Asymmetric Total Syntheses of Schizozygane Alkaloids

Stereodivergent Total Synthesis of Tacaman Alkaloids

This paper describes a concise, asymmetric and stereodivergent total synthesis of tacaman alkaloids. A key step in this synthesis is the biocatalytic Baeyer-Villiger oxidation of cyclohexanone, which was developed to produce seven-membered lactones and establish the required stereochemistry at the C14 position (92 % yield, 99 % ee, 500 mg scale). Cis- and trans-tetracyclic indoloquinolizidine scaffolds were rapidly synthesized through an acid-triggered, tunable acyl-Pictet-Spengler type cyclization cascade, serving as the pivotal reaction for building the alkaloid skeleton. Computational results revealed that hydrogen bonding was crucial in stabilizing intermediates and inducing different addition reactions during the acyl-Pictet-Spengler cyclization cascade. By strategically using these two reactions and the late-stage diversification of the functionalized indoloquinolizidine core, the asymmetric total syntheses of eight tacaman alkaloids were achieved. This study may potentially advance research related to the medicinal chemistry of tacaman alkaloids.

Catalytic asymmetric intramolecular propargylation of cyclopropanols to access the cuparane core

The catalytic asymmetric propargylation of enol(ate) intermediates is a well-established method for the synthesis of α-propargyl-substituted carbonyl compounds. However, the propargylation of homo-enol(ate) or its equivalents for the synthesis of β-propargyl-substituted carbonyl compounds remains underdeveloped. A catalytic enantioselective decarboxylative intramolecular propargylation of cyclopropanols has been developed using a PyBox-complexed copper catalyst. This reaction offers an effective approach to assemble a cyclopentanone skeleton bearing an all-carbon quaternary stereogenic center and an adjacent quaternary gem-dimethyl carbon center, which is the core scaffold of the naturally occurring cuparenoids. Key to the success of this protocol is the use of a new structurally optimized PyBox ligand. This study represents the first example of catalytic asymmetric intramolecular propargylation of cyclopropanols.

Vallesamidine and schizozygane alkaloids: rearranged monoterpene indole alkaloids and synthetic endeavours

Covering 1963 to 2023Monoterpene indole alkaloids are the main sub-family of indole alkaloids with fascinating structures, stereochemistry, and diverse bioactivities (e.g., anticancer, anti-malarial and anti-arrhythmic etc.). Vallesamidine alkaloids and structurally more complex schizozygane alkaloids are small groups of rearranged monoterpene indole alkaloids with a unique 2,2,3-trialkylated indoline scaffold, while schizozygane alkaloids can generate a further rearranged skeleton, isoschizozygane, possessing a tetra-substituted, bridged tetrahydroquinoline core. In this review, the origin and structural features of vallesamidine and schizozygane alkaloids are introduced, and a discussion on the relationship of these alkaloids with aspidosperma alkaloids and a structural rearrangement hypothesis based on published studies is followed. Moreover, uncommon skeletons and potential bioactivities, such as anti-malarial and anti-tumour activities, make such alkaloids important synthetic targets, attracting research groups globally to accomplish total synthesis, resulting in impressive works on novel total synthesis, formal synthesis, and construction of key intermediates. These synthetic endeavours are systematically reviewed and highlighted with key strategies and efficiencies, providing different viewpoints on molecular structures and promoting the extension of chemical space and mining of new active scaffolds.

Ring-Opening Coupling of Cyclopropanol with 1,2,3-Triazole for the Synthesis of Fused Triazoles

A radical ring-opening arylation of cyclopropanol with 1,2,3-triazole has been achieved. This synthetic protocol provides straightforward access to a wide range of structurally diverse and chiral 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyridines with high efficiency from readily available chiral cyclopropanols.

Enantioselective Palladium-Catalyzed Domino Carbonylative Heck Esterification of o-Iodoalkenylbenzenes with Arylboronic Acids

The current investigation presents an innovative palladium-catalyzed asymmetric carbonylative Heck esterification method. This approach facilitates the efficient synthesis of various chiral γ-ketoacid esters by utilizing o-alkenyliodobenzenes and arylboronic acids as primary substrates. This reaction achieves the creation of three carbon-carbon bonds, two carbon-oxygen bonds, and the establishment of a quaternary carbon center within a single step. The α-chiral γ-ketoacid esters were obtained in yields ranging from good to high yields, displaying enantiomeric excesses (ee's) levels up to 92% under mild reaction conditions.

A Homo-Mannich Reaction Strategy Enables Collective Access to Ibophyllidine, Aspidosperma, Kopsia, and Melodinus Alkaloids

We report here a homo-Mannich reaction of cyclopropanol with an iminium ion, generated by an asymmetric allylic dearomatization of indole, to construct a tricyclic hydrocarbazole core, which is shared by a variety of monoterpenoid indole alkaloids across families. Through this approach, an all-carbon quaternary stereogenic center as well as an allyl and a ketone group were installed. Using this functionalized hydrocarbazole as the structural platform, D ring and E rings of different sizes (i.e., five-, six-, and seven-membered) were successively or simultaneously assembled, leading to a collective asymmetric synthesis of seven alkaloids belonging to the ibophyllidine, Aspidosperma, Kopsia, and Melodinus alkaloid families.

