Total Synthesis of Septedine and 7-Deoxyseptedine

Biomimetic total synthesis of the reported structure of (+)-selaginedorffone B

The first enantioselective total synthesis of the reported structure of the structurally unique aromatic tetraterpenoid of anti-cancer potential, (+)-selaginedorffone B (2), has been accomplished from two modified abietane diterpenoids through an intermolecular Diels-Alder reaction between a bio-inspired diene 3 (HOMO counterpart) and dienophile 4 (corresponding LUMO counterpart) in a 23-step sequence, whereas the core framework of the monomeric abietane diterpenoid was constructed via alkyne-activated ene-cyclization. Computational analysis was conducted to reveal the intricate regio and diastereoselectivity of this novel Diels-Alder reaction, strengthening the experimental results. The absolute configuration of the synthesized molecule was validated through X-ray studies of late-stage intermediates as well as comprehensive 2D NMR analysis.

Enantioselective total synthesis of atisane diterpenoids: (+)-sapinsigin H, (+)-agallochaol C, and (+)-16α, 17-dihydroxy-atisan-3-one

Enantioselective total synthesis of (+)-sapinsigin H, (+)-agallochaol C, and (+)-16α, 17-dihydroxy-atisan-3-one has been accomplished starting from enantiopure Wieland-Miescher ketone. Key features of the syntheses include a benzannulation step to construct the tricyclic core, an oxidative dearomatization step to generate the diene, and a Diels-Alder reaction with ethylene gas to establish the bicyclo[2.2.2]octane framework. Efficient late-stage functionalisation of the A-ring by aerobic oxidation and Baeyer-Villiger oxidation completed the atisane target molecules.

Scalable Total Synthesis of Acremolactone B

Acremolactone B is a pyridine-containing azaphilone-type polyketide. The first total synthesis of this molecule was achieved on a gram scale, based on an aza-6π electrocyclization-aromatization strategy for construction of the tetra-substituted pyridine ring. A bicyclic intermediate was expeditiously prepared by using [2+2] photocycloaddition and chemoselective Baeyer-Villiger oxidation, which was further elaborated to a densely substituted aza-triene. An electrocyclization-aromatization cascade was utilized to forge the tetracyclic core of this natural product, and the side chain was introduced through diastereoselective acylation and reduction.

Enantioselective Divergent Syntheses of Diterpenoid Pyrones

Capitalizing a synergy between late-stage C(sp3)-H alkynylation and a series of transition metal-catalyzed alkyne functionalization reactions, we reported herein enantioselective divergent synthesis of 10 diterpenoid pyrones within 14-16 steps starting from chiral pool enoxolone, including the first enantioselective synthesis of higginsianins A, B, D, E, and metarhizin C. Our synthesis also highlights an unprecedented biomimetic oxidative rearrangement of α-pyrone into 3(2H)-furanone, as well as applications of Echavarren C(sp3)-H alkynylation reaction and Toste chiral counterion-mediated Au-catalyzed intramolecular allene hydroalkoxylation in natural product synthesis.

Controllable skeletal reorganizations in natural product synthesis

Covering: 2016 to 2023The synthetic chemistry community is always in pursuit of efficient routes to natural products. Among the many available general strategies, skeletal reorganization, which involves the formation, cleavage, and migration of C-C and C-heteroatom bonds, stands out as a particularly useful approach for the efficient assembly of molecular skeletons. In addition, it allows for late-stage modification of natural products for quick access to other family members or unnatural derivatives. This review summarizes efficient syntheses of steroid, terpenoid, and alkaloid natural products that have been achieved by means of this strategy in the past eight years. Our goal is to illustrate the strategy's potency and reveal the spectacular human ingenuity demonstrated in its use and development.

Catalytic Asymmetric Total Synthesis of (-)-Garryine via an Enantioselective Heck Reaction

The first asymmetric total synthesis of the hexacyclic veatchine-type C20-diterpenoid alkaloid (-)-garryine is presented. Key steps include a Pd-catalyzed enantioselective Heck reaction, a radical cyclization, and a photoinduced C-H activation/oxazolidine formation sequence. Of note, a highly enantioselective Heck reaction developed in this work provides efficient access to 6/6/6 tricyclic compounds, in particular, containing a C19-functionalitiy, which is useful for diverse transformations.

