Asymmetric synthesis of spiro[4H-chromene-3,3'-oxindoles] via a squaramide-organocatalytic three-component cascade Knoevenagel/Michael/cyclization sequence

Asymmetric synthesis of spiro[4H-chromene-3,3'-oxindole] derivatives was realized through an organocatalytic cascade Knoevenagel/Michael/cyclization reaction using a quinidine-derived squaramide. Under the optimized conditions, the reactions of isatins, malononitrile, and sesamol yield the desired spirooxindoles in good yields (75-87%) and moderate to high ee values (up to 90% ee).

Multiple-cycle photochemical cascade reactions

Cascade reactions represent an efficient and economical synthetic approach, enabling the rapid synthesis of a wide array of structurally complex organic compounds. These compounds, previously inaccessible, can now be synthesized in a remarkably limited number of steps. Concurrently, the photochemical reactions of organic molecules have gained prominence as a potent strategy for accessing a diverse range of radical species and intermediates. This is achieved in a controlled manner under mild conditions. Owing to the relentless endeavors of chemists, significant strides have been made in the realm of photochemical cascade reactions. These advancements have facilitated the synthesis of novel molecular structures with high complexity, structures that are typically challenging to generate under thermal conditions. In this review, we comprehensively summarize and underscore the recent pivotal advancements in visible-light-induced cascade reactions. Our focus is on the elucidation of multiple photochemical catalytic cycles, emphasizing the catalytic activation modes and the types of reactions involved.

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.

From Conventional to Sustainable Catalytic Approaches for Heterocycles Synthesis

Synthesis of heterocyclic compounds is fundamental for all the research area in chemistry, from drug synthesis to material science. In this framework, catalysed synthetic methods are of great interest to effective reach such important building blocks. In this review, we will report on some selected examples from the last five years, of the major improvement in the field, focusing on the most important conventional catalytic systems, such as transition metals, organocatalysts, to more sustainable ones such as photocatalysts, iodine-catalysed reaction, electrochemical reactions and green innovative methods.

Asymmetric Synthesis of Hydrophenanthrenones Bearing Multiple Stereogenic Centers via Squaramide-Catalyzed Domino 1,4-/1,4-Addition Desymmetrization Sequence

An unprecedented stereoselective route for procuring tetrahydrophenanthren-2(1H)-ones bearing up to five contiguous stereogenic centers has been accomplished. A bifunctional squaramide enables a domino 1,4-/1,4-addition desymmetrization reaction sequence between the newly developed β-nitrostyrene-tethered 2,5-cyclohexadienones and trisubstituted enolizable nucleophiles to provide direct access to the polyfunctionalized hydrophenanthren-2(1H)-ones in excellent enantio- (up to >99%) and diastereoselectivity (up to >20:1). By using a psedoenantiomeric squaramide, the enantiomers of hydrophenanthrene derivatives were also accessed with excellent stereocontrol.

Influence of Coordination to Silver(I) Centers on the Activity of Heterocyclic Iodonium Salts Serving as Halogen-Bond-Donating Catalysts

Kinetic data based on 1 H NMR monitoring and computational studies indicate that in solution, pyrazole-containing iodonium triflates and silver(I) triflate bind to each other, and such an interplay results in the decrease of the total catalytic activity of the mixture of these Lewis acids compared to the separate catalysis of the Schiff condensation, the imine-isocyanide coupling, or the nucleophilic attack on a triple carbon-carbon bond. Moreover, the kinetic data indicate that such a cooperation with the silver(I) triflate results in prevention of decomposition of the iodonium salts during the reaction progress. XRD study confirms that the pyrazole-containing iodonium triflate coordinates to the silver(I) center via the pyrazole N atom to produce a rare example of a pentacoordinated trigonal bipyramidal dinuclear silver(I) complex featuring cationic ligands.

Diastereo- and Enantioselective Synthesis of Functionalized Dihydropyrans via an Organocatalytic Claisen Rearrangement/Oxa-Michael Addition Tandem Sequence

A straightforward diastereo- and enantioselective Claisen rearrangement/oxa-Michael addition tandem sequence with a cinchona squaramide catalyst was described, which afforded a practical and atom-economical approach to access a range of valuable dihydropyrans in good to excellent yields with excellent stereoselectivities. The organo-bifunctional catalyst played a key role in enhancing stereoselectivity in this asymmetric tandem sequence. Moreover, the asymmetric catalytic sequential processes of the hydroalkoxylation/Claisen rearrangement/cyclization sequence and Claisen rearrangement/aza-Michael addition tandem sequence have also been afforded good yields and moderate stereoselectivities.

