Total synthesis of the strychnos alkaloid (+)-minfiensine: tandem enantioselective intramolecular Heck-iminium ion cyclization

Total Synthesis of the Norcembranoid Scabrolide B and Its Transformation into Sinuscalide C, Ineleganolide, and Horiolide

It was recognized only recently that the sister norcembranoids scabrolides A and B have notably different carbotricyclic scaffolds. Therefore, our synthesis route leading to scabrolide A could not be extended to its sibling. Rather, a conceptually new approach had to be devised that relied on a challenging intramolecular alkenylation of a ketone to forge the congested central cycloheptene ring at the bridgehead enone site; the required cyclization precursor was attained by a lanthanide-catalyzed Mukaiyama-Michael addition. The dissonant 1,4-oxygenation pattern was then installed by allylic rearrangement/oxidation of the enone, followed by suprafacial 1,3-transposition. Synthetic scabrolide B was transformed into sinuscalide C by dehydration and into ineleganolide by base-mediated isomerization/oxa-Michael addition, which has potential biosynthetic implications; under basic conditions, the latter compound converts into horiolide by an intricate biomimetic cascade.

Base-Controlled Palladium-Catalyzed Intramolecular 'One Substrate - Five Reactions' of 5-Benzyl-1-(2-halobenzyl)-2-phenyl-1H-pyrazol-3(2H)-ones

Achieving site-selectivity and chemoselectivity is enormously challenging for substrates with multi-reactive sites in organic reactions. One kind of model substrates, 5-benzyl-1-(2-halobenzyl)-2-phenyl-1H-pyrazol-3(2H)-ones with six reactive sites were chosen as the examples to probe their intramolecular four kinds of five reactions including C(sp³ )-H arylation, C(sp² )-H arylation, reductive Heck reaction, and domino Heck reaction/alkylation of aryl C(sp² )-H bonds through variation of the reaction conditions. Screening of the reaction conditions showed that the different bases controlled the palladium-catalyzed intramolecular site-selectivity and chemoselectivity of the substrates: (i) Cesium carbonate (Cs₂ CO₃ ) promoted the benzyl C(sp³ )-H arylation of the substrates providing dihydropyrazolo[1,5-b]isoquinolin-2(1H)-ones at 100 °C, and isomerization of the dihydropyrazolo[1,5-b]isoquinolin-2(1H)-ones gave isoquinoline derivatives at a higher temperature (140 °C); (ii) Sodium acetate (NaOAc) mediated the aryl C(sp² )-H arylation of the substrates affording seven-membered biphenyl N-heterocycles; (iii) Sodium dichloroacetate (Cl₂ HCCO₂ Na) facilitated occurrence of the reductive Heck reaction of the substrates affording 1H-pyrazolo[5,1-a]isoindol-2(8H)-ones; (iv) Sodium trifluoroacetate (F₃ CCO₂ Na) assisted performance of the domino Heck reaction/aryl C(sp² )-H alkylation of the substrates leading to the spiro heterocycles. The 'one substrate - multiple reactions - multiple products' strategy greatly reduces cost, increases diversity of products, provides more opportunity for screening of pharmaceutical molecules, and enriches modern organic synthetic chemistry.

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp³-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

Enantioselective Total Synthesis of 10-Desoxy Analogue of a Previously Reported Natural Peroxyguaidiol

Described herein is an enantioselective synthesis of an analogue of a previously reported guaiane endoperoxide isolated from aerial parts of Croton arboreous. The polycyclic framework of the target structure was constructed with the C-7 stereogenic center derived from L-(-)-carvone and other stereogenic centers installed via substrate chirality-induced asymmetric reactions, starting with the synthesis of the seven-membered ring through regioselective enolization of carvone, ring-expansion, and installation of a conjugated C═C bond. Further functionalization was then achieved through regioselective enolization, triflation, and installation of an isopropenyl group. During the synthesis, some exceptions to the well-known rules of "thermodynamic control" and "kinetic control" in enolization of asymmetric cyclic ketones were observed. In construction of the bridged five-membered and endoperoxy rings, a peroxycarbenium [3 + 2] cycloaddition reaction with alkenes was carried out with several alkenes-silyl-gem-dihydroperoxides of different relative configurations. However, no expected [3 + 2] products were observed. Finally, the five-membered ring was smoothly installed through an intramolecular Darzens reaction, and the peroxy functionality was introduced via a carbon-centered radical-mediated reaction with triplet oxygen, followed by an intramolecular etherification under acidic conditions. Comparison of the ¹H and ¹³C NMR spectra of the synthetic analogue and the natural product revealed that the latter was definitely not an endoperoxide.