Finding activity through rigidity: syntheses of natural products containing tricyclic bridgehead carbon centers

Covering: up to 2022Tricyclic bridgehead carbon centers (TBCCs) are a synthetically challenging substructure found in many complex natural products. Here we review the syntheses of ten representative families of TBCC-containing isolates, with the goal of outlining the strategies and tactics used to install these centers, including a discussion of the evolution of the successful synthetic design. We provide a summary of common strategies to inform future synthetic endeavors.

Divergent Total Syntheses of Napelline-Type C20-Diterpenoid Alkaloids: (-)-Napelline, (+)-Dehydronapelline, (-)-Songorine, (-)-Songoramine, (-)-Acoapetaldine D, and (-)-Liangshanone

The napelline-type alkaloids possess an azabicyclo[3.2.1]octane moiety and an ent-kaurane-type tetracyclic skeleton (6/6/6/5) along with varied oxidation patterns embedded in the compact hexacyclic framework. Herein, we disclose a divergent entry to napelline-type alkaloids that hinges on convergent assembly of the ent-kaurane core using a diastereoselective intermolecular Cu-mediated conjugate addition and subsequent intramolecular Michael addition reaction as well as rapid construction of the azabicyclo[3.2.1]octane motif via an intramolecular Mannich cyclization. The power of this strategy has been demonstrated through efficient asymmetric total syntheses of eight napelline-type alkaloids, including (-)-napelline, (-)-12-epi-napelline, (+)-dehydronapelline, (+)-12-epi-dehydronapelline, (-)-songorine, (-)-songoramine, (-)-acoapetaldine D, and (-)-liangshanone.

The diterpenoid alkaloids

The diterpenoid alkaloids are a family of extremely important natural products that have long been a research hotspot due to their myriad of intricate structures and diverse biological properties. This chapter systematically summarizes the past 11 years (2009-2019) of studies on the diterpenoid alkaloids, including the "so-called" atypical ones, covering the classification and biogenetic relationships, phytochemistry together with 444 new alkaloids covering 32 novel skeletons and the corrected structures, chemical reactions including conversion toward toxoids, synthetic studies, as well as biological activities. It should be noted that the synthetic studies, especially the total syntheses of various diterpenoid alkaloids, are for the first time reviewed in this treatise. This chapter, in combination with our four previous reviews in volumes 42, 59, 67, and 69, will present to the readers a more completed and updated profile of the diterpenoid alkaloids.

Enantioselective Total Synthesis of (-)-Spiroxins A, C, and D

Spiroxins A, C, and D are metabolites that have been identified in the marine fungal strain LL-37H248. Their unique polycyclic structures and intriguing biological activities make them attractive targets for the synthetic community. Based on a scalable enantioselective epoxidation of 5-substituted naphthoquinone, an oxidation/spiroketalization cascade, ortho-selective chlorination of the phenol unit, and oxime-ester-directed acetoxylation, an enantioselective total synthesis of (-)-spiroxins A and C and the first total synthesis of (-)-spiroxin D have been achieved.

Asymmetric Total Syntheses of (+)-Davisinol and (+)-18-Benzoyldavisinol: A HAT-Initiated Transannular Redox Radical Approach

The first and asymmetric total syntheses of two C11-oxygenated hetisine-type diterpenoid alkaloids, namely, (+)-davisinol and (+)-18-benzoyldavisinol, is described. The concise synthetic approach features a HAT-initiated transannular redox radical cyclization, an ODI-Diels-Alder cycloaddition, and an acylative kinetic resolution. By incorporating an efficient late-stage assembly of the azabicycle, our strategy would streamline the synthetic design of C₂₀-diterpenoid alkaloids and pave the way for their modular syntheses.

