Stereoselective Total Synthesis of Eburnane-Type Alkaloids Enabled by Conformation-Directed Cyclization and Rearrangement

Total Synthesis of (+)-Eburnamonine Using Asymmetric Alkene Cyanoamidation through C-CN Bond Activation

We report the total synthesis of (+)-eburnamonine using enantioselective alkene cyanoamidation to form the all-carbon quaternary stereocenter. Palladium, phosphoramidite ligand, and a Lewis acid combine to form a co-catalyst that promotes C-CN activation of a cyanoformamide, followed by intramolecular alkene cyanoamidation. Overall, the synthesis of (+)-eburnamonine is accomplished in 8 steps from 4-methylene hexanoic acid and tryptamine, providing an example of asymmetric aliphatic-tethered alkene cyanoamidation and its use in total synthesis.

Arylative Ring Expansion of 3-Vinylazetidin-3-Ols and 3-Vinyloxetan-3-Ols to Dihydrofurans by Dual Palladium and Acid Catalysis

In contrast to the well-studied 1-vinylcyclobutanols, the reactivity of 3-vinylazetidin-3-ols 1 and 3-vinyloxetan-3-ols 2 under transition metal catalysis remains largely unexplored. We report herein their unique reactivity under dual palladium and acid catalysis. In the presence of a catalytic amount of Pd(OAc)2(PPh3)2, AgTFA and triflic acid, the reaction of 1 or 2 with aryl iodides affords 2,3,4-trisubstituted dihydrofurans, which are valuable heterocycles in organic synthesis. Mechanistic studies reveal that this arylative ring-expansion reaction proceeds via a domino process involving Heck arylation of alkene, acid-catalyzed transposition of allylic alcohol and ring opening of the azetidine/oxetane by an internal hydroxyl group.

Direct S0 → Tn Transition under Visible Light Irradiation Enabling Synthesis of a Pseudoindoxyl Scaffold

Pseudoindoxyl is a partial skeleton found in various natural products. Its light-absorption properties make it useful for the design of functional molecules. However, versatile synthesis methods have not yet been reported. In this report, we present a versatile synthetic method for pseudoindoxyls using the direct S0 → Tn transition under visible light irradiation. We also discuss the application of pseudoindoxyls as photocatalysts.

Iridium-Catalyzed Reductive (3+2) Annulation of Lactams Enabling the Rapid Total Synthesis of (±)-Eburnamonine

A reductive (3+2) annulation of lactams through iridium-catalyzed hydrosilylation and photoredox coupling with α-bromoacetic acid was developed. The iridium-catalyzed hydrosilylation of the lactam carbonyl group and subsequent elimination provide a transient cyclic enamine, which undergoes iridium-catalyzed photoredox coupling with α-bromoacetic acid in a one-pot process. The developed conditions show high functional-group tolerance and provide cyclic N,O-acetals containing a quaternary carbon center. The resulting N,O-acetals undergo a variety of acid-mediated nucleophilic addition reactions via iminium ions to give substituted cyclic amines. The developed sequence including reductive (3+2) annulation and acid-mediated nucleophilic addition was successfully applied to the four-step total synthesis of (±)-eburnamonine.

BiBr3 -Mediated Intramolecular Aza-Prins Cyclization of Aza-Achmatowicz Rearrangement Products: Asymmetric Total Synthesis of Suaveoline and Sarpagine Alkaloids

An intramolecular aza-Prins cyclization of aza-Achmatowicz rearrangement products was developed in which bismuth tribromide (BiBr3 ) plays a dual role as an efficient Lewis acid and source of the bromide nucleophile. This approach enables the facile construction of highly functionalized 9-azabicyclo[3.3.1]nonanes (9-ABNs), which are valuable synthetic building blocks and a powerful platform for the synthesis of a variety of alkaloid natural products and drug molecules. Suitable substrates for the aza-Prins cyclization include 1,1-disubstituted alkenes, 1,2-disubstituted alkenes, alkynes, and allenes, with good to excellent yields observed. Finally, we showcase the application of this new approach to the enantioselective total synthesis of six indole alkaloids: (-)-suaveoline (1), (-)-norsuaveoline (2), (-)-macrophylline (3), (+)-normacusine B (4), (+)-Na -methyl-16-epipericyclivine (5) and (+)-affinisine (6) in a total of 9-14 steps. This study significantly expands the synthetic utility of the aza-Achmatowicz rearrangement, and the strategy (aza-Achmatowicz/aza-Prins) is expected to be applicable to the total synthesis of other members of the big family of macroline and sarpagine indole alkaloids.

