Organocatalytic Enantioselective Cascade Aza-Michael/Michael Addition Sequence for Asymmetric Synthesis of Chiral Spiro[pyrrolidine-3,3′-oxindole]s

Brønsted-Acid-Catalyzed In Situ Formation of Acyclic Tertiary Enamides and Its Application to the Preparation of Diverse Nitrogen-Containing Heterocyclic Compounds

A Brønsted acid-catalyzed cascade process, involving in situ formation of acyclic tertiary enamides and intramolecular Michael reaction, is developed for the synthesis of functionalized cyclic tertiary enamides. Based on the dual reactivities of the enamide moiety, several reaction sequences were realized by using rationally designed substrates, leading to biologically relevant nitrogen-containing heterocyclic compounds with diverse structural skeletons in a concise and diastereocontrolled manner.

Synthesis of 5,6-Dihydropyrazolo[5,1-a]isoquinolines through Tandem Reaction of C,N-Cyclic Azomethine Imines with α,β-Unsaturated Ketones

An innovative and efficient approach has been developed for the synthesis of 5,6-dihydropyrazolo[5,1-a]isoquinolines. This one-pot tandem reaction involves the reaction of C,N-cyclic azomethine imines with α,β-unsaturated ketones, using K₂CO₃ as the base and DDQ as the oxidant. The process results in functionalized 5,6-dihydropyrazolo[5,1-a]isoquinolines with good yields. This convenient one-step method encompasses a tandem [3 + 2]-cycloaddition, detosylation, and oxidative aromatization.

Chemodivergent 1,3-Dipolar Cycloadditions of C,N-Cyclic Azomethine Imines with γ-Sulfonamido-α,β-Unsaturated Ketones to Synthesize Tricyclic Dinitrogen-Fused Heterocycles

1,3-Dipolar cycloadditions of C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones were developed to synthesize tricyclic dinitrogen-fused heterocycles. Highly functionalized tricyclic tetrahydroisoquinolines were readily obtained in good to high yields in the [3 + 2]-cycloaddition reaction of N-Bz-protected C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones under mild reaction conditions. Moreover, DABCO-catalyzed cycloaddition of N-Ts-protected C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones followed by cleavage of the tosyl group is a convenient route to synthesize tetrahydropyrazolo [5,1-a]isoquinolines in good yields with excellent diastereoselectivities.

Organocatalytic Asymmetric [3 + 2]-Annulations of γ-Sulfonamido/γ-Hydroxy-α,β-Unsaturated Ketones with Cyclic N-Sulfimines: Synthesis of Chiral Polyheterotricyclic Imidazolidines and Oxazolidines

The first organocatalytic asymmetric [3 + 2]-annulation of γ-sulfonamido-α,β-unsaturated ketones with cyclic N-sulfimines has been developed, and enantioenriched functionalized polyheterotricyclic imidazolidines were obtained in good yields and with excellent enantioselectivities. This approach was also extended to the asymmetric [3 + 2]-annulation of γ-hydroxy-α,β-unsaturated ketones, affording enantioenriched polyheterotricyclic oxazolidines. In addition, base-catalyzed [3 + 2]-annulations of γ-sulfonamido/γ-hydroxy-α,β-unsaturated ketones with cyclic N-sulfimines were re-investigated under mild reaction conditions for the synthesis of racemic polyheterotricyclic imidazolidines and oxazolidines with excellent diastereoselectivities.

Enantioselective synthesis of isoxazole-containing spirooxindole tetrahydroquinolines via squaramide-catalysed cascade reactions

A class of o-sulfonylaminostyryl isoxazole synthons were designed and demonstrated to be useful building blocks in asymmetric cascade aza-Michael/Michael reaction with 3-olefinic oxindoles. This squaramide-catalysed cascade reaction afforded structurally complex isoxazole-containing spirooxindole tetrahydroquinolines bearing three contiguous stereocenters in good to excellent yields (up to 99%) with high diastereoselectivities (up to >20 : 1 dr) and enantioselectivities (up to 88% ee). Moreover, the gram-scale synthesis and synthetic transformations were also demonstrated.