Asymmetric Total Synthesis of Hetidine-Type C20-Diterpenoid Alkaloids: (+)-Talassimidine and (+)-Talassamine

Here, we report the first asymmetric total synthesis of (+)-talassimidine and (+)-talassamine, two hetidine-type C₂₀-diterpenoid alkaloids. A highly regio- and diastereoselective 1,3-dipolar cycloaddition of an azomethine ylide yielded a chiral tetracyclic intermediate in high enantiopurity, thus providing the structural basis for asymmetric assembly of the hexacyclic hetidine skeleton. In this key step, the introduction of a single chiral center induces four new continuous chiral centers. Another key transformation is the dearomative cyclopropanation of the benzene ring and subsequent S_N2-like ring opening of the resultant cyclopropane ring with water as a nucleophile, which not only establishes the B ring but also precisely installs the difficult-to-achieve equatorial C7-OH group.

Contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids

Many natural products have intriguing biological properties that arise from their fascinating chemical structures. However, the intrinsic complexity of the structural skeleton and the reactive functional groups on natural products pose tremendous challenges to chemical syntheses. Semi-synthesis uses chemical compounds isolated from natural sources as the starting materials to produce other novel compounds with distinct chemical and medicinal properties. In particular, advancements in various types of sp3 C-H bond functionalization reactions and skeletal rearrangement methods have contributed to the re-emergence of semi-synthesis as an efficient approach for the synthesis of structurally complex bioactive natural products. Here, we begin with a brief discussion of several bioactive natural products that were obtained via a semi-synthetic approach between 2008 and 2015 and we then discuss in-depth contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids reported during 2016-2020.

Pivotal Reactions in the Creation of the Polycyclic Skeleton of Cryptotrione

Three pivotal reactions, namely, enyne cycloisomerization, polyene cyclization, and quinone methide formation, are applied to synthesize the complex polycyclic skeleton of cryptotrione. This review summarizes the most prominent applications of these three reactions to the total syntheses of natural products, covering results published in the literature between 2011 and 2020.

1 Introduction

2 Three Pivotal Reactions Applied to Create the Polycyclic Framework of Cryptotrione

2.1 Enyne Cycloisomerization

2.2 Polyene Cyclization

2.3 Quinone Methide Formation

3 Conclusion

Synthesis of Functionalized Bicyclo[2.2.2]octan-2-ones Skeleton via Tandem Process of Amino Acid Involved Formal [4 + 2] and Decarboxylative-Mannich Sequence

An efficient approach to a functionalized bicyclo[2.2.2]octan-2-one scaffold has been developed through a one-pot cascade process including amino acid involved successive Michael addition and decarboxylative-Mannich sequence. Starting from α,β-unsaturated ketones and amino acids, a series of desired products 7a-7m and 8a-8o were obtained with moderate yields. In addition, the tandem process was reasonably explained by the results of DFT calculations.

Synthesis of Three-Dimensionally Fascinating Diterpenoid Alkaloids and Related Diterpenes

