L-Amino acid ester as a biomimetic reducing agent for the reduction of unsaturated CC bonds

The first example of an efficient protocol for the reduction of disubstituted methyleneindolinones, isoindigos and tetrasubstituted olefins for the synthesis of 3-substituted 2-oxindoles, dihydroisoindigos and tetrasubstituted ethane derivatives using an L-amino acid ester as an attractive biomimetic reducing agent has been developed. This new protocol has the advantages of mild reaction conditions without the need for any metal catalysts, a broad substrate scope (31 examples), excellent yields (90-98%) and good functional group tolerance, providing an operationally simple and practically useful methodology for reductive reactions. The L-amino acid derivative, which is cheap, nontoxic and easy to handle, serves as a new biomimetic reducing agent for use in organic chemistry, providing a novel and promising approach for future applications in reductive reactions.

Total Synthesis of (+)- and (-)-Calycanthidine and Formal Synthesis of (-)-Idiospermuline via Key Pd(0)-Catalyzed Asymmetric Allylations

We envisioned a novel asymmetric strategy to access unsymmetrically substituted dimeric 2-oxindoles [(S,S)-8 and (R,R)-8] for the total synthesis of calycanthidine (4a). The key to success is the development of efficient Pd(0)-catalyzed asymmetric sequential allylations [via a highly enantioselective [up to 94% enantiomeric excess (ee)] and diastereoselective (up to ∼13:1) process] of unsymmetrically protected dimeric 2-oxindoles at the 3,3' position [such as (S,S)-8 and (R,R)-8]. Gratifyingly, a mixture of bis-ester (±)-10a, ester-carbonates (±)-10b and (±)-10c, and bis-carbonate 10d could afford (S,S)-8 and (R,R)-8 in highly stereoselective fashion, thereby culminating in the total synthesis of (+)-calycanthidine [ent-(4a)] and (-)-calycanthidine (4a). This effort also culminated in the formal total synthesis of idiospermuline (5).

New Entries in Organocatalysts from an Alkaloid Library; Development of Aminal Catalysis for a Michael Reaction Based on Calycanthine

Natural products have historically been actively evaluated for their biological activity in the development of pharmaceuticals, while their evaluation as asymmetric catalysts has rarely been explored. In this study, we evaluated the catalytic activity of the natural product library. Three naturally occurring alkaloids, gardnerine, spiradine A, and calycanthine, were found to catalyze an asymmetric Michael reaction using oxindole and nitrostyrene. We further studied (+)-calycanthine, which is characterized by its aminal structure. Concise synthetic and extraction protocols were developed to provide both enantiomers of calycanthine. Further derivatization of this alkaloid led to improved enantioselectivity in a model reaction. Computational studies suggested that the aminal moiety of the catalyst activated nucleophiles and electrophiles through multiple hydrogen bonding interactions, including nonclassical hydrogen bonds between carboxylic acid and the aminal C-H.

Total Synthesis of (+)- and (-)-Calycanthine by Means of Thio-Urea-Catalyzed Sequential Michael Reactions of Bis-oxindoles

A unified catalytic asymmetric approach to naturally occurring piperidinoindoline and pyrrololidinoindoline alkaloids has been realized via the development of a thio-urea-catalyzed sequential Michael addition of bis-oxindole onto nitroethylene (up to 93% ee and >20:1 dr). This strategy offers the total syntheses of either enantiomers of naturally occurring calycanthine.

Decarboxylative Allylic Alkylation of Phthalides: Stabilized Benzylic Nucleophiles for sp3-sp3 Coupling

A new family of stabilized benzylic nucleophiles for the palladium-catalyzed decarboxylative allylic alkylation reaction has been developed. Allyl esters derived from 3-carboxyphthalides were found to undergo palladium-catalyzed deallylation and decarboxylation under mild reaction conditions, a process facilitated by the formation of a stabilized aromatic anion. The regioselective allylic coupling of this intermediate afforded a variety of functionalized phthalides in 73-96% yields.

Concise total syntheses of bis(cyclotryptamine) alkaloids via thio-urea catalyzed one-pot sequential Michael addition

Naturally occurring bis(cyclotryptamine) alkaloids feature vicinal all-carbon quaternary stereocenters with an elongated labile C-3a-C-3a' Sigma bond with impressive biological activities. In this report, we have developed a thio-urea catalyzed one-pot sequential Michael addition of bis-oxindole onto selenone to access enantioenriched dimeric 2-oxindoles with vicinal quaternary stereogenic centers at the pseudobenzylic position (up to 96% ee and >20 : 1 dr). This strategy has been successfully applied for the total syntheses of either enantiomers of chimonanthine, folicanthine, and calycanthine.

