Total Synthesis of the Diterpenoid (+)-Harringtonolide

Divergent Synthesis of 17-nor-Cephalotane Diterpenoids through Developed Ynol-diene Cyclization

On the basis of a novel ynol-diene cyclization developed as a rapid access to tropone unit, the first divergent strategy to 17-nor-cephalotane diterpenoids has been successfully established. Combining with a bioinspired stereoselective dual hydrogenation, the divergent total synthesis of (+)-3-deoxyfortalpinoid F, (+)-harringtonolide, (-)-fortalpinoids M/N/P, and analog (-)-20-deoxocephinoid P have been achieved in 14-17 linear longest steps starting from commercially available materials.

Cephalotane diterpenoids: structural diversity, biological activity, biosynthetic proposal, and chemical synthesis

Covering: up to the end of 2023Cephalotane diterpenoids are a unique class of natural products exclusive to the genus Cephalotaxus, featuring a rigid 7,6,5,6-fused tetracyclic architecture. The study of cephalotanes dates back to the 1970s, when harringtonolide (1), a Cephalotaxus troponoid with a peculiar norditerpenoid carbon skeleton, was first discovered. In recent years, prototype C20 diterpenoids proposed as cephalotane were disclosed, which triggered intense studies on this diterpenoid family. To date, a cumulative total of 105 cephalotane diterpenoids with great structural diversity and biological importance have been isolated. In addition, significant advances have been made in the field of total synthesis and biosynthesis of cephalotanes in recent years. This review provides a complete overview of the chemical structures, bioactivities, biosynthetic aspects, and completed total synthesis of all the isolated cephalotane diterpenoids, which will help guide future research on this class of compounds.

Bioinspired Total Synthesis of Cephalotaxus Diterpenoids and Their Structural Analogues

Herein, we present a unified chemical synthesis of three subgroups of cephalotaxus diterpenoids. Key to the success lies in adopting a synthetic strategy that is inspired by biosynthesis but is opposite in nature. By employing selective one-carbon introduction and ring expansion operations, we have successfully converted cephalotane-type C18 dinorditerpenoids (using cephanolide B as a starting material) into troponoid-type C19 norditerpenoids and intact cephalotane-type C20 diterpenoids. This synthetic approach has enabled us to synthesize cephinoid H, 13-oxo-cephinoid H, 7-oxo-cephinoid H, fortalpinoid C, 7-epi-fortalpinoid C, cephanolide E, and 13-epi-cephanolide E. Furthermore, through the development of an intermolecular asymmetric Michael reaction between β-oxo esters and β-substituted enones, we have achieved the enantioselective synthesis of advanced intermediates within our synthetic sequence, thus formally realizing the asymmetric total synthesis of the cephalotaxus diterpenoids family.

Late-stage benzenoid-to-troponoid skeletal modification of the cephalotanes exemplified by the total synthesis of harringtonolide

Skeletal modifications enable elegant and rapid access to various derivatives of a compound that would otherwise be difficult to prepare. They are therefore a powerful tool, especially in the synthesis of natural products or drug discovery, to explore different natural products or to improve the properties of a drug candidate starting from a common intermediate. Inspired by the biosynthesis of the cephalotane natural products, we report here a single-atom insertion into the framework of the benzenoid subfamily, providing access to the troponoid congeners - representing the reverse of the proposed biosynthesis (i.e., a contra-biosynthesis approach). Computational evaluation of our designed transformation prompted us to investigate a Büchner-Curtius-Schlotterbeck reaction of a p-quinol methylether, which ultimately results in the synthesis of harringtonolide in two steps from cephanolide A, which we had previously prepared. Additional computational studies reveal that unconventional selectivity outcomes are driven by the choice of a Lewis acid and the nucleophile, which should inform further developments of these types of reactions.