Three-dimensional cage-like natural products represent astounding and long-term challenges in the research endeavors of total synthesis. A central issue that synthetic chemists need to address lies in how to efficiently construct the polycyclic frameworks as well as to install the requisite substituent groups. The diterpenoid alkaloids that biogenetically originate from amination of diterpenes and diversify through late-stage skeletal reorganization belong to such a natural product category. As the characteristic components of the Aconitum and Delphinium species, these molecules display a rich array of biological activities, some of which are used as clinical drugs. More strikingly, their intricate and beautiful architectures have rendered the diterpenoid alkaloids elusive targets in the synthetic community. The successful preparation of these intriguing compounds relies on the development of innovative synthetic strategies.Our laboratory has explored the total synthesis of a variety of diterpenoid alkaloids and their biogenetically related diterpenes over the past decade. In doing so, we have accessed 6 different types of skeletons (atisine-, denudatine-, arcutane-, arcutine-, napelline-, and hetidine-type) and achieved the total synthesis of 6 natural products (isoazitine, dihydroajaconine, gymnandine, atropurpuran, arcutinine, and liangshanone). Strategically, an oxidative dearomatization/Diels-Alder (OD/DA) cycloaddition sequence was widely employed in our synthesis to form the ubiquitous [2.2.2]-bicyclic ring unit and its related ring-distorted derivatives in these complex target molecules. This protocol, in combination with additional bond-forming key steps, allowed us to prepare the corresponding polycyclic alkaloids and a biogenetically associated diterpene. For example, bioinspired C-H activation, aza-pinacol, and aza-Prins cyclizations were used toward a unified approach to the atisine-, denudatine-, and hetidine-type alkaloids via ajaconine intermediates in our first work. To pursue the synthesis of atropurpuran and related arcutine alkaloids, we harnessed a ketyl-olefin radical cyclization to assemble the carbocycle and an aza-Wacker cyclization to construct the unusual pyrrolidine ring. Furthermore, a one-pot alkene cleavage/Mannich cyclization tactic, sequential Robinson annulation, and intramolecular aldol addition were developed, which facilitated the formation of the napelline alkaloid scaffold and the first total synthesis of liangshanone. Finally, the utility of the Mannich cyclization and enyne cycloisomerization reactions allowed for access to the highly functionalized A/E and C/D ring fragments of aconitine (regarded as the "Holy Grail" of diterpenoid alkaloids). This Account provides insight into our synthetic designs and approaches used toward the synthesis of diterpenoid alkaloids and relevant diterpenes. These endeavors lay a foundation for uncovering the biological profiles of associated molecules and also serve as a reference for preparing other three-dimensionally fascinating natural products.

Innovation of hypervalent(iii) iodine in the synthesis of natural products

The focus article discusses the innovation of hypervalent(iii) iodine regarding skeletal rearrangement, cycloaddition and cyclization, and sp³ C–H functionalization in natural product synthesis.

Structure, property, biogenesis, and activity of diterpenoid alkaloids containing a sulfonic acid group from Aconitum carmichaelii

Three new C20-diterpenoid alkaloids with a sulfonic acid unit, named aconicarmisulfonines B and C (1 and 2) and chuanfusulfonine A (3), respectively, were isolated from the Aconitum carmichaelii lateral roots ("fu zi" in Chinese). Structures of 1-3 were determined by spectroscopic data analysis. Intriguing chemical properties and reactions were observed for the C20-diterpenoid alkaloids: (a) specific selective nucleophilic addition of the carbonyl (C-12) in 1 with CD3OD; (b) interconversion between 1 and 2 in D2O; (c) stereo- and/or regioselective deuterations of H-11α in 1-3 and both H-11α and H-11β in aconicarmisulfonine A (4); (d) TMSP-2,2,3,3-d 4 promoted cleavage of the C-12-C-13 bond of 4 in D2O; (e) dehydrogenation of 4 in pyridine-d 5, and (f) Na2SO3-assisted dehydrogenation and N-deethylation of songorine (5, a putative precursor of 1-4). Biogenetically, 1 and 2 are correlated with 4, for which the same novel carbon skeleton is proposed to be derived from semipinacol rearrangements via migrations of C-13-C-16 and C-15-C-16 bonds of the napelline-type skeleton, respectively. Meanwhile, 3 is a highly possible precursor or a concurrent product in the biosynthetic pathways of 1, 2, and 4. In the acetic acid-induced mice writhing assay, at 1.0 mg/kg (i.p.), compounds 1, 2, 5, 5a, and 5b exhibited analgesic effects against mice writhing.

A Case for Bond-Network Analysis in the Synthesis of Bridged Polycyclic Complex Molecules: Hetidine and Hetisine Diterpenoid Alkaloids

A key challenge in the synthesis of diterpenoid alkaloids lies in identifying strategies that rapidly construct their multiply bridged polycyclic skeletons. Existing approaches to these structurally intricate secondary metabolites are discussed in the context of a "bond-network analysis" of molecular frameworks, which was originally devised by Corey some 40 years ago. The retrosynthesis plans that emerge from a topological analysis of the highly bridged frameworks of the diterpenoid alkaloids are discussed in the context of eight recent syntheses of hetidine and hetisine natural products and their derivatives. This Minireview highlights the extent to which network analyses of the type described here sufficed for designing synthesis plans, as well as areas where they had to be amalgamated with functional group oriented synthetic planning considerations.