Total Synthesis of Sarpagine Alkaloid (-)-Normacusine B

An asymmetric total synthesis of the sarpagine alkaloid (-)-normacusine B is presented. Salient features of this synthesis include a photocatalytic nitrogen-centered radical cascade reaction to assemble the tetrahydrocarbolinone skeleton, a titanium-mediated intramolecular amide-alkene coupling to construct the bridged azabicyclo[3.3.1]nonane moiety, and a nickel-catalyzed reductive Heck coupling to assemble the azabicyclo[2.2.2]octane ring system.

Structure units oriented approach towards collective synthesis of sarpagine-ajmaline-koumine type alkaloids

Sarpagine-Ajmaline-Koumine type monoterpenoid indole alkaloids represent a fascinating class of natural products with polycyclic and cage-like structures, interesting biological activities, and related biosynthetic origins. Herein we report a unified approach towards the asymmetric synthesis of these three types of alkaloids, leading to a collective synthesis of 14 natural alkaloids. Among them, akuammidine, 19-Z-akuammidine, vincamedine, vincarine, quebrachidine, vincamajine, alstiphylianine J, and dihydrokoumine are accomplished for the first time. Features of our synthesis are a new Mannich-type cyclization to construct the key indole-fused azabicyclo[3.3.1]nonane common intermediate, a SmI2 mediated coupling to fuse the aza-bridged E-ring, stereoselective olefinations to install either the 19-E or 19-Z terminal alkenes presented in the natural alkaloids, and an efficient iodo-induced cyclization to establish the two vicinal all-carbon quaternary centers in the Koumine-type alkaloids.

Asymmetric Organocatalytic Syntheses of Bioactive Compounds

Background:

The total syntheses of complex natural products have evolved to include new methodologies to save time, simplifying the form to achieve these natural compounds.

Objective:

In this review, we have described the asymmetric synthesis of different natural products and biologically active compounds of the last ten years until the current day.

Results:

An asymmetric organocatalytic reaction is a key to generate stereoselectively the main structure with the required stereochemistry.

Conclusion:

Even more remarkable, the organocatalytic cascade reactions, which are carried out with high stereoselectivity, as well as a possible approximation of the organocatalysts activation with sub-strates are also described.

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.

Palladium-Catalyzed Asymmetric Tandem Denitrogenative Heck/Tsuji-Trost of Benzotriazoles with 1,3-Dienes

The asymmetric denitrogenative cycloaddition has emerged as a powerful tool to build chiral aza-heterocyles. However, only one example of asymmetric denitrogenative cycloaddition of benzotriazole with unsaturated hydrocarbons has been explored so far, because the ring-opening of benzotriazole to generate α-imino metal carbenoid species is a thermodynamically unfavorable process. We herein report an efficient asymmetric denitrogenative cycloaddition of benzotriazoles with cyclic and acyclic 1,3-dienes enabled by Pd and chiral sulfonamide phosphine ligand. A variety of substituted hexahydrocarbazoles and indolines were delivered in good yields with high ee values. Interestingly, a pair of enantiomers could be obtained with the use of Xu1 and PC2 with the same absolute configuration. The synthetic utilities of the optically active hexahydrocarbazoles were also showcased.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Total Synthesis of (+)-Alsmaphorazine C and Formal Synthesis of (+)-Strictamine: A Photo-Fries Approach

A bioinspired photo-Fries/imine capture cascade reaction was developed in continuous-flow mode, which facilitated the rapid construction of a series of diversely functionalized 2,7-heterocycle-fused tetrahydrocarbazoles, the ubiquitous core structures embedded in strychnos and akuammiline-type monoterpene indole alkaloids. The synthetic utility of this novel method has been preliminarily explored by the first total synthesis of (+)-alsmaphorazine C and formal synthesis of (+)-strictamine in a concise and efficient manner.

Enantioselective formation of quaternary carbon stereocenters in natural product synthesis: a recent update

Covering: 2013-2018 Natural products bearing quaternary carbon stereocenters have attracted tremendous interest from the synthetic community due to their diverse biological activities and fascinating molecular architectures. However, the construction of these molecules in an enantioselective fashion remains a long-standing challenge because of the lack of efficient asymmetric catalytic methods for installing these motifs. The rapid progress in the development of new-generation efficient chiral catalysts has opened the door for several asymmetric reactions, such as Michael addition, dearomative cyclization, polyene cyclization, α-arylation, cycloaddition, allylation, for the construction of quaternary carbon stereocenters in a highly enantioselective fashion. These asymmetric catalytic methods have greatly facilitated the synthesis of complex natural products with improved output and overall efficiency. In this concise review, we highlight the progress in the last six years in complex natural product synthesis, in which at least one quaternary carbon stereocenter has been constructed via asymmetric catalytic technologies, with particular emphasis on the analysis of the stereochemical model of each enantioselective transformation.