Streamlined Strategy for Scalable and Enantioselective Total Syntheses of the Eburnane Alkaloids

A general, concise, and efficient strategy for the enantioselective synthesis of the eburnane alkaloid family of natural products is disclosed. Specifically, 13 members of the natural product family were prepared from commercially available and inexpensive starting materials. The brevity and modularity of the route are largely on account of a two-phase synthesis logic and a key catalytic enantioconvergent cross-coupling to establish the C20 stereogenic center. The strategies described here are expected to facilitate in-depth biological studies and provide access to new anticancer eburnane analogues.

Synthesis of 3-Acyloxyindolenines by TiCl3-Mediated Reductive Cyclization of 2-(ortho-Nitroaryl)-Substituted Enol Esters

In the presence of TiCl₃, the reductive cyclization of tetrasubstituted enol esters bearing a 2-(ortho-nitroaryl) substituent affords 3-acyloxy-2,3-disubstituted indolenines in good yields. A domino process involving the partial reduction of nitro to a nitroso group followed by 5-center-6π-electrocyclization, 1,2-acyloxy migration, and the further reduction of the resulting nitrone intermediate accounts for the reaction outcome. The so-obtained indolenines are converted smoothly to 2,2-disubstituted oxindoles via a sequence of saponification and semipinacol rearrangement.

Rapid construction of indole-fused 8-10 membered lactones via a tandem reaction

An intramolecular anaerobic Mukaiyama hydration-initiated tandem reduction/condensation/acyl migration/aromatization reaction was developed, which enabled the rapid construction of indole-fused 8-10 membered lactones starting from cyclic 2-allyl-2-(2-nitrophenyl)-1,3-diketones. A nitro substituent in the substrates acted as both an oxygen source in the Mukaiyama hydration step and a nitrogen source in a tandem indole ring construction step. Our reaction features mild conditions, atom economy, and inexpensive reagents and it can be conveniently scaled up to a gram scale in modest yields. A rational reaction mechanism was also proposed based on previous reports and control experiments.

Stereoselective Total Syntheses of C18-Oxo Eburnamine-Vincamine Alkaloids

Here, we disclose the divergent total syntheses of representative C18-oxo eburnamine-vincamine alkaloids (+)-eburnaminol, (-)-larutenine, and (-)-cuanzine. Key to the approach is a substrate-controlled iridium-catalyzed asymmetric hydrogenation/lactamization cascade that leads to the formation of the common tetracyclic skeleton with essential cis-C20/C21 stereochemistry (93% yield, 98% ee, >20:1 dr, gram scale). Access to the targeted alkaloids is effected late in the synthesis by implementation of a number of diversity-oriented transformations and late-stage modifications.

Monoterpene indole alkaloids from Vinca minor L. (Apocynaceae): Identification of new structural scaffold for treatment of Alzheimer's disease

One undescribed indole alkaloid together with twenty-two known compounds have been isolated from aerial parts of Vinca minor L. (Apocynaceae). The chemical structures of the isolated alkaloids were determined by a combination of MS, HRMS, 1D, and 2D NMR techniques, and by comparison with literature data. The NMR data of several alkaloids have been revised, corrected, and missing data have been supplemented. Alkaloids isolated in sufficient quantity were screened for their in vitro acetylcholinesterase (AChE; E.C. 3.1.1.7) and butyrylcholinesterase (BuChE; E.C. 3.1.1.8) inhibitory activity. Selected compounds were also evaluated for prolyl oligopeptidase (POP; E.C. 3.4.21.26), and glycogen synthase 3β-kinase (GSK-3β; E.C. 2.7.11.26) inhibition potential. Significant hBuChE inhibition activity has been shown by (-)-2-ethyl-3[2-(3-ethylpiperidinyl)-ethyl]-1H-indole with an IC₅₀ value of 0.65 ± 0.16 μM. This compound was further studied by enzyme kinetics, along with in silico techniques, to reveal the mode of inhibition. This compound is also predicted to cross the blood-brain barrier (BBB) through passive diffusion.

