Total Synthesis of (+)-Arborisidine

Gold-catalyzed cyclization and cycloaddition in natural product synthesis

Covering: 2016 to mid 2023Transition metal catalysis, known for its remarkable capacity to expedite the assembly of molecular complexity from readily available starting materials in a single operation, occupies a central position in contemporary chemical synthesis. Within this landscape, gold-catalyzed reactions present a novel and versatile paradigm, offering robust frameworks for accessing diverse structural motifs. In this review, we highlighted a curated selection of publications in the past 8 years, focusing on the deployment of homogeneous gold catalysis in the ring-forming step for the total synthesis of natural products. These investigations are categorized based on the specific ring formations they engender, accentuating the prevailing gold-catalyzed methodologies applied to surmount intricate challenges in natural products synthesis.

A Formal Synthesis of (±)-Arborisidine

Herein, we report a formal synthesis of (±)-arborisidine via the creation of Jiao's intermediate with the critical caged structure. Starting from tryptamine, a Pictet-Spengler cyclization forged the piperidine ring, a Pd-catalyzed indole allylation and ring-closing metathesis protocol afforded a bridged aza-bicyclo[3.3.1]nonane moiety, and an intramolecular N-alkylation closed the final pyrrolidine ring. This study provides a new approach to the unique caged framework of arborisidine and relevant alkaloids.

Asymmetric Synthesis of Arboduridine

The first asymmetric total synthesis of the monoterpenoid indole alkaloid arboduridine has been accomplished. The tricyclic A/B/D ring system was constructed by an enantioselective Michael reaction followed by intramolecular nucleophilic addition. Intramolecular α-amination of a ketone forged the piperidine ring, while a Horner-Wadsworth-Emmons (HWE) reaction was used to form the pyrrolidine ring. A reduction cyclization cascade led to formation of the tetrahydrofuran ring.

A New Reaction Mode of 3-Halooxindoles: Acting as C-C-O Three-Atom Components for (3+3) Cycloaddition to Access Indolenine-Fused 2H-1,4-Oxathiines

Herein, we report an unprecedented implementation of 3-halooxindoles as C-C-O three-atom components for (3+3) cycloaddition with pyridinium 1,4-zwitterionic thiolates, affording structurally diverse indolenine-fused 2H-1,4-oxathiines in moderate to high yields. A combined experimental and computational mechanistic study suggests that the reaction proceeds through addition of a S conjugate to the o-azaxylylene intermediate, followed by O-Michael addition and a sequential retro-Michael addition/pyridine extrusion pathway.

Kinetics of H· Transfer from CpCr(CO)3H to Various Enamides: Application to Construction of Pyrrolidines

The rate constants kH (kD) have been determined at 27 °C for H· (D·) transfer from CpCr(CO)3H(D) to the C=C bonds of various enamides. This process leads to the formation of α-amino radicals. Vinyl enamides with N-alkyl and N-phenyl substituents have proven to be good H· acceptors, with rate constants close to those of styrene and methyl methacrylate. A methyl substituent on the incipient radical site decreases kH by a factor of 4; a methyl substituent on the carbon that will receive the H· decreases kH by a factor of 380. The measured kH values indicate that these α-amino radicals can be used for the cyclization of enamides to pyrrolidines. A vanadium hydride, HV(CO)4(dppe), has proven more effective at the cyclization of enamides than Cr or Co hydrides-presumably because the weakness of the V-H bond leads to faster H· transfer. The use of the vanadium hydride is operationally simple, employs mild reaction conditions, and has a broad substrate scope. Calculations have confirmed that H· transfer is the slowest step in these cyclization reactions.

Short, Scalable Access to Pyrrovobasine

A concise gram-scale synthesis of pyrrovobasine (1) is reported. Key transformations include a three-step decagram-scale synthesis of the tetracyclic compound, Mn-mediated direct radical cyclization, and the introduction of a naturally rare pyrraline structure. The synthesis is designed to be applicable to gram-scale synthesis using inexpensive and readily available reagents.