Highly modified cephalotane-type diterpenoids from Cephalotaxus fortunei var. alpina and C. sinensis

Cephalotanes are a rare class of diterpenoids occurring exclusively in Cephalotaxus plants. The intriguing structures and promising biological activities for this unique compound class prompt us to investigate C. fortunei var. alpina and C. sinensis, leading to the isolation of six undescribed cephalotane-type diterpenoids and/or norditerpenoids, ceforloids A-F (1-6). Their structures were elucidated by comprehensive analysis of spectroscopic data, including ECD and single-crystal X-ray diffraction studies, as well as quantum chemical calculations. Compound 1 possesses an unprecedented norditerpenoid skeleton featuring an unusual acetophenone moiety, and originated putatively from a disparate biogenetic pathway. Compounds 4 and 5 incorporate a unique 12,13-p-hydroxybenzylidene acetal motif. Compound 6 is a rare cephalotane-type diterpenoid glycoside. Immunosuppressive assays showed that compounds 2 and 6 exhibited mild suppressive activity against the activated T and B lymphocytes proliferation. These findings not only expanded the structural diversity of this small group of diterpenoids, but also explored their potential as novel structures for the development of immunosuppressive agents.

Enantioselective Total Synthesis of (-)-Cephalotanin B

Cephalotaxus diterpenoids are attractive natural products with intriguing molecular frameworks and promising biological features. As a structurally unusual member, (-)-cephalotanin B possesses an extraordinarily congested heptacyclic skeleton, three lactone units, and nine consecutive stereocenters. Herein, we report an enantioselective total synthesis of (-)-cephalotanin B based on a divergent asymmetric Michael addition reaction, a novel Pauson-Khand/deacyloxylation process discovered in the development of a second-generation stereoselective Pauson-Khand reaction protocol, and an epoxide-opening/elimination/dual-lactonization cascade to construct the challenging propeller-shaped A-B-C ring system as key transformations.

Functional Characterization and Cyclization Mechanism of a Diterpene Synthase Catalyzing the Skeleton Formation of Cephalotane-Type Diterpenoids

CsCTS, a new diterpene synthase from Cephalotaxus sinensis responsible for forming cephalotene, the core skeleton of cephalotane-type diterpenoids with a highly rigid 6/6/5/7 tetracyclic ring system, was functionally characterized. The stepwise cyclization mechanism is proposed mainly based on structural investigation of its derailment products, and further demonstrated through isotopic labeling experiments and density functional theory calculations. Homology modeling and molecular dynamics simulation combined with site-directed mutagenesis revealed the critical amino acid residues for the unique carbocation-driven cascade cyclization mechanism of CsCTS. Altogether, this study reports the discovery of the diterpene synthase that catalyzes the first committed step of cephalotane-type diterpenoid biosynthesis and delineates its cyclization mechanism, laying the foundation to decipher and artificially construct the complete biosynthetic pathway of this type diterpenoids.

Asymmetric Total Synthesis of Cephalotaxus Diterpenoids: Cephinoid P, Cephafortoid A, 14-epi-Cephafortoid A and Fortalpinoids M-N, P

The asymmetric total syntheses of cephalotaxus C19 diterpenoids, bearing a unique cycloheptene A ring with a chiral methyl group at C-12, were disclosed based on a universal strategy. Six members, including cephinoid P, cephafortoid A, 14-epi-cephafortoid A and fortalpinoids M-N, P, were accomplished for the first time. The concise approach relies on two crucial steps: (1) a Nicholas/Hosomi-Sakurai cascade reaction was developed to efficiently generate the cycloheptene ring bearing a chiral methyl group; (2) an intramolecular Pauson-Khand reaction was followed to facilitate the construction of the complete skeleton of target molecules. Our studies provide a new strategy for the synthetic analysis of cephalotaxus diterpenoids and structurally related polycyclic natural products.

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.

Carbocycloaddition Strategies for Troponoid Synthesis

Tropone is the prototypical aromatic 7-membered ring, and can be found in virtually any undergraduate textbook as a key example of non-benzenoid aromaticity. Aside from this important historical role, tropone is also of high interest as a uniquely reactive synthon in complex chemical synthesis as well as a valuable chemotype in drug design. More recently, there has been growing interest in the utility of tropones for catalysis and material science. Thus, synthetic strategies capable of synthesizing functional tropones are key to fully exploiting the potential of this aromatic ring system. Cycloaddition reactions are particularly powerful methods for constructing carbocycles, and these strategies in turn have proven to be powerful for generating troponoids. The following review article provides an overview of strategies for troponoids wherein the 7-membered carbocycle is generated through a cycloaddition reaction. Representative examples of each strategy are also provided.

