Adventures in bridgehead substitution chemistry: synthesis of polycyclic polyprenylated acylphloroglucinols (PPAPs)

A review of nemorosone: Chemistry and biological properties

Nemorosone is a bicyclic polyprenylated acylphloroglucinol derivative originally isolated from Clusia spp. and it can be obtained through chemical synthesis employing different synthetic strategies. Since its discovery, it has attracted great attention both from a biological and chemical viewpoint. In the present article, we attempted to review various chemical and biological topics around nemorosone, with an emphasis on its antiproliferative activities. For this purpose, relevant data was collected from different scientific databases including Google Scholar, PubMed, Scopus and ISI Web of Knowledge. This natural compound has shown activity against several types of malignancies such as leukemia, human colorectal, pancreatic, and breast cancer because it modulates multiple molecular pathways. Nemorosone has both cytostatic and cytotoxic activity and it also seems to induce apoptosis and ferroptosis. Additionally, it has antimicrobial capabilities against Gram-positive bacteria and parasites belonging to genus Leishmania. Its promising antiproliferative pre-clinical effects deserve further attention for anticancer and anti-parasitic drug development and translation to the clinic.

Strategies towards endo-type B polycyclic polyprenylated acylphloroglucinols: total synthesis of regio-hyperibone L and (+)-epi-clusianone

The first and general method for the total synthesis of regio-hyperibone L and (+)-epi-clusianone via domino Dieckmann cyclization was developed.

Programmable meroterpene synthesis

The bicyclo[3.3.1]nonane architecture is a privileged structural motif found in over 1000 natural products with relevance to neurodegenerative disease, bacterial and parasitic infection, and cancer among others. Despite disparate biosynthetic machinery, alkaloid, terpene, and polyketide-producing organisms have all evolved pathways to incorporate this carbocyclic ring system. Natural products of mixed polyketide/terpenoid origins (meroterpenes) are a particularly rich and important source of biologically active bicyclo[3.3.1]nonane-containing molecules. Herein we detail a fully synthetic strategy toward this broad family of targets based on an abiotic annulation/rearrangement strategy resulting in a 10-step total synthesis of garsubellin A, an enhancer of choline acetyltransferase and member of the large family of polycyclic polyprenylated acylphloroglucinols. This work solidifies a strategy for making multiple, diverse meroterpene chemotypes in a programmable assembly process involving a minimal number of chemical transformations.

Me2AlSEt-Promoted Domino Dieckmann Cyclization Enables the Total Synthesis of Polycyclic Polyprenylated Acylphloroglucinols

A bioinspired, Me₂AlSEt-promoted domino Dieckmann cyclization via an 8-membered ring intermediate to construct bicyclo[3.3.1]nonanes was developed, and the divergent syntheses of nine complex polycyclic polyprenylated acylphloroglucinols were achieved. This novel domino cyclization tolerates a series of congested substrates, providing a very efficient way to construct diverse polycyclic structures. The selectivity and the advantages of the domino cyclization were studied. Moreover, the structure-activity relationship study leads to the identification of three simplified potent antitumor agents.

Regiodivergent Photocyclization of Dearomatized Acylphloroglucinols: Asymmetric Syntheses of (-)-Nemorosone and (-)-6-epi-Garcimultiflorone A

Regiodivergent photocyclization of dearomatized acylphloroglucinol substrates has been developed to produce type A polycyclic polyprenylated acylphloroglucinol (PPAP) derivatives using an excited-state intramolecular proton transfer (ESIPT) process. Using this strategy, we achieved the enantioselective total syntheses of the type A PPAPs (-)-nemorosone and (-)-6-epi-garcimultiflorone A. Diverse photocyclization substrates have been investigated leading to divergent photocyclization processes as a function of tether length. Photophysical studies were performed, and photocyclization mechanisms were proposed based on investigation of various substrates as well as deuterium-labeling experiments.