Natural product total synthesis using rearrangement reactions

The synthetic utility of rearrangement reactions in total synthesis for the rapid construction of core skeletons, the precise control of stereochemistry, and the identification of suitable synthons has been discussed.

Asymmetric Total Syntheses of Schizozygane Alkaloids

The concise, collective, and asymmetric total syntheses of four schizozygane alkaloids, which feature a "Pan lid"-like hexacyclic core scaffold bearing up to six continuous stereocenters, including two quaternary ones, are described. A new method of dearomative cyclization of cyclopropanol onto the indole ring at C2 was developed to build the ABCF ring system of the schizozygane core with a ketone group. Another key skeleton-building reaction, the Heck/carbonylative lactamization cascade, ensured the rapid assembly of the hexacyclic schizozygane core and concurrent installation of an alkene group. By strategic use of these two reactions and through late-stage diversifications of the functionalized schizozygane core, the first and asymmetric total syntheses of (+)-schizozygine, (+)-3-oxo-14α,15α-epoxyschizozygine, and (+)-α-schizozygol and the total synthesis of (+)-strempeliopine have been accomplished in 11-12 steps from tryptamines.

α-Ketol and α-iminol rearrangements in synthetic organic and biosynthetic reactions

In the presence of a suitable acid or base, α-hydroxyaldehydes, ketones, and imines can undergo isomerization that features the 1,2-shift of an alkyl or aryl group. In the process, the hydroxy group is converted to a carbonyl and the aldehyde/ketone or imine is converted to an alcohol or amine. Such α-ketol/α-iminol rearrangements are used in a wide variety of synthetic applications including asymmetric synthesis, tandem reactions, and the total synthesis and biosynthesis of natural products. This review explores the use of α-ketol rearrangements in these contexts over the past two decades.

Total synthesis of the pseudoindoxyl class of natural products

The pseudoindoxyl sub-structural motif, amongst the large set of the indole class of alkaloids, represents a unique subset of the oxygenated indole class of the alkaloid family. A majority of this class of natural products contains complex bridged/polycyclic scaffolds with interesting biological profiles. They are thus attractive synthetic targets. Starting from 1963, twenty-eight natural products having the pseudoindoxyl scaffold have been isolated, among which the synthesis of 13 natural products has been accomplished. In this review, we highlight the completed as well as the formal total synthesis of the natural products with a spiro-pseudoindoxyl ring, with a focus on their development. The challenges and the future perspective based on the recent developments in the field will also be discussed. We strongly believe that this review will not only update but also attract the attention of researchers in dealing with the synthesis of pseudoindoxyl compounds.

Highly Enantioselective Iridium(I)-Catalyzed Hydrocarbonation of Alkenes: A Versatile Approach to Heterocyclic Systems Bearing Quaternary Stereocenters

We report a versatile, highly enantioselective intramolecular hydrocarbonation reaction that provides a direct access to heteropolycyclic systems bearing chiral quaternary carbon stereocenters. The method, which relies on an iridium(I)/bisphosphine chiral catalyst, is particularly efficient for the synthesis of five-, six- and seven-membered fused indole and pyrrole products, bearing one and two stereocenters, with enantiomeric excesses of up to >99 %. DFT computational studies allowed to obtain a detailed mechanistic profile and identify a cluster of weak non-covalent interactions as key factors to control the enantioselectivity.

The Enantioselective Synthesis of Eburnamonine, Eucophylline, and 16'-epi-Leucophyllidine

A synthetic approach to the heterodimeric bisindole alkaloid leucophyllidine is disclosed herein. An enantioenriched lactam building block, synthesized through palladium-catalyzed asymmetric allylic alkylation, served as the precursor to both hemispheres. The eburnamonine-derived fragment was synthesized through a Bischler-Napieralski/hydrogenation approach, while the eucophylline-derived fragment was synthesized by Friedländer quinoline synthesis and two sequential C-H functionalization steps. A convergent Stille coupling and phenol-directed hydrogenation united the two monomeric fragments to afford 16'-epi-leucophyllidine in 21 steps from commercial material.