Total Synthesis of (-)-Ceforalide B and (-)-Cephanolides B-D

(-)-Ceforalide B (1) and (-)-cephanolides B-D (2-4) are benzenoid cephanolide diterpenoids possessing the same pentacyclic skeleton, which contains three C13-C15 substituent patterns and different benzylic oxidation states. An olefination/6π-electrocyclization/oxidative aromatization cascade has been verified as divergent access to three C13-C15 patterns. The benzylic aerobic oxidations enabled by the Co(OAc)2·4H2O/bromide salt/O2/PPh3/N-hydroxyphthalimide system have been developed to deliver expected site-selectivity and different oxidation states. Through the divergent strategy, total synthesis of (-)-ceforalide B and (-)-cephanolides B-D is accomplished.

Unified Total Syntheses of Benzenoid Cephalotane-Type Norditerpenoids: Cephanolides and Ceforalides

Detailed herein are our synthetic studies toward the preparation of the C₁₈- and C₁₉-benzenoid cephalotane-type norditerpenoids. Guided by chemical network analysis, the core structure of this natural product family was constructed in a concise manner using an iterative cross-coupling, followed by a formal inverse-electron-demand [4 + 2] cycloaddition. Initial efforts to functionalize an alkene group in the [4 + 2] cycloadduct using a Mukaiyama hydration and a subsequent olefination led to the complete C₁₈-carbon framework. While effective, this approach proved lengthy and prompted the development of a direct alkene difunctionalization that relies on borocupration to advance the cycloadduct to the natural products. Late-stage peripheral C-H functionalization facilitated access to all of the known cephanolides in 6-10 steps as well as five recently isolated ceforalides in 8-13 steps.

Experimental and Computational Mechanistic Study of Carbonazidate-Initiated Cascade Reactions

A variety of Huisgen cyclization or nitrene/carbene alkyne cascade reactions with different types of termination were investigated. Accessible nitrene precursors were assessed, and carbonazidates were found to be the only effective initiators. Solvents, terminal alkynyl substituents, and catalysts can all impact the reaction outcome. Study of the mechanism both computationally (by density functional theory) and experimentally revealed relevant intermediates and plausible reaction pathways.

Asymmetric Total Syntheses of Cephalotane-Type Diterpenoids Cephanolides A-D

Cephanolides A-D are cephalotane-type diterpenoids featuring a novel 6/6/6/5 tetracyclic core embedded with a bridged δ-lactone. The asymmetric and divergent total syntheses of cephanolides A-D have been accomplished, proceeding in 11-14 steps from a known alcohol. The salient features of the present work include (i) a substrate-controlled diastereoselective intermolecular Diels-Alder reaction to form the 6-6 cis-fused rings, (ii) a palladium-catalyzed formal bimolecular [2 + 2 + 2] cycloaddition reaction via a partially intermolecular cascade reaction sequence involving multiple carbometalations to rapidly install the key tetracyclic skeleton, and (iii) lactonization and late-stage oxidative diversification to complete total syntheses of the four benzenoid cephanolides.

Total syntheses of strained polycyclic terpenes

Terpenoids constitute a broad class of natural compounds with tremendous variability in structure and bioactivity, which resulted in a strong interest of the chemical community to this class of natural products over the last 150 years. The presence of strained small rings renders the terpenoid targets interesting for chemical synthesis, due to limited number of available methods and stability issues. In this feature article, a number of recent examples of total syntheses of terpenoids with complex carbon frameworks featuring small rings are discussed. Specific emphasis is given to the new developments in strategical and tactical approaches to construction of such systems.

