Palladium-catalyzed asymmetric construction of vicinal all-carbon quaternary stereocenters and its application to the synthesis of cyclotryptamine alkaloids

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.

Regiodivergent (3 + 2) annulation reactions of oxyallyl cations

We report a new method for the regiodivergent dearomative (3 + 2) reaction between 3-substituted indoles and oxyallyl cations. Access to both regioisomeric products is possible and is contingent on the presence or absence of a bromine atom on the substituted oxyallyl cation. In this way, we are able to prepare molecules that contain highly-hindered, stereodefined, vicinal, quaternary centers. Detailed computational studies employing energy decomposition analysis (EDA) at the DFT level establishes that regiochemical control arises from either reactant distortion energy or orbital mixing and dispersive forces, depending on the oxyallyl cation. Examination of the Natural Orbitals for Chemical Valence (NOCV) confirms that indole acts as the nucleophilic partner in the annulation reaction.

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.

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).

Copper-Catalyzed Cyclization/Dimerization of Tryptamines with O2/Air as the Sole Oxidant: Direct Access to Complex Bispyrrolidino[2,3-b]indoline

The first transition metal catalytic one-step synthesis of the 3a, 3a'-bispyrrolidino [2,3-b] indoline scaffold via tandem cyclization/dimerization of tryptamines has been realized with the environmentally friendly O₂/air as the sole oxidant. Different from the traditional direct oxidation of indole "N-H" group by excess amount of metal salts, a copper-catalyzed oxidative cyclization reaction is developed for the formation of the radical pyrrolidinoindoline intermediate in the current strategy. The robustness and practicality of this methodology is demonstrated by the step-economic, divergent total synthesis of natural products (±)-folicanthine and meso-folicanthine.

Diastereoselective Pd-Catalyzed Decarboxylative Allylation To Construct Quaternary Stereocenters Using Sulfinimine as the ­Directing Group

A robust Pd(0)-catalyzed decarboxylation alkylation has been developed to construct quaternary stereocenters through the use of chiral tert-butanesulfinamide as the directing group. This strategy provides a simple and efficient route to the construction of quaternary chiral centers at the α-position of chiral sulfinamides, with high diastereoselectivities and high yields.

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 Synthesis of Quaternary Carbon Stereocenters by Asymmetric Allylic Alkylation: A Review

Quaternary stereocenters are of great importance to the three-dimensionality and enhanced properties of new molecules, but the synthetic challenges in creating quaternary stereocenters greatly hinder their wide use in drug discovery, organic material design, and natural product synthesis. The asymmetric allylic alkylation (AAA) of allylic substrates has proven to be a powerful methodology for enantioselective formation of structure skeletons bearing single or more quaternary carbon centers in modern asymmetric organocatalysis. AAA has certain advantages in constructing the tetrasubstituted stereocenters, including but not limited to mild reactive conditions, effective reaction rates, new functional group introduction, and carbon chains length extension. This review outlines the key considerations in the application of AAA reactions and summarizes the recent progress of AAA reactions in the enantioselective synthesis of products containing quaternary stereocenters. Meanwhile, a detailed discussion of the AAA reactions such as ligands, scope of substrates, transformations and the general reaction mechanisms is also provided. We hope this review could stimulate further advances in much broader areas, including organic synthesis, asymmetric catalysis, C-H activation, and symmetrical pharmaceutical chemistry.

Construction of Vicinal Quaternary Carbon Stereocenters Through Diastereo- and Enantioselective Oxidative 1,6-Conjugate Addition

The asymmetric construction of vicinal quaternary carbon stereocenters with at least one moiety in acyclic systems is a formidable challenge. We disclose a solution involving diastereo- and enantioselective oxidative 1,6-conjugate addition. The practical asymmetric cross-dehydrogenative coupling of 2,2-diarylacetonitriles and diverse α-substituted cyclic 1,3-dicarbonyls proceeds, for vicinal quaternary carbon stereocenters with one center in acyclic systems, in excellent yields and stereoselectivities. The generality of the approach is further demonstrated by the stereoselective creation of vicinal quaternary carbon stereocenters with both centers in acyclic systems using acyclic β-ketoesters as coupling partners. Computational studies elucidate the origins of both diastereo- and enantioselectivity.

Construction of polycyclic structures with vicinal all-carbon quaternary stereocenters via an enantioselective photoenolization/Diels-Alder reaction

All-carbon quaternary stereocenters are ubiquitous in natural products and significant in drug molecules. However, construction of all-carbon stereocenters is a challenging project due to their congested chemical environment. And, when vicinal all-carbon quaternary stereocenters are present in one molecule, they will dramatically increase its synthetic challenge. A chiral titanium promoted enantioselective photoenolization/Diels-Alder (PEDA) reaction allows largely stereohindered tetra-substituted dienophiles to interact with highly active photoenolized hydroxy-o-quinodimethanes, delivering fused or spiro polycyclic rings bearing vicinal all-carbon quaternary centers in excellent enantiomeric excess through one-step operation. This newly developed enantioselective PEDA reaction will inspire other advances in asymmetric excited-state reactions, and could be used in the total synthesis of structurally related complex natural products or drug-like molecules for drug discovery.

Phosphine-Catalyzed Asymmetric Allylic Alkylation of Achiral MBH Carbonates with 3,3'-Bisindolines: Enantioselective Construction of Quaternary Stereogenic Centers

The asymmetric allylic alkylation (AAA) of achiral Morita-Baylis-Hillman (MBH) carbonates with 3,3'-bisindolines under the catalysis of amino-acid-derived bifunctional phosphines was accomplished. With the AAA approach introduced herein, challenging 3,3'-bisindolines bearing an all-carbon quaternary stereocenter (C3a) have been constructed in good yields with good to excellent enantioselectivties. In addition, the synthetic value of this protocol was demonstrated by the facile synthesis of the core skeleton of gliocladin C.

