Allylative Approaches to the Synthesis of Complex Guaianolide Sesquiterpenes from Apiaceae and Asteraceae

Expanding Natural Diversity: Tailored Enrichment of the 8,12-Sesquiterpenoid Lactone Chemical Space through Divergent Synthesis

The divergent synthesis of a non-natural 8,12-sesquiterpenoid lactone collection is described. The synthesis relies on a rationally designed guaianolide scaffold bearing a tertiary hydroxyl as the pinpoint for inducing its selective diversification. Key reactions include an unprecedented Suarez-type CH lactonization and a highly diastereoselective oxy-Cope/ene cascade that allows the introduction of three stereocenters in a single operation. Selective oxidative/reductive and redox neutral transformations follow to highlight the synthesis of naturally unpresented highly substituted 8,12-guaianolides.

Hypocretenolides: collective total syntheses and activities toward metastatic colon cancer

A concise and collective synthetic route to hypocretenolides was developed for the first time. This route features one-pot addition-alkylation and intramolecular 1,3-dipolar cycloaddition to efficiently assemble the 5/7/6 ring system. Our syntheses enabled multigram preparation of hypocretenolide which facilitated further biological evaluation. Preliminary CCK-8 cytotoxic results of hypocretenolide indicated its IC50 values within 1 μM against 4 colon cancer cell lines. Wound healing and transwell assays suggested the promising inhibitory activities of hypocretenolide toward the migratory capabilities of colon cancer cells in vitro. The animal results confirmed that hypocretenolide can inhibit metastasis of colon cancer cells.

Six undescribed guaianolide-type sesquiterpenes from the aerial parts of Daphne penicillata

Six undescribed guaianolide sesquiterpenes (1-6) were obtained from the aerial parts of Daphne penicillata. Their structures and absolute configuration were elucidated by HRESIMS, NMR analyses, ECD calculations and single-crystal X-ray diffraction analysis. Structurally, all compounds possess the typical 5,7-fused system of 8,12-guaianolides and this guaianolide-type was first reported to be isolated from Daphne penicillata. All compounds (1-6) were evaluated for anti-inflammatory and cytotoxic activity. Among them, compounds 1 and 5 showed moderate inhibitory effects on LPS-induced NO production in BV2 cells and 4 displayed potential inhibition against Hep3B cells with an IC50 value of 7.33 μM.

Deciphering the quest in the divergent total synthesis of natural products

The divergent synthesis of natural products is rapidly developing towards achieving the goal of efficiency and economy in total synthesis. However, presently, the sustainable development of the synthesis of natural products does not permit the linear synthesis of a single target. In this case, divergent total synthesis is based on the identification of an advanced intermediate with structural features that can be mapped in more than two molecules. However, the identification of this intermediate and its scalable synthesis in enantiopure form are challenging. Herein, we present the details of the ingenious efforts by researchers in the last six years toward the divergent synthesis of two to as many as eight natural products initially via a single route, and then diverging from a common intermediate and further branching out toward several natural products. The planning and strategies adopted can serve as guidelines for the future development of efficient divergent routes aimed at achieving higher efficiency toward multiple targets, causing divergent synthesis to become an accepted common practice.

Recent advances in the syntheses of guaianolides

Sesquiterpene lactones, especially guaianolides representing a bigger class of natural products, have served as appealing candidates for total synthesis due to their varied bio- and pharmaceutical activities. This tutorial review delineates the creative efforts of many researchers in the total syntheses of different complex guaianolides recently published in the literature. Many of the syntheses display meticulous interplay between new methods and the ingenuity of strategies achieved through well-planned routes. In some cases, the Chiron approach has come in quite handy, wherein the structural features and stereochemistry of select molecules could map well with naturally available starting materials. A few catalytic methods like diastereoselective aldol reaction, enediyne or dienyne metathesis, or photochemical methods have been efficiently used. This compilation also aims to enhance the diversity space based on these natural products and further interest in the sustainable total synthesis of this class of compounds and related molecules.

