Pallambins A and B, Unprecedented Hexacyclic 19-nor-Secolabdane Diterpenoids from the Chinese Liverwort Pallavicinia ambigua

Bioinspired Total Synthesis of Natural Products

Currently, the frontier challenges in total synthesis pertain to increasing the synthetic efficiency and enabling the divergent synthesis of a number of natural products. Bioinspired synthesis has been well recognized as an effective approach to increasing synthetic efficiency. Especially, when bioinspired synthesis was applied at late-stage skeletal diversification to generate various natural products with distinct carbon skeletons, it held special promise for achieving both goals. In our laboratory, bioinspired synthesis has served as one of two long-standing principles for facilitating the efficient synthesis of natural products. In this Account, we summarize our endeavors and journeys in the bioinspired synthesis of natural products. We categorize our work into three parts based on the imitation of biosynthetic reactions and processes. (1) To mimic the key cyclization steps. Inspired by the biosynthetic process that formed the core skeleton, we developed new synthetic methods to enable the rapid and efficient construction of the core skeletons of the targeted molecules, ultimately leading to their concise total synthesis, for example, seven-step total synthesis of lamellarins D and H featuring three bioinspired oxidative coupling reactions, seven-step total synthesis of clavicipitic acid highlighted by a C-H activation/aminocyclization cascade reaction, eight-step total synthesis of phalarine via a bioinspired oxidative coupling, seven-step total synthesis of α-cyclopiazonic acid, and ten-step total synthesis of speradine C through a bioinspired cascade cyclization reaction initiated by the benzylic carbocation of indole. (2) To mimic the revised biosynthetic pathway proposed by us. In some cases, the proposed biosynthetic processes may be flawed, as they contradict some basic principles of chemistry. Thus, an alternative biosynthetic process must be proposed and investigated. We showcase the total synthesis of euphorikanin A through a bioinspired benzilic acid-type rearrangement and bipolarolides A and B via a bioinspired Prins reaction/ether formation cascade cyclization. (3) To mimic the skeletal diversification process. Nature usually synthesizes a multitude of products from a key common intermediate in a divergent manner. Biogenic skeletal diversification to generate various natural products with distinct carbon skeletons has also drawn our attention. Compared with single-target-oriented synthesis, skeletal-diversity-oriented synthesis of natural products remains underexplored due to its high synthetic challenges. We showcased the divergent total syntheses of ten pallavicinia diterpenoids with three distinct skeletons and six grayanane diterpenoids with three distinct skeletons, which were achieved with unprecedented ease and high efficiency by imitation of the proposed biogenic skeletal diversification process. These two successful projects can serve as inspiration for the application of the bioinspired skeletal diversification strategy to other skeletally diverse natural products.

Synthesis of the bicyclic butenolide core of pallamolide A: a biomimetic approach

Pallamolide A is a 7,8-seco-labdane terpenoid possessing a unique bicyclo[2.2.2]octane core and a spiro-butenolide moiety. A biomimetic synthesis of the bicyclic butenolide core over 10 steps is reported, featuring an unexpected autoxidation ring opening, and a vinylogous Mukaiyama aldol reaction which was spontaneously followed by an unusual intramolecular vinylogous aldol reaction to assemble the spiro-butenolide moiety and bicyclic core of pallamolide A.

Unique Reactivity of the 1,4-Bis(silyloxy)-1,3-cyclopentadiene Moiety: Application to the Synthesis of 7-Norbornanone Derivatives

We describe a method for the synthesis of various 2-silyloxy-2-norbornen-7-ones by exploiting the specific reactivity of the 1,4-bis(silyloxy)-1,3-cyclopentadiene framework, which is generated by the silylation of a 2,2-disubstituted-1,3-cyclopentanedione bearing a picolinoyloxy group at the 2' position of its C-2 side chain. The release of the acyloxy group during the reaction generates carbocations that are then attacked by silyloxy-substituted carbons in the 1,4-bis(silyloxy)-1,3-cyclopentadiene moiety skeleton, forming a 4,5-cis-fused ring skeleton. Skeletal rearrangement of the bicyclic core results in the formation of the corresponding 2-silyloxy-2-norbornen-7-one. This novel transformation of 1,3-cyclopentanedione moieties can be used to synthesise other cyclopentenone-fused bicyclic frameworks.

Total Synthesis of Pallamolides A-E

We have achieved the first total synthesis of pallamolides A-E. Of these compounds, pallamolides B-E possess intriguing tetracyclic skeletons with novel intramolecular transesterifications. Key transformations include highly diastereoselective sequential Michael addition reactions to construct the bicyclo[2.2.2]octane core with the simultaneous generation of two quaternary carbon centers, a one-pot SmI2-mediated intramolecular ketyl-enoate cyclization/ketone reduction to generate the key oxabicyclo[3.3.1]nonane moiety, and an acid-mediated deprotection/oxa-Michael addition/β-hydroxy elimination cascade sequence to assemble the tetracyclic pallamolide skeleton. Kinetic resolution of ketone 14 through Corey-Bakshi-Shibata reduction enabled the asymmetric synthesis of pallamolides A-E.