Recent Advances in the Total Synthesis of Cephalotane-Type Norditerpenoids from Cephalotaxus sinensis

Cephalotaxus diterpenoids are well known for their unique structures and biological activities. Cephanolides, as new cephalotane-type norditerpenoids isolated from Cephalotaxus sinensis, have attracted considerable attention from the synthetic community. The present Short Review summarizes strategic approaches toward the total synthesis of cephanolides from 2018 to 2021.

1 Introduction

2 Synthetic Approaches toward Cephalotane-Type Norditerpenoids

2.1 First Total Synthesis of Cephanolides B and C by Zhao (2018)

2.2 Total Synthesis of Cephanolides A–D by Sarpong (2021)

2.3 Total Synthesis of Cephanolide B by Yang (2021)

2.4 Asymmetric Total Synthesis of Cephanolide A by Gao (2020)

2.5 Asymmetric Total Synthesis of Cephanolide B by Gao (2021)

2.6 Asymmetric Total Synthesis of Cephanolides A and B by Cai (2021)

3 Conclusion and Perspectives

Photoaffinity Probe Reveals the Potential Target of Harringtonolide for Cancer Cell Migration Inhibition

Harringtonolide (HO, 1) is a bioactive diterpenoid tropone isolated from Cephalotaxus harringtonia with antiproliferation activity. Until now there have been no reports to elucidate its anticancer mechanism. Herein we report the synthesis of HO-derived probes (10, 11, and 12) to identify the possible target of HO. As a result, the application of a novel photoaffinity alkyne-tagged probe from HO (compound 12) showed direct engagement between HO and receptor for activated C kinase 1 (RACK1). Furthermore, HO could suppress the epithelial-mesenchymal transition (EMT) process and inhibit activation of the FAK/Src/STAT3 signaling pathway in A375 cells. This study provides a groundwork for HO as an effective antitumor agent that targets RACK1 to suppress cancer cell migration.

Progress in structure, synthesis and biological activity of natural cephalotane diterpenoids

The Cephalotaxus genus is well-known owing to the numerous complex, biologically relevant natural products that can be obtained from its constituent species. The successful identification of various Cephalotaxus alkaloids and natural, structurally diverse cephalotane diterpenoids that exhibit antitumor activities and excellent pharmacological properties has encouraged the discovery of previously undescribed compounds from this genus. The present review summarizes the different strategies for the total synthesis of cephalotane diterpenoids as well as their diverse chemical structures, antitumor activities, structure-activity relationships (SARs), and biosynthetic pathways.

Scalable Birch reduction with lithium and ethylenediamine in tetrahydrofuran

[Figure: see text].

Catalytic Asymmetric Inverse-Electron-Demand Diels-Alder Reactions of 2-Pyrones with Indenes: Total Syntheses of Cephanolides A and B

An inverse-electron-demand Diels-Alder (IEDDA) reaction could complement the conventional normal-electron-demand Diels-Alder reaction in the synthesis of six-membered carbocycles. However, catalytic asymmetric all-carbon-based IEDDA reactions are underdeveloped. Herein, we disclosed a copper-catalyzed asymmetric IEDDA reaction using electron-deficient 3-carboalkoxyl-2-pyrones and electronically unbiased indenes as reactants. This method enables the rapid and enantioselective construction of a wide range of hexahydrofluorenyl bridged-lactone scaffolds. Using this method, asymmetric total syntheses of cephanolides A and B were accomplished.

Asymmetric Total Synthesis of (+)-Mannolide C

(+)-Mannolide C is a complex hexacyclic C₂₀ cephalotane-type diterpenoid featuring a highly strained 7/6/6/5 tetracyclic core containing eight consecutive stereocenters and two bridging lactones. The first asymmetric total synthesis of (+)-mannolide C has been accomplished by lipase-mediated resolution, Ru-complex-catalyzed double ring-closing metathesis (RCM) reactions, Ni^II -catalyzed diastereoselective Michael addition, and Mn^III -catalyzed allylic oxidation as the key transformations.

