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

3,4-seco-Prenyllabdane sesterterpenoids and 3,4-seco-labdane diterpenoids with Zika virus inhibitory potential from Callicarpa nudiflora

Two rearranged prenyllabdane sesterterpenoids nudiflorawus A-B (1-2) with a previously unreported carbon skeleton, and five undescribed 3,4-seco-labdane diterpenoids, nudiflorawus C-G (3-7), along with two known diterpenoids (8-9), were isolated from the leaves of Callicarpa nudiflora. Compounds 1-2 exhibited the first example of 3,4-seco-prenyllabdane sesterterpenoids with a unique six-membered ring in the side chain. Their structures were established via various spectroscopic methods. NMR calculations with DP4+ analysis and ECD were further adopted to confirm their relative and absolute configurations. Compound 6 showed significant Zika virus (ZIKV) inhibitory activity with an EC50 value of 25.35 ± 0.742 μM. Western blot, quantitative real-time PCR, and immunofluorescence results further indicated that compound 6 could block ZIKV infection and replication by inhibiting the expression of ZIKV-envelope protein.

The role and mechanisms of canonical and non-canonical tailoring enzymes in bacterial terpenoid biosynthesis

Covering: up to April 2024Terpenoids represent the largest and structurally most diverse class of natural products. According to textbook knowledge, this diversity arises from a two-step biosynthetic process: first, terpene cyclases generate a vast array of mono- and polycyclic hydrocarbon scaffolds with multiple stereocenters from a limited set of achiral precursors, a process extensively studied over the past two decades. Subsequently, tailoring enzymes further modify these complex scaffolds through regio- and stereocontrolled oxidation and other functionalization reactions, a topic of increasing interest in recent years. The resulting highly functionalized terpenoids exhibit a broad spectrum of unique biological activities, making them promising candidates for drug development. Recent advances in genome sequencing technologies along with the development and application of sophisticated genome mining tools have revealed bacteria as a largely untapped resource for the discovery of complex terpenoids. Functional characterization of a limited number of bacterial terpenoid biosynthetic pathways, combined with in-depth mechanistic studies of key enzymes, has begun to reveal the versatility of bacterial enzymatic processes involved in terpenoid modification. In this review, we examine the various tailoring reactions leading to complex bacterial terpenoids. We first discuss canonical terpene-modifying enzymes, that catalyze the functionalization of unactivated C-H bonds, incorporation of diverse functional groups, and oxidative and non-oxidative rearrangements. We then explore non-canonical terpene-modifying enzymes that facilitate oxidative rearrangement, cyclization, isomerization, and dimerization reactions. The increasing number of characterized tailoring enzymes that participate in terpene hydrocarbon scaffold fomation, rather than merely decorating pre-formed scaffolds suggests that a re-evaluation of the traditional two-phase model for terpenoid biosynthesis might be warranted. Finally, we address the potential and challenges of mining bacterial genomes to identify terpene biosynthetic gene clusters and expand the bacterial terpene biosynthetic and chemical space.

Isolation and Bioinspired Synthesis of Lauenones A and B, Skeleton-Rearranged Diterpenoids with Antiadipogenic Activity from Croton laui

Two skeleton-rearranged labdane diterpenoids, lauenones A (1) and B (2) were isolated from Croton laui. Their bioinspired synthesis was accomplished in 9 and 10 steps, respectively, without using any protecting groups. Key steps include a semipinacol rearrangement and a substrate-controlled one-pot reaction cascade involving photooxidation and aldol condensation. Notably, both lauenones A and B exhibit antiadipogenesis in 3T3-L1 adipocytes by downregulating the differentiation factors and liposynthesis enzymes at mRNA transcription and protein expression levels.

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.

Beyond Essential Oils: Diterpenes, Lignans, and Biflavonoids from Juniperus communis L. as a Source of Multi-Target Lead Compounds

Common Juniper (Juniperus communis L.) is a gymnosperm that stands out through its fleshy, spherical female cones, often termed simply "berries". The cone berries and various vegetative parts (leaves, twigs and even roots) are used in traditional phytotherapy, based on the beneficial effects exerted by a variety of secondary metabolites. While the volatile compounds of Juniperus communis are known for their aromatic properties and have been well-researched for their antimicrobial effects, this review shifts focus to non-volatile secondary metabolites-specifically diterpenes, lignans, and biflavonoids. These compounds are of significant biomedical interest due to their notable pharmacological activities, including antioxidant, anti-inflammatory, antimicrobial, and anticancer effects. The aim of this review is to offer an up-to-date account of chemical composition of Juniperus communis and related species, with a primary emphasis on the bioactivities of diterpenes, lignans, and biflavonoids. By examining recent preclinical and clinical data, this work assesses the therapeutic potential of these metabolites and their mechanisms of action, underscoring their value in developing new therapeutic options. Additionally, this review addresses the pharmacological efficacy and possible therapeutic applications of Juniperus communis in treating various human diseases, thus supporting its potential role in evidence-based phytotherapy.

