Total Synthesis of (+)-Haplophytine

Total Synthesis of Njaoamine C by Concurrent Macrocycle Formation

In conceptual terms, the first total synthesis of the cytotoxic marine natural product njaoamine C differs from all known approaches toward related alkaloids of the manzamine superfamily in that both macrocyclic rings enveloping the diazatricyclic core are concomitantly formed; this goal was reached by double ring closing alkyne metathesis (dRCAM). The success of this maneuver does not merely reflect a favorable preorientation of the four alkyne chains that need to be concatenated in the proper pairwise manner but is also the outcome of dynamic covalent chemistry involving error correction by the chosen "canopy" molybdenum alkylidyne catalyst. The end game downstream of dRCAM capitalizes on the striking chemoselectivity of palladium-catalyzed hydrostannation, which selects for (hetero)arylalkynes even in the presence of sterically much more accessible dialkylalkynes or alkenes; for this preference, the method complements the classical repertoire of hydrometalation and semireduction reactions.

Cyclic Allene Approach to the Manzamine Alkaloid Keramaphidin B

We report an approach to the core of the manzamine alkaloid keramaphidin B that relies on the strain-promoted cycloaddition of an azacyclic allene with a pyrone trapping partner. The cycloaddition is tolerant of nitrile and primary amide functional groups and can be complemented with a subsequent retro-Diels-Alder step. These efforts demonstrate that strained cyclic allenes can be used to build significant structural complexity and should encourage further studies of these fleeting intermediates.

Concise Total Synthesis of (+)-Pleiocarpamine and Convergent Total Syntheses of (+)-Voacalgine A and (+)-Bipleiophylline via an Aerobic Oxidative Coupling

The stereocontrolled total synthesis of (+)-pleiocarpamine and the total syntheses of (+)-voacalgine A and (+)-bipleiophylline have been achieved. The scalable and concise 10-step synthesis of (+)-pleiocarpamine features construction of stereochemistry at the C16 position by radical cyclization and that of the highly strained cage-like structure via Pd-catalyzed intramolecular aromatic C-H functionalization. By modifying the biomimetic aerobic oxidative coupling of tryptophane derivatives catalyzed by FePc(CO₂H)₈, the oxidative coupling of the synthesized (+)-pleiocarpamine with pyrocatechuic acid was established to produce (+)-voacalgine A. The total synthesis of (+)-bipleiophylline was completed by the second coupling of (+)-voacalgine A with (+)-pleiocarpamine or one-pot couplings of 2 equiv of (+)-pleiocarpamine with pyrocatechuic acid.

Iron-Catalyzed Cross-Coupling of Thioesters and Organomanganese Reagents

We report a Fukuyama-type coupling of thioesters with aliphatic organomanganese reagents utilizing a cheap and easily available iron(III) precatalyst. The reactions exhibit a wide tolerance of solvents and functional groups, allowing for the conversion of thioesters derived from natural products and pharmaceutical compounds. A strong steric impact from each reaction component (carboxylic moiety, thiol substituent and manganese reagent) was displayed, which enabled regioselective transformation of dithioesters. Mechanistic investigations showed that the released thiolate does not act as a mere spectator ligand, but rather positively influences the stability of intermediate alkyl(II)ferrates.

Formal Total Syntheses of (+)- and (-)-Aspidophytine from a Common, Homochiral Precursor

A formal total synthesis of (-)-aspidophytine (2), a key substructure associated with the heterodimeric indole alkaloid haplophytine (1) and itself a natural product, has been established by employing the homochiral and enzymatically derived cis-1,2-dihydrocatechol 8 as a starting material. Specifically, compound 8 has been converted into the pentacyclic product 26, an advanced intermediate associated with a previously reported synthesis of aspidophytine (2). Simple modifications to the reaction sequence have also allowed for the identification of a synthetic pathway leading from dihydrocatechol 8 to (+)-aspidophytine (ent-2).

