Additive and Emergent Catalytic Properties of Dimeric Unnatural Amino Acid Derivatives: Aldol and Conjugate Additions

Different densely substituted L- and D-proline esters were prepared by asymmetric (3+2) cycloaddition reactions catalyzed by conveniently selected EhuPhos chiral ligands. The γ-nitro-2-alkoxycarbonyl pyrrolidines thus obtained in either their endo or exo forms were functionalized and coupled to yield the corresponding γ-dipeptides. The catalytic properties of these latter dimers were examined using aldol and conjugate additions as case studies. When aldol reactions were analyzed, an additive behavior in terms of stereocontrol was observed on going from the monomers to the dimers. In contrast, in the case of the conjugate additions between ketones and nitroalkenes, the monomers did not catalyze this reaction, whereas the different γ-dipeptides promoted the formation of the corresponding Michael adducts. Therefore, in this latter case emergent catalytic properties were observed for these novel γ-dipeptides based on unnatural proline derivatives. Under certain conditions stoichiometric amounts of ketone, acid and nitroalkene), formation of N-acyloxy-2-oxooctahydro-1H-indoles was observed.

Diaminomethylenemalononitrile as a Chiral Single Hydrogen Bond Catalyst: Application to Enantioselective Conjugate Addition of α-Branched Aldehydes

An improved diaminomethylenemalononitrile organocatalyst, bearing a N,N-disubstituted structure, promoted enantioselective conjugate addition reaction of α-branched aldehydes with vinyl sulfone, affording adducts with excellent enantioselectivities (up to 96% ee). Mechanistic studies revealed that the diaminomethylenemalononitrile motif holds the vinyl sulfone substrate using a single hydrogen bond accompanied by multiple weak interactions, including electrostatic C-H⋅⋅⋅O interactions.

Syntheses and reactivity of spiro-epoxy/aziridine oxindole cores: developments in the past decade

This review highlights various reactions to afford spiro-epoxy/aziridine oxindoles and their potential synthetic transformations.

Pyrrolidine-based chiral porous polymers for heterogeneous organocatalysis in water

The “bottom-up” reticulation of chiral pyrrolidine into POPs for heterogeneous organocatalysis in pure water.

Asymmetric Michael Addition Organocatalyzed by α,β-Dipeptides under Solvent-Free Reaction Conditions

The application of six novel α,β-dipeptides as chiral organocatalysts in the asymmetric Michael addition reaction between enolizable aldehydes and N-arylmaleimides or nitroolefins is described. With N-arylmaleimides as substrates, the best results were achieved with dipeptide 2 as a catalyst in the presence of aq. NaOH. Whereas dipeptides 4 and 6 in conjunction with 4-dimethylaminopyridine (DMAP) and thiourea as a hydrogen bond donor proved to be highly efficient organocatalytic systems in the enantioselective reaction between isobutyraldehyde and various nitroolefins.

Recent Advances in Asymmetric Organocatalyzed Conjugate Additions to Nitroalkenes

The asymmetric conjugate addition of carbon and heteroatom nucleophiles to nitroalkenes is a very interesting tool for the construction of highly functionalized synthetic building blocks. Thanks to the rapid development of asymmetric organocatalysis, significant progress has been made during the last years in achieving efficiently this process, concerning chiral organocatalysts, substrates and reaction conditions. This review surveys the advances in asymmetric organocatalytic conjugate addition reactions to α,β-unsaturated nitroalkenes developed between 2013 and early 2017.

A stereolibrary of conformationally restricted amino acids based on pyrrolidinyl/piperidinyloxazole motifs

A stereolibrary of conformationally restricted oxazole-containing amino acids, namely all isomers of 5–pyrrolydinyl- and 5-piperidinyloxazole-4-carboxylic acids, were designed and synthesized in three steps by the reaction of the corresponding N-Boc-protected amino acids and ethyl isocyanoacetate. These natural products-inspired amino acids are valuable building blocks for the synthesis of peptidomimetics and potential lead compounds for drug discovery.

A one-pot ‘click’ reaction from spiro-epoxides catalyzed by Cu(i)-pyrrolidinyl-oxazole-carboxamide

A sustainable green methodology for the ‘one-pot’ syntheses of 1,2,3-triazolo 3-hydroxy-2-oxindoles from isatin–epoxides has been employed via a CuAAC reaction.