Catalytic asymmetric preparation of pyrroloindolines: strategies and applications to total synthesis

Pyrroloindolines are widely present in natural products. In this review, we summarize state-of-the-art of catalytic asymmetric synthesis of pyrroloindolines, as well as related applications to natural products total synthesis.

Reductive Cross-Coupling of Vinyl Electrophiles

The synthesis of alkenes (olefins) is a central subject in the synthetic community. The transition-metal-catalyzed reductive cross-coupling of vinyl electrophiles has emerged as a promising tool to produce alkenes with improved flexibility, structural complexity, and functionality tolerance. In this review, we summarized the progress in this field with respect to cross-electrophile couplings and reductive Heck reactions using vinyl electrophiles.

1 Introduction

2 Cross-Electrophile Coupling of Vinyl Electrophiles

3 Reductive Heck Reaction of Vinyl Electrophiles

4 Summary and Outlook

Palladium-catalyzed asymmetric dearomative cyclization in natural product synthesis

Asymmetric catalysis is a rapidly growing field in modern organic chemistry and has been indispensable for the synthesis of enantioenriched materials to meet demands from the academies to pharmaceutical industries. Asymmetric dearomative cyclization catalyzed by transition metals has been a hot research area in the last decade. Fascinated by its ability to construct sterically hindered quaternary stereogenic center(s) through dearomatization and simultaneously forging new ring structure(s) through cyclization, palladium-catalyzed asymmetric dearomative cyclization has been applied to the synthesis of structurally complicated natural products and it is increasingly prevalent in the literature. In particular, the resultant product from dearomative cyclization, which usually carries one or more unsaturated C-C bond(s), allows further functional group transformations. Previously reported applications of palladium-catalyzed asymmetric dearomative cyclization in natural product synthesis are presented here and discussed in depth.

Enantioselective Total Synthesis of (+)-Corymine and (-)-Deformylcorymine

We report herein the first enantioselective total synthesis of akuammiline alkaloids (+)-corymine and (-)-deformylcorymine. Starting from commercially available N-nosyltryptamine, the target molecules are both achieved in 11 steps. Key elements of the design include (a) a copper-catalyzed enantioselective addition of dimethyl malonate to a 3-bromooxindole to secure the C7 all-carbon quaternary stereocenter, (b) a one-step construction of cyclohexyl and pyrrolidinyl rings via intramolecular nucleophilic C- and N-addition, and (c) a nickel-promoted 7-endo cyclization of alkenyl bromide to furnish the azepanyl ring. The strategy is further extended to the synthesis of another three members of the akuammiline family, namely, (-)-10-demethoxyvincorine, (-)-2(S)-cathafoline, and (-)-3-epi-dihydrocorymine 17-acetate.

Palladium-Catalyzed Reductive Heck Coupling of Alkenes

The Mizoroki-Heck reaction is one of the most studied palladium-catalyzed cross-coupling reactions, representing a powerful method for forming C-C bonds between diverse substrates with broad functional group compatibility. However, the reductive variant has received considerably less attention. In this Review, we summarize distinct mechanistic aspects of the reductive Heck reaction, highlight recent contributions to the field, and discuss potential applications of the reductive Heck reaction in the pharmaceutical industry. With the potential to have a large impact in both academic and industrial settings, further development of the reductive Heck reaction is a promising area of future investigation.

Efficient Enantioselective Syntheses of Chiral Natural Products Facilitated by Ligand Design

The employment of enantioselective transition-metal-catalyzed transformations as key steps in asymmetric natural product syntheses have attracted considerable attention in recent years owing to their versatile synthetic utilities, mild conditions and high efficiency in chirality generation. The chiral catalysts or supporting ligands are believed to be crucial for the requisite reactivity and enantioselectivity. Therefore, the rational design of chiral ligands is at the heart of developing new asymmetric transition-metal catalyzed reactions and provides an avenue to the asymmetric synthesis of natural products. Our group has been engaged in the development of transition-metal-catalyzed enantioselective cross-coupling, cyclization and other related reactions and the application of these methodologies to natural product syntheses. In this account, we summarized our recent synthetic efforts towards the efficient total syntheses of several different types of natural products including terpenes, alkaloids and polyketides facilitated by the design of a series of versatile P-chiral phosphorous ligands.