Enantioselective Total Synthesis of (-)-Limaspermidine and (-)-Kopsinine by a Nitroaryl Transfer Cascade Strategy

We report an intramolecular conjugate addition/Truce-Smiles/E1cb cascade of 2-nitrobenzenesulfonamide-functionalized cyclohexenones as a new entry to the core scaffold of monoterpene indole alkaloids. The method was applied to the asymmetric total synthesis of (-)-limaspermidine, (-)-kopsinilam, and (-)-kopsinine, as well as the framework of the kopsifoline alkaloids, thus highlighting its complementarity to existing approaches involving the use of indole-based starting materials or the interrupted Fischer indole synthesis. Furthermore, we show that the cascade tolerates various substituents on the nitroarene, opening the way to other natural products as well as non-natural analogues.

Ortho-selective C–H arylation of phenols with N-carboxyindoles under Brønsted acid- or Cu(i)-catalysis

Control over chemo- and regioselectivity is a critical issue in the heterobiaryl synthesis via C–H oxidative coupling. To address this challenge, a strategy to invert the normal polarity of indoles in the heterobiaryl coupling was developed. With N-carboxyindoles as umpoled indoles, an exclusively ortho-selective coupling with phenols has been realized, employing a Brønsted acid- or Cu(i)-catalyst (as low as 0.01 mol%). A range of phenols and N-carboxyindoles coupled with exceptional efficiency and selectivity at ambient temperature and the substrates bearing redox-active aryl halides (–Br and –I) smoothly coupled in an orthogonal manner. Notably, preliminary examples of atropselective heterobiaryl coupling have been demonstrated, based on a chiral disulfonimide or a Cu(i)/chiral bisphosphine catalytic system. The reaction was proposed to occur through S_N2′ substitution or a Cu(i)–Cu(iii) cycle, with Brønsted acid or Cu(i) catalysts, respectively.

Control over chemo- and regioselectivity is a critical issue in the heterobiaryl synthesis via C–H oxidative coupling. To address this challenge, a strategy to invert the normal polarity of indoles was developed.

Visible-Light-Mediated Cross Dehydrogenative Coupling of Thiols with Aldehydes: Metal-Free Synthesis of Thioesters at Room Temperature

Thioesters play a crucial role in biological systems and serve as important building blocks for organic synthesis. Herein, Eosin Y and TBHP mediated photochemical cross dehydrogenative coupling (PCDC) between feedstock aldehydes and thiols has been described at room temperature to synthesize thioesters. This thioesterification protocol proceeds smoothly to give the desired products in good to excellent yields by the suitable PCDC of both alkyl/aryl- aldehydes with a variety of alkyl/aryl-thiols and generates water and ^tBuOH as green byproducts. This method is also found to be scalable with good efficiency. Mechanistic investigations reveal that under this photochemical condition, the formation of acyl radical can be achieved from aldehyde. This acyl radical was further intercepted with an intermediate disulfide, generated in situ via the dehydrogenation of thiol to give the desired thioester. Moreover, disulfides, which are relatively easier to handle, also provided good to excellent yields in the optimized reaction condition. This protocol was further extended toward the more challenging direct transformation of alcohols to thioesters.

Synthesis of C5-Allylindoles through an Iridium-Catalyzed Asymmetric Allylic Substitution/Oxidation Reaction Sequence of N-Alkyl Indolines

Iridium/Brønsted acid cooperative catalyzed asymmetric allylic substitution reactions at the C5 position of indolines have been reported for the first time. The highly efficient protocol allows rapid access to various C5-allylated products in good to high yields (48-97%) and enantioselectivities (82% to >99% ee) with wide functional group tolerance. The transformations allow not only the formation of C5-allylindoline derivatives but also the synthesis of C5-allylindole analogues in good yields and excellent stereoselectivities via an allylation/oxidation reaction sequence.