Strategies for total synthesis of bispyrrolidinoindoline alkaloids

Covering up to 2021Complex cyclotryptamine alkaloids with a bispyrrolidino[2,3-b]indoline (BPI) skeleton are an intriguing family of natural products, exhibiting wide systematic occurrences, large structural diversity, and multiple biological activities. Based on their structural characteristics, BPI alkaloids can be classified into chimonanthine-type BPI alkaloids, BPI diketopiperazines, and BPI epipolythiodiketopiperazines. These intricate molecules have captivated great attention soon after their isolation and identification in the 1960s. Due to the structural complexity, the total synthesis of these cyclotryptamine alkaloids is challenging. Nevertheless, remarkable progress has been achieved in the last six decades; in particular, several methods have been successfully established for the construction of vicinal all-carbon quaternary stereocenters. In this review, the structural diversity and chemical synthesis of these BPI alkaloids were summarized. BPI alkaloids are mainly synthesized by the methods of oxidative dimerization, reductive dimerization, and alkylation of bisoxindole. The purpose of this review is to present overall strategies for assembling the BPI skeleton and efforts towards controlling the stereocenters.

Electrocatalysis as a key strategy for the total synthesis of natural products

Electrochemical strategies have been a powerful approach for the synthesis of valuable intermediates, in particular heterocyclic motifs. Because of the mild nature, a wide range of nonclassical bond disconnections have been achieved via in situ-generated radical intermediates in a highly efficient manner. In particular, anodic electrochemical oxidative strategies have been utilized for the total synthesis of many structurally intriguing natural products. In this review article, we have discussed a number of total syntheses of structurally intriguing alkaloids and terpenoids in which electrochemical processes play an important role as a key methodology.

The catalytic decarboxylative allylation of enol carbonates: the synthesis of enantioenriched 3-allyl-3'-aryl 2-oxindoles and the core structure of azonazine

The catalytic asymmetric synthesis of 3-allyl-3'-aryl 2-oxindoles has been shown via the Pd(0)-catalyzed decarboxylative allylation of allylenol carbonates. This methodology provides access to a variety of 2-oxindole substrates (5a-v) with all-carbon quaternary stereocenters (up to 94% ee) at the pseudobenzylic position under additive-free and mild conditions. The synthetic potential of this method was shown by the asymmetric synthesis of the tetracyclic core of the diketopiparazine-based alkaloid azonazine (11).

Pd(0)-Catalyzed Deacylative Allylations (DaA) Strategy and Application in the Total Synthesis of Alkaloids

Natural product synthesis has been the prime focus for the development of new carbon-carbon bond forming transformations. In particular, the construction of molecules with all-carbon quaternary centers remain one of the most facinating targets. In this regard, transition-metal catalyzed processes have gained imporatnce owing to their mild nature. Towards this, Pd(0)-catalyzed decarboxylative allylations (DcA) is worth mentioning and has emerged as a convenient method for synthesis of molecules even in their enantioenriched form. However, in order to have a flexible approach that facilitate rapid production of derivatives by utilizing commercially available allyl alcohols, the concept of Pd(0)-catalyzed deacylative allylations (DaA) methodology gains popularity. In these reactions, the transfer of an acyl group has a functional role in activating the allylic alcohol (proelectrophile) toward reaction with Pd(0)-catalysts. We present here an Account on newly conceptualized deacylative allylations (DaA) methodology and its applications in the synthesis of various intermediates and building blocks. Further, its potential in the total synthesis of naturally occurring alkaloids have been summarized in this personal account.

Oxidative electro-organic synthesis of dimeric hexahydropyrrolo-[2,3-b]indole alkaloids involving PCET: total synthesis of (±)-folicanthine

An efficient electrochemical oxidation strategy for the total synthesis of a dimeric hexahydropyrrolo[2,3-b]indole alkaloid, (±)-folicanthine (1b), has been envisioned. Control experiments suggest that a PCET pathway involving stepwise electron transfer followed by proton transfer (ET-PT) was involved in the key oxidative dimerization process.

Catalytic Enantioselective Alkylation of Prochiral Enolates

The asymmetric alkylation of enolates is a particularly versatile method for the construction of α-stereogenic carbonyl motifs, which are ubiquitous in synthetic chemistry. Over the past several decades, the focus has shifted to the development of new catalytic methods that depart from classical stoichiometric stereoinduction strategies (e.g., chiral auxiliaries, chiral alkali metal amide bases, chiral electrophiles, etc.). In this way, the enantioselective alkylation of prochiral enolates greatly improves the step- and redox-economy of this process, in addition to enhancing the scope and selectivity of these reactions. In this review, we summarize the origin and advancement of catalytic enantioselective enolate alkylation methods, with a directed emphasis on the union of prochiral nucleophiles with carbon-centered electrophiles for the construction of α-stereogenic carbonyl derivatives. Hence, the transformative developments for each distinct class of nucleophile (e.g., ketone enolates, ester enolates, amide enolates, etc.) are presented in a modular format to highlight the state-of-the-art methods and current limitations in each area.