Total Synthesis of (+)-3-Deoxyfortalpinoid F, (+)-Fortalpinoid A, and (+)-Cephinoid H

3-Deoxyfortalpinoid F, fortalpinoid A, and cephinoid H are members of the Cephalotaxus diterpenoids class of natural products, which feature diverse chemical structures and valuable biological activities. We report herein the development of a diastereoselective Pauson-Khand reaction as an effective pathway to access the core tetracyclic skeleton, which is found widely in Cephalotaxus diterpenoids. Furthermore, we enabled the construction of the tropone moiety through a ring-closing metathesis/elimination protocol. Based on the developed strategy, asymmetric synthesis of the title compounds has been achieved for the first time.

Cephalodiones A-D: Compound Characterization and Semisynthesis by [6+6] Cycloaddition

Cephalodiones A-D (1-4), the first example of C₁₉ -norditerpenoid dimers, were isolated and fully characterized from a Cephalotaxus plant. These new skeletal natural products shared a unique tricyclo[6.4.1.1^2,7 ]tetradeca-3,5,9,11-tetraene-13,14-dione core that was capped in both ends with rigid multicyclic ring systems either C₂ -symmetrically or asymmetrically. Compounds 1-4 were proposed to be biosynthetically produced by the [6+6]-cycloaddition of two identical C₁₉ -norditerpenoid troponoids, which was validated by the semisyntheses of dimers 2-4. Moreover, some compounds showed significant inhibition on Th17 cell differentiation.

Semi-Synthesis of Harringtonolide Derivatives and Their Antiproliferative Activity

Harringtonolide (HO), a natural product isolated from Cephalotaxus harringtonia, exhibits potent antiproliferative activity. However, little information has been reported on the systematic structure-activity relationship (SAR) of HO derivatives. Modifications on tropone, lactone, and allyl positions of HO (1) were carried out to provide 17 derivatives (2-13, 11a-11f). The in vitro antiproliferative activity against four cancer cell lines (HCT-116, A375, A549, and Huh-7) and one normal cell line (L-02) was tested. Amongst these novel derivatives, compound 6 exhibited comparable cell growth inhibitory activity to HO and displayed better selectivity index (SI = 56.5) between Huh-7 and L-02 cells. The SAR results revealed that the tropone and lactone moieties are essential for the cytotoxic activities, which provided useful suggestions for further structural optimization of HO.

Discovery and Surprises with Cyclizations, Cycloadditions, Fragmentations, and Rearrangements in Complex Settings

We discuss a number of synthesis routes to complex natural products recently reported from our group. Although the structures are quite varied, we demonstrate the research endeavor as a setting to examine the implementation of cyclizations, cycloadditions, rearrangements, and fragmentations. We showcase how the various transformations enabled access to key core structures and thereby allowed the rapid introduction of complexity. Two different routes to (-)-mitrephorone A, the first case discussed, led to the use of Koser's reagent to effect oxetane formation from diosphenol derivatives. Even though the Diels-Alder cycloaddition reaction represents one of the workhorses of complex molecule synthesis, there are opportunities provided by the complexity of secondary metabolites for discovery, study, and development. In our first approach to (-)-mitrephorone A, Diels-Alder cycloaddition provided access to fused cyclopropanes, while the second synthesis underscored the power of diastereoselective nitrile oxide cycloadditions to access hydroxy ketones. The successful implementation of the second approach required the rigorous stereocontrolled synthesis of tetrasubstituted olefins; this was accomplished by a highly stereoselective Cr-mediated reduction of dienes. The diterpenoid (+)-sarcophytin provided a stage for examining the Diels-Alder cycloaddition of two electron-deficient partners. The study revealed that in the system this unusual combination works optimally with the E,Z-dienoate and proceeds through an exo transition state to provide the desired cycloadduct. Our reported pallambin synthesis showcased the use of fulvene as a versatile building block for the core structure. Fulvene decomposition could be outcompeted by employing it as a diene and using a highly reactive dienophile, which affords a bicyclic product that can in turn be subjected to chemo- and stereoselective manipulations. The synthesis route proceeds with a C-H insertion providing the core structure en route to pallambin A and B. The studies resulting in our synthesis of gelsemoxonine highlight the use of the acid-catalyzed rearrangement/chelotropic extrusion of oxazaspiro[2.4]heptanes to access complex β-lactams, which are otherwise not readily prepared by extant methods in common use. Mechanistic investigations of the intriguing ring contraction supported by computational studies indicate that the reaction involves a concerted cleavage of the N-O bond and cyclopropane ring opening under the extrusion of ethylene. The synthesis of guanacastepenes focused on the use of cyclohexyne in [2+2]-cycloadditions with enolates. The resulting cyclobutene can be enticed to undergo ring opening to give a fused six-seven ring system. The cycloinsertion reaction of cyclohexyne developed for the first time proves useful as a general approach to complex fused ring systems.