Intramolecular Diels–Alder Reactions of Oxazoles, Imidazoles, and Thiazoles

The development of the intramolecular Diels–Alder cycloaddition­ of azole heterocycles, i.e. oxazoles (IMDAO), imidazoles (IMDAI), and thiazoles (IMDAT), has had a significant impact on the efficient preparation of heterocyclic intermediates and natural products. In particular, highly efficient and versatile IMDAO reactions have been utilized as a key step in several synthetic schemes to provide alkaloids and terpenoid target molecules. More limited studies have been performed on IMDAI and IMDAT cycloadditions. Some drawbacks, such as the occasionally­ challenging preparation of IMDA precursors, are also highlighted in this review. Perspectives are provided on how IMDAI and IMDAT­ transformations can be further expanded for target-directed syntheses.

1 Introduction

2 Oxazoles

2.1 IMDAO Approaches to Furanosesquiterpenes and Furanosteroids

2.1.1 Syntheses of Highly Oxygenated Sesquiterpenes

2.1.2 Syntheses of (±)-Gnididione and (±)-Isognididione

2.1.3 Synthesis of (±)-Stemoamide

2.1.4 Synthesis of (±)-Paniculide A

2.1.5 Syntheses of (+)- and (–)-Norsecurinine

2.1.6 Synthesis of Evodone

2.1.7 Syntheses of (±)-Ligularone and (±)-Petasalbine

2.1.8 Syntheses of Imerubrine, Isoimerubrine, and Grandirubrine

2.1.9 Syntheses of Furanosteroids

2.1.10 Syntheses of Substituted Indolines and Tetrahydroquinolines

2.2 IMDAO Approaches to Pyridines: the Kondrat’eva Reaction

2.2.1 Syntheses of Suaveoline and Norsuaveoline

2.2.2 Synthesis of Eupolauramine

2.2.3 Syntheses of (–)-Plectrodorine and (+)-Oxerine

2.2.4 Synthesis of Amphimedine

2.2.5 Synthetic Approach to the Western Segment of Haplophytine

2.2.6 Synthesis of Marinoquinoline A

2.2.6.1 IMDAO Approach to Marinoquinoline A

2.2.6.2 Scope of Allenyl IMDAO Cycloaddition

2.3 Lewis Acid Catalysis in IMDAO Reactions

2.3.1 Effects of Europium Catalysts on IMDAO Reactions

2.3.2 Effects of Copper Catalysts on IMDAO Reactions

3 Imidazoles

4 Thiazoles

4.1 Syntheses of Menthane and Eremophilane

4.2 Further Comments on the Intramolecular Cycloadditions of Thiocarbonyl Ylides

5 Conclusions and Outlook

Pd-Catalyzed Asymmetric Dearomatization of Indoles via Decarbonylative Heck-Type Reaction of Thioesters

We report herein a palladium-catalyzed ligand-promoted asymmetric dearomatization of indoles via the decarbonylation of thioesters and the subsequent reductive Heck reaction. This protocol provides a facile and efficient way to construct an aza-quaternary stereocenter at the C2 position of indolines. A variety of functional groups and substitutions could be well tolerated, affording the substituted indolines with high enantioselectivities.

Synthetic Approaches to Tricyclic Aminoketones in the Total Synthesis of Aspidosperma and Kopsia Alkaloids

Aspidosperma and kopsia alkaloids are significant functional molecules because of their potent biological activities. Their intricate structures present an intrinsic synthetic challenge and thus attract significant attention from synthetic organic academic community. Over the past decades, a series of elegant strategies has been developed, in particular, the Stork's original Fischer indolization of tricyclic aminoketones 1. Herein, we report a comprehensive review on various synthetic approaches access to tricyclic aminoketones 1 and provide a practical guidance to readers whose are interested in employing tricyclic aminoketones 1 as versatile building blocks in the realm of total synthesis of aspidosperma, kopsia and structurally related alkaloids.

