Enantioselective Conversion of Oligoprenol Derivatives to Macrocycles in the Germacrene, Cembrene, and 18-Membered Cyclic Sesterterpene Series

Total Synthesis of an Immunosuppressive C25 Macrocyclic Terpenoid Produced by Terpene Synthase (LcTPS2)

Macrocyclic natural products, particularly those with no functionalities except unsaturation, are recognized for their therapeutic potential but are notoriously challenging to synthesize. In this study, we report the first total synthesis of an unconventional 18-membered, C25 macrocyclic terpenoid, which has demonstrated substantial immunosuppressive activity. This synthesis was achieved through strategic modifications and innovative reaction engineering, utilizing α-terpineol and geraniol as starting materials, highlighting a novel approach in macrocyclic terpenoid synthesis.

Climbing the Oxidase Phase Ladder by Using Dioxygen as the Sole Oxidant: The Case Study of Costunolide

Natural sesquiterpenoid lactones are prominent scaffolds in drug discovery. Despite the progress made in their synthesis, their extensive oxidative decoration makes their chemo- and stereoselective syntheses highly challenging. Herein, we report our effort to mimic part of the oxidase phase used in the costunolide pathway to achieve the protecting-group-free total synthesis of santamarine, dehydrocostus lactone, estafiatin, and nine more related natural sesquiterpenoid lactones by using dioxygen as the sole oxidant.

Neurotrophic Natural Products

Neurotrophins (NGF, BDNF, NT3, NT4) can decrease cell death, induce differentiation, as well as sustain the structure and function of neurons, which make them promising therapeutic agents for the treatment of neurodegenerative disorders. However, neurotrophins have not been very effective in clinical trials mostly because they cannot pass through the blood-brain barrier owing to being high-molecular-weight proteins. Thus, neurotrophin-mimic small molecules, which stimulate the synthesis of endogenous neurotrophins or enhance neurotrophic actions, may serve as promising alternatives to neurotrophins. Small-molecular-weight natural products, which have been used in dietary functional foods or in traditional medicines over the course of human history, have a great potential for the development of new therapeutic agents against neurodegenerative diseases such as Alzheimer's disease. In this contribution, a variety of natural products possessing neurotrophic properties such as neurogenesis, neurite outgrowth promotion (neuritogenesis), and neuroprotection are described, and a focus is made on the chemistry and biology of several neurotrophic natural products.

A Cryptic Plant Terpene Cyclase Producing Unconventional 18- and 14-Membered Macrocyclic C25 and C20 Terpenoids with Immunosuppressive Activity

A versatile terpene synthase (LcTPS2) producing unconventional macrocyclic terpenoids was characterized from Leucosceptrum canum. Engineered Escherichia coli and Nicotiana benthamiana expressing LcTPS2 produced six 18-/14-membered sesterterpenoids including five new ones and two 14-membered diterpenoids. These products represent the first macrocyclic sesterterpenoids from plants and the largest sesterterpenoid ring system identified to date. Two variants F516A and F516G producing approximately 3.3- and 2.5-fold, respectively, more sesterterpenoids than the wild-type enzyme were engineered. Both 18- and 14-membered ring sesterterpenoids displayed significant inhibitory activity on the IL-2 and IFN-γ production of T cells probably via inhibition of the MAPK pathway. The findings will contribute to the development of efficient biocatalysts to create bioactive macrocyclic sesterterpenoids, and also herald a new potential in the well-trodden territory of plant terpenoid biosynthesis.

Synthesis of α-exo-Methylene-γ-butyrolactones: Recent Developments and Applications in Natural Product Synthesis

The α-exo-methylene-γ-butyrolactone moiety is present in a vast array of structurally diverse natural products and is often central to their biological activity. In this short review, we summarize new approaches to α-exo-methylene-γ-butyrolactones developed over the past decade as well as their applications in total synthesis.

1 Introduction

2 Approaches to α-exo-Methylene-γ-butyrolactones

2.1 Enantioselective Synthesis via Lactonization Approaches

2.2 Enantioselective Halolactonizations

2.3 Enantioselective Barbier-Type Allylation

2.4 C–H Insertion/Olefination Sequences

2.5 Alkene Cyclization

2.6 Strain-Driven Dyotropic Rearrangement

3 β-(Hydroxymethylalkyl)-α-exo-methylene-γ-butyrolactones

4 Applications in Total Synthesis

4.1 Sesquiterpene Lactones

4.2 Lignans

4.3 Other Monocyclic Natural Products

4.4 Choice of Methodology in Recent Total Syntheses

5 Summary and Outlook

Chirality-matched catalyst-controlled macrocyclization reactions

Macrocycles, formally defined as compounds that contain a ring with 12 or more atoms, continue to attract great interest due to their important applications in physical, pharmacological, and environmental sciences. In syntheses of macrocyclic compounds, promoting intramolecular over intermolecular reactions in the ring-closing step is often a key challenge. Furthermore, syntheses of macrocycles with stereogenic elements confer an additional challenge, while access to such macrocycles are of great interest. Herein, we report the remarkable effect peptide-based catalysts can have in promoting efficient macrocyclization reactions. We show that the chirality of the catalyst is essential for promoting favorable, matched transition-state relationships that favor macrocyclization of substrates with preexisting stereogenic elements; curiously, the chirality of the catalyst is essential for successful reactions, even though no new static (i.e., not "dynamic") stereogenic elements are created. Control experiments involving either achiral variants of the catalyst or the enantiomeric form of the catalyst fail to deliver the macrocycles in significant quantity in head-to-head comparisons. The generality of the phenomenon, demonstrated here with a number of substrates, stimulates analogies to enzymatic catalysts that produce naturally occurring macrocycles, presumably through related, catalyst-defined peripheral interactions with their acyclic substrates.