Total Synthesis of the Cephalotaxus Norditerpenoids (±)-Cephanolides A-D

Concise syntheses of the Cephalotaxus norditerpenoids cephanolides A-D (8-14 steps from commercial material) using a common late-stage synthetic intermediate are described. The success of our approach rested on an early decision to apply chemical network analysis to identify the strategic bonds that needed to be forged, as well as the efficient construction of the carbon framework through iterative Csp²-Csp³ cross-coupling, followed by an intramolecular inverse-demand Diels-Alder cycloaddition. Strategic late-stage oxidations facilitated access to all congeners of the benzenoid cephanolides isolated to date.

Asymmetric total synthesis of cephanolide B

The asymmetric synthesis of cephanolide B, a complex C₁₈ Cephalotaxus dinorditerpenoid, is presented for the first time.

Recyclable heterogeneous gold(I)-catalyzed oxidative ring expansion of alkynyl quinols: a practical access to tropone and its analogues

The heterogeneous gold(i)-catalyzed oxidative ring expansion of alkynyl quinols has been achieved by using a benzyldiphenylphosphine-modified MCM-41-immobilized gold(i) complex [MCM-41-BnPh2P-AuNTf2] as the catalyst and 8-methylquinoline N-oxide as the oxidant under mild reaction conditions, yielding a variety of functionalized tropone derivatives in good to excellent yields. Extension of this methodology allows for facile construction of other seven- or six-membered ring systems including dibenzotropones, dibenzooxepines, phenanthrenes, and quinolin-2(1H)-ones. This new heterogeneous gold(i) complex can be readily recovered through a simple filtration process and recycled at least eight times without any apparent decrease in catalytic efficiency.

Asymmetric Total Synthesis of Cephanolide A

The first asymmetric total synthesis of cephanolide A, a complex hexacyclic C₁₈ dinorditerpenoid from cephalotaxus sinensis, was achieved. The synthesis features a convergent strategy, which provides a flexible approach to prepare the biogenetically cephalotaxus diterpenoids and structurally related derivatives for biological studies. A mild intramolecular Prins cyclization was developed to construct the central hexahydrofluorenol skeleton (A-B-C ring), which relies on the originally proposed hydroacylation strategy. A remote hydroxy group directed hydrogenation was applied to stereospecifically reduce the tetra-substituted enone unit. A sequence of ring forming steps, including lactonization, cation mediated etherification and Friedel-Crafts cyclization, was efficiently utilized to forge the cage-like skeleton.

Diterpenoids and Flavonoids from the Twigs of Cephalotaxus fortunei var. alpina

Three new diterpenoids (a cephalotane, an abietane and a 9(10→20)-abeo-abietane) and one new flavonoid, together with 11 known compounds, were isolated from the twigs of Cephalotaxus fortunei var. alpina. The new compounds were identified by comprehensive spectroscopic (including 1D and 2D-NMR and HR-ESI-MS) analysis. Anti-inflammatory, immunosuppressive and cytotoxic activities of three new compounds were evaluated. 3β,20-epoxyabieta-8,11,13-triene-3α,12-diol showed weak cytotoxicity against tumor cell lines NCI-H1975, HepG2, MCF-7, while fortalpinoid R and 3-acetonyl-3,5,7,4'-tetrahydroxy-2-methoxyflavanone were not active at 80 μM. None of these compounds showed anti-inflammatory and immunosuppressive activities.

