Biomimetic Synthesis of Biologically Active Natural Products: An Updated Review

BACKGROUND

Natural products have optical activities with unusual structural characteristics or specific stereoselectivity, mostly including spiro-ring systems or quaternary carbon atoms. Expensive and time-consuming methods for natural product purification, especially natural products with bioactive properties, have encouraged chemists to synthesize those compounds in laboratories. Due to their significant role in drug discovery and chemical biology, natural products have become a major area of synthetic organic chemistry. Most medicinal ingredients available today are healing agents derived from natural resources, such as plants, herbs, and other natural products.

METHODS

Materials were compiled using the three databases of ScienceDirect, PubMed, and Google Scholar. For this study, only English-language publications have been evaluated based on their titles, abstracts, and full texts.

RESULTS

Developing bioactive compounds and drugs from natural products has remained challenging despite recent advances. A major challenge is not whether a target can be synthesized but how to do so efficiently and practically. Nature has the ability to create molecules in a delicate but effective manner. A convenient method is to imitate the biogenesis of natural products from microbes, plants, or animals for synthesizing natural products. Inspired by the mechanisms occurring in the nature, synthetic strategies facilitate laboratory synthesis of natural compounds with complicated structures.

CONCLUSION

In this review, we have elaborated on the recent syntheses of natural products conducted since 2008 and provided an updated outline of this area of research (Covering 2008-2022) using bioinspired methods, including Diels-Alder dimerization, photocycloaddition, cyclization, and oxidative and radical reactions, which will provide an easy access to precursors for biomimetic reactions. This study presents a unified method for synthesizing bioactive skeletal products.

Discovery of Unusual Sesterterpenoids from Colquhounia coccinea var. mollis and Their Metabolic Implications

Three unusual sesterterpenoids featuring unprecedented rearranged colquhounane (C25) and tetranorcolquhounane (C21) frameworks, colquhounoids E (1) and F (3) and norcolquhounoid F (2), were isolated from a Lamiaceae medicinal plant Colquhounia coccinea var. mollis. Their structures were elucidated by spectroscopic analysis and quantum chemical calculations. A biomimetic inspired regioselective cyclopropane cleavage was achieved under acidic conditions. The immunosuppressive activities of these new sesterterpenoids were also evaluated.

Total Synthesis of Terpenes and Their Biological Significance: A Critical Review

Terpenes are a group of natural products made up of molecules with the formula (C5H8)n that are typically found in plants. They are widely employed in the medicinal, flavor, and fragrance industries. The total synthesis of terpenes as well as their origin and biological potential are discussed in this review.

Spectanoids A-H: Eight undescribed sesterterpenoids from Aspergillus spectabilis

Ten sesterterpenoids, including eight undescribed ones named spectanoids A-H and two known analogs, were obtained from Aspergillus spectabilis. Their structures, including absolute configurations, were determined based on HRESIMS, NMR, ECD calculations and single-crystal X-ray diffraction analyses. Spectanoids A-G are tricyclic sesterterpenoids with an unusual 5/12/5 ring system, while spectanoid H possesses a 5/8/6/5 ring system. All of these compounds were evaluated for their cytotoxic activities against three human cancer cells, and spectanoid A, spectanoid C and spectanoid F exhibited moderate cytotoxic activities with IC₅₀ values ranging from 12.1 to 26.1 μM.

Synthetic Studies toward Leucosceptroid Family of Natural Products

Leucosceptroids are sesterterpenoids with potent antifeedant and antifungal activities. In this paper, efforts on two synthetic strategies toward stereoselective total synthesis of the leucosceptroid family of natural products are reported. Intramolecular addition cyclization strategy could lead to a stereochemically mismatched core structure, while intermolecular addition/ring-closing metathesis cyclization strategy successfully furnished an advanced common intermediate bearing eight contiguous stereogenic centers, including three tetra-substituted ones, which fully matches all the stereochemistry on the tricyclic framework in leucosceptroid H. Late-stage transformation of this intermediate to leucosceptroid H encountered difficulty in oxidizing the secondary hydroxyl group to a carbonyl group in the target. Instead of the desired oxidation, an interesting tricyclic spiral product originating from a C-C bond cleavage was observed.