Enantioselective [4 + 2] Cycloaddition/Cyclization Cascade Reaction and Total Synthesis of cis-Bis(cyclotryptamine) Alkaloids

The asymmetric catalytic synthesis of 3-cyclotryptamine substituted oxindoles through formal [4 + 2] cycloaddition/cyclization cascade is described. A wide range of cyclotryptamine derivatives were obtained in enantioenriched form under mild reaction conditions and were found to have potential anticancer activity. The strategy enables ready assembly of cyclotryptamine subunits at the C_3a-C_3a' positions with two quaternary stereogenic centers in cis-selectivity, leading to the concise synthesis of optically active cis-bis(hexahydropyrroloindole) and others of the cyclotryptamine alkaloid family.

Fragment Coupling Reactions in Total Synthesis That Form Carbon-Carbon Bonds via Carbanionic or Free Radical Intermediates

Fragment coupling reactions that form carbon-carbon bonds are valuable transformations in synthetic design. Advances in metal-catalyzed cross-coupling reactions in the early 2000s brought a high level of predictability and reliability to carbon-carbon bond constructions involving the union of unsaturated fragments. By comparison, recent years have witnessed an increase in fragment couplings proceeding via carbanionic and open-shell (free radical) intermediates. The latter has been driven by advances in methods to generate and utilize carbon-centered radicals under mild conditions. In this Review, we survey a selection of recent syntheses that have implemented carbanion- or radical-based fragment couplings to form carbon-carbon bonds. We aim to highlight the strategic value of these disconnections in their respective settings and to identify extensible lessons from each example that might be instructive to students.

KOH-mediated stereoselective alkylation of 3-bromooxindoles for the synthesis of 3,3′-disubstituted oxindoles with two contiguous all carbon quaternary centres

The stereoselective synthesis of 3,3′-disubstituted oxindoles having all-carbon quaternary stereocenters has been achieved using KOH as a base with an excellent diastereomeric ratio (98 : 2).

Catalytic enantioselective construction of vicinal quaternary carbon stereocenters

This review summarizes the advances in the catalytic enantioselective construction of vicinal quaternary carbon stereocenters, introduces major synthetic strategies and discusses their advantages and limitations, highlights the application of known protocols in the total synthesis of natural products, and outlines the synthetic opportunities.

This review summarizes the advances in catalytic enantioselective construction of vicinal quaternary carbon stereocenters, introduces major synthetic strategies and discusses their advantages and limitations, and outlines the synthetic opportunities.

Oxidative cyanation of 2-oxindoles: formal total synthesis of (±)-gliocladin C

Efficient oxidative direct cyanations of 3-alkyl/aryl 2-oxindoles using Cyano-1,2-BenziodoXol-3(1H)-one (CBX) (2a) have been reported under 'transition metal-free' conditions to synthesize a wide variety of 3-cyano 3-alkyl/aryl 2-oxindoles sharing an all-carbon quaternary center under additive-free conditions. The application of this process is shown by the formal total synthesis of (±)-gliocladin C (11c) in a few steps.

Thiourea-Catalyzed Enantioselective Malonate Addition onto 3-Sulfonyl-3'-indolyl-2-oxindoles: Formal Total Syntheses of (-)-Chimonanthine, (-)-Folicanthine, and (+)-Calycanthine

A general approach to bispyrroloindolines via a key thiourea-catalyzed addition of malonates to 3-sulfonyl-3'-indolyl-2-oxindoles is reported. The enantioselelective process is found to be highly effective (up to 94% ee), where a C-C bond formation leads to the synthesis of a number of 2-oxindoles with an all-carbon quaternary stereocenter.

Pd(0)-Catalyzed Chemoselective Deacylative Alkylations (DaA) of N-Acyl 2-Oxindoles: Total Syntheses of Pyrrolidino[2,3- b]indoline Alkaloids, (±)-Deoxyeseroline, and (±)-Esermethole

We report an efficient Pd(0)-catalyzed deacylative allylation of N-acyl 3-substituted 2-oxindoles via the coupling of in situ generated nucleophiles (3 and 4) with allyl electrophiles for the synthesis of a variety of 2-oxindoles with C3-quaternary centers. Gratifyingly, this alkylation process is found to be highly chemoselective in nature, where a C-C bond formation is completely predominant over a C-N bond formation. A variety of key intermediates were synthesized utilizing an aforementioned methodology.