Recent developments in one-pot stepwise synthesis (OPSS) of small molecules

One-pot synthesis is an active topic in organic chemistry due to its intrinsic advantages of simple operation, high mass efficiency, low cost, and less amount of waste disposal. Among three kinds of one-pot syntheses, 1) cascade reactions, 2) multicomponent reactions (MCRs), and 3) one-pot stepwise synthesis (OPSS), OPSS could be more flexible and practical since it is carried out stepwisely and have variable reaction conditions for different steps. This perspective article uses selected examples to highlight the recent development in OPSS involving cyclization, cycloaddition, rearrangement, and catalytic reactions for the synthesis of heterocyclic scaffolds, asymmetric molecules, natural products, and bioactive compounds.

Asymmetric Synthesis of Cyclohexenone-Fused Isochromans via Quinidine-Catalyzed Domino Peroxyhemiacetalization/Oxa-Michael Addition/Desymmetrization Sequence

A highly enantio- and diastereoselective synthesis of highly functionalized isochromans was achieved through an organocatalyzed domino reaction. Quinidine as the catalyst initiates a peroxyhemiacetalization/oxa-Michael/desymmetrization domino sequence between various 2,5-cyclohexadienone-tethered aryl aldehydes with hydroperoxides to generate the single diastereomers of isochromans appended with a cyclohexenone ring bearing three vicinal stereocenters in good yields and high enantioselectivities under ambient reaction conditions.

Recent development in asymmetric organocatalytic domino reactions involving 1,6-addition as a key step

This article highlights the significant development in stereoselective domino reactions involving 1,6-addition as a crucial step.

Brønsted acid catalyzed enantioselective addition of hydrazones to 3-indolylmethanols

A metal-free synthesis of enantiopure β-substituted tryptophan derivatives was developed. A chiral Brønsted acid enabled the addition of donor-substituted hydrazones to 3-indolylmethanols in excellent yields and enantioselectivities.

Domino Michael/Michael reaction catalyzed by switchable modularly designed organocatalysts

The domino Michael/Michael reaction between (E)-7-aryl-7-oxohept-5-enals and trans-cinnamaldehydes was investigated by using modularly designed organocatalysts (MDOs). It was found that both the enamine and iminium catalytic modes of the MDOs are switchable and can be individually switched on and off by using appropriate combinations of the precatalyst modules and the reaction conditions. When both the enamine and iminium catalysis modes of the MDOs are switched on, the desired domino reaction products can be obtained in good yields and stereoselectivities under optimized conditions.

An Iterative Phosphate Tether Mediated Approach for the Synthesis of Complex Polyol Subunits

A pot-economical approach to advanced polyol subunits is reported. The key reactions involved are iterative use of a phosphate tether-mediated one-pot sequential RCM/CM/H₂ with subsequent utilization of either a regio-/diasteroselective cuprate addition or a Pd-catalyzed reductive allylic transposition. This method highlights the asymmetric synthesis of 12 complex polyol subunits in 4-6 one-pot sequential operations with a total of 12-14 reactions, of which 4-5 are catalytic, with minimal workup and purification procedures.

Serendipitous One-Step Synthesis of Cyclopentene Derivatives from 5'-Deoxy-5'-heteroarylsulfonylnucleosides as Nucleoside-Derived Julia-Kocienski Reagents

A serendipitous one-step transformation of 5'-deoxy-5'-heteroarylsulfonylnucleosides into cyclopentene derivatives is reported. This unique transformation likely proceeds via a domino reaction initiated by α-deprotonation of the heteroaryl sulfone and subsequent elimination reaction to generate a nucleobase and an α,β-unsaturated sulfone that contains a formyl group. The Michael addition of the nucleobase to the α,β-unsaturated sulfone and the subsequent intramolecular Julia-Kocienski reaction eventually generate the cyclopentene ring. Heteroarylthio and acylthio groups can be incorporated into the cyclopentene core in place of the nucleobase by conducting this reaction in the presence of a heteroarylthiol and a thiocarboxylic acid, respectively. cis,cis-Trisubstituted cyclopentene derivatives are obtained as a single stereoisomer from ribonucleoside-derived Julia-Kocienski sulfones.