Recent development and applications of semipinacol rearrangement reactions

As has been well-recognized, semipinacol rearrangement functions as an exceptionally useful methodology in the synthesis of β-functionalized ketones, creation of quaternary carbon centers, and construction of challenging carbocycles. Due to their versatile utilities in organic synthesis, development of novel rearrangement reactions has been a vibrant topic that continues to shape the research field. Recent breakthroughs in novel electrophiles, tandem processes, and enantioselective catalytic transformations further enrich the toolbox of this chemistry and spur the strategic applications of this methodology in natural product synthesis. These achievements will be discussed in this minireview.

Natural Product Synthesis Enabled by Domino Processes Incorporating a 1,2-Rearrangement Step

The art of natural product total synthesis is closely associated with two major determinants: the development/application of novel chemical reactions and the innovation in strategic use of classic organic reactions. While purposely seeking/applying a new synthetic methodology allowing nonconventional bond disconnections could shorten the synthetic route, the development of domino processes composed of a series of well-established reactions could also lead to a concise, practical, and aesthetically appealing synthesis. As an important class of textbook reactions, the 1,2-anionotropic rearrangements discovered at the dawn of modern organic chemistry have important bearings not only on chemical synthesis but also on the conceptual breakthroughs in the field. In its basic form, the 1,2-shift affords nothing but a constitutional isomer of the starting material and is therefore not a complexity-generating transformation. However, such a simple 1,2-shift could in fact change the molecular topology if the precursor is cleverly designed. More dramatically, it can metamorphosize the structure of the substrate when it is combined with other transformations in a domino sequence. In this Outlook, we highlight recent examples of natural product synthesis featuring a key domino process incorporating a 1,2-anionotropic rearrangement. Specifically, domino reactions integrating Wagner-Meerwein, pinacol, α-ketol, α-aminoketone, α-iminol, or benzilic acid rearrangements will be discussed.

An invocation for computational evaluation of isomerization transforms: cationic skeletal reorganizations as a case study

Covering: 2010 to 2020This review article describes how cationic rearrangement reactions have been used in natural product total synthesis over the last decade as a case study for the many productive ways by which isomerization reactions are enabling for synthesis. This review argues that isomerization reactions in particular are well suited for computational evaluation, as relatively simple calculations can provide significant insight.

α-Iminol Rearrangement Triggered by Pd-Catalyzed C-H Addition to Nitriles Sequences: Synthesis of Functionalized α-Amino Cyclopentanones

A α-iminol rearrangement triggered by Pd-catalyzed C-H addition of electronic-rich heteroarenes to cyclobutanone-derived O-acyl cyanohydrins was described, which provided a practical and efficient protocol for the preparation of functionalized α-amino cyclopentanones in an atom- and step-economic fashion. In addition, further synthetic transformations of products have also been demonstrated.

Unified divergent strategy towards the total synthesis of the three sub-classes of hasubanan alkaloids

Elegant asymmetric synthesis of hasubanan alkaloids have been developed over the past decades. However, a divergent approach leading to all three sub-classes of this family of natural products remains unknown. We report herein the realization of such an endeavor by accomplishing enantioselective total syntheses of four representative members. The synthesis is characterized by catalytic enantioselective construction of the tricyclic compounds from which three different intramolecular C-N bond forming processes leading to three topologically different hasubanan alkaloids are developed. An aza-Michael addition is used for the construction of the aza-[4.4.3]-propellane structure of (-)-cepharamine, whereas an oxidation/double deprotection/intramolecular hemiaminal forming sequence is developed to forge the bridged 6/6/6/6 tetracycle of (-)-cepharatines A and C and a domino bromination/double deprotection/cyclization sequence allows the build-up of the 6/6/5/5 fused tetracyclic structure of (−)-sinoracutine.

Several Hasubanan alkaloids have been synthesized in the past decades, however a divergent approach to access the 3 subclasses of such natural products has not been reported yet. Here, the authors show the enantioselective total syntheses of four representative members via a unified strategy leading to the three topologically different classes of alkaloids.

Harnessing the chemistry of 4aH-carbazoles: a consecutive rearrangements approach to carbazoles

We report a mild method for the synthesis of 4aH-carbazole intermediates with significant synthetic flexibility and variability, and demonstrate the controllable rearrangements of such intermediates to carbazoles.