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.

Pallamins A-C, ent-labdane and pallavicinin based dimers from the Chinese liverwort Pallavicinia ambigua (mitt.) stephani

Three unprecedented ent-labdane and pallavicinin based dimers pallamins A-C formed via [4 + 2] Diels-Alder cycloaddition, together with eight biosynthetically related monomers were isolated from Pallavicinia ambigua. Their structures were determined by the extensive analysis of HRESIMS and NMR spectra. The absolute configurations of the labdane dimers were determined by single crystal X-ray diffraction of the homologous labdane units, ¹³C NMR, and ECD calculations. Moreover, a preliminary evaluation of the anti-inflammatory activities of the isolated compounds was performed using the zebrafish model. Three of the monomers demonstrated significant anti-inflammatory activity.

Diels–Alder adducts of a labdane diterpenoid from the Chinese liverwort Pallavicinia subciliata

A 7,8-seco-2,8-cyclolabdane diterpenoid, pallasubcin A (1), and three pallasubcin A-derived dimers, pallasubcins B–D (2–4), formed via a Diels–Alder reaction, were isolated from the Chinese liverwort Pallavicinia subciliata.

The Total Synthesis of Diquinane-Containing Natural Products

Diquinane or bicyclo[3.3.0]octane is a conspicuous structural unit existing in the carbo-frameworks of a wide range of natural products such as alkaloids and terpenoids. These diquinane-containing molecules not merely exhibit intriguing architectures, but also showcase a broad spectrum of significant bioactivities, which draw widespread attention from the global synthetic community. During the past decade, with an aim to accomplish the total syntheses of such specified cornucopias of natural products, a variety of elegant strategies for construction of the diquinane ring system have been disclosed. In this Minireview, the achievements on this subject in the timeline from 2010 to June 2020 are demonstrated and it is discussed how the diquinane unit is strategically forged in the context of the specific target structure. In addition, impacts of the selected works to the field of natural product total synthesis is highlighted and the particular outlook of diquinane-containing natural product synthesis is provided.

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.

seco-Labdanes: A Study of Terpenoid Structural Diversity Resulting from Biosynthetic C-C Bond Cleavage

The cleavage of a C-C bond is a complexity generating process, which complements oxidation and cyclisation events in the biosynthesis of terpenoids. This process leads to increased structural diversity in a cluster of related secondary metabolites by modification of the parent carbocyclic core. In this review, we highlight the diversifying effect of C-C bond cleavage by examining the literature related to seco-labdanes-a class of diterpenoids arising from such C-C bond cleavage events.

Approaches for the isolation and identification of hydrophilic, light-sensitive, volatile and minor natural products

Water-soluble, volatile, minor and photosensitive natural products are yet poorly known, and this review discusses the literature reporting the isolation strategies for some of these metabolites.

Plagiochianins A and B, Two ent-2,3- seco-Aromadendrane Derivatives from the Liverwort Plagiochila duthiana

Two novel ent-aromadendrane derivatives, plagiochianin A (1), possessing an unprecedented 2,3:6,7-di- seco-6,8-cyclo-aromadendrane carbon scaffold conjugated with three cyclic acetals, and plagiochianin B (2), an exceptional pyridine type aromadendrane alkaloid, were isolated from the Chinese liverwort Plagiochila duthiana. Their structures were established by comprehensive spectroscopic analysis coupled with single-crystal X-ray diffraction and electronic circular dichroism calculations. A plausible biogenetic pathway of these two compounds is presented, and their acetylcholinesterase inhibitory activities are preliminarily tested using TLC-bioautographic assays.

Dual Lewis Acid/Photoredox-Catalyzed Addition of Ketyl Radicals to Vinylogous Carbonates in the Synthesis of 2,6-Dioxabicyclo[3.3.0]octan-3-ones

A combined Lewis acid/photoredox catalyst system enabled the intramolecular umpolung addition of ketyl radicals to vinylogous carbonates in the synthesis of 2,6-dioxabicyclo[3.3.0]octan-3-ones. This reaction proceeded on a variety of aromatic ketones to provide THF rings in good yield (up to 95%). Although diastereoselectivity was found to be modest (1.4-5:1) for the C-C bond forming reaction, the minor diastereomers were converted to 2,6-dioxabicyclo[3.3.0]octan-3-ones by an efficient Lewis acid-mediated epimerization cascade in up to 90% yield.

Strategies to diversify natural products for drug discovery

Natural product libraries contain specialized metabolites derived from plants, animals, and microorganisms that play a pivotal role in drug discovery due to their immense structural diversity and wide variety of biological activities. The strategies to greatly extend natural product scaffolds through available biological and chemical approaches offer unique opportunities to access a new series of natural product analogues, enabling the construction of diverse natural product-like libraries. The affordability of these structurally diverse molecules has been a crucial step in accelerating drug discovery. This review provides an overview of various approaches to exploit the diversity of compounds for natural product-based drug development, drawing upon a series of examples to illustrate each strategy.