Tandem construction of biological relevant aliphatic 5-membered N-heterocycles

Nature often uses cascade reactions in a highly stereocontrolled manner for assembly structurally diverse nitrogen-containing heterocyclic scaffolds, i.e. secondary metabolites, important for medicinal chemistry and pharmacy. Five-membered nitrogen-containing heterocycles as standalone rings, as well as spiro and polycyclic systems are pharmacophores of drugs approved in various therapies, i.a. antibacterial or antiviral, antifungal, anticancer, antidiabetic, as they target many key enzymes. Furthermore, a large number of pyrrolidine derivatives are currently considered as drug candidates. Cascade transformations, also known as domino or tandem reactions, offer straightforward methods to build N-heterocyclic libraries of the great structural variety desired for drawing SAR conclusions. The tandem transformations are often atom economic and time-saving because they are performed as the one-pot, so no need for purification after each 'virtual' step and the limited necessity of protective groups are characteristic for these processes. Thus, the same results as in classical multistep synthesis can be achieved at markedly lower costs and shorter time, which is in line with modern green chemistry rules. Great advantage of cascade reactions is often reflected in their high regio- and stereoselectivities, enabling the preparing of the heterocyclic compound better fitted to the expected target in cells. This review reveals the biological relevance of N-heterocyclic scaffolds based on saturated 5-membered rings since we showed a number of examples of approved drugs together with the recent biologically attractive leading structures of drug candidates. Next, novel cascade synthetic procedures, taking into account the structure of the reactants and reaction mechanisms, enabling to obtain biological-relevant heterocyclic frameworks with good yields and relatively high stereoselectivity, were reviewed and compared. The review covers the advances of designing biological active N-heterocycles mainly from 2018 to 2021, whereas the synthetic part is focused on the last 7 years.

Recent advances in organocatalytic asymmetric aza-Michael reactions of amines and amides

Nitrogen-containing scaffolds are ubiquitous in nature and constitute an important class of building blocks in organic synthesis. The asymmetric aza-Michael reaction (aza-MR) alone or in tandem with other organic reaction(s) is an important synthetic tool to form new C-N bond(s) leading to developing new libraries of diverse types of bioactive nitrogen compounds. The synthesis and application of a variety of organocatalysts for accomplishing highly useful organic syntheses without causing environmental pollution in compliance with 'Green Chemistry" has been a landmark development in the recent past. Application of many of these organocatalysts has been extended to asymmetric aza-MR during the last two decades. The present article overviews the literature published during the last 10 years concerning the asymmetric aza-MR of amines and amides catalysed by organocatalysts. Both types of the organocatalysts, i.e., those acting through non-covalent interactions and those working through covalent bond formation have been applied for the asymmetric aza-MR. Thus, the review includes the examples wherein cinchona alkaloids, squaramides, chiral amines, phase-transfer catalysts and chiral bifunctional thioureas have been used, which activate the substrates through hydrogen bond formation. Most of these reactions are accompanied by high yields and enantiomeric excesses. On the other hand, N-heterocyclic carbenes and chiral pyrrolidine derivatives acting through covalent bond formation such as the iminium ions with the substrates have also been included. Wherever possible, a comparison has been made between the efficacies of various organocatalysts in asymmetric aza-MR.

Phosphine-catalyzed [3 + 2] annulation of β-sulfonamido-substituted enones with trans-α-cyano-α,β-unsaturated ketones for the synthesis of highly substituted pyrrolidines

To synthesize highly substituted pyrrolidines, we developed a phosphine-catalyzed [3 + 2] annulation of β-sulfonamido-substituted enones with trans-α-cyano-α,β-unsaturated ketones. We prepared a series of pyrrolidines under mild conditions with high yields and moderate-to-good diastereoselectivities. A catalytic mechanism for this reaction is suggested.

To synthesize highly substituted pyrrolidines, we developed a phosphine-catalyzed [3 + 2] annulation of β-sulfonamido-substituted enones with trans-α-cyano-α,β-unsaturated ketones.

Asymmetric Synthesis of 3,3'-Tetrahydrofuryl Spirooxindoles via Palladium-Catalyzed [3+2] Cycloadditions of Methyleneindolinones with Vinylethylene Carbonates

A palladium-catalyzed asymmetric [3+2] cycloaddition reaction of methyleneindolinones with vinylethylene carbonates has been successfully developed, which provides a highly efficient method for the synthesis of structurally diverse 3,3'-tetrahydrofuryl spirooxindoles in high yields (≤99%) with excellent stereoselectivities (>20:1 dr, ≤99% ee). Furthermore, this methodology shows a wide substrate scope and high utility in diversity-oriented synthesis.