Efficient Construction of (±)-epi-Costunolide through a Chromium(II)-Mediated Nozaki–Hiyama–Kishi Reaction

(±)-epi-Costunolide has been synthesized through a seven-step procedure starting from (E,E)-farnesol. The key step includes an intramolecular allylation of an aldehyde through a chromium(II)-mediated Nozaki–Hiyama–Kishi reaction, in which more than one equivalent of CrCl2 has been recognized as the most effective reagent to promote the conversion. An anti-inflammatory screen showed that epi-costunolide is a moderate inhibitor of B lymphocyte proliferation.

Proton-accelerated Lewis acid catalysis for stereo- and regioselective isomerization of epoxides to allylic alcohols

The isomerization of epoxides to allylic alcohols was developed via proton-accelerated Lewis acid catalysis. The addition of tBuOH as a proton source is the key to the efficient catalytic cycle. Trisubstituted epoxides, including enantioenriched derivatives, were selectively converted to secondary-allylic alcohols without loss of enantiopurity.

Accelerating Biphasic Biocatalysis through New Process Windows

Process intensification through continuous flow reactions has increased the production rates of fine chemicals and pharmaceuticals. Catalytic reactions are accelerated through an unconventional and unprecedented use of a high-performance liquid/liquid counter current chromatography system. Product generation is significantly faster than in traditional batch reactors or in segmented flow systems, which is exemplified through stereoselective phase-transfer catalyzed reactions. This methodology also enables the intensification of biocatalysis as demonstrated in high yield esterifications and in the sesquiterpene cyclase-catalyzed synthesis of sesquiterpenes from farnesyl diphosphate as high-value natural products with applications in medicine, agriculture and the fragrance industry. Product release in sesquiterpene synthases is rate limiting due to the hydrophobic nature of sesquiterpenes, but a biphasic system exposed to centrifugal forces allows for highly efficient reactions.

Diterpenoids from Euphorbia royleana reverse P-glycoprotein-mediated multidrug resistance in cancer cells

Eight previously undescribed diterpenoids, euphoroyleans A-H, including two cembranes, three ingenanes, two ent-atisanes, and one ent-kaurane, along with 22 known analogues were isolated from the whole plants of Euphorbia royleana. The structures of euphoroyleans A-H, including the absolute configurations, were elucidated by extensive spectroscopic analyses, chemical transformation, and single crystal X-ray diffractions. All the isolates were screened for their chemoreversal abilities on P-glycoprotein (P-gp)-mediated multidrug resistance (MDR) cancer cell line HepG2/DOX, and eight compounds exhibited significant activities. Among them, ingol-3,7,12-triacetate-8-benzoate, the most active MDR modulator with no obvious cytotoxicity, could enhance the efficacy of anticancer drug DOX to ca. 105 folds at 10 μM, being stronger than the positive drug verapamil. Mechanistic study revealed that ingol-3,7,12-triacetate-8-benzoate could inhibit the transport activity of P-gp rather than its expression, and the possible recognition mechanism between compounds and P-gp was predicted by molecular docking.

Advances in chemistry and bioactivity of parthenolide

(−)-Parthenolide is a germacrane sesquiterpene lactone, available in ample amounts from the traditional medical plant feverfew (Tanacetum parthenium).

Stereoselective total synthesis of parthenolides indicates target selectivity for tubulin carboxypeptidase activity

The 2-(silyloxymethyl)allylboration of aldehydes was established to enable stereoselective access to α-(exo)-methylene γ-butyrolactones under mild conditions. Acid-labile functionality and chiral carbonyl compounds are tolerated. Excellent asymmetric induction was observed for β,β'-disubstituted α,β-epoxy aldehydes. These findings led to the enantioselective total synthesis of the sesquiterpene natural product (-)-parthenolide, its unnatural (+)-enantiomer, and diastereoisomers. Among all the isomers tested in cell culture, only (-)-parthenolide showed potent inhibition of microtubule detyrosination in living cells, confirming its exquisite selectivity on tubulin carboxypeptidase activity. On the other hand, the anti-inflammatory activity of the parthenolides was weaker and less selective with regard to compound stereochemistry.

Addressing the Chemistry of Germacrene A by Isotope Labeling Experiments

Despite the central role of germacrene A in sesquiterpene biosynthesis and its widespread occurrence in nature, its complete NMR characterization is still pending. This problem was solved through enzymatic preparation of germacrene A isotopomers that allowed for a full signal assignment to all three conformers. The obtained materials gave insights into the stereochemical course of the Cope rearrangement to β-elemene and uncovered the Cope rearrangement as a new EI-MS fragmentation reaction.

Stereoconfining macrocyclizations in the total synthesis of natural products

This review covers selected examples of point chirality-forming macrocyclizations in natural product total synthesis in the past three decades.