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.

Concise synthesis of piperarborenine B

A concise synthesis of piperarborenine B is reported. Organocatalytic electrophilic amination of pyrrolidines, stereospecific oxidative ring contraction and an original diastereoselective Krapcho dealkoxycarbonylation/transmethylation contribute to a novel synthetic strategy to the preparation of a non-symmetrical cyclobutane core. Being transition-metal-free, directing-group-free and protecting-group-free, a five-step synthesis of piperarborenine B was accomplished.

Manganese-Catalyzed Asymmetric Hydrogenation of 3H-Indoles

The asymmetric hydrogenation (AH) of 3H-indoles represents an ideal approach to the synthesis of useful chiral indoline scaffolds. However, very few catalytic systems based on precious metals have been developed to realize this challenging reaction. Herein, we report a Mn-catalyzed AH of 3H-indoles with excellent yields and enantioselectivities. The kinetic resolution of racemic 3H-indoles by AH was also achieved with high s-factors to construct quaternary stereocenters. Many acid-sensitive functional groups, which cannot be tolerated when using a state-of-the-art ruthenium catalyst, were compatible with manganese catalysis. This new process expands the scope of this transformation and highlights the uniqueness of earth-abundant metal catalysis. The reaction could proceed with catalyst loadings at the parts per million (ppm) level with an exceptional turnover number of 72 350. This is the highest value yet reported for an earth-abundant metal-catalyzed AH reaction.

Asymmetric total synthesis of (1S,2S,4S)-β-elemene

Elemenes are sesquiterpene natural products extracted from Chinese medicinal herbs and have been used as an important antitumor drug in China. Here, we report the first stereoselective total synthesis of (1S,2S,4S)-β-elemene using (R)-carvone as a chiral pool starting material. The isopropenyl moiety was achieved in a highly stereoselective manner through 1,4-Michael conjugate addition. The following transformations like regio- and stereoselective aldol condensation, Wittig olefination have been employed as the key steps, resulting in a concise total synthesis of (1S,2S,4S)-β-elemene. Our accomplishment will allow further biological investigations of this natural product and open opportunities for developing a new potentially promising antitumor drug.

The first asymmetric total synthesis of (1S,2S,4S)-β-elemene was accomplished in five steps or eight steps by using the chiral pool strategy.

Total synthesis of monoterpenoid indole alkaloid (–)-arbophyllidine

The first asymmetric total synthesis of (–)-arbophyllidine, an unusual pentacyclic monoterpenoid indole alkaloid, has been achieved.

Assembling the carbon skeleton of A-74528

The 2′-phosphodiesterase inhibitor A-74528, which combines an intriguing biosynthesis with unusual biological activity, is one of the most complex type II polyketides. As a synthetic target, it represents a significant challenge due to its size but also due to a unique carbon skeleton that features a hexacarbocyclic core with an appended pyrone. Here we report our efforts toward the synthesis of A-74528, which culminated in the construction of the full carbon skeleton and the correct installation of all but one stereocenter. Our strategy employs a molybdenum-catalyzed branched allylation to establish the central quaternary carbon and relies on establishing the remaining stereocenters in a substrate-controlled manner. Carbocycles were established using a spiro epoxide annulation, a 1,3-dipolar cycloaddition, followed by an aldol condensation, and a gold-catalyzed hydroarylation. The pyrone was appended to an aldehyde branching off the quaternary stereocenter by a one-carbon homologation and Mukaiyama aldol addition.

We report our efforts toward the synthesis of A-74528, which culminated in the construction of the full carbon skeleton.