A Bischler-Napieralski and homo-Mannich sequence enables diversified syntheses of sarpagine alkaloids and analogues

Sarpagine alkaloids offer signicant opportunities in drug discovery, yet the efficient total syntheses and diverse structural modifications of these natural products remain highly challenging due to the architectural complexity. Here we show a homo-Mannich reaction of cyclopropanol with imines generated via a Bischler-Napieralski reaction enables a protecting-group-free, redox economic, four-step access to the tetracyclic sarpagine core from L-tryptophan esters. Based on this advancement, diversified syntheses of sarpagine alkaloids and analogues are achieved in a short synthetic route. The systematic anticancer evaluation indicates that natural products vellosimine and Na-methyl vellosimine possess modest anticancer activity. Intensive structural optimization of these lead molecules and exploration of the structure-activity relationship lead to the identification of analogue 15ai with an allene unit showing a tenfold improvement in anticancer activities. Further mechanism studies indicate compound 15ai exertes antiproliferation effects by inducing ferroptosis, which is an appealing non-apoptotic cell death form that may provide new solutions in future cancer therapies.

Rapid Access to 2,2-Disubstituted Indolines via Dearomative Indolic-Claisen Rearrangement: Concise, Enantioselective Total Synthesis of (+)-Hinckdentine A

The construction of 2,2-disubstituted indolines has long presented a synthetic challenge without any general solutions. Herein, we report a robust protocol for the dearomative Meerwein-Eschenmoser-Claisen rearrangement of 3-indolyl alcohols that provides efficient access to 2-substituted and 2,2-disubstituted indolines. These versatile subunits are useful for natural product synthesis and medicinal chemistry. The title [3,3] sigmatropic rearrangement proceeds in generally excellent yield and transfers the C3-indolic alcohol chirality to the C2 position with high fidelity, thus providing a reliable method for the construction of enantioenriched 2,2-disubstituted indolines. The power of this methodology is demonstrated through the concise and strategically unique total synthesis of the marine natural product hinckdentine A, which features a dearomative Claisen rearrangement, a diastereocontrolled hydrogenation of the alkene product, a one-pot amide-to-oxime conversion using Vaska's complex, and a regioselective late-stage tribromination.

Heck Macrocyclization in Forging Non-Natural Large Rings including Macrocyclic Drugs

The intramolecular Heck reaction is a well-established strategy for natural product total synthesis. When constructing large rings, this reaction is also referred to as Heck macrocyclization, which has proved a viable avenue to access diverse naturally occurring macrocycles. Less noticed but likewise valuable, it has created novel macrocycles of non-natural origin that neither serve as nor derive from natural products. This review presents a systematic account of the title reaction in forging this non-natural subset of large rings, thereby addressing a topic rarely covered in the literature. Walking through two complementary sections, namely (1) drug discovery research and (2) synthetic methodology development, it demonstrates that beyond the well-known domain of natural product synthesis, Heck macrocyclization also plays a remarkable role in forming synthetic macrocycles, in particular macrocyclic drugs.

Arylation of Cyclopropanol with Pyrrole: Asymmetric Synthesis of Indolizidine 167B, Indolizidine 209D, and Monomorine I

A Fe(NO₃)₃-mediated ring-opening arylation of cyclopropanol with the electron-rich pyrrole has been developed, which might proceed through oxidative radical ring opening of cyclopropanol followed by cyclization to the pyrrole motif and then aromatization. This method enables direct arylation of cyclopropanol without prefunctionalization and thus allows rapid access to a diverse array of chiral 5,6,7,8-tetrahydroindolizines from easily available chiral amino acid esters. The synthetic utility has been demonstrated by the asymmetric synthesis of alklaoids (-)-indolizidine 167B, (+)-indolizidine 209D, (+)-monomorine I, and a natural product analogue.

Intramolecular trapping of spiro radicals to produce unusual cyclization products from usual migration substrates

A conceptually new methodology to give unusual cyclization products from usual migration substrates was disclosed. The highly complex and structurally important and valuable spirocyclic compounds were produced through radical addition, intramolecular cyclization and ring opening instead of usual migration to the di-functionalization products of olefins. Furthermore, a plausible mechanism was proposed based on a series of mechanistic studies including radical trapping, radical clock, verification experiments of intermediates, isotope labeling and KIE experiments.

Palladium-Catalyzed Carbonylations: Application in Complex Natural Product Total Synthesis and Recent Developments

Carbon monoxide is a cheap and abundant C1 building block that can be readily incorporated into organic molecules to rapidly build structural complexity. In this Perspective, we outline several recent (since 2015) examples of palladium-catalyzed carbonylations in streamlining complex natural product total synthesis and highlight the strategic importance of these carbonylation reactions in the corresponding synthesis. The selected examples include spinosyn A, callyspongiolide, perseanol, schizozygane alkaloids, cephanolides, and bisdehydroneostemoninine and related stemona alkaloids. We also provide our perspective about the recent advancements and future developments of palladium-catalyzed carbonylations.

Pd-Catalyzed Asymmetric 5-exo-trig Cyclization/Cyclopropanation/Carbonylation of 1,6-Enynes for the Construction of Chiral 3-Azabicyclo[3.1.0]hexanes

We herein disclose a mild and efficient access to chiral 3-azabicyclo[3.1.0]hexanes via a Pd-catalyzed asymmetric 5-exo-trig cyclization/cyclopropanation/carbonylation of 1,6-enynes. Various nucleophiles, such as alcohols, phenols, amines and water, are well compatible with the reaction system. This reaction forms three C-C bonds, two rings, two adjacent quaternary carbon stereocenters as well as one C-O/C-N bond with excellent regio- and enantioselectivities. The products could be further functionalized to generate a library of 3-azabicyclo[3.1.0]hexane frameworks.

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.

Asymmetric β-Arylation of Cyclopropanols Enabled by Photoredox and Nickel Dual Catalysis

The enantioselective functionalization and transformation of readily available cyclopropyl compounds are synthetically appealing yet challenging topics in organic synthesis. Here we report an asymmetric β-arylation of cyclopropanols with aryl bromides...