Retrosynthetic strategies and their impact on synthesis of arcutane natural products

Decisions, decisions, decisions: the interplay between different retrosynthetic strategies in the synthesis of the highly bridged, polycyclic arcutane natural products.

Retrosynthetic analysis is a cornerstone of modern natural product synthesis, providing an array of tools for disconnecting structures. However, discussion of retrosynthesis is often limited to the reactions used to form selected bonds in the forward synthesis. This review details three strategies for retrosynthesis, focusing on how they can be combined to plan the synthesis of polycyclic natural products, such as atropurpuran and the related arcutane alkaloids. Recent syntheses of natural products containing the arcutane framework showcase how these strategies for retrosynthesis can be combined to plan the total synthesis of highly caged scaffolds. Comparison of multiple syntheses of the same target provides a unique opportunity for detailed analysis of the impact of retrosynthetic disconnections on synthesis outcomes.

Cobalt-Catalyzed Hydrogen-Atom Transfer Induces Bicyclizations that Tolerate Electron-Rich and Electron-Deficient Intermediate Alkenes

A novel CoII -catalyzed polyene cyclization was developed that is uniquely effective when performed in hexafluoroisopropanol as the solvent. The process is presumably initiated by metal-catalyzed hydrogen-atom transfer (MHAT) to 1,1-disubstituted or monosubstituted alkenes, and the reaction is remarkable for its tolerance of internal alkenes bearing either electron-rich methyl or electron-deficient nitrile substituents. Electron-rich aromatic terminators are required in both cases. Terpenoid scaffolds with different substitution patterns are obtained with excellent diastereoselectivities, and the bioactive C20-oxidized abietane diterpenoid carnosaldehyde was made to showcase the utility of the nitrile-bearing products. Also provided are the results of several mechanistic experiments that suggest the process features an MHAT-induced radical bicyclization with late-stage oxidation to regenerate the aromatic terminator.

Iridium-Catalyzed Asymmetric Synthesis of Functionally Rich Molecules Enabled by (Phosphoramidite,Olefin) Ligands

The catalytic, asymmetric synthesis of complex molecules has been a core focus of our research program for some time because developments in the area can have an immediate impact on the identification of novel strategies for the synthesis of value-added molecules. In concert with this central interest, we have emphasized the design of ligand scaffolds as a tactic to discover and develop novel chemistry and overcome well-recognized synthetic challenges. Based on our group's work on chiral pool-derived diolefin ligands, we designed and implemented a class of hybrid (phosphoramidite,olefin) ligands, which combines the properties of both phosphoramidite and olefin motifs to impact, fine-tune, and even override the inherent reactivity of the metal center. Specifically, we have utilized these unique modifying ligands to address several recognized limitations in the field of iridium-catalyzed, asymmetric allylic substitution. The methods we have documented typically employ branched, unprotected allylic alcohols as substrates and obviate the need for rigorous exclusion of air and moisture. Following Takeuchi's seminal report demonstrating the high aptitude of Ir(I)-phosphite catalysts for  branch-selective allylic substitution, concerted efforts from numerous research laboratories have led to a broadening of the synthetic utility of this reaction class. The first section of this Account outlines the process leading to our discovery of an unprecedented (phosphoramidite,olefin) ligand and its validation in the first iridium-catalyzed amination of branched, unprotected allylic alcohols. This section continues with our work involving heteroatom-based nucleophiles within inter- and intramolecular etherification, thioetherification and spiroketalization processes. The second section highlights the use of readily available carbon nucleophiles possessing sp, sp², and sp³ hybridization in a series of enantioselective carbon-carbon bond-forming reactions. We describe how alkylzinc, allylsilane, and several classes of organotrifluoroborate nucleophiles can be coupled enantioselectively to enable construction of several key motifs including 1,5-dienes, 1,4-dienes, and 1,4-enynes. Since the unique electronic and steric properties of this class of ligands renders the (η³-allyl)-Ir(III) intermediate highly electrophilic, even weak nucleophiles such as alkyl olefins can be used. We also show that more nucleophilic alkene motifs such as enamines and in situ generated ketene acetals smoothly participate in substitution reactions with allylic alcohols to yield valuable piperidines and γ,δ-unsaturated esters, respectively. The concept of stereodivergent dual catalysis, which synergistically combines chiral amine catalysis with iridium catalysis to furnish α-allylated aldehydes containing two independently controllable stereocenters is then discussed. This process has enabled the independent, stereoselective synthesis of all four possible product stereoisomers from a single set of starting materials, and was highlighted in the stereodivergent synthesis of Δ⁹-tetrahydrocannabinol. This Account concludes with an overview of our organometallic mechanistic studies regarding relevant intermediates within the catalytic cycle of this class of allylic substitution. These studies have allowed us to better understand the origin of the unique characteristics exhibited by this catalyst in comparison to related systems.

