Transannular Acylation Facilitates C5-C9 Bond Formation in Hyperforin Total Synthesis

Hyperforin is considered the flagship congener among polycyclic polyprenylated acylphloroglucinols due to its compelling and complex molecular architecture, coupled with remarkable biological activity, thus rendering it an appealing synthetic target for chemists over the past two decades. Herein, an innovative linear total synthesis of hyperforin is reported. Our synthesis relies on the formation of the bicyclo[3.3.1]nonane-2,4,9-trione framework via transannular acylation of a decorated eight-membered ring, followed by late stage bridgehead substitution.

Photoenolization of α,β-Unsaturated Esters Enables Enantioselective Contra-Thermodynamic Positional Isomerization to α-Tertiary β,γ-Alkenyl Esters

The enantioselective protonation of prochiral enolates is an ideal and straightforward platform to synthesize stereodefined α-tertiary esters, which are recurring motifs in a myriad of biorelevant molecules and important intermediates thereof. However, this approach remains onerous, particularly when dealing with α-unactivated esters and related acids, as enantioinduction on the nascent nucleophile necessitates peremptory reaction conditions, thus far only achieved via preformed enolates. A complementary and contra-thermodynamic catalytic strategy is herein described, where a transient photoenol, in the form of a ketene hemiacetal, is enantioselectively protonated with a chiral phosphoric acid (CPA). The prochiral photoketene hemiacetals are procured from excited α,β-unsaturated esters, specifically from the Z-geometric isomer through [1,5]-hydride shift as a chemically productive nonradiative relaxation pathway. Tautomerization via formal 1,3-proton transfer in the photoketene hemiacetal with CPA as a proton shuttle delivers α-branched β,γ-alkenyl esters in good to excellent yields and enantioselectivity under mild conditions. Furthermore, the current protocol was coupled to functional group interconversion experiments, as well as in a formal total synthesis of a known marine γ-butyrolactone-type metabolite. Performing the reaction in a continuous photoflow setup also enabled a gram-scale synthesis of a β,γ-alkenyl ester with up to 92% ee.

Asymmetric Total Synthesis of (+)-Hyperbeanol A

A bioinspired alkylation dearomatization reaction was developed to construct the 5/6/6 fused-spiro tricyclic core framework and spiro-quaternary carbon chiral center. The usage of this approach for assembling these natural products of spirocyclic polycyclic polyprenylated acylphloroglucinols with an octahydrospiro-[cyclohexan-1,5'-indene] core is demonstrated by the first asymmetric total synthesis of highly oxidized hyperbeanol A.

Single-cell RNA sequencing facilitates the elucidation of the complete biosynthesis of the antidepressant hyperforin in St. John's wort

Hyperforin is the compound responsible for the effectiveness of St. John's wort (Hypericum perforatum) as an antidepressant, but its complete biosynthetic pathway remains unknown. Gene discovery based on co-expression analysis of bulk RNA-sequencing data or genome mining failed to discover the missing steps in hyperforin biosynthesis. In this study, we sequenced the 1.54-Gb tetraploid H. perforatum genome assembled into 32 chromosomes with the scaffold N50 value of 42.44 Mb. By single-cell RNA sequencing, we identified a type of cell, "Hyper cells", wherein hyperforin biosynthesis de novo takes place in both the leaves and flowers. Through pathway reconstitution in yeast and tobacco, we identified and characterized four transmembrane prenyltransferases (HpPT1-4) that are localized at the plastid envelope and complete the hyperforin biosynthetic pathway. The hyperforin polycyclic scaffold is created by a reaction cascade involving an irregular isoprenoid coupling and a tandem cyclization. Our findings reveal how and where hyperforin is biosynthesized, enabling synthetic-biology reconstitution of the complete pathway. Thus, this study not only deepens our comprehension of specialized metabolism at the cellular level but also provides strategic guidance for elucidation of the biosynthetic pathways of other specializied metabolites in plants.