Synthesis and sequential diastereoselective incorporation of hydroxyl groups into hexahydrofuro[3,2-f]indolizin-7(2H)-one to give mono-, di- and tetra-hydroxyfuroindolizidines

Dihydrofuro[2,3-f]indolizidinone obtained from biosourced reagents even at multigram-scale was used as an advanced building-block with up to five points of chemical diversification. This resulted in the sequential synthesis of a series of mono-, di- and tetra-hydroxyfuranoindolizidines belonging to a very scarce and elaborate tetrahydrofuran-fused indolizidine family with up to six controlled stereogenic centers. These sequences include, among others, diastereoselective olefin epoxidation, stereoselective epoxide ring opening into tetrahydrofuran trans-diols, their protection as an ester or acetonide, and lactam carbonyl reduction ultimately followed by acetate or acetonide deprotection.

Synthetic Studies toward Dimeric Indole Alkaloids Based on Convergent Synthetic Strategy

The total syntheses of dimeric indole alkaloids, haplophytine, and T988s are described. These dimeric compounds comprising two structurally different indole units are ubiquitous in nature, and many possess pharmaceutically important activities. To realize an efficient chemical synthesis of these dimeric indole alkaloids, the establishment of convergent synthetic strategies and development of new coupling methods are indispensable. The linkage of two highly functionalized units at a late stage of the synthesis frequently induces synthetic problems such as chemoselectivity and steric repulsion. Moreover, although transition metal-catalyzed reactions are usually an effective method for the cross-coupling of two units, the application of these cross-coupling reactions to bond formation involving a sterically hindered C(sp³) is often difficult. Thus, even with precise modern synthetic methods, it is currently difficult to realize convergent syntheses of dimeric indole alkaloids possessing a quaternary carbon linking two units. To combat these synthetic problems, we developed a synthetic method to link two indole units using an Ag-mediated nucleophilic substitution reaction. In this review, we provide a detailed discussion of convergent synthetic strategies and coupling methods for dimeric indole alkaloids.

Regio- and diastereoselective dearomatizations of N-alkyl activated azaarenes: the maximization of the reactive sites

An unprecedented base-promoted multi-component one-pot dearomatization of N-alkyl activated azaarenes was developed, which enabled the synthesis of complex and diverse bridged cyclic polycycles with multiple stereocenters in a highly regio- and diastereoselective manner. Besides, we realized the step-controlled dearomative bi- and trifunctionalization of quinolinium salts. These transformations not only achieved the maximization of the reaction sites of pyridinium, quinolinium and isoquinolinium salts to enhance structural complexity and diversity, but also opened up a new reaction mode of these N-activated azaarenes. A unique feature of this strategy is the use of easily accessible and bench-stable N-alkyl activated azaarenes to provide maximum reactive sites for dearomative cascade cyclizations. In addition, the salient characteristics including high synthetic efficiency, short reaction time, mild conditions and simple operation made this strategy particularly attractive.

An unprecedented base-promoted multi-component one-pot dearomatization of N-alkyl activated azaarenes was developed to construct complex and diverse bridged cyclic polycycles with multiple stereocenters in a highly regio- and diastereoselective manner.

Applications of Friedel-Crafts reactions in total synthesis of natural products

Over the years, Friedel-Crafts (FC) reactions have been acknowledged as the most useful and powerful synthetic tools for the construction of a special kind of carbon-carbon bond involving an aromatic moiety. Its stoichiometric and, more recently, its catalytic procedures have extensively been studied. This reaction in recent years has frequently been used as a key step (steps) in the total synthesis of natural products and targeted complex bioactive molecules. In this review, we try to underscore the applications of intermolecular and intramolecular FC reactions in the total syntheses of natural products and complex molecules, exhibiting diverse biological properties.