Catalytic Asymmetric Acyloin Rearrangements of α-Ketols, α-Hydroxy Aldehydes, and α-Iminols by N,N'-Dioxide-Metal Complexes

A highly enantioselective acyloin rearrangement of cyclic α-ketols has been developed with a chiral Al(III)-N,N'-dioxide complex as catalyst. This strategy provided an array of optically active 2-acyl-2-hydroxy cyclohexanones in moderate to good yields with high enantioselectivities. The asymmetric isomerizations of acyclic α-hydroxy aldehydes and α-iminols were achieved as well under modified conditions, affording the corresponding chiral α-hydroxy ketones and α-amino ketones in moderate results. Moreover, further transformations of product to enantioenriched diols were carried out.

Towards the Sarpagine-Ajmaline-Macroline Family of Indole Alkaloids: Enantioselective Synthesis of an N-Demethyl Alstolactone Diastereomer

the strategy involving the use of functionalized tetrahydro-6H-cycloocta[b]indol-6-one is reported as a key intermediate for synthesis of members of the sarpagine-ajmaline-macroline family of monoterpene indole alkaloids. The desired tricycle was synthesized through the following key steps: 1) Evans' syn-selective aldolization; 2) Liebeskind-Srogl cross-coupling using the phenylthiol ester of 3-chloropropanoic acid as a surrogate of acrylic thioester for the synthesis of 2,3-disubstituted indoles; and 3) ring-closing metathesis (RCM) for the formation of the eight-membered ring. An N-allylation followed by intramolecular 1,4-addition was planned for synthesis of the vobasine class of natural products. However, attempted cyclizations under a diverse set of conditions involving anionic, radical, and organopalladium/organonickel species failed to produce the bridged ring system. On the other hand, esterification of the pendant primary alcohol function with acetoacetic acid, followed by intramolecular Michael addition, afforded the desired tetracycle with excellent diastereoselectivity. Subsequent functional group manipulation and transannular cyclization of the amino alcohol afforded the N(1)-demethyl-3,5-diepi-alstolactone. We believe that the same synthetic route would afford the alstolactone should the amino alcohol with appropriate stereochemistry be used as the starting material.

Stereocontrolled Synthesis of (±)-Melokhanine E via an Intramolecular Formal [3 + 2] Cycloaddition

A convergent sequence to access the indole alkaloid (±)-melokhanine E in 12-steps (8-step longest linear sequence) and an 11% overall yield is reported. The approach utilizes two cyclopropane moieties as reactive precursors to a 1,3-dipole and imine species to enable stereoselective construction of the core scaffold through a formal [3 + 2] cycloaddition. The natural product was evaluated for its antimicrobial activity based on isolation reports; however, no activity was observed. The reported efforts serve as a synthetic platform to prepare an array of alkaloids bearing this core structural motif.

Visible-light-induced intramolecular radical cascade of α-bromo-N-benzyl-alkylamides: a new strategy to synthesize tetracyclic N-fused indolo[2,1-a]isoquinolin-6(5H)-ones

Polycyclic indole scaffolds are ubiquitous in pharmaceuticals and natural products and in materials science. Here, we present a visible-light-initiated intramolecular aryl migration/desulfonylation/cyclization cascade reaction for the synthesis of tetracyclic indolo[2,1-a]isoquinolin-6(5H)-ones. This protocol not only exhibited a wide substrate scope but also provided a mild route to access a variety of tetracyclic N-fused indoles.

Synthesis of 2,3-disubstituted indoles via a tandem reaction

A wide array of 2,3-disubstituted indoles were accessed in modest to good yields via a tandem reduction/condensation/fragmentation/cyclization sequence. Differential fragmentation made the reaction more complicated.

One-pot synthesis of 3-hydroxy-2-oxindoles via acyloin rearrangements of 2-hydroxy-indolin-3-ones generated in situ from 2-alkynyl arylazides

A novel one-pot method to prepare 3-hydroxy-2-oxindoles via acyloin rearrangements of 2-hydroxy-indolin-3-ones generated in situ from 2-alkynyl arylazides has been described.