Talaroterpenoids A-F: Six New Seco-Terpenoids from the Marine-Derived Fungus Talaromyces aurantiacus

Six new highly oxidized seco-terpenoids, including three 3-nor-labdane type diterpenes, talaroterpenoids A-C (1-3), and three meroterpenoids containing an orthoester group, talaroterpenoids D-F (6-8), together with five known compounds (4-5 and 9-11), were isolated from the marine-derived fungus Talaromyces aurantiacus. Their chemical structures were elucidated through 1D, 2D NMR, HRESIMS, J-based configuration analysis (JBCA), computational ECD calculations, and single-crystal X-ray diffraction analysis. Compounds 1 and 2 contain an unusual 6,20-γ-lactone-bridged scaffold. Compounds 10 and 11 presented inhibitory effects on NO release in lipopolysaccharide (LPS)-induced BV-2 cells with IC50 values of 11.47 and 11.32 μM, respectively. Talaroterpenoid C (3) showed moderate antifungal activity against A. alternata and P. theae Steyaert.

Isolation and identification of 3,4-seco-labdane diterpenoids from Callicarpa nudiflora and investigation of their cytotoxicity against HepG2 cells

Twenty-one previously undescribed compounds, including nineteen 3,4-seco-labdanes (nudiflopenes P-W, Y, A_I-J_I), one 3,4-seco-pimarane (nudiflopene X), and one labdane (nudiflopene Z), along with nine known compounds (one 3,4-seco-pimarane and eight 3,4-seco-labdanes) were isolated from the leaves of Callicarpa nudiflora Hook. Et Arn. The structures of these compounds were elucidated by high-resolution electrospray ionization mass spectrometry and one- and two-dimensional nuclear magnetic resonance spectroscopy. In addition, configurations of the isolated compounds were determined by electronic circular dichroism, DP4+ probability analysis, and single-crystal X-ray diffraction experiments. All undescribed compounds were evaluated for their cytotoxicity against HepG2 cells in vitro, among which compound 12 exhibited a moderate activity with an IC₅₀ value of 27.8 μM.

Anti-inflammatory labdane diterpenoids from the aerial parts of Leonurus japonicus

Twenty-two labdane-type diterpenoids, including ten pairs of 15-epimers and a pair of 13,15-epimers, were obtained from the aerial parts of a well-known medicinal plant Leonurus japonicus Houtt. While these epimers were separated by chiral HPLC, their structures were established mainly via spectroscopic methods especially NMR, X-ray crystallography and ECD techniques. Among them, seventeen compounds, encompassing three pairs of solvolysis artefacts likely due to the use of ethanol as extracting solvent, were reported for the first time in the current work. Our preliminary anti-inflammatory screening demonstrated that seven diterpenoids displayed noteworthy inhibitory effect on the NO production in LPS-induced RAW264.7 cells. In addition, the release of pro-inflammatory factors TNF-α, IL-1β and IL-6, as well as the expression of iNOS and COX-2 proteins, was also suppressed by the unreported 15,16-epoxy-6β-hydroxy-15α-methoxy-7,16-dioxolabd-8,13-diene. Further investigation into the preliminary anti-inflammatory mechanism of this compound indicated that it could block the activation of NF-κB signaling pathway.

Nudifloids A-N, structurally diverse 3,4-seco-labdane diterpenoids from Callicarpa nudiflora with inflammatory inhibitory activity

Fourteen undescribed seco-type diterpenoids, named nudifloids A-N, together with ten known analogs, were isolated from the leaves of Callicarpa nudiflora. Nudifloids A-N had a characteristic 3,4-seco-labdane-type diterpenoid skeleton, whereas nudifloids A-C and K-N were 3,4-seco-norditerpenoids. Nudifloid A was the first example of a 3,4-seco-12,13,14,15,16-quartnor-labdane diterpenoid, with a seven-membered lactone ring formed through esterification between C-3 and C-11. Nudifloids B and C were a pair of highly modified 3,4-seco-labdane nor-diterpenoid epimers, of which C-2 and C-18 were cyclized together to form a cyclohexene fragment. The structures of these undescribed diterpenoids were established by spectroscopic analysis and reference data. The anti-inflammatory activity of diterpenoids in rich yield was evaluated by analyzing the inhibition of lipopolysaccharide plus nigericin-induced pyroptosis in J774A.1 cells. Nudifloids D and E exhibited prominent anti-NLRP3 inflammasome activity, with IC₅₀ values of 1.80 and 1.59 μM, respectively. Cell permeability assays revealed that nudifloid D inhibited pyroptosis, which could ameliorate inflammation by blocking the activation of the NLRP3 inflammasome.

Remote B-Ring Oxidation of Sclareol with an Engineered P450 Facilitates Divergent Access to Complex Terpenoids

Though chiral pool synthesis is widely accepted as a powerful strategy in complex molecule synthesis, the effectiveness of the approach is intimately linked to the range of available chiral building blocks and the functional groups they possess. To date, there is still a pressing need for new remote functionalization methods that would allow the installation of useful chemical handles on these building blocks to enable a broader spectrum of synthetic manipulations. Herein, we report the engineering of a P450_BM3 variant for the regioselective C-H oxidation of sclareol at C6. The synthetic utility of the resulting product was demonstrated in a formal synthesis of ansellone B, the first total synthesis of the 2,3-seco-labdane excolide B, and a model study toward (+)-pallavicinin.