Enantioselective Total Syntheses of Methanoquinolizidine-Containing Akuammiline Alkaloids and Related Studies

The akuammiline alkaloids are a structurally diverse class of bioactive natural products isolated from plants found in various parts of the world. A particularly challenging subset of akuammiline alkaloids are those that contain a methanoquinolizidine core. We describe a synthetic approach to these compounds that has enabled the first total syntheses of (+)-strictamine, (-)-2( S)-cathafoline, (+)-akuammiline, and (-)-Ψ-akuammigine. Our strategy relies on the development of the reductive interrupted Fischer indolization reaction to construct a common pentacyclic intermediate bearing five contiguous stereocenters, in addition to late-stage formation of the methanoquinolizidine framework using a deprotection-cyclization cascade. The total syntheses of (-)-Ψ-akuammigine and (+)-akuammiline mark the first preparations of akuammiline alkaloids containing both a methanoquinolizidine core and vicinal quaternary centers. Lastly, we describe the bioinspired reductive rearrangements of (+)-strictamine and (+)-akuammiline to ultimately provide (-)-10-demethoxyvincorine and a new analogue thereof.

Synthesis of a Highly Functionalised and Homochiral 2-Iodocyclohexenone Related to the C-Ring of the Polycyclic, Indole Alkaloids Aspidophytine and Haplophytine

The enzymatically-derived and enantiomerically pure (1S,2S)-3-bromocyclohexa-3,5-diene-1,2-diol (7) has been elaborated over 10 steps into cyclohexenone 8. The latter compound embodies the enantiomeric form of the C-ring associated with the hexacyclic framework of the alkaloid aspidophytine (2). As such, this work sets the stage for effecting the conversion of the enantiomeric metabolite ent-7 into compound ent-8, and thence, through previously established protocols, including a palladium-catalysed Ullmann cross-coupling reaction, into the title alkaloids.

Metamorphosis of cycloalkenes for the divergent total synthesis of polycyclic indole alkaloids

This review summarizes the divergent synthesis of monoterpene indole alkaloids using cycloalkene as the turning point of structural diversity.

Understanding and Interrupting the Fischer Azaindolization Reaction

Experimental and computational studies pertaining to the Fischer azaindolization reaction are reported. These studies explain why pyridylhydrazines are poorly reactive in Fischer indolization reactions, in addition to the origin of hydrazine substituent effects. Additionally, an interrupted variant of Fischer azaindolization methodology is disclosed, which provides a synthetic entryway into fused azaindoline scaffolds.

Catalytic asymmetric chemodivergent arylative dearomatization of tryptophols

The first catalytic asymmetric arylative dearomatization of tryptophols has been established. By using quinone imine ketals as aryl group surrogates and modulating the reaction conditions, the cascade reaction of tryptophols with quinone imine ketals afforded two series of arylative dearomatization products in generally high yields, and excellent diastereo- and enantioselectivities.

Pardon the Interruption: A Modification of Fischer's Venerable Reaction for the Synthesis of Heterocycles and Natural Products

This account provides an overview of our laboratory's studies of an unusual variant of the Fischer indolization reaction. We describe the discovery of the so-called 'interrupted Fischer indolization' and the development of the reaction from a methodological standpoint. In addition, our efforts to evaluate and apply this methodology in the context of akuammiline alkaloid total synthesis are discussed.

Studies Directed toward the Synthesis of Aspidophytine: Construction of Its Perhydroquinoline Core

We have developed an efficient route for the synthesis of the perhydroquinoline core of the indole alkaloid aspidophytine (2), starting from commercially available and inexpensive 3-acetylpyridine. This densely functionalized perhydroquinoline core displays four contiguous stereocenters including an all-carbon quaternary center. The synthetic sequence features a highly effective Diels-Alder reaction using a carbamate-substituted siloxy diene accompanied by a spontaneous intramolecular substitution of the newly formed 3°-alkyl bromide with a carbamate group. The installation of the electron-rich aniline moiety was accomplished via a TBSOTf-mediated intramolecular aza-Michael reaction, and the relative stereochemistry of the aza-Michael product (30) was confirmed by X-ray crystallographic analysis. Among the useful transformations that were developed through this study is a highly enantioselective Diels-Alder reaction of a versatile cyclic carbamate siloxy diene.