Visible light organic photoredox catalytic cascade reactions

Over the past years, impressive progress has been made in the development of organic photoredox catalytic cascade reactions without the participation of expensive and toxic transition metals under visible light irradiation. These transformations highly depend on the in situ generation of various radical species in the photoredox catalytic cycles. Numerous chemically and biomedically valuable building blocks have been synthesized through this efficient and sustainable protocol. In this review, we highlight the recent progress in this blooming area by presenting a series of new catalytic cascade reactions mediated by organic photoredox catalysts and describe their mechanisms and applications which have appeared in the recent literature.

Recent Advance of Domino Michael Reaction in Natural Product Synthesis

Recent advances in the total syntheses of cyclic natural products and related compounds from 2005 to 2021, which employ domino Michael reactions as key steps, have been reviewed, focusing mainly on the domino Michael reactions catalyzed by organocatalysts.

Asymmetric Sequential Michael Addition and Cyclization Reactions of 2-(2-Nitrovinyl)phenols Catalyzed by Bifunctional Amino-Squaramides

In this work, we describe the Michael addition–cyclization reaction of 2-(2-nitrovinyl)phenol with two different reactive Michael donors, which lead to chiral benzopyran derivatives. Specifically, bifunctional amino-squaramides with one or two chiral units in the side chains were evaluated as catalysts in these transformations. Furthermore, the utility of selected green solvents as reaction media for these processes was also tested. The best result was achieved with methyl-cyclopentanone-2-carboxylate as the Michael donor in ethyl (–)-l-lactate with quinine-based amino-squaramide as catalyst (yield 72%, dr >99:1, ee 99%).

Recyclable fluorous cinchona organocatalysts for asymmetric synthesis of biologically interesting compounds

Organocatalysis has unique modes of activation, mild reaction conditions, and good catalyst structural amenability. The integration of green techniques such as catalyst recovery and one-pot reactions makes organocatalysis more efficient and attractive. Presented in this article are the recyclable cinchona alkaloid-catalyzed reactions including fluorination and Michael addition-initiated cascade reactions in asymmetric synthesis of functionalized compounds of biological interest.

Enantioselective Construction of Spirooxindole-Fused Cyclopentanes

The present study reports an asymmetric organocatalytic cascade reaction of oxindole derivates with α,β-unsaturated aldehydes efficiently catalyzed by simple chiral secondary amine. Spirooxindole-fused cyclopentanes were produced in excellent isolated yields (up to 98%) with excellent enantiopurities (up to 99% ee) and moderate to high diastereoselectivities. The synthetic utility of the protocol was exemplified on a set of additional transformations of the corresponding spiro compounds. In addition, a study showing the promising biological activity of selected enantioenriched products was accomplished.

Predicting the catalytic activity of azolium-based halogen bond donors: an experimentally-verified theoretical study

This report demonstrates the successful application of electrostatic surface potential distribution analysis for evaluating the relative catalytic activity of a series of azolium-based halogen bond donors. A strong correlation (R² > 0.97) was observed between the positive electrostatic potential of the σ-hole on the halogen atom and the Gibbs free energy of activation of the model reactions (i.e., halogen abstraction and carbonyl activation). The predictive ability of the applied approach was confirmed experimentally. It was also determined that the catalytic activity of azolium-based halogen bond donors was generally governed by the structure of the azolium cycle, whereas the substituents on the heterocycle had a limited impact on the activity. Ultimately, this study highlighted four of the most promising azolium halogen bond donors, which are expected to exhibit high catalytic activity.

Organocatalytic total synthesis of bioactive compounds based on one-pot methodologies

The combination of one-pot methodologies to asymmetric organocatalysis allow a green and direct access to many types of complex highly functionalized chiral products, including important key intermediates in total syntheses of important bioactive compounds. A series of chiral organocatalysts have already been successfully applied to such syntheses. This report collects major developments in the total synthesis of biologically active products based on the use of enantioselective organocatalytic domino/tandem reactions as key steps. It is divided into two parts dealing successively with reactions based on the use of proline-derived catalysts and other organocatalysts.