In Pursuit of Synthetic Efficiency: Convergent Approaches

The field of total synthesis has reached a stage in which emphasis has been increasingly focused on synthetic efficiency rather than merely achieving the synthesis of a target molecule. The pursuit of synthetic efficiency, typically represented by step count and overall yield, is a rich source of inspiration and motivation for synthetic chemists to invent innovative strategies and methods. Among them, convergent strategy has been well recognized as an effective approach to improve efficiency. This strategy generally involves coupling of fragments with similar complexity to furnish the target molecule via subsequent cyclization or late-stage functionalization. Thus, methodologies that enable effective connection of fragments are critical to devising a convergent plan. In our laboratory, convergent strategy has served as a long-standing principle for pursuing efficient synthesis during the course of planning and implementing synthetic projects. In this Account, we summarize our endeavors in the convergent synthesis of natural products over the last ten years. We show how we identify reasonable bond disconnections and employ enabling synthetic methodologies to maximize convergency, leading to the efficient syntheses of over two-dozen highly complex molecules from eight disparate families.In detail, we categorize our work into three parts based on the diverse reaction types for fragment assembly. First, we demonstrate the application of a powerful single-electron reducing agent, SmI₂, in a late-stage cyclization step, forging the polycyclic skeletons of structurally fascinating Galbulimima alkaloids and Leucosceptrum sesterterpenoids. Next, we showcase how three different types of cycloaddition reactions can simultaneously construct two challenging C-C bonds in a single step, providing concise entries to three distinct families, namely, spiroquinazoline alkaloids, gracilamine, and kaurane diterpenoids. In the third part, we describe convergent assembly of ent-kaurane diterpenoids, gelsedine-type alkaloids, and several drug molecules via employing some bifunctional synthons. To access highly oxidized ent-kaurane diterpenoids, we introduce the hallmark bicyclo[3.2.1]octane ring system at an early stage, and then execute coupling and cyclization by means of a Hoppe's homoaldol reaction and a Mukaiyama-Michael-type addition, respectively. Furthermore, we showcase how the orchestrated combination of an asymmetric Michael addition, a tandem oxidation-aldol reaction and a pinacol rearrangement can dramatically improve the efficiency in synthesizing gelsedine-type alkaloids, with nary a protecting group. Finally, to address the supply issue of several drugs, including anti-influenza drug zanamivir and antitumor agent Et-743, we exploit scalable and practical approaches to provide advantages over current routes in terms of cost, ease of execution, and efficiency.

Selective ring-rearrangement or ring-closing metathesis of bicyclo[3.2.1]octenes

Explored was the competitive ring-closing metathesis vs. ring-rearrangement metathesis of bicyclo[3.2.1]octenes prepared by a simple and convergent synthesis from bicyclic alkylidenemalono-nitriles and allylic electrophiles. It was uncovered that ring-closing metathesis occurs exclusively on the tetraene-variant, yielding unique, stereochemically and functionally rich polycyclic bridged frameworks, whereas the reduced version (a triene) undergoes ring-rearrangement metathesis to 5-6-5 fused ring systems resembling the isoryanodane core.

Stereoselective Construction of the Highly Congested Tricyclic Core Structure in Leucosceptroid H

Leucosceptroids are sesterterpenoids with potent antifeedant and antifungal activities. An efficient stereoselective construction of the highly congested [5,6,5] tricyclic framework of leucosceptroid H is presented. This framework bearing eight contiguous stereogenic centers, including three tetrasubstituted ones, could serve as a common intermediate for the collective total synthesis of the leucosceptroid family of natural products.

Leucosceptroid B from glandular trichomes of Leucosceptrum canum reduces fat accumulation in Caenorhabditis elegans through suppressing unsaturated fatty acid biosynthesis

Obesity that is highly associated with numerous metabolic diseases has become a global health issue nowdays. Plant sesterterpenoids are an important group of natural products with great potential; thus, their bioactivities deserve extensive exploration. RNA-seq analysis indicated that leucosceptroid B, a sesterterpenoid previously discovered from the glandular trichomes of Leucosceptrum canum, significantly regulated the expression of 10 genes involved in lipid metabolism in Caenorhabditis elegans. Furthermore, leucosceptroid B was found to reduce fat storage, and downregulate the expression of two stearoyl-CoA desaturase (SCD) genes fat-6 and fat-7, and a fatty acid elongase gene elo-2 in wild-type C. elegans. In addition, leucosceptroid B significantly decreased fat accumulation in both fat-6 and fat-7 mutant worms but did not affect the fat storage of fat-6; fat-7 double mutant. These findings indicated that leucosceptroid B reduced fat storage depending on the downregulated expression of fat-6, fat-7 and elo-2 and thereby inhibiting the biosynthesis of the corresponding unsaturated fatty acid. These findings provide new insights into the development and utilization of plant sesterterpenoids as potential antilipemic agents.