Synthesis of cyclobutane-fused oxygen-containing tricyclic framework via thermally promoted intramolecular cycloaddition of cyclohexadienone-tethered allenes

We have developed a unique approach for the thermally promoted cycloaddition of cyclohexadienone-tethered allenes to form a versatile cyclobutane-fused oxygen-containing tricyclic framework in an environmentally friendly and atomic economic fashion with high regioselectivity. The reaction encompasses a broad substrate scope and functional group tolerance of cyclohexadienone moieties. Moreover, the cycloaddition was also applicable to the late-stage functionalization of pharmaceutically relevant compounds.

17- nor-Cephalotane-Type Diterpenoids from Cephalotaxus fortunei

Seventeen new 17- nor-cephalotane-type diterpenoids, fortalpinoids A-Q (1-17), were isolated from the seeds of Cephalotaxus fortunei var. alpine. Compound 12 represents the first 17- nor-cephalotane-type diterpenoid featuring an 8-oxabicyclo[3.2.1]oct-2-ene moiety. The absolute configuration of fortunolide A (18) was determined for the first time, and the structure of cephinoid Q was revised to 14- epi-cephafortoid A (24) by X-ray crystallographic data analysis. Some of the compounds showed significant cytotoxicity against A549 and HL-60 cells, and the structure-activity relationship of this compound class is discussed.

Cascade cyclization and intramolecular nitrone dipolar cycloaddition and formal synthesis of 19-hydroxyibogamine

A cascade or domino sequence of condensation of hydroxylamine and an aldehyde to give an oxime, cyclization to a nitrone, and intramolecular 1,3-dipolar cycloaddition has been successfully employed where there is branching at C-4 as a route to the iboga alkaloids. Cyclization occurs with displacement of chloride as a leaving group and intramolecular cycloaddition occurs with an alkene as a dipolarophile. The reaction gives an azabicyclo[2.2.2]octane product containing a fused isoxazolidine as a single stereoisomer and this was converted to an isoquinuclidine that completed a formal synthesis of the alkaloid (±)-19-hydroxyibogamine.

Tropolone natural products

This review on natural products containing a tropolonoid motif highlights analytical methods applied for structural identification and biosynthetic pathway analysis, the ecological context and the pharmacological potential of this compound class.

Bioactive norditerpenoids from Cephalotaxus fortunei var. alpina and C. lanceolata

Twenty-eight naturally occurring Cephalotaxus tropone analogues, including 19 previously undescribed ones, were identified from Cephalotaxus fortunei Hook. var. alpina H. L. Li and C. lanceolata K. M. Feng. The presence of the C20 cephinoids A-E revealed that these tropones were assigned to the norditerpenoids and were perhaps derived from labdane-type diterpenoids. These norditerpenoids showed excellent cytotoxicity against human cancer cells (IC₅₀, 20-0.1 μM) in vitro. The SAR (structure-activity relationship) analysis disclosed that the tropone moiety and the lactone ring were crucial structural features for the observed activities. Further SAR analyses led to a new candidate, cephinoid H, which demonstrated an inhibition of 49.0% by administration to zebrafish at a dose of 60.0 ng/mL, compared to cisplatin (DDP, 22.4%) at 15.0 μg/mL. These compounds might affect the NF-κB signaling pathway rather than binding to microtubules. Additionally, the isolated norditerpenoids showed almost equal anti-inflammatory activities compared to the positive control, MG132.

Expedient Construction of the Hexacycle of Franchetine

Franchetine, a unique 7,17-seco type of norditerpenoid alkaloid, possesses a highly congested polycyclic architecture coupled with nine stereogenic centers. Here we present an efficient synthetic approach for the intact hexacyclic framework of franchetine from the known tricyle 16 in 20 steps. The synthesis features a diastereoselective 6-exo-tet radical cyclization for construction of ring A and a unique oxidative Wagner-Meerwein-type rearrangement to realize the functionalized [3.2.1] bridging ring CD.