Asymmetric organocatalysis: an enabling technology for medicinal chemistry

The efficacy and synthetic versatility of asymmetric organocatalysis have contributed enormously to the field of organic synthesis since the early 2000s. As asymmetric organocatalytic methods mature, they have extended beyond the academia and undergone scale-up for the production of chiral drugs, natural products, and enantiomerically enriched bioactive molecules. This review provides a comprehensive overview of the applications of asymmetric organocatalysis in medicinal chemistry. A general picture of asymmetric organocatalytic strategies in medicinal chemistry is firstly presented, and the specific applications of these strategies in pharmaceutical synthesis are systematically described, with a focus on the preparation of antiviral, anticancer, neuroprotective, cardiovascular, antibacterial, and antiparasitic agents, as well as several miscellaneous bioactive agents. The review concludes with a discussion of the challenges, limitations and future prospects for organocatalytic asymmetric synthesis of medicinally valuable compounds.

Revealing the Similarities of α,β-Unsaturated Iminiums and Acylazoliums in Organocatalysis

The secondary amine-catalyzed reactions proceeding via α,β-unsaturated iminiums and the N-heterocyclic carbene (NHC)-catalyzed transformations taking place via α,β-unsaturated acylazoliums are the two widely used electrophilic intermediates in organocatalysis. Over the last two decades, these two intermediates are extensively utilized for the enantioselective construction of valuable molecules. Both intermediates are generated by the covalent binding of catalysts to the substrates leading to LUMO activation of α,β-unsaturated carbonyls. A variety of soft nucleophiles are known to add to the α,β-unsaturated iminiums and acylazoliums in a conjugate fashion, and in many cases, striking similarity in reactivity has been observed. Having said this, there are few cases where these intermediates exhibit difference in reactivity. This Minireview is aimed at highlighting the resemblances in reactivity between α,β-unsaturated iminiums and acylazoliums thereby shedding light on the unnoticed parallels of the two intermediates in organocatalysis.

20 Years of Forging N-Heterocycles from Acrylamides through Domino/Cascade Reactions

Acrylamides are versatile building blocks that are easily obtained from readily available starting materials. During the last 20 years, these valuable substrates bearing a nucleophilic nitrogen atom and an electrophilic double bond have proven to be efficient domino partners, leading to a wide variety of complex aza-heterocycles of synthetic relevance. In this non-exhaustive review, metal-free and metal-triggered reactions followed by an annulation will be presented; these two approaches allow good modulation of the reactivity of the polyvalent acrylamides.

1 Introduction

2 Metal-Free Annulations

2.1 Domino Reactions Triggered by a Michael Addition

2.2 Domino Reactions Triggered by an Aza-Michael Addition

2.3 Domino Processes Triggered by an Acylation Reaction

2.4 Domino Reactions Triggered by a Baylis–Hillman Reaction

2.5 Cycloadditions and Domino Reactions

2.6 Miscellaneous Domino Reactions

3 Metal-Triggered/Mediated Annulations

3.1 Zinc-Promoted Transformations

3.2 Rhodium-Catalyzed Functionalization/Annulation Cascades

3.3 Cobalt-Catalyzed Functionalization/Annulation Cascades

3.4 Ruthenium-Catalyzed Functionalization/Annulation Cascades

3.5 Iron-Catalyzed Functionalization/Annulation Cascades

3.6 Palladium-Catalyzed Functionalization/Annulation Cascades

3.7 Copper-Catalyzed Transformations

3.8 Transition Metals Acting in Tandem in Domino Processes

4 Radical Cascade Reactions

5 Conclusion

Isoindolinone Synthesis via One-Pot Type Transition Metal Catalyzed C-C Bond Forming Reactions

Isoindolinone structure is an important privileged scaffold found in a large variety of naturally occurring as well as synthetic, biologically and pharmaceutically active compounds. Owing to its crucial role in a number of applications, the synthetic methodologies for accessing this heterocyclic skeleton have received significant attention during the past decade. In general, the synthetic strategies can be divided into two categories: First, direct utilization of phthalimides or phthalimidines as starting materials for the synthesis of isoindolinones; and second, construction of the lactam and/or aromatic rings by different catalytic methods, including C-H activation, cross-coupling, carbonylation, condensation, addition and formal cycloaddition reactions. Especially in the last mentioned, utilization of transition metal catalysts provides access to a broad range of substituted isoindolinones. Herein, the recent advances (2010-2020) in transition metal catalyzed synthetic methodologies via formation of new C-C bonds for isoindolinones are reviewed.