Divergent Synthesis of Antiviral Diterpenes Wickerols A and B

Wickerols A and B are diterpene natural products that have a novel fused 6-5-6-6 ring framework and exhibit potent antiviral activity against the H1N1 type A influenza virus. Herein, we report a divergent synthesis of wickerols A and B in 16 and 15 steps, respectively, from commercial sitolactone. The key reactions of the synthesis are a SmI₂-mediated intramolecular ketone-allylic acetate reductive cyclization, a Claisen rearrangement, and an intramolecular alkylation/aldol reaction that rapidly assembled the compact tetracyclic core framework in a stereocontrolled manner. The work described herein allowed us to confirm the absolute configurations of wickerols A and B.

Total synthesis of sesterterpenoids

Total syntheses of biologically and structurally fascinating sesterterpenoids published between Jan. 2012 and Jan. 2018 are summarized and discussed here.

Asymmetric total synthesis of (+)-astellatol and (−)-astellatene

Here we describe the full account of the total synthesis of (+)-astellatol, as well as the first total synthesis of (−)-astellatene.

Stereoselective Synthesis of the ABC Ring System of Aspterpenacids

Aspterpenacids A and B are sesterterpenoids that possess a unique and highly congested 5/3/7/6/5 fused ring system. These compounds also contain a sterically encumbered isopropyl trans-hydrindane motif and a cyclopropane motif bearing two quaternary centers, which make them remarkably challenging synthetic targets. Herein, we report the successful construction of the key highly substituted ABC ring system in a stereoselective manner.

Total synthesis of complex terpenoids employing radical cascade processes

Covering: 2011-2017Radical cyclizations have a rich history in organic chemistry and have been particularly generous to the field of natural product synthesis. Owing to their ability to operate in highly congested molecular quarters, and with significant functional group compatibility, these transformations have enabled the synthesis of numerous polycyclic terpenoid natural products over the past several decades. Moreover, when programmed accordingly into a synthetic plan, radical cascade processes can be used to rapidly assemble molecular complexity, much in the same way nature rapidly constructs terpene frameworks through cationic cyclization pathways. This review highlights recent total syntheses of complex terpenoids (from 2011-2017) employing C-C bond-forming radical cascade sequences.

Intramolecular Diels-Alder Reactions of Tethered Enoate Substituted Furans Induced by Dialkylaluminum Chloride

Gold(I)-catalyzed cycloisomerization of enynols 11 and 17, obtained by Sonogashira coupling, led to the tethered enoate-substituted furans 14 and 19. While attempts at thermal and several Lewis acid induced intramolecular Diels-Alder reactions remained fruitless, dialkylaluminum chloride led to the formation of hexahydroindene and octahydronaphthalene derivatives 20-23. Their formation can be explained by Lewis acid induced opening of the epoxy bridge with transfer of one alkyl group to the intermediate cycloadduct.

Navigating the Chiral Pool in the Total Synthesis of Complex Terpene Natural Products

The pool of abundant chiral terpene building blocks (i.e., "chiral pool terpenes") has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.

Recent Advances in Substrate-Controlled Asymmetric Cyclization for Natural Product Synthesis

Asymmetric synthesis of naturally occurring diverse ring systems is an ongoing and challenging research topic. A large variety of remarkable reactions utilizing chiral substrates, auxiliaries, reagents, and catalysts have been intensively investigated. This review specifically describes recent advances in successful asymmetric cyclization reactions to generate cyclic architectures of various natural products in a substrate-controlled manner.

Bioinspired total syntheses of terpenoids

Some recent examples of conceptually different bioinspired total syntheses of complex terpenoids are presented that highlight the numerous benefits of pursuing bioinspired strategies.

The direct oxidative diene cyclization and related reactions in natural product synthesis

The direct oxidative cyclization of 1,5-dienes is a valuable synthetic method for the (dia)stereoselective preparation of substituted tetrahydrofurans. Closely related reactions start from 5,6-dihydroxy or 5-hydroxyalkenes to generate similar products in a mechanistically analogous manner. After a brief overview on the history of this group of transformations and a survey on mechanistic and stereochemical aspects, this review article provides a summary on applications in natural product synthesis. Moreover, current limitations and future directions in this area of chemistry are discussed.

Stereoselective synthesis of a highly oxygenated decahydrocyclopenta[g]chromene derivative: the common tricyclic framework of leucosceptrine and leucosesterterpenone

Stereoselective construction of the highly oxygenated decahydrocyclopenta[g]chromene skeleton, which is the tricyclic core of leucosceptrine, which possesses prolylendopeptidase inhibitory activity, and leucosesterterpenone, which exhibits anti-angiogenic activity, from Leucosceptrum canum, was achieved.