Short Scalable Route to Apiaceae Sesquiterpene Scaffolds: Total Synthesis of 4-epi-Epiguaidiol A

The oxy-Cope/ene reaction cascade to form a locked elemane conformer allows the short scalable synthesis of versatile Apiaceae scaffolds. The divergent fate of the obtained macrocyclic germacrane is surveyed under cationic and dioxygen-induced Prins-type reaction conditions to allow the diastereoselective synthesis of oxidized Apiaceae guaiane congeners and the total synthesis of 4-epi-epiguaidiol A. Additionally, the unprecedented reduction of a hydrogen-bond-biased guaiane substrate permits the chemoselective synthesis of desoxo-jungiaguaiane.

Asymmetric Total Synthesis of Eupalinilide E, a Promoter of Human HSPC Expansion

A concise and scalable asymmetric total synthesis of eupalinilde E from (R)-(-)-carvone in 12 steps is reported with an overall yield of 20%. The key steps of the synthesis are a tandem Favorskii rearrangement-elimination reaction in the chromatography-free synthesis of carvone-derived 2-cyclopentene carbaldehyde and its catalyst-free stereospecific tandem allylboration-lactonization using recyclable trifluoroethanol as a promoter and solvent affording β-hydroxymethyl-α-methylene-γ-butyrolactone.

Total Synthesis of Natural Terpenoids Enabled by Cobalt Catalysis

Transition metal catalysis plays an essential role in the total synthesis of natural products. Cobalt-mediated asymmetric catalysis has successfully been used as a primary or a secondary step in the total synthesis of natural products, especially terpenoids. Terpenoids represent one of the most prominent families among various categories of natural products, attracting immense attention due to their promising physiological activities. This review summarizes the recent advances toward the total synthesis of terpenoids by cobalt-mediated asymmetric catalysis, which may shed some light on their future synthetic efforts toward natural pesticides such as celanguline, azadirachtin, etc.

Total Syntheses of Proposed Structures of 4,10-Dihydroxy-8,12-guaianolides

The first total syntheses of two 4,10-dihydroxy-8,12-guaianolides that were reported to be natural products were achieved. Toward the syntheses of a collection of related guaianolides, the typical 5,7-fused system of 8,12-guaianolides was constructed by a ring expansion reaction of a hydroxylated coronafacic acid analogue that can be practically synthesized and optically resolved. The total syntheses of these compounds revealed that the previously reported structures of both natural products were incorrect.

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

Function-Oriented and Modular (+/-)-cis-Pseudoguaianolide Synthesis: Discovery of New Nrf2 Activators and NF-κB Inhibitors

Described herein is a function-oriented synthesis route and biological evaluation of pseudoguaianolide analogues. The 10-step synthetic route developed retains the topological complexity of the natural product, installs functional handles for late-stage diversification, and forges the key bioactive Michael acceptors early in the synthesis. The analogues were found to be low-micromolar Nrf2 activators and micromolar NF-κB inhibitors and dependent on the local environment of the Michael acceptor moieties.

Total Synthesis of ent-Plagiochianin B

An enantioselective total synthesis of plagiochianin B is described that employs (+)-3-carene as its point of departure and delivers the enantiomer of the natural product. Key features of the synthesis include a palladium-mediated regioselective oxidative cleavage of an olefin residing on a pyridine derived from a 6π-azatriene electrocyclization.

Evolution of a Synthetic Strategy for Complex Polypyrrole Alkaloids: Total Syntheses of Curvulamine and Curindolizine

Structurally unprecedented antibacterial alkaloids containing multiple electron-rich pyrrole units have recently been isolated from Curvularia sp. and Bipolaris maydis fungi. This article documents the evolution of a synthetic program aimed at accessing the flagship metabolites curvulamine and curindolizine which are presumably a dimer and trimer of a C₁₀N biosynthetic building block, respectively. Starting with curvulamine, we detail several strategies to merge two simple, bioinspired fragments, which while ultimately unsuccessful, led us toward a pyrroloazepinone building block-based strategy and an improved synthesis of this 10π-aromatic heterocycle. A two-step annulation process was then designed to forge a conserved tetracyclic bis-pyrrole architecture and advanced into a variety of late-stage intermediates; unfortunately, however, a failed decarboxylation thwarted the total synthesis of curvulamine. By tailoring our annulation precursors, success was ultimately found through the use of a cyanohydrin nucleophile which enabled a 10-step total synthesis of curvulamine. Attempts were then made to realize a biomimetic coupling of curvulamine with an additional C₁₀N fragment to arrive at curindolizine, the most complex family member. Although unproductive, we developed a 14-step total synthesis of this alkaloid through an abiotic coupling approach. Throughout this work, effort was made to harness and exploit the innate reactivity of the pyrrole nucleus, an objective which has uncovered many interesting findings in the chemistry of this reactive heterocycle.