Photo-biocatalytic Cascades: Combining Chemical and Enzymatic Transformations Fueled by Light

In the field of green chemistry, light - an attractive natural agent - has received particular attention for driving biocatalytic reactions. Moreover, the implementation of light to drive (chemo)enzymatic cascade reactions opens up a golden window of opportunities. However, there are limitations to many current examples, mostly associated with incompatibility between the enzyme and the photocatalyst. Additionally, the formation of reactive radicals upon illumination and the loss of catalytic activities in the presence of required additives are common observations. As outlined in this review, the main question is how to overcome current challenges to the exploitation of light to drive (chemo)enzymatic transformations. First, we highlight general concepts in photo-biocatalysis, then give various examples of photo-chemoenzymatic (PCE) cascades, further summarize current synthetic examples of PCE cascades and discuss strategies to address the limitations.

Seven-Step Synthesis of All-Nitrogenated Sugar Derivatives Using Sequential Overman Rearrangements

All-nitrogenated sugars (ANSs), in which all hydroxy groups in a carbohydrate are replaced with amino groups, are anticipated to be privileged structures with useful biological activities. However, ANS synthesis has been challenging due to the difficulty in the installation of multi-amino groups. We report herein the development of a concise synthetic route to peracetylated ANSs in seven steps from commercially available monosaccharides. The key to success is the use of the sequential Overman rearrangement, which enables formal simultaneous substitution of four or five hydroxy groups in monosaccharides with amino groups. A variety of ANSs are available through the same reaction sequence starting from different initial monosaccharides by chirality transfer of secondary alcohols. Transformations of the resulting peracetylated ANSs such as glycosylation and deacetylation are also demonstrated. Biological studies reveal that ANS-modified cholesterol show cytotoxicity against human cancer cell lines, whereas each ANS and cholesterol have no cytotoxicity.

Iodonium salts as efficient iodine(iii)-based noncovalent organocatalysts for Knorr-type reactions

Hypervalent iodine(iii)-derivatives display higher catalytic activity than other aliphatic and aromatic iodine(i)- or bromine(i)-containing substrates for a Knorr-type reaction of N-acetyl hydrazides with acetyl acetone to give N-acyl pyrazoles. The highest activity was observed for dibenziodolium triflate, for which 10 mol% resulted in the generation of N-acyl pyrazole from acyl hydrazide and acetyl acetone typically at 50 °C for 3.5-6 h with up to 99% isolated yields. 1H NMR titration data and DFT calculations indicate that the catalytic activity of the iodine(iii) is caused by the binding with a ketone.

Recent advances in organocatalytic asymmetric multicomponent cascade reactions for enantioselective synthesis of spirooxindoles

Catalytic asymmetric MCCRs for enantioselective synthesis of spirooxindoles by using chiral phosphoric acids, amines, bifunctional thiourea/squaramides and metal-based reagents as catalysts.

Enantioselective Synthesis of Pyrrolizidinone Scaffolds through Multiple-Relay Catalysis

A triple-tandem protocol for the synthesis of the pyrrolizidinone skeleton has been devised. It involves a cross metathesis-intramolecular aza-Michael reaction-intramolecular Michael addition tandem sequence, starting from N-pentenyl-4-oxo-2-alkenamides and conjugated ketones. In the presence of two cooperative catalysts, namely the second-generation Hoveyda-Grubbs catalyst and (R)-TRIP-derived BINOL phosphoric acid, this multiple-relay catalytic process takes place in good yields and outstanding levels of diastero- and enantioselectivity with the simultaneous generation of three contiguous stereocenters.

Combining photo-redox and enzyme catalysis for the synthesis of 4H-pyrimido[2,1-b] benzothiazole derivatives in one pot

A novel strategy combining visible-light and enzyme catalysis in one pot for the synthesis of 4H-pyrimido[2,1-b] benzothiazole derivatives from alcohols is described for the first time. Fourteen 4H-pyrimido[2,1-b] benzothiazole derivatives were prepared with yields of up to 98% under mild reaction conditions by a simple operation. The photoorgano catalyst rose Bengal (rB) was employed to oxyfunctionalise alcohols to aldehydes. Compared with aldehydes, alcohols with more stable properties and lower cost, thus we used photocatalysis to oxidize alcohols into aldehydes. Next, the enzyme was used to further catalyze the reaction of Biginelli to produce the target product of 4H-pyrimidine [2,1-b] benzothiazole. Experimental results show that this method provides a more efficient and eco-friendly strategy for the synthesis of 4H-pyrimido[2,1-b] benzothiazole derivatives.