Lewis-Acid-Mediated Thiocyano Semipinacol Rearrangement of Allylic Alcohols for Construction of α-Quaternary Center β-Thiocyano Carbonyls

An electrophilic thiocyano semipinacol rearrangement of allylic alcohols has been achieved for the first time by using N-thiocyano-dibenzenesulfonimide (NTSI). This approach provides a direct, simple, and efficient strategy for the formation of thiocyano carbonyl compounds with moderate to excellent yields. Meanwhile, an all-carbon quaternary center was rapidly constructed. In addition, an asymmetric version of this tandem reaction was preliminarily investigated.

Synthesis of (+)-Tacamonine via Stereoselective Radical Cyclization

A concise, asymmetric synthesis of the indole alkaloid (+)-tacamonine is reported involving a stereoselective radical cyclization of a 1-phenylsulfanyl tetrahydro-β-carboline bearing a pendant enoate ester side chain as a key step. In this process, a single stereocenter in the side chain allows for the formation of two stereocenters of the natural product in a highly diastereoselective fashion. Computational investigations of this key cyclization support the experimentally observed outcome and shed light on the factors impacting its stereoselectivity.

Lewis Base/Brønsted Acid Co-catalyzed Enantioselective Sulfenylation/Semipinacol Rearrangement of Di- and Trisubstituted Allylic Alcohols

An enantioselective sulfenylation/semipinacol rearrangement of 1,1-disubstituted and trisubstituted allylic alcohols was accomplished with a chiral Lewis base and a chiral Brønsted acid as cocatalysts, generating various β-arylthio ketones bearing an all-carbon quaternary center in moderate to excellent yields and excellent enantioselectivities. These chiral arylthio ketone products are common intermediates with many applications, for example, in the design of new chiral catalysts/ligands and the total synthesis of natural products. Computational studies (DFT calculations) were carried out to explain the enantioselectivity and the role of the chiral Brønsted acid. Additionally, the synthetic utility of this method was exemplified by an enantioselective total synthesis of (-)-herbertene and a one-pot synthesis of a chiral sulfoxide and sulfone.

Indole Alkaloid Synthesis Facilitated by Photoredox Catalytic Radical Cascade Reactions

The monoterpene indole alkaloids, containing over 3000 known members and more than 40 structural types, represent one of the largest natural product families that have proven to be an important drug source. Their complex chemical structures and significant biological activities have rendered these alkaloids attractive targets in the synthetic community for decades. While chemists have developed many synthetic methodologies and tactics toward this end, general strategies allowing divergent access to a large variety of structural types and members of monoterpene indole alkaloids are still limited and highly desirable. Photoredox catalysis has emerged in recent years as a powerful tool to realize chemical transformations via single electron transfer (SET) processes that would otherwise be inaccessible. In particular, when the radical species generated by the visible light photoinduced approach is involved in well-designed cascade reactions, the formation of multiple chemical bonds and the assembly of structurally complex molecules would be secured in a green and economic manner. This protocol might serve to remodel the way of thinking for the preparation of useful pharmaceuticals and complex natural products. Due to a long-standing interest in the synthesis of diverse indole alkaloids, our group previously developed a cyclopropanation strategy ( Qin , Y. Acc. Chem. Res. 2011 , 44 , 447 ) that was versatile to access several intriguing indole alkaloid molecules. With an idea of developing more general synthetic approaches to as many members of various indole alkaloids as possible, we recently disclosed new radical cascade reactions enabled by photoredox catalysis, leading to the collective asymmetric total synthesis of 42 monoterpene indole alkaloids belonging to 7 structural types. Several important discoveries deserve to be highlighted. First, the use of photocatalytic technology allowed us to achieve an unusual reaction pathway that reversed the conventional reactivity between two nucleophilic amine and enamine groups. Second, a crucial nitrogen-centered radical, directly generated from a sulfonamide N-H bond, triggered three types of cascade reactions to deliver indole alkaloid cores with manifold functionalities and controllable diastereoselectivities. Moreover, expansion of this catalytic, scalable, and general methodology permitted the total synthesis of a large collection of indole alkaloids. In this Account, we wish to provide a complete picture of our studies concerning the original synthetic design, method development, and applications in total synthesis. It is anticipated that the visible-light-driven cascade strategy will find further utility in the realm of natural product synthesis.