Metal-hydride hydrogen atom transfer (MHAT) reactions in natural product synthesis

Functionalization of olefins has been an important transformation in synthetic chemistry. This review will focus on the natural product synthesis employing the MHAT reaction as the key strategy.

Evolution of Pauson-Khand Reaction: Strategic Applications in Total Syntheses of Architecturally Complex Natural Products (2016–2020)

Metal-mediated cyclizations are important transformations in a natural product total synthesis. The Pauson-Khand reaction, particularly powerful for establishing cyclopentenone-containing structures, is distinguished as one of the most attractive annulation processes routinely employed in synthesis campaigns. This review covers Co, Rh, and Pd catalyzed Pauson-Khand reaction and summarizes its strategic applications in total syntheses of structurally complex natural products in the last five years. Additionally, the hetero-Pauson-Khand reaction in the synthesis of heterocycles will also be discussed. Focusing on the panorama of organic synthesis, this review highlights the strategically developed Pauson-Khand reaction in fulfilling total synthetic tasks and its synthetic attractiveness is aimed to be illustrated.

Syntheses of Complex Terpenes from Simple Polyprenyl Precursors

From structure elucidation and biogenesis to synthetic methodology and total synthesis, terpene natural products have profoundly influenced the development of organic chemistry. Moreover, their myriad functional attributes range from fragrance to pharmaceuticals and have had great societal impact. Ruzicka's formulation of the "biogenetic isoprene rule," a Nobel Prize winning discovery now over 80 years old, allowed for identification of higher order terpene (aka "isoprenoid") structures from simple five-carbon isoprene fragments. Notably, the isoprene rule still holds pedagogical value to students of organic chemistry today. Our laboratory has completed syntheses of over two dozen terpene and meroterpene structures to date, and the isoprene rule has served as a key pattern recognition tool for our synthetic planning purposes. At the strategic level, great opportunity exists in finding unique and synthetically simplifying ways to connect the formal C₅ isoprene fragments embedded in terpenes. Biomimetic cationic polyene cyclizations represent the earliest incarnation of this idea, which has facilitated expedient routes to certain terpene polycycle classes. Nonetheless, a large swath of terpene chemical space remains inaccessible using this approach.In this Account, we describe strategic insight into our endeavors in terpene synthesis published over the last five years. We show how biosynthetic understanding, combined with a desire to utilize abundant and inexpensive [C₅]_n building blocks, has led to efficient, abiotic syntheses of multiple complex terpenes with disparate ring systems. Informed by nature, but unconstrained by its processes, our synthetic assembly exploits chemical reactivity across diverse reaction types-including radical, anionic, pericyclic, and metal-mediated transformations.First, we detail an eight-step synthesis of the cembrane diterpene chatancin from dihydrofarnesal using a bioinspired-but not -mimetic-cycloaddition. Next, we describe the assembly of the antimalarial cardamom peroxide using a polyoxygenation cascade to fuse multiple units of molecular oxygen onto a dimeric skeleton. This three-to-four-step synthesis arises from (-)-myrtenal, an inexpensive pinene oxidation product. We then show how a radical cyclization cascade can forge the hallmark cyclooctane ring system of the complex sesterterpene 6-epi-ophiobolin N from two simple polyprenyl precursors, (-)-linalool and farnesol. To access the related, more complex metabolite 6-epi-ophiobolin A, we exploited the plasticity of our synthetic route and found that use of geraniol (C₁₀) rather than farnesol (C₁₅) gave us the flexibility needed to address the additional oxidation found in this congener. Following this work, we describe two strategies to access several guaianolide sesquiterpenes. Retrosynthetic disconnection to monoterpenes, carvone or (-)-linalool, coupled with a powerful allylation strategy allowed us to address guaianolides with disparate stereochemical motifs. Finally, we examine a semisynthetic approach to the illicium sesquiterpenes from the abundant 15-carbon feedstock terpene (+)-cedrol using an abiotic ring shift and multiple C-H oxidation reactions inspired by a postulated biosynthesis of this natural product class.