Asymmetric Total Synthesis of Arcutinidine, Arcutinine, and Arcutine

We have accomplished the asymmetric total synthesis of arcutinidine, arcutinine, and arcutine, three arcutine-type C20-diterpenoid alkaloids. A pentacyclic intermediate was rapidly assembled by using two Diels-Alder reactions. We developed a cascade sequence of Prins cyclization and Wagner-Meerwein rearrangement to construct the core of arcutinidine, which was then elaborated into an oxygenated pentacycle through a scalable route. Chemoselective reductive amination followed by spontaneous imine formation furnished the pyrroline motif in the final stage. We clarified the S configuration of the α-carbon of the acyl group within arcutine through chemical synthesis and crystallographic analysis.

Development of a Terpene Feedstock-Based Oxidative Synthetic Approach to the Illicium Sesquiterpenes

The Illicium sesquiterpenes are a family of natural products containing over 100 highly oxidized and structurally complex members, many of which display interesting biological activities. This comprehensive account chronicles the evolution of a semisynthetic strategy toward these molecules from (+)-cedrol, seeking to emulate key aspects of their presumed biosynthesis. An initial route generated lower oxidation state analogs but failed in delivering a crucial hydroxy group in the final step. Insight gathered during these studies, however, ultimately led to a synthesis of the pseudoanisatinoids along with the allo-cedrane natural product 11- O-debenzoyltashironin. A second-generation strategy was then developed to access the more highly oxidized majucinoid compounds including jiadifenolide and majucin itself. Overall, one dozen natural products can be accessed from an abundant and inexpensive terpene feedstock. A multitude of general observations regarding site-selective C(sp³)-H bond functionalization reactions in complex polycyclic architectures are reported.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

Contemporary Synthetic Strategies toward seco-Prezizaane Sesquiterpenes from Illicium Species

Since the elucidation of the structure of anisatin in the late 1960s, sesquiterpene lactones from various Illicium species of plants have captivated synthetic chemists worldwide, resulting in a large body of synthetic work. In particular, Illicium sesquiterpenes containing the seco-prezizaane carbon framework have seen immense interest in recent years owing to desirable structural and medicinal attributes. This synopsis will focus on recently developed synthetic strategies to access these compact, highly oxidized terpenoids.

A Benzyne Insertion Approach to Hetisine-Type Diterpenoid Alkaloids: Synthesis of Cossonidine (Davisine)

The hetisine-type natural products exhibit one of the most complex carbon skeletons within the diterpenoid alkaloid family. The use of network analysis has enabled a synthesis strategy to access alkaloids in this class with hydroxylation on the A-ring. Key transformations include a benzyne acyl-alkylation to construct a key fused 6-7-6 tricycle, a chemoselective nitrile reduction, and sequential C-N bond formations using a reductive cyclization and a photochemical hydroamination to construct an embedded azabicycle. Our strategy should enable access to myriad natural and unnatural products within the hetisine-type.