A catalytic N-deacylative alkylation approach to hexahydropyrrolo[2,3-b]indole alkaloids

A versatile unprecedented strategy to diversely functionalized hexahydropyrrolo[2,3-b]indole alkaloids is described in high chemical yields. The synthesis features a key Pd(0)-catalyzed deacylative alkylation of N-acyl 3-substituted indoles using only 1 mol% of Pd(PPh3)4. The scope of this methodology is further defined in the asymmetric synthesis of pyrroloindolines using a diastereoselective approach.

Cu(ii)-tBu-PHOX catalyzed enantioselective malonate addition onto 3-hydroxy 2-oxindoles: application in the synthesis of dimeric pyrroloindoline alkaloids

An efficient Cu(ii)-PHOX-catalyzed malonate addition onto 3-hydroxy 3-indolyl-2-oxindoles is envisioned to afford excellent enantioselectivities (up to >99% ee) in high chemical yields. Detailed characterization techniques including X-ray, NMR, CV and EPR experiments suggest that a Cu(ii)-complex is involved as an active species in this process. Applying this strategy, an advanced intermediate of cyclotryptamine alkaloids has been synthesized in few steps for a general approach to bis-cyclotryptamine alkaloids.

Enantioselective Synthesis of Vicinal All-Carbon Quaternary Centers via Iridium-Catalyzed Allylic Alkylation

The development of the first enantioselective transition-metal-catalyzed allylic alkylation providing access to acyclic products bearing vicinal all-carbon quaternary centers is disclosed. The iridium-catalyzed allylic alkylation reaction proceeds with excellent yields and selectivities for a range of malononitrile-derived nucleophiles and trisubstituted allylic electrophiles. The utility of these sterically congested products is explored through a series of diverse chemo- and diastereoselective product transformations to afford a number of highly valuable, densely functionalized building blocks, including those containing vicinal all-carbon quaternary stereocenters.

Development of catalytic deacylative alkylations (DaA) of 3-acyl-2-oxindoles: total synthesis of meso-chimonanthine and related alkaloids

We present an effective deacylative alkylation strategy for the construction of a variety of 2-oxindoles bearing an all-carbon quaternary center at the pseudobenzylic position. A wide variety of products with quaternary centers could be accessed by employing simple Pd(0) catalysis under mild reaction conditions. Importantly, the same strategy works equally well for the dimeric 2-oxindole system, furnishing products with a vicinal quaternary center in favour of meso-isomer as the major product. Eventual application to the total syntheses of meso-chimonanthine and meso-folicanthine very well demonstrates the synthetic potential of this strategy.

Synthesis of 2-Oxindoles Sharing Vicinal All-Carbon Quaternary Stereocenters via Organocatalytic Aldol Reaction

An organocatalytic enantioselective aldol reaction using paraformaldehyde as C1-unit has been developed for the synthesis of 2-oxindoles sharing vicinal all-carbon quaternary stereocenters. The methodology is eventually employed in the formal total synthesis of (+)-folicanthine (1b).

Synthesis of 2-oxindoles via 'transition-metal-free' intramolecular dehydrogenative coupling (IDC) of sp(2) C-H and sp(3) C-H bonds

The synthesis of a variety of 2-oxindoles bearing an all-carbon quaternary center at the pseudo benzylic position has been achieved via a ‘transition-metal-free’ intramolecular dehydrogenative coupling (IDC). The construction of 2-oxindole moieties was carried out through formation of carbon–carbon bonds using KOt-Bu-catalyzed one pot C-alkylation of β-N-arylamido esters with alkyl halides followed by a dehydrogenative coupling. Experimental evidences indicated toward a radical-mediated path for this reaction.

Total Synthesis of Dimeric HPI Alkaloids

In this paper, we report a full account of the synthesis of dimeric hexahydropyrroloindole alkaloids and its analogues. The key feature of our new strategy is the novel catalytic copper (10 %) mediated intramolecular arylations of o-haloanilides followed by intermolecular oxidative dimerization of the resulting oxindoles in one pot. This sequential reaction leads to the key intermediates for the synthesis of (+)-chimonanthine, (+)-folicanthine, (-)-calycanthine and (-)-ditryptophenaline. In the presence of catalytic amount of cuprous iodide (10 %), an intramolecular arylation of o-haloanilides followed by an intermolecular oxidative dimerization of the resulting oxindoles leads to a common intermediate for the synthesis of (+)-chimonanthine, (+)-folicanthine and (-)-calycanthine. Based on this cascade sequence, we also developed a flexible strategy towards the asymmetric syntheses of dimeric HPI alkaloids (-)-ditryptophenaline and its analogues.