Azetidine-Derived Dinuclear Zinc-Catalyzed Asymmetric Conjugate Addition of Bioactive Heterocycles to β,γ-Unsaturated α-Keto esters

Dinuclear Zn-Catalytic System as Brønsted Base and Lewis Acid for Enantioselectivity in Same Chiral Environment

Zinc (Zn) is a crucial element with remarkable significance in organic transformations. The profusion of harmless zinc salts in the Earth's outer layer qualifies zinc as a noteworthy contender for inexpensive and eco-friendly reagents and catalysts. Recently, widely recognized uses of organo-Zn compounds in the field of organic synthesis have undergone extensive expansion toward asymmetric transformations. The ProPhenol ligand, a member of the chiral nitrogenous-crown family, exhibits the spontaneous formation of a dual-metal complex when reacted with alkyl metal (R-M) reagents, e.g., ZnEt2. The afforded Zn complex possesses two active sites, one Lewis acid and the other Brønsted base, thereby facilitating the activation of nucleophiles and electrophiles simultaneously within the same chiral pocket. In this comprehensive analysis, we provide a thorough account of the advancement and synthetic potential of these diverse catalysts in organic synthesis, while emphasizing the reactivity and selectivities, i.e., dr and ee due to the design/structure of the ligands employed.

New Binaphthyl-Proline-Based Chiral Ligands Bearing Imidazoline Groups: Design, Synthesis, and Their Application in Enantioselective Conjugate Addition of 4-Hydroxycoumarin and Related Nucleophiles to β,γ-Unsaturated α-Ketoesters

This paper describes the design and application of new binaphthyl-proline-based chiral ligands bearing imidazoline functional groups. These chiral ligands incorporate the advantages of both the binaphthyl and proline skeletons, they are featured with regulatable electronic and steric properties for the imidazoline functional groups, and form chiral complexes with different metal salts such as cuprous acetate. In the presence of an appropriate amount of a chiral catalyst, enantioselective conjugate addition of 4-hydroxycoumarin or related nucleophiles to different β,γ-unsaturated α-ketoesters proceeded readily, giving the desired products in high yield (up to 99%) and excellent enantiomeric excess (up to 99%).

Chiral tertiary amine-squaramide catalyzed highly enantioselective Michael addition of cyclic diketones to β,γ-unsaturated α-keto Esters

Abstract:

A novel tertiary amine squaramide has successfully developed and applied to catalyze the asymmetric Michael addition between cyclic diketones and β,γ-unsaturated α-ketoesters. The catalyst system performed well with a low catalyst loading of 1 mol% under mild reaction conditions. A series of synthetically and pharmaceutically useful chiral bicyclic compounds were obtained with both high yields (up to 97%) and excellent enantioselectivities (up to 97% ee).

Enantioselective Synthesis of Coumarins Catalyzed by Chiral Squaramide

Abstract:

A newly chiral tertiary amine-squaramide has successfully developed and applied into the asymmetric Michael addition of 4-hydroxycoumarin to β,γ-unsaturated α-ketoesters. The catalyst system performed well with a low catalyst loading of 1 mol% under mild reaction conditions. A series of coumarin derivatives were obtained with good yields and enantioselectivities

Enantioselective synthesis of coumarins catalyzed by an isosteviol-derived tertiary amine-squaramide catalyst

A newly tertiary amine-squaramide organocatalyst has been successfully developed and applied into the asymmetric Michael addition of 4-hydroxycoumarin to β,γ-unsaturated α-ketoesters. The catalyst system performed well with a low catalyst loading of 1 mol% under mild reaction conditions. A series of coumarin derivatives were obtained in good to high yields (up to 97%) with high enantioselectivities (up to 96% ee).

Recent advances in the synthesis and reactivity of azetidines: strain-driven character of the four-membered heterocycle

Azetidines represent one of the most important four-membered heterocycles used in organic synthesis and medicinal chemistry. The reactivity of azetidines is driven by a considerable ring strain, while at the same the ring is significantly more stable than that of related aziridines, which translates into both facile handling and unique reactivity that can be triggered under appropriate reaction conditions. Recently, remarkable advances in the chemistry and reactivity of azetidines have been reported. In this review, we provide an overview of the synthesis, reactivity and application of azetidines that have been published in the last years with a focus on the most recent advances, trends and future directions. The review is organized by the methods of synthesis of azetidines and the reaction type used for functionalization of azetidines. Finally, recent examples of using azetidines as motifs in drug discovery, polymerization and chiral templates are discussed.

A new asymmetric activation strategy for hydrazones as acyl anion equivalents in the bimetallic catalyzed carbonyl-ene reaction

A new asymmetric activation strategy for hydrazones as acyl anion equivalents is developed in the bimetallic catalyzed carbonyl-ene reaction of isatins and hydrazones. Under mild conditions, optically active functionalized 3-hydroxy-2-oxindoles were furnished in up to 98% yield with up to 97% enantioselectivity. In this process, formaldehyde tert-butylhydrazone which is seldom employed in asymmetric carbonyl-ene reactions accelerated by a metallic catalyst can be activated well by a Brønsted base. A possible catalytic cycle is proposed.

The renaissance of strained 1-azabicyclo[1.1.0]butanes as useful reagents for the synthesis of functionalized azetidines

Since their discovery in the late 1960s, 1-azabicyclo[1.1.0]butanes have demonstrated to be interesting precursors of azetidines, because of the peculiar reactivity of the C3–N bond that allows double functionalization in the 1,3 positions.