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

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.

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.

A Comprehensive Review of the Classification, Sources, Phytochemistry, and Pharmacology of Norditerpenes

Norditerpenes are considered to be a common and widely studied class of bioactive compounds in plants, exhibiting a wide array of complex and diverse structural types and originating from various sources. Based on the number of carbons, norditerpenes can be categorized into C19, C18, C17, and C16 compounds. Up to now, 557 norditerpenes and their derivatives have been found in studies published between 2010 and 2023, distributed in 51 families and 132 species, with the largest number in Lamiaceae, Euphorbiaceae, and Cephalotaxaceae. These norditerpenes display versatile biological activities, including anti-tumor, anti-inflammatory, antimicrobial, and antioxidant properties, as well as inhibitory effects against HIV and α-glucosidase, and can be considered as an important source of treatment for a variety of diseases that had a high commercial value. This review provides a comprehensive summary of the plant sources, chemical structures, and biological activities of norditerpenes derived from natural sources, serving as a valuable reference for further research development and application in this field.

Photosensitizer-free visible-light-promoted glycosylation enabled by 2-glycosyloxy tropone donors

Photochemical glycosylation has attracted considerable attention in carbohydrate chemistry. However, to the best of our knowledge, visible-light-promoted glycosylation via photoactive glycosyl donor has not been reported. In the study, we report a photosensitizer-free visible-light-mediated glycosylation approach using a photoactive 2-glycosyloxy tropone as the donor. This glycosylation reaction proceeds at ambient temperature to give a wide range of O-glycosides or oligosaccharides with yields up to 99%. This method is further applied in the stereoselective preparation of various functional glycosyl phosphates/phosphosaccharides, the construction of N-glycosides/nucleosides, and the late-stage glycosylation of natural products or pharmaceuticals on gram scales, and the iterative synthesis of hexasaccharide. The protocol features uncomplicated conditions, operational simplicity, wide substrate scope (58 examples), excellent compatibility with functional groups, scalability of products (7 examples), and high yields. It provides an efficient glycosylation method for accessing O/N-glycosides and glycans.

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.

Diterpenoids from Euphorbia lactea and their anti-HIV-1 activity

Nine undescribed diterpenoids, euphlactenoids A-I (1-9), including four ingol-type diterpenoids (1-4) with a 5/3/11/3-tetracyclic framework and five ent-pimarane-type diterpenoids (5-9), together with thirteen known diterpenoids (10-22), were identified from the leaves and stems of Euphorbia lactea Haw. The structures and absolute configurations of compounds 1-9 were unequivocally elucidated on the basis of spectroscopic analysis, ECD calculations and single crystal X-ray diffraction. Compounds 3 and 16 showed anti-HIV-1 effects with IC₅₀ values of 1.17 μM (SI = 16.54) and 13.10 μM (SI = 1.93), respectively.

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.

Survey of natural products reported by Asian research groups in 2021

The new natural products reported in 2021 in peer-reviewed articles in journals with good reputations were reviewed and analyzed. The advances made by Asian research groups in the field of natural products chemistry in 2021 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on their structural classification.

Cephalotaxine-type and homoerythrina-type alkaloids with antiproliferative effects from Cephalotaxus fortunei

Twenty-two cephalotaxine-type and ten homoerythrina-type alkaloids, including seven previously undescribed ones, were isolated from the twigs and leaves and the seed kernels of Cephalotaxus fortunei. Their structures were established by spectroscopic analysis, single crystal X-ray diffraction, and ECD calculation methods. Cephalofortunine A β-N-oxide (1) is the first nitrogen-oxidized homoerythrina-type alkaloid. The isolated compounds were evaluated for their in vitro antiproliferative effects against two human leukemia cell lines (THP-1 and K562). All compounds showed different levels of antiproliferation in THP-1 and K562 cells with GI₅₀ values of 0.24-29.55 μM. Hainanensine (31) was the most active against two cancer cell lines with GI₅₀ values of 0.24 ± 0.07, and 0.29 ± 0.01 μM, respectively.

Biogenetic and biomimetic synthesis of natural bisditerpenoids: hypothesis and practices

Covering: up to March 2022Bisditerpenoids, or diterpenoid dimers, are a group of natural products with high structural variance, deriving from homo- or hetero-dimeric coupling of two diterpenoid units. They usually possess complex architectures resulting from the diversity of monomeric diterpenoids as building blocks and the dimerization processes. These compounds have attracted the attention of synthetic and biological scientists owing to the rarity of their natural origin and their significant biological activities. Herein, we provide a review highlighting some of the interesting bisditerpenoids reported since 1961 and showcase the chemical diversity in both their structures and biosynthesis, as well as their biological functions. This review focuses on the biosynthetic dimerization pathways of interesting molecules and their biomimetic synthesis, which may act as useful inspiration for the discovery and synthesis of more bisditerpenoids and further pharmacological investigations.

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.

Phytochemical and biological studies on rare and endangered plants endemic to China. Part XXII. Structurally diverse diterpenoids from the leaves and twigs of the endangered conifer Torreya jackii and their bioactivities

A phytochemical investigation on the MeOH extract of the leaves and twigs of the endangered conifer Torreya jackii Chun led to the isolation and characterization of 21 structurally diverse diterpenoids. Among them, six are previously undescribed, including four abietane-type (torreyins A-D, resp.) and two labdane-type diterpenoids (torreyins E and F). Their structures and absolute configurations were determined by a combination of spectroscopic methods, calculated/experimental electronic circular dichroism (ECD) data, and single-crystal X-ray diffraction analyses. In particular, torreyins A-C are rare 11,12-seco-abietane type diterpenoids possessing a dilactone moiety, and their biosynthetic pathway starting from a co-occurring abietane derivative (i.e., cyrtophyllone B) was briefly proposed. Among the isolates, 7-oxo-dehydroabietic acid and 15-methoxy-7,13-abietadien-18-oic acid showed considerable inhibitory effects against acetyl-coenzyme A carboxylase 1 (ACC1) and protein tyrosine phosphatase 1 B (PTP1B), with IC₅₀ values of 3.1 and 6.8 μM, respectively.

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.

Discovery of four modified classes of triterpenoids delineated a metabolic cascade: compound characterization and biomimetic synthesis

Chemical studies on Dichapetalum gelonioides have afforded 18 highly modified complex triterpenoids belonging to four compound classes as defined by the newly adapted functional motifs associated with the A ring of the molecules. Their structures were determined by solid data acquired by diverse methods. The biosynthetic pathway for the four compound classes was rationalized via cascade modifications involving diverse chemical events. The subsequent biomimetic syntheses afforded all the desired products, including compounds 16 and 19 that were not obtained in our purification, which validated the proposed biosynthetic pathway. Besides, some compounds exhibited strong cytotoxic activities, especially 2 and 4 showed nanomolar potency against the NAMALWA tumor cell line, and a gross structure–activity relationship (SAR) of these compounds against the tested tumor cell lines was delineated.

Characterization of four classes of highly modified triterpenoids from Dichapetalum gelonioides sheds light on an unprecedented biosynthetic cascade, which was validated by the subsequent biomimetic syntheses. Moreover, some isolates exhibited nanomolar cytotoxic activities.

Nine sesquiterpenoid dimers with four unprecedented types of carbon skeleton from Chloranthus henryi var. hupehensis

Chlorahupetones A–I (1–9), nine sesquiterpenoid dimers, were isolated from Chloranthus henryi var. hupehensis.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as neopetrothiazide from a Neopetrosia species.