Catalytic Asymmetric and Divergent Synthesis of Tricyclic and Tetracyclic Spirooxindoles: Controllable Site-Selective Electrophilic Halocyclization of 1,6-Enynes

Catalytic asymmetric and divergent assembly of tricyclic and tetracyclic 3,3'-pyrrolidonyl spirooxindoles was developed, involving a one-pot chiral Brønsted base catalyzed asymmetric propargylation for the synthesis of oxindole 1,6-enynes and a subsequent switchable site-selective and highly diastereoselective electrophilic iodocyclization of 1,6-enynes. In addition, antitumor properties of the newly synthesized compounds were evaluated.

Recent advances in the asymmetric synthesis of pharmacology-relevant nitrogen heterocycles via stereoselective aza-Michael reactions

In this review, recent applications of a stereoselective aza-Michael reaction for asymmetric synthesis of naturally occurring N-containing heterocyclic scaffolds and their usefulness to pharmacology are summarized.

Chiral bifunctional bisphosphine enabled enantioselective tandem Michael addition of tryptamine-derived oxindoles to ynones

A chiral phosphine-catalyzed tandem Michael addition of tryptamine-derived oxindoles to ynones was developed to synthesize spiro[pyrrolidine-3,3′-oxindole] in good yields, good to excellent enantio- and diastereoselectivities.

Organocatalytic asymmetric synthesis of highly substituted pyrrolidines bearing a stereogenic quaternary centre at the 3-position

An organocatalytic asymmetric cascade reaction has been developed for the synthesis of highly substituted pyrrolidines having a stereogenic quaternary centre at the 3-position. N-Tosyl aminomethyl enone and trans-α-cyano-α,β-unsaturated ketone were utilized as the reaction partners in this method. Cinchonidine derived bifunctional amino-squaramide catalysts were the best to obtain the products in high enantio- and diastereoselectivities.

Organocatalytic asymmetric synthesis of 2,4-disubstituted imidazolidines via domino addition-aza-Michael reaction

The first highly diastereo- and enantioselective synthesis of 2,4-disubstituted imidazolidines has been developed via a formal [3+2] cyclization reaction. Bidentate aminomethyl enones and N-tosyl imines were used as the reaction partners in the reaction. Bifunctional squaramide catalysts were found to be efficient for this reaction and few transformations of the products have been demonstrated.

Asymmetric Catalytic Double Michael Additions for the Synthesis of Spirooxindoles

Asymmetric cascade double Michael additions to construct 2'-substituted 3,3'-spirooxindoles by using a chiral guanidine organocatalyst has been developed. A series of spirooxindole derivatives containing dihydrofuran or pyrrolidine subunits were obtained with good to excellent diastereo- and enantioselectivities. The method showed great tolerance of a number of aromatic and aliphatic alkynones. The strategy gave access to the asymmetric synthesis of (-)-salacin for the first time.

Catalytic asymmetric synthesis of spirooxindoles: recent developments

The past four years have witnessed significant developments in the field of the catalytic asymmetric synthesis of spirooxindoles, and this feature article outlines the recent progress in this area, including the contributions of our group. This article is divided into sections according to the size and type of the generated spiro-ring fused at the C3-position of the oxindole core.

Non-Covalent Organocatalyzed Domino Reactions Involving Oxindoles: Recent Advances

The ubiquitous presence of spirooxindole architectures with several functionalities and stereogenic centers in bioactive molecules has been appealing for the development of novel methodologies seeking their preparation in high yields and selectivities. Expansion and refinement in the field of asymmetric organocatalysis have made possible the development of straightforward strategies that address these two requisites. In this review, we illustrate the current state-of-the-art in the field of spirooxindole synthesis through the use of non-covalent organocatalysis. We aim to provide a concise overview of very recent methods that allow to the isolation of unique, densely and diversified spirocyclic oxindole derivatives with high structural diversity via the use of cascade, tandem and domino processes.

Organocatalytic asymmetric Michael addition of α-alkylidene succinimides to nitrostyrenes

A squaramide-catalyzed asymmetric Michael addition of α-alkylidene succinimides to nitrostyrenes for the synthesis of chiral succinimide derivatives was described.

Squaramide-catalysed asymmetric cascade aza-Michael/Michael addition reaction for the synthesis of chiral trisubstituted pyrrolidines

A squaramide catalysed aza-Michael/Michael cascade reaction between nitroalkenes and tosylaminomethyl enones or enoates afforded highly functionalized chiral pyrrolidines in good yields (up to 99%) with good diastereoselectivities (up to 91 : 9 dr) and excellent enantioselectivities (up to >99% ee) under mild conditions.