Stereoselective Synthesis of Cyclobutanes by Contraction of Pyrrolidines

Here we report a contractive synthesis of multisubstituted cyclobutanes containing multiple stereocenters from readily accessible pyrrolidines using iodonitrene chemistry. Mediated by a nitrogen extrusion process, the stereospecific synthesis of cyclobutanes involves a radical pathway. Unprecedented unsymmetrical spirocyclobutanes were prepared successfully, and a concise, formal synthesis of the cytotoxic natural product piperarborenine B is reported.

Divergent Synthesis of Skeletally Distinct Arboridinine and Arborisidine

A divergent synthesis of skeletally distinct arboridinine and arborisidine was achieved. The central divergent strategy was inspired by the divergent biosynthetic cyclization mode of arboridinine and arborisidine and their hidden topological connection. The branch point was reached through a Michael and Mannich cascade process. A site-selective intramolecular Mannich reaction was developed to construct the tetracyclic core of arboridinine, while a site-selective intramolecular α-amination of ketone was used to access the tetracyclic core of arborisidine. A strategic Peterson olefination through intramolecular nucleophile delivery was able to set up the exocyclic olefin of arboridinine.

Highly Selective Hydrogenation of C═C Bonds Catalyzed by a Rhodium Hydride

Under mild conditions (room temperature, 80 psi of H₂) Cp*Rh(2-(2-pyridyl)phenyl)H catalyzes the selective hydrogenation of the C═C bond in α,β-unsaturated carbonyl compounds, including natural product precursors with bulky substituents in the β position and substrates possessing an array of additional functional groups. It also catalyzes the hydrogenation of many isolated double bonds. Mechanistic studies reveal that no radical intermediates are involved, and the catalyst appears to be homogeneous, thereby affording important complementarity to existing protocols for similar hydrogenation processes.

Total Synthesis of (-)-Arborisidine

An asymmetric total synthesis of cage-like indole alkaloid arborisidine is presented. The new synthetic strategy features a catalytic parallel kinetic resolution based on ambident nucleophilicity (C3/N) of indole to set the absolute configurations of the two quaternary chiral centers, and a 5-exo-trig radical cyclization to form the bridged nitrogen-containing five-membered ring.

Asymmetric Total Synthesis of Norzoanthamine

We report herein the asymmetric total synthesis of norzoanthamine using radical reactions as key steps for rapid access to the congested carbocyclic core, which is the major synthetic challenge for most zoanthamine alkaloids. (1) The Ueno-Stork radical cyclization was applied to construct the adjacent quaternary centers at the C-9 and C-22 positions; (2) a Co-catalyzed HAT radical reaction was successfully applied to construct the quaternary center at C-12 via Csp³ -Csp² bond formation; (3) a Mn-catalyzed HAT radical reaction was used to stereospecifically reduce the tetra-substituted olefin (C13=C18) and install the contiguous stereocenters in proximity to the quaternary center. A one-pot bio-inspired cyclization step was finally applied to forge the unstable bis-amino acetal skeleton. Our approach can precisely control the stereochemistry of seven vicinal stereocenters and effectively construct the highly congested heptacyclic skeleton.

Synthesis of aza-quaternary centers via Pictet-Spengler reactions of ketonitrones

Despite the array of advances that have been made in Pictet-Spengler chemistry, particularly as it relates to the synthesis of β-carboline derivatives of both natural and designed origin, the ability to use such reactions to generate aza-quaternary centers remains limited. Herein, we report a simple procedure that enables the synthesis of a variety of such products by harnessing the distinct reactivity profiles of ketonitrones as activated by commercially available acyl chlorides. Notably, the reaction process is mild, fast, and high-yielding (54-97%) for a diverse collection of substrates, including some typically challenging ones, such as indole cores with electron-deficient substituents. In addition, by deploying an acyl bromide in combination with a thiourea promoter, a catalytic, asymmetric version has been established, leading to good levels of enantioselectivity (up to 83% ee) for several ketonitrones. Finally, the resultant N-O bonds within the products can also be functionalized in several unique ways, affording valuable complementarity to existing Pictet-Spengler variants based on the use of imines.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Intramolecular Sakurai Allylation of Geminal Bis(silyl) Enamide with Indolenine. A Diastereoselective Cyclization To Form Functionalized Hexahydropyrido[3,4-b]Indole