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.

NaI/TBHP-promoted reaction of indole-2-thiones with arylsulfonyl hydrazides: construction of achiral axial 3,3′-biindole-2,2′-dibenzenesulfonothioate derivatives

A new method for the synthesis of achiral axial 3,3′-biindole-2,2′-dibenzenesulfonothioate derivatives from indole-2-thiones with arylsulfonyl hydrazides promoted by NaI/TBHP is disclosed.

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).

Synthesis of Polycyclic Fused Indoline Scaffolds through a Substrate-Guided Reactivity Switch

Zn(II)-catalyzed divergent synthesis of functionalized polycyclic indolines through formal [3 + 2] and [4 + 2] cycloadditions of indoles with 1,2-diaza-1,3-dienes (DDs) is reported. The nature and type of substituents of substrates are found to act as a chemical switch to trigger two distinct reaction pathways and to obtain two different types of products upon the influence of the same catalyst. The mechanism of both [4 + 2] and [3 + 2] cycloadditions was investigated and fully rationalized by density functional theory (DFT) calculations.

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.

Discovery and Total Synthesis of a Bis(cyclotryptamine) Alkaloid Bearing the Elusive Piperidinoindoline Scaffold

Bis(cyclotryptamine) alkaloids have been popular topics of study for many decades. Five possible scaffolds for bis(cyclotryptamine) alkaloids were originally postulated in the 1950s, but only four of these scaffolds have been observed in natural products to date. We describe synthetic access to the elusive fifth scaffold, the piperidinoindoline, through syntheses of compounds now termed "dihydropsychotriadine" and "psychotriadine". The latter of these compounds was subsequently identified in extracts of the flower Psychotria colorata. Our synthetic route features a stereospecific solid-state photodecarbonylation reaction to introduce the key vicinal quaternary stereocenters.

Catalytic Asymmetric Allylic Amination with Isatins, Sulfonamides, Imides, Amines, and N-Heterocycles

A generally useful palladium-catalyzed method for the asymmetric allylic amination with a large variety of isatins, sulfonamides, imides, amines, and N-heterocycles is introduced. A single protocol with a readily available catalyst accomplishes this reaction at room temperature with high yields and enantioselectivities often exceeding 90%, which is demonstrated with 31 examples.

Enantioselective synthesis of mixed 3,3'-bisindoles via a phosphine-catalyzed umpolung γ-addition of 3'-indolyl-3-oxindoles to allenoates

An enantioselective umpolung γ-addition reaction of 3'-indolyl-3-oxindoles to allenoates catalyzed by amino acid-derived bifunctional phosphine catalysts has been developed. A wide range of chiral mixed 3,3'-bisindole scaffolds containing an all-carbon quaternary stereogenic center were obtained in high yields and with excellent enantioselectivities. 3,3'-Bisindoles are valuable synthetic intermediates, the employment of which led to formal total syntheses of (+)-Chimonanthine, (+)-Folicanthine and (-)-Calycanthine, as well as facile creation of useful pyrrolidinoindoline core structure.

Construction of 1-Tetralols Bearing Two Contiguous Quaternary Chiral Centers through a Rhodium-Catalyzed Enantioselective Desymmetrization Cascade Reaction

A novel and efficient access to polyfunctionnalized chiral 1-tetralols, bearing two contiguous quaternary carbon stereocenters, has been developed from various and easily accessible alkynyl-1,3-diketones, through a cascade process including a regioselective alkyne insertion, a 1,4-Rh shift, and a nucleophilic addition step via the desymmetrization of the 1,3-diketone moiety thanks to an appropriate rhodium-chiral diene complex in the presence of arylboronic acids.

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.

Rh-Catalyzed diastereo- and linear-selective α-allylation of chiral cycloenamines

In this paper, a Rh-catalyzed diastereoselective α-allylation of cycloenamines is demonstrated.

Synthetic Entry to Polyfunctionalized Molecules through the [3+2]-Cycloaddition of Thiocarbonyl Ylides

Here we present a comprehensive study on the [3+2]-cycloaddition of thiocarbonyl ylides with a wide variety of alkenes and alkynes. The obtained dihydro- and tetrahydrothiophene products serve as exceptionally versatile intermediates providing access to thiophenes, dienes, dendralenes, and vic-quarternary carbon centers. The use of high-pressure conditions enables thermally unstable, sterically encumbered or moderately reactive substrates to undergo the cycloaddition under mild conditions, thereby increasing the yield by up to 58%. In addition, we showcase its utility by the formal syntheses of the pharmaceuticals NGB 4420 and tenilapine.

Diastereoselective Rh-catalyzed decarboxylative allylation to form quaternary stereocenters using sulfinimine as the directing group

In this paper, we reported for the first time that the diastereoselective Rh-catalyzed decarboxylative allylation of chiral sulfinimines could be used to form quaternary stereocenters.

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.

N-Sulfonyl Bisimidazoline Ligands and Their Applications in Pd(II)-Catalyzed Asymmetric Addition toward α-Tertiary Amines

A new class of chiral N-sulfonyl bisimidazoline (Bim) ligands have been designed, prepared, and applied in Pd(II)-catalyzed asymmetric addition of arylboronic acids to isatin-derived N-Boc ketimines. The combination of Pd(OCOCF₃)₂ and N-tosyl Bim ligand shows high catalytic activity and excellent asymmetric induction, enabling asymmetric addition to offer α-tertiary amines with generally good to high yields and excellent enantioselectivity (up to 96% yield, 96% ee). This asymmetric Pd(II) catalysis can tolerate air conditions, providing a practical and operationally simple protocol toward the construction of an enantioenriched α-tertiary stereocenter.

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.