The Total Synthesis of Hyperfirin via a Cyclooctadiene Strategy

Polycyclic polyprenylated acylphloroglucinols (PPAPs) combine compelling structural complexity with effective biological activity. The total synthesis of Hyperfirin is reported as one linear sequence. Key to this novel modular strategy is to access the bicyclo[3.3.1]nonane-2,4,9-trione framework via transannular acylation of a decorated eight-membered ring, followed by late stage bridgehead substitution. The described route adds flexibility to PPAP construction and broadens the scope of eight-membered ring chemistry.

Total Synthesis of Hypersampsone M

We report the first total synthesis of hypersampsone M, an archetypal member of the homoadamantane polycyclic polyprenylated acylphloroglucinols (PPAPs). Commencing from cyclohexenone, a key cyclopentene annulation followed by ring-expansion results in an elusive hydrazulene that undergoes a series of unexpected late-stage transformations, ultimately enabling completion of the synthesis. The route detailed herein represents a potentially general strategy for the synthesis of related homoadamantane PPAPs.

Sequential Dieckmann cyclization enables the total synthesis of 7-epi-clusianone and 18-hydroxy-7-epi-clusianone

A unified approach for the construction of the bicyclo[3.3.1]nonane-2,4,9-trione core of polycyclic polyprenylated acylphloroglucinols (PPAPs) was reported. This approach involves a sequential process of two distinct Dieckmann condensation reactions from the linear precursor. Using this method, the divergent total synthesis of the natural products 7-epi-clusianone and 18-hydroxy-7-epi-clusianone and the formal synthesis of sampsonione P were achieved. Additionally, other key steps to realize this strategy include RuCl3-catalyzed oxidative olefin cleavage and Pd-catalyzed Tsuji-Trost decarboxylative allylation. The synthesis indicated that bicyclo[3.3.1]nonane-2,4,9-triones could also be constructed via 6-membered intermediates.

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.

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.

Asymmetric Intramolecular Hydroalkylation of Internal Olefin with Cycloalkanone to Directly Access Polycyclic Systems

An asymmetric intramolecular hydroalkylation of unactivated internal olefins with tethered cyclic ketones was realized by the cooperative catalysis of a newly designed chiral amine (SPD-NH2 ) and PdII complex, providing straightforward access to either bridged or fused bicyclic systems containing three stereogenic centers with excellent enantioselectivity (up to 99 % ee) and diastereoselectivity (up to >20 : 1 dr). Notably, the bicyclic products could be conveniently transformed into a diverse range of key structures frequently found in bioactive terpenes, such as Δ6 -protoilludene, cracroson D, and vulgarisins. The steric hindrance between the Ar group of the SPD-NH2 catalyst and the branched chain of the substrate, hydrogen-bonding interactions between the N-H of the enamine motif and the C=O of the directing group MQ, and the counterion of the PdII complex were identified as key factors for excellent stereoinduction in this dual catalytic process by density functional theory calculations.

Bioinspired Total Synthesis of Hyperireflexolides A and B

Hyperireflexolides A and B were synthesized in six steps via the dearomatization and fragmentation of a simple acylphloroglucinol starting material. The dearomatized acylphloroglucinol undergoes a sequence of oxidative radical cyclization, retro-Dieckmann fragmentation, stereodivergent intramolecular carbonyl-ene reactions, and final α-hydroxy-β-diketone rearrangements to give the target natural products. This sequence is based on a biosynthetic proposal that claims the hyperireflexolides as highly rearranged polycyclic polyprenylated acylphloroglucinols (PPAPs), which is supported by the structural revision of hyperireflexolide B.

Intramolecular Tricarbonyl-Ene Reactions and α-Hydroxy-β-Diketone Rearrangements Inspired by the Biosynthesis of Polycyclic Polyprenylated Acylphloroglucinols

Structurally unique natural products pose biosynthetic puzzles whose solution can inspire new chemical reactions. Herein, we propose a unified biosynthetic pathway towards some complex meroterpenoids-the hyperireflexolides, biyoulactones, hybeanones and hypermonones. This hypothesis led to the discovery of uncatalyzed, intramolecular carbonyl-ene reactions that are spontaneous at room temperature. We also developed an anionic cascade reaction featuring an α-hydroxy-β-diketone rearrangement and an intramolecular aldol reaction to access four distinct natural product scaffolds from a common intermediate.