselective cascade process, transition metal-free, synthetically useful yields, and green conditions by using ethanol as a solvent.

Enantio- and Diastereoselective Michael Addition of Cyclic Ketones/Aldehydes to Nitroolefins in Water as Catalyzed by Proline-Derived Bifunctional Organocatalysts

New l-proline-derived bifunctional secondary amine organocatalysts were synthesized for enantioselective Michael reactions in water as a solvent. Application of these catalysts in Michael additions provided high yield (up to 97%) with high stereoselectivity (dr up to 99:1 and ee up to 99%). The effect of phenyl group at (R)-C₆ in the catalyst was investigated and played a key role in successful catalysis by density functional theory computational calculations. The synthetic utility of this reaction was demonstrated by the formal synthesis of Sch 50971, which is a novel histamine H₃ receptor agonist.

A triple cascade approach towards the diastereoselective synthesis of spiro trans-decalinol scaffolds

A [2+2+2] annulation reaction between cyclohexanone, β-nitrostyrene and 2-arylidene-1,3-indanedione afforded multisubstituted spiro trans-decalinol derivatives in high chemical yields (up to 75%) and excellent diastereoselectivity (up to >20 : 1) at room temperature. This one-pot three-component system follows a triple cascade sequence via the Michael/nitro-Michael/Aldol process, resulting in the formation of three C-C bonds, five contiguous stereocenters as well as a spiro quaternary carbon center.

Optimizing the Local Chemical Environment on a Bifunctional Helical Peptide Scaffold Enables Enhanced Enantioselectivity and Cooperative Catalysis

We describe a proof-of-concept study in which peptide-bound enamine and thiourea catalysts are used to facilitate the conjugate addition of cyclohexanone to nitroolefins. Our bifunctional peptide scaffold is modified to optimize the local environment around both catalysts to enhance both reactivity and enantioselectivity, affording selectivities of ≤95% ee. Circular dichroism, nuclear magnetic resonance nuclear Overhauser effect studies, and molecular dynamics simulations verify the helical structure of our catalyst in solution and the importance of the secondary structure in catalysis.

Application of Proline-Derived (Thio)squaramide Organocatalysts in Asymmetric Diels–Alder and Conjugate Addition Reactions

The synthesis of chiral proline-derived squaramide and thiosquaramide organocatalysts, which are capable of the dual activation in asymmetric reactions is reported. The (thio)squaramide moiety can form hydrogen bonds to activate the substrates and to stereocontrol the reaction, while the pyrrolidine unit can form enamines to activate carbonyl compounds via aminocatalysis. Comparing the performance of thiosquaramide to squaramide, the Diels–Alder reaction of (anthracen-9-yl)acetaldehyde and trans-β-nitrostyrene was examined, which has been investigated in the literature using quantum chemical calculations. Both squaramide and thiosquaramide gave excellent yields (up to 99%) and enantiomeric excess values (up to 98%). Moreover, their catalytic performance was compared in conjugate addition of lawsone to β,γ-unsaturated α-keto ester.

Synthesis and Catalytic Activities of 3-Decyl-β-proline for Michael Reactions in Water without an Organic Solvent

3-Decyl-β-proline, which has a highly lipophilic substituent, was synthesized, and its catalytic activities in Michael addition using water as the solvent were investigated. The decyl substituent promoted the reaction by hydrophobic interactions to afford the Michael adduct in a high yield and with high diastereoselectivity under low catalyst loading.