A fluoride-promoted intramolecular Sakurai allylation of geminal bis(silyl) enamide with indolenine has been developed. The reaction facilitates an efficient cyclization to give hexahydropyrido[3,4-b]indoles in good yields with high diastereoselectivity. The resulted cis, trans-stereochemistry further enables the ring-closing metathesis (RCM) reaction of two alkene moieties, giving a tetracyclic N-hetereocycle widely found as the core structure in akuammiline alkaloids.

Metal-hydride hydrogen atom transfer (MHAT) reactions in natural product synthesis

Functionalization of olefins has been an important transformation in synthetic chemistry. This review will focus on the natural product synthesis employing the MHAT reaction as the key strategy.

Asymmetric Biocatalytic Synthesis of 1-Aryltetrahydro-β-carbolines Enabled by "Substrate Walking"

Stereoselective catalysts for the Pictet-Spengler reaction of tryptamines and aldehydes may allow a simple and fast approach to chiral 1-substituted tetrahydro-β-carbolines. Although biocatalysts have previously been employed for the Pictet-Spengler reaction, not a single one accepts benzaldehyde and its substituted derivatives. To address this challenge, a combination of substrate walking and transfer of beneficial mutations between different wild-type backbones was used to develop a strictosidine synthase from Rauvolfia serpentina (RsSTR) into a suitable enzyme for the asymmetric Pictet-Spengler condensation of tryptamine and benzaldehyde derivatives. The double variant RsSTR V176L/V208A accepted various ortho-, meta- and para-substituted benzaldehydes and produced the corresponding chiral 1-aryl-tetrahydro-β-carbolines with up to 99 % enantiomeric excess.

Asymmetric Total Synthesis and Assignment of Absolute Configuration of Arbornamine

The asymmetric total synthesis of arbornamine was accomplished in 13 steps, leading to the assignment of its absolute configuration. The key features of the strategy include construction of the C16 quaternary carbon center by a highly diastereoselective Grignard reagent addition to N-tert-butanesulfinylimine, sequential site-selective amidation and N-alkylation to form the C and E rings, and [Ni(COD)₂]-mediated Michael addition to close the D ring.

Acyclic nitronate olefin cycloaddition (ANOC): regio- and stereospecific synthesis of isoxazolines

We report the first demonstrations of intra- and intermolecular acyclic nitronate olefin cycloaddition (ANOC) reactions that enable the highly efficient syntheses of isoxazolines bearing various functional groups. This general approach to accessing γ-lactone fused isoxazolines was hitherto unprecedented. The room temperature transformations reported herein exhibit wide substrate scopes, as evidenced by more than 70 examples, including the generation of five tricyclic isoxazolines. The robustness of this methodology was confirmed by a series of trials that afforded highly functionalized isoxazolines. Both experimental results and density functional theory calculations indicate that these transformations proceed via the in situ formation of acyclic nitronates together with concerted [3+2] cycloaddition and tert-butyloxy group elimination processes to give regio- and stereospecificity.

Enantioselective Total Syntheses of Pentacyclic Homoproaporphine Alkaloids

Herein we report the first enantioselective total syntheses of pentacyclic homoproaporphine alkaloids by means of a route, which includes a tandem retro-oxa-Michael addition and nucleophilic substitution to generate the oxa-benzobicyclco[3.3.1]nonane core structure, a Pictet-Spengler cyclization to construct the fused B and C rings, and sequential Baeyer-Villiger oxidation and pinacol-type cyclization to install the hydroxyl-lactol moiety of D ring. With this unified route, six pentacyclic homoproaporphine alkaloids have been synthesized enantioselectively.