Total Syntheses of Echitamine, Akuammiline, Rhazicine, and Pseudoakuammigine

Echitamine (1) and akuammiline (2) are representative members of a fascinating class of monoterpenoid indole alkaloids. We report the syntheses of 2 and its congener deacetylakuammiline (3). The azabicyclo[3.3.1]nonane motif was assembled through silver-catalyzed internal alkyne cyclization, and one-pot C-O bond cleavage/C-N bond formation furnished the pentacyclic scaffold. Compound 3 then served as a common intermediate for preparing a series of structurally diverse and synthetically challenging congeners including 1. A position-selective Polonovski-Potier reaction followed by formal N-4 migration built the core of N-demethylechitamine (4) and 1. An alternative route featuring Meisenheimer rearrangement gave 4 as well. Oxidation of the alcohol within 3 gave rhazimal (5), which underwent tandem indolenine hydrolysis, hemiaminalization, and hemiketalization to form rhazicine (6). A sequence of N,O-ketalization and reductive amination secured the chemoselectivity of N-methylation, leading to pseudoakuammigine (7).

A 5 + 1 Protic Acid Assisted Aza-Pummerer Approach for Synthesis of 4-Chloropiperidines from Homoallylic Amines

We report that HCl·DMPU induces the formation of (thiomethyl)methyl carbenium ion from DMSO under mild conditions. Homoallylic amines react with this electrophile to generate 4-chloropiperidines in good yields. The method applies to both aromatic and aliphatic amines. The use of HCl·DMPU as both non-nucleophilic base and chloride source constitutes an environmentally benign alternative for piperidine formation. The reaction has a broad substrate scope, and the conditions offer good chemical yields with high functional group tolerance and scalability.

Palladium-Catalyzed Asymmetric Intramolecular Reductive Heck Desymmetrization of Cyclopentenes: Access to Chiral Bicyclo[3.2.1]octanes

A palladium-catalyzed asymmetric reductive Heck reaction of unactivated aliphatic alkenes, with eliminable β-hydrogen atoms, has been realized for the first time. A series of optically active bicyclo[3.2.1]octanes bearing chiral quaternary and tertiary carbon stereocenters were obtained in good yields with excellent enantioselectivities, exhibiting good functional-group tolerance and scalability. Moreover, deuterated optically active bicyclo[3.2.1]octanes were also obtained in high efficiency.

A metal-free desulfurizing radical reductive C–C coupling of thiols and alkenes

An intermolecular reductive C–C coupling of electrophilic alkyl radicals and alkenes has been developed.

Axially Chiral Cyclic Diphosphine Ligand-Enabled Palladium-Catalyzed Intramolecular Asymmetric Hydroarylation

In transition metal-catalyzed asymmetric synthesis, enantioselectivity strongly depends on the structures of chiral ligands, so the development of new chiral ligands is crucial. Here, an efficient and highly enantioselective palladium-catalyzed intramolecular hydroarylation has been developed, and a new kind of N-heterocycles, 1H-pyrazolo[5,1-a]isoindol-2(8H)-ones containing a quaternary stereocenter, was prepared in high yields and excellent enantiomeric excess values. The reaction was effectively catalyzed by palladium-diphosphine complexes with numerous functional group tolerance, in which the newly developed axially chiral cyclic diphosphine ligands played key roles in the reactivity and enantioselectivity of the substrates. We believe that the cyclic diphosphine ligands with adjustable dihedral angles will find wide application in asymmetric synthesis.

Recent applications of the 1,2-carbon atom migration strategy in complex natural product total synthesis

1,2-Carbon atom rearrangement has been broadly applied as a guiding strategy in complex molecule assembly. As it entails the carbon-carbon or carbon-heteroatom bond migration between two vicinal atoms, this type of reaction is capable of generating structural complexity through a molecular skeletal reorganization. This review will focus on recent employment of this strategy in the total synthesis of natural products, highlighting the exceptional utility of such synthetic methodologies in the construction of intricate carbocycles, heterocycles or structurally complex motifs from synthetically more accessible precursors.

Total Synthesis of (±)-Minfiensine via a Formal [3+2] Cycloaddition

(±)-Minfiensine (1) was synthesized in 10 steps in 26% overall yield with the 1,2,3,4-tetrahydro-9a,4a-iminoethanocarbazole core constructed through a [3+2] cycloaddition reaction between indole and an azaoxyallylic cation.

Cycloaddition/annulation strategies for the construction of multisubstituted pyrrolidines and their applications in natural product synthesis

Pyrrolidines are privileged substructures of numerous bioactive natural products and drugs.

Hydrotalcite-supported palladium nanoparticles as catalysts for the hydroarylation of carbon–carbon multiple bonds

Palladium nanoparticles supported on hydrotalcites catalyze the hydroarylation reaction of carbon–carbon multiple bonds.

Engaging 2-methyl indolenines in a tandem condensation/1,5-hydride transfer/cyclization process: construction of a novel indolenine–tetrahydroquinoline assembly

An efficient BF₃·Et₂O-catalyzed diastereoselective tandem condensation/1,5-hydride transfer/cyclization strategy for the synthesis of a novel indolenine–tetrahydroquinoline assembly was reported.