Concise Total Syntheses of (+)-Haplocidine and (+)-Haplocine via Late-Stage Oxidation of (+)-Fendleridine Derivatives

We report the first total syntheses of (+)-haplocidine and its N1-amide congener (+)-haplocine. Our concise synthesis of these alkaloids required the development of a late-stage and highly selective C-H oxidation of complex aspidosperma alkaloid derivatives. A versatile, amide-directed ortho-acetoxylation of indoline amides enabled our implementation of a unified strategy for late-stage diversification of hexacyclic C19-hemiaminal ether structures via oxidation of the corresponding pentacyclic C19-iminium ions. An electrophilic amide activation of a readily available C21-oxygenated lactam, followed by transannular cyclization and in situ trapping of a transiently formed C19-iminium ion, expediently provided access to hexacyclic C19-hemiaminal ether alkaloids (+)-fendleridine, (+)-acetylaspidoalbidine, and (+)-propionylaspidoalbidine. A highly effective enzymatic resolution of a non-β-branched primary alcohol (E = 22) allowed rapid preparation of both enantiomeric forms of a C21-oxygenated precursor for synthesis of these aspidosperma alkaloids. Our synthetic strategy provides succinct access to hexacyclic aspidosperma derivatives, including the antiproliferative alkaloid (+)-haplocidine.

Total syntheses of (±)-spiroindimicins B and C enabled by a late-stage Schöllkopf-Magnus-Barton-Zard (SMBZ) reaction

The spiroindimicins are a family of structurally unprecedented alkaloids isolated from the deep-sea-derived marine actinomycete Streptomyces sp. SCSIO 03032. The total syntheses of (±)-spiroindimicins B and C are disclosed, the first of any member of this family. Central to the successful strategy was installing the spirocentre using a mild intramolecular Heck reaction, the assembly of a pentacyclic spirobisindole by Fischer indolization and a late-stage Schöllkopf-Magnus-Barton-Zard (SMBZ) reaction to construct the trisubstituted pyrrole.

Trends in applying C-H oxidation to the total synthesis of natural products

Covering: 2006 to 2015C-H functionalization remains one of the frontier challenges in organic chemistry and drives quite an active area of research. It has recently been applied in various novel strategies for the synthesis of natural products. It can dramatically increase synthetic efficiency when incorporated into retrosynthetic analyses of complex natural products, making it an essential part of current trends in organic synthesis. In this Review, we focus on selected case studies of recent applications of C-H oxidation methodologies in which the C-H bond has been exploited effectively to construct C-O and C-N bonds in natural product syntheses. Examples of syntheses representing different types of C-H oxidation are discussed to illustrate the potential of this approach and inspire future applications.

Enantioselective Total Syntheses of Akuammiline Alkaloids (+)-Strictamine, (-)-2(S)-Cathafoline, and (-)-Aspidophylline A

The akuammiline alkaloids are a family of natural products that have been widely studied for decades. Although notable synthetic achievements have been made recently, akuammilines that possess a methanoquinolizidine core have evaded synthetic efforts. We report an asymmetric approach to these alkaloids, which has culminated in the first total syntheses of (-)-2(S)-cathafoline and the long-standing target (+)-strictamine. Moreover, the first enantioselective total synthesis of aspidophylline A is described.

Synthesis of benzofuro[3,2-b]indoline amines via deamination-interrupted Fischer indolization and their unexpected reactivity towards nucleophiles

We report the access to the benzofuro[3,2-b]indoline framework via an interrupted Fischer indolization and its unexpected reactivity towards allyl nucleophiles.

Monoterpenoid Bisindole Alkaloids

This chapter covers the literature on bisindole alkaloids consisting of monoterpenoid indoles, published up to June 2015. Bisindole alkaloids isolated from plants belonging to the families Apocynaceae and Loganiaceae, including Iboga-vobasine type, Aspidosperma-Aspidosperma type, eburnan-Aspidosperma type, Strychnos-Strychnos type, macroline-macroline type, and so on, are described. Some recent syntheses of monoterpenoid bisindole alkaloids are outlined as well.