Catalytic Efficiency of Primary α-Amino Amides as Multifunctional Organocatalysts in Recent Asymmetric Organic Transformations

Chiral primary α-amino amides, consisting of an adjacent enamine bonding site (Bronsted base site), a hydrogen bonding site (Bronsted acid site), and flexible bulky substituent groups to modify the steric factor, are proving to be extremely valuable bifunctional organocatalysts for a wide range of asymmetric organic transformations. Primary α-amino amides are less expensive alternatives to other primary amino organocatalysts, such as chiral diamines and cinchona-alkaloid-derived primary amines, as they are easy to synthesize, air-stable, and allow for the incorporation of a variety of functional groups. In recent years, we have demonstrated the catalytic use of simple primary α-amino amides and their derivatives as organocatalysts for the aldol reaction, Strecker reaction, Michael tandem reaction, allylation of aldehydes, reduction of N-Aryl mines, opening of epoxides, hydrosilylation, asymmetric hydrogen transfer, and N-specific nitrosobenzene reaction with aldehydes.

Environmental Modulation of Chiral Prolinamide Catalysts for Stereodivergent Conjugate Addition

Synthetic chiral catalysts generally rely on proximal functional groups or ligands for chiral induction. Enzymes often employ environmental chirality to achieve stereoselectivity. Environmentally controlled catalysis has benefits such as size and shape selectivity but is underexplored by chemists. We here report molecularly imprinted nanoparticles (MINPs) that utilized their environmental chirality to either augment or reverse the intrinsic selectivity of a chiral prolinamide cofactor. The latter ability allowed the catalyst to produce products otherwise disfavored in the conjugate addition of aldehyde to nitroalkene. The catalysis occurred in water at room temperature and afforded γ-nitroaldehydes with excellent yields (up to 94%) and ee (>90% in most cases). Up to 25:1 syn/anti and 1:6 syn/anti ratios were achieved through a combination of catalyst-derived and environmentally enabled selectivity. The high enantioselectivity of the MINP also made it possible for racemic catalysts to perform asymmetric catalysis, with up to 80% ee for the conjugate addition.

Divergent Electrochemical Carboamidation of Cyclic Amines

We developed an electrochemical carboamidation sequence that affords either cyclic β-amidoamine products via direct functionalization or linear hydroxybisamide products via a ring opening pathway. The reaction pathway was dependent on the nature of the N-acyl activating group, with carbamate groups favoring direct isocyanide addition to the N-acyliminium ion intermediate and the benzoyl activating group favoring the ring opening-functionalization pathway. Both protocols are one-pot reaction sequences, have general applicability, and lead to peptide-like products of greatly increased molecular complexity.

Organocatalysis for the Asymmetric Michael Addition of Aldehydes and α,β-Unsaturated Nitroalkenes

Michael addition is an important reaction because it can be used to synthesize a wide range of natural products or complex compounds that exhibit biological activities. In this study, a mirror image of an aldehyde and α,β-unsaturated nitroalkene were reacted in the presence of (R,R)-1,2-diphenylethylenediamine (DPEN). Herein, thiourea was introduced as an organic catalyst, and a selective Michael addition reaction was carried out. The primary amine moiety of DPEN reacts with aldehydes to form enamines, which is activated by the hydrogen bond formation between the nitro groups of α,β-unsaturated nitroalkenes and thiourea. Our aim was to obtain an asymmetric Michael product by adding 1,4-enamine to an alkene to form a new carbon–carbon bond. As a result, the primary amine of the chiral diamine was converted to an enamine. The reaction proceeded with a relatively high degree of enantioselectivity, which was achieved using double activation via hydrogen bonding of the nitro group and thiourea. Michael products with a high degree of enantioselectivity (97–99% synee) and diastereoselectivity (syn/anti = 9/1) were obtained in yields ranging from 94–99% depending on the aldehydes.

Chiral anthranilic pyrrolidine as custom-made amine catalyst for enantioselective Michael reaction of nitroalkenes with carbonyl compounds

A chiral anthranilic pyrrolidine catalyst as a custom-made amine-catalyst was developed for the enantio- and diastereo selective Michael reaction of nitroalkenes with carbonyl compounds. In particular, a peptide-like catalyst in which an α-amino acid is attached to the anthranilic acid skeleton induced the high stereoselectivity of the reaction with aldehydes. Studies of the reaction mechanism indicated that the catalyst exhibits a divergent stereocontrol in the reaction, namely, steric control by a 2-substituted group on the catalyst and hydrogen-bonding control by a carboxylic acid group on the catalyst.

Michael addition of carbonyl compounds to nitroolefins under the catalysis of new pyrrolidine-based bifunctional organocatalysts

Novel bifunctional pyrrolidine-based organocatalysts for the asymmetric Michael addition of carbonyl compounds to nitroolefins have been synthesised from homoallylamines, which are easily obtained from (R)-glyceraldehyde as a chiral precursor. Under optimal reaction conditions, these bifunctional organocatalysts showed a high catalytic efficiency (almost quantitative yield in most cases) and stereoselectivity in the Michael addition reactions of a variety of aldehydes (up to 98 : 2 dr and 97% ee) and ketones (up to 98 : 2 dr and 99% ee) to nitroolefins.

Green synthesis of new pyrrolidine-fused spirooxindoles via three-component domino reaction in EtOH/H2O

An efficient, green and sustainable approach for the synthesis of novel polycyclic pyrrolidine-fused spirooxindole compounds was developed. The synthesis included a one-pot, three-component, domino reaction of (E)-3-(2-nitrovinyl)-indoles, isatins and chiral polycyclic α-amino acids under catalyst-free conditions at room temperature in EtOH–H₂O. The salient features of this methodology are eco-friendliness, high yields and the ease of obtaining target compounds without the involvement of toxic solvents and column chromatography. These novel polycyclic pyrrolidine-fused spirooxindoles provide a collection of structurally diverse compounds that show promise for future bioassays and medical treatments.

An efficient, green and sustainable approach was developed for the synthesis of novel polycyclic pyrrolidine-fused spirooxindole compounds.

Catalytic dehydrogenative aromatization of cyclohexanones and cyclohexenones

Prompted by the scant attention paid by published literature reviews to the comprehensive catalytic dehydrogenative aromatization of cyclohexa(e)nones, this review describes recent methods developed to-date involving transition-metal-catalyzed oxidative aromatization and metal-free strategies for the transformation of cyclohexa(e)nones to substituted phenols.

Functionalised Cookson's Diketones in Chlorosulfonic Acid: Towards Polysubstituted D3-Trishomocubanes

A number of Cookson's diketone ( Cs-trishomocubane-8,11-dione) derivatives were synthesised and introduced into the reaction with chlorosulfonic acid to give previously unknown polysubstituted D3-trishomocubanes (pentacyclo[6.3.0.02,6.03,10.05,9]undecanes) – chiral analogues of the parent cubanes. In the case of 1,9-dibromo- Cs-trishomocubane-8,11-dione, the reaction proceeded with conservation of both carbonyl groups selectively affording 3-bromo-6-chloro- D3-trishomocubane-4,7-dione whose distinct substitution pattern was confirmed by means of X-ray structural analysis.

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.