Hapmnioides A-C, Rearranged Labdane-Type Diterpenoids from the Chinese Liverwort Haplomitrium mnioides

Many exceptional labdane-type diterpenoids have been exclusively found in liverworts, which serve as taxonomic molecules or play important ecological roles in interactions among organisms. Three unprecedented labdane-type diterpenoids hapmnioides A (1), B (2), and C (3) formed through cascade rearrangement from the Chinese liverwort Haplomitrium mnioides are reported. Their structures were established by comprehensive spectroscopic analysis coupled with single-crystal X-ray diffraction, and their anti-inflammatory activities were also preliminarily tested.

11-Step Total Synthesis of Pallambins C and D

The structurally intriguing terpenes pallambins C and D have been assembled in only 11 steps from a cheap commodity chemical: furfuryl alcohol. This synthesis, which features a redox-economic approach free of protecting-group manipulations, assembles all four-ring systems via a sequential cyclization strategy. Of these four-ring constructing operations, two are classical (Robinson annulation and Mukaiyama aldol) and two are newly devised. During the course of this work a method for the difunctionalization of enol ethers was developed, and the scope of this transformation was explored.

Highly Rigid Labdane-Type Diterpenoids from a Chinese Liverwort and Light-Driven Structure Diversification

Two unprecedented labdane-type diterpenoids haplomintrins A (1) and B (2) with six rings system were isolated from a Chinese liverwort Haplomitrium mnioides. Light-driven reaction of homologous haplomitrenonolides C (6), A (4), and D (3) afforded haplomintrins A-C (1, 2, and 7), respectively, while 4 was converted to more complex congeners haplomintrins D-G (8-11) through intramolecular cyclization. Formation of 1 and 2 from compounds 6 and 4, respectively, helps us to postulate that a photochemical reaction is involved in the biosynthetic pathway. These structure features can be used as molecular markers of H. mnioides, and their allelopathic effects are also preliminarily tested.

Notolutesins A-J, dolabrane-type diterpenoids from the Chinese liverwort Notoscyphus lutescens

Ten new dolabrane-type diterpenoids, notolutesins A-J (1-10), were isolated from the Chinese liverwort Notoscyphus lutescens, along with four known compounds. The structures of the new compounds were established on the basis of extensive spectroscopic data, and that of 1 was confirmed by single-crystal X-ray crystallography. The absolute configuration of 1 was determined by comparing its experimental and calculated electronic circular dichroism spectra. All of the isolates were evaluated for their cytotoxicity against a small panel of human cancer cell lines, and compound 1 exhibited an IC50 value of 6.2 μM against the PC3 human prostate cancer cell line.

Synthesis of Novel Spiro[Cyclopropane-Indazole] Derivatives Via Magnesium-Mediated Conjugate Addition of Bromoform

A novel series of 2,2-dibromo-3-aryl-1'-phenyl-6’,7'-dihydrospiro[cyclopropane-1,5'-indazol]-4'(1' H)-ones derivatives were prepared in moderate to good yields by conjugate addition of bromoform to ( E)-5-arylmethylene-1-phenyl-6,7-dihydro-1 H-indazol-4(5 H)-ones in the presence of magnesium under N2. The structures of all the products were characterised by 1H NMR, 13C NMR, IR spectroscopy, elemental analysis and mass spectrometry.

Diterpenoids from the Chinese liverwort Heteroscyphus tener and their antiproliferative effects

Four new ent-labdane diterpenoids, heteroscyphins A-D (1-4), and four known diterpenoids (5-8) were isolated from the Chinese liverwort Heteroscyphus tener (Steph.) Schiffn. The absolute configuration of compound 1 was defined by single-crystal X-ray diffraction using Cu Kα radiation. Cytotoxicity tests revealed that compounds 3 and 5 exhibited modest activity against seven cancer cell lines. Compound 5 showed inhibitory effects on prostate cancer (PCa) cell proliferation but with less inhibition on non-neoplastic prostate epithelial cells. Compound 5 markedly caused cell growth arrest at the G0/G1 phase and induced cellular apoptosis through ROS-mediated DNA damage in PCa cells.

Secondary metabolites from the Chinese liverwort Cephaloziella kiaeri

Sixteen new clerodane diterpenoids, cephaloziellins A-P (1-16), and two known analogues (17 and 18) were isolated from an EtOH extract of the Chinese liverwort Cephaloziella kiaeri. The structures of the new compounds were elucidated from extensive spectroscopic data (IR, UV, HRESIMS, 1D NMR, and 2D NMR), and the structures of 5, 9, and 15 were confirmed by single-crystal X-ray diffraction analyses. The absolute configurations of all new compounds were established by comparing experimental and calculated electronic circular dichroism spectra.