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

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.

Cephaliverins A-D, four new lignans from Cephalotaxus oliveri Mast

Four new lignans named cephaliverins A-D (1-4), along with seven known analogues (5-11), were isolated from Cephalotaxus oliveri Mast. Their structures were elucidated on the basis of HR-ESI-MS and NMR analyses, and their absolute configurations were determined by ECD comparison. Cephaliverin A (1), herpetotriol (5) and hedyotol A (6) exhibited moderate antitumor activity against HepG2 and A549 cell lines.

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.

Diterpenolignans and cephalotane diterpenoids from Cephalotaxus oliveri mast. with antitumor activity

Four undescribed naturally diterpenolignans, and two cephalotane diterpenoids, along with seven known compounds, including two pairs of enantiomers, were isolated from the twigs and leaves of Cephalotaxus oliveri Mast. Their structures were elucidated via spectroscopic data interpretation, chiral-phase HPLC analysis, NMR calculations, and electronic circular dichroism analysis. All the isolated compounds were evaluated for their cytotoxic activities against three kinds of human tumor cell lines. Among them, compound 8 exhibited the most potent activities against MCF-7, HepG2 and A549 cell lines with IC50 values of 2.83, 4.75 and 2.77 μM, respectively.

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.

An integrative view on the systematic position of the cupressophyte Cephalotaxus

We made an in-depth review of historical studies of the cupressophyte conifer genus Cephalotaxus Siebold & Zucc. with an emphasis on its systematic position. We suggest that the systematic position of the genus is better understood using an integrative approach, so the evolution of phenetic characters is discussed within the context of recent phylogenomics. We propose that the genus should be classified as a separate family Cephalotaxaceae belonging to the clade consisting of Cupressaceae, Cephalotaxaceae, and Taxaceae; the family Cephalotaxaceae is sister to the Taxaceae but not nested within the Taxaceae and is characterized by a unique set of characters including morphology, anatomy, embryology, and chemistry. The family Cephalotaxaceae shows transitional characters between the Cupressaceae and the Taxaceae; the family possesses female cones with a primary cone axis bearing 5-8 pairs of decussate bracts, which is similar to the typical female cones of the Cupressaceae, on the one hand, and may have given rise to the reduced female cone of the Taxaceae with one terminal ovule partially or completely enclosed in a fleshy aril. In parallel, the compound male cone of the Cephalotaxaceae evolved into the seemingly "simple" male cones of the Taxaceae by means of reduction, elimination, and fusion.

Cytotoxic alkaloids from the twigs and leaves of Cephalotaxus sinensis

Twelve new Cephalotaxus alkaloids (1-12) and nine known analogues (13-21) were isolated and identified from the twigs and leaves of Cephalotaxus sinensis. The structures of the new compounds (1-12) were elucidated by extensive spectroscopic analysis and single-crystal X-ray diffraction analysis. Cephalosine H (8) is the third example of an alkaloid containing the cephalolancine skeleton. Cephalosines J and K (10 and 11) are the rare natural Δ(2)1-alkene-6-hydroxyl homoerythrina-type alkaloids isolated from the Cephalotaxus genus. The racemization of cephalotaxine-type alkaloids is discussed. Alkaloids 6, 7, 11, 16, 18 and 19 exhibited broad and potent cytotoxicities against five human cancer cell lines, with IC₅₀ values ranging from 0.053 to 10.720 μM, highlighting these compounds as promising leads for the development of new antitumor agents.

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.

New Diterpenes with Potential Antitumoral Activity Isolated from Plants in the Years 2017-2022

Diterpenes represent a wider class of isoprenoids, with more than 18,000 isolated compounds, and are present in plants, fungi, bacteria, and animals in both terrestrial and marine environments. Here, we report on the fully characterised structures of 251 new diterpenes, isolated from higher plants and published from 2017, which are shown to have antitumoral activity. An overview on the most active compounds, showing IC50 < 20 μM, is provided for diterpenes of different classes. The most active compounds were extracted from 29 different plant families; particularly, Euphorbiaceae (69 compounds) and Lamiaceae (54 compounds) were the richest sources of active compounds. A better activity than the positive control was obtained with 33 compounds against the A549 cell line, 28 compounds against the MCF-7 cell line, 9 compounds against the HepG2 cell line, 8 compounds against the Hep3B cell line, 19 compounds against the SMMC-7721 cell line, 9 compounds against the HL-60 cell line, 24 compounds against the SW480 cell line, and 19 compounds against HeLa.

Abietane diterpenes from the twigs and leaves of Cephalotaxus oliveri Mast. with antitumor activity

Twenty one abietane diterpenes were isolated from Cephalotaxus oliveri Mast. The structures of 7 undescribed diterpenoids, named cephaloliverins A-G, were elucidated via spectroscopic data interpretation and electronic circular dichroism (ECD) analysis. The isolated diterpenoids were evaluated for their cytotoxicity against three kinds of human tumor cell lines (MCF-7, HepG2, and A549). Metaglyptin A was the most active with IC₅₀ values of 16.34, 15.63 and 21.33 μM and was further investigated for colony formation and apoptosis in HepG2 cells.

Cephaloliverols A and B, two sterol-hybrid meroterpenoids from Cephalotaxus oliveri

Cephaloliverols A (1) and B (2), two meroterpenoids based on a sterol and an abietane diterpene possessing a dioxane ring, were isolated from the twigs and leaves of Cephalotaxus oliveri. Their structures were established by spectroscopic analysis and quantum chemical calculation. 1 and 2 represent the first sterol-hybrid meroditerpenoids. The two compounds and their precursors decreased NO production in a dose-dependent manner in LPS-stimulated RAW 264.7 macrophages.

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

Cephalotane-type C20 diterpenoids from Cephalotaxus fortunei var. alpina

Seventeen new cephalotane-type diterpenoids were isolated from Cephalotaxus fortunei var. alpina. Compounds 14 and 15 contain an unusual 7-oxabicyclo[4.1.1]octane moiety.