Silver-Catalyzed, Chemo- and Enantioselective Intramolecular Dearomatization of Indoles to Access Sterically Congested Azaspiro Frameworks

An asymmetric dearomatization of indoles bearing α-diazoacetamide functionalities was developed for synthesizing high-value spiro scaffolds. A silver phosphate chemoselectively catalyzed the sterically challenging dearomatization, whereas more typically used metal catalysts for carbene transfer reactions, such as a rhodium complex, were not effective and instead resulted in a Büchner ring expansion or cyclopropanation. Mechanistic studies indicated that the spirocyclization occurred through a silver-assisted asynchronous concerted process and not via a silver-carbene intermediate. Analyses based on natural bond orbital population and a distortion/interaction model indicated that the degree of C-Ag mutual interaction is crucial for achieving a high level of enantiocontrol. In addition, an oxidative disconnection of a C(sp³)-C(sp²) bond in the product provided unconventional access to the corresponding chiral spirooxindole.

The indole-based subincanadine alkaloids and their biogenetic congeners

The tryptamine-derived polycyclic bridged bioactive indole alkaloids subincanadines A-G were isolated in 2002 by Ohsaki and coworkers from the bark of the Brazilian medicinal plant Aspidosperma subincanum. Kobayashi proposed that subincanadines D-F could be biosynthetically resulting from stemmadenine via two different pathways and, furthermore, that the subincanadines A-C could be biogenetically resulting from subincanadines D and E. Kam and coworkers, in their focused efforts, isolated five indole alkaloids from Malaysian Kopsia arborea species, namely valparicine, apparicine, arboridinine, arborisidine, and arbornamine in combination with subincanadine E. On the basis of structural features, it has been proposed and proved in some examples that subincanadine E is a biogenetic precursor of these five different bioactive indole alkaloids bearing complex structural architectures. All important information on isolation, characterization, bioactivity, probable biogenetic pathways, and more specifically racemic and enantioselective total synthesis of subincanadine alkaloids and their biogenetic congeners are summarized in the present chapter. Special importance is given to the total synthesis and the synthetic strategies intended therein, comprising a set of main reactions.

Gold-Catalyzed Intramolecular Dearomatization Reactions of Indoles for the Synthesis of Spiroindolenines and Spiroindolines

A gold-catalyzed dearomatization reaction of indole derivatives was realized in the presence of JohnPhosAuCl/AgOMs to afford a series of spiroindolenines in excellent yields (≤99%). In addition, when the Hantzsch ester was used as the hydrogen transfer reagent, various spiroindolines were obtained in a cascade fashion starting from readily available indole derivatives in modest to good yields (≤79%). Both reactions feature readily available substrates, mild conditions, and good functional group tolerance.

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.

Synthetic Diversity from a Versatile and Radical Nitrating Reagent

We leverage the slow liberation of nitrogen dioxide from a newly discovered, inexpensive succinimide-derived reagent to allow for the C-H diversification of alkenes and alkynes. Beyond furnishing a library of aryl β-nitroalkenes, this reagent provides unparalleled access to β-nitrohydrins and β-nitroethers. Detailed mechanistic studies strongly suggest that a mesolytic N-N bond fragmentation liberates a nitryl radical. Using in situ photo-sensitized, electron paramagnetic resonance spectroscopy, we observed direct evidence of a nitryl radical in solution by nitrone spin-trapping. To further exhibit versatility of N-nitrosuccinimide under photoredox conditions, the late-stage diversification of an extensive number of C-H partners to prepare isoxazolines and isoxazoles is presented. This approach allows for the formation of an in situ nitrile oxide from a ketone partner, the presence of which is detected by the formation of the corresponding furoxan when conducted in the absence of a dipolarophile. This 1,3-dipolar cycloaddition with nitrile oxides and alkenes or alkynes proceeds in a single-operational step using a mild, regioselective, and general protocol with broad chemoselectivity.