Research Progress of Polycyclic Polyprenylated Acylphloroglucinols

Polycyclic polyprenylated acylphloroglucinols (PPAPs) are a class of hybrid natural products sharing the mevalonate/methylerythritol phosphate and polyketide biosynthetic pathways and showing considerable structure and bioactivity diversity. This review discusses the progress of research into the chemistry and biological activity of 421 natural PPAPs in the past 11 years as well as in-depth studies of biological activities and total synthesis of some PPAPs isolated before 2006. We created an online database of all PPAPs known to date at http://www.chem.uky.edu/research/grossman/PPAPs . Two subclasses of biosynthetically related metabolites, spirocyclic PPAPs with octahydrospiro[cyclohexan-1,5'-indene]-2,4,6-trione core and complicated PPAPs produced by intramolecular [4 + 2] cycloadditions of MPAPs, are brought into the PPAP family. Some PPAPs' relative or absolute configurations are reassigned or critically discussed, and the confusing trivial names in PPAPs investigations are clarified. Pharmacologic studies have revealed a new molecular mechanism whereby hyperforin and its derivatives regulate neurotransmitter levels by activating TRPC6 as well as the antitumor mechanism of garcinol and its analogues. The antineoplastic potential of some type B PPAPs such as oblongifolin C and guttiferone K has increased significantly. As a result of the recent appearances of innovative synthetic methods and strategies, the total syntheses of 22 natural PPAPs including hyperforin, garcinol, and plukenetione A have been accomplished.

Skeleton Reassignment of Type C Polycyclic Polyprenylated Acylphloroglucinols

The previous assignment of the type C skeleton of polycyclic polyprenylated acylphloroglucinols (PPAPs) was controversial and proved to be incorrect in this study. The structures of the type C PPAPs (3-6) were revised to corresponding type A structures (3a-6a) via ¹³C NMR spectroscopic analysis and a quantum computational chemistry method. Therefore, only types A and B PPAPs are likely present in plants of the family Clusiaceae.

Stereoselective synthesis of bicyclo[3.n.1]alkenone frameworks by Lewis acid-catalysis

An indium-catalysed α,α′-annulation of cyclic ketones and alkynyl enones, leading to bicyclo[3.n.1]alkenones, is presented.

Furanmonogones A and B: two rearranged acylphloroglucinols with a 4,5-seco-3(2H)-furanone core from the flowers of Hypericum monogynum

Rearranged acylphloroglucinols with an unprecedented 4,5-seco-3(2H)-furanone skeleton, furanmonogones A (1) and B (2), were isolated from the flowers of Hypericum monogynum.

Norsampsone E, an unprecedented decarbonyl polycyclic polyprenylated acylphloroglucinol with a homoadamantyl core from Hypericum sampsonii

Norsampsone E, an unprecedented polycyclic polyprenylated acylphloroglucinol from Hypericum sampsonii, featured a homoadamantyl skeleton with the loss of the C-4 carbonyl.

Annulative Methods Enable a Total Synthesis of the Complex Meroterpene Berkeleyone A

Synthetic pathways to complex meroterpenes derived from 3,5-dimethylorsellinic acid (DMOA) and farnesyl pyrophosphate have not been reported despite heavy biosynthetic and medicinal interest. Herein we report the first total synthesis of berkeleyone A, a potential gateway compound to a plethora of fungal-derived meroterpenes, in 13 steps. In addition, we have further developed a novel annulation reaction for the synthesis of hydroxylated 1,3-cyclohexadiones in a single step.

Chiral separation and absolute configurations of two pairs of racemic polyprenylated benzophenones from Hypericum sampsonii

(±) Sampsonins A-B (1-2), two pairs of racemic polyprenylated benzophenones, were isolated from the aerial parts of Hypericum sampsonii and successfully separated by chiral HPLC column. Their structures were elucidated by spectroscopic analyses, X-ray diffraction analysis, and quantum chemical calculation of ECD method. Besides, the plausible biogenetic pathways of 1-2 were proposed, and all of them were evaluated for RXRα transcriptional-inhibitory activities and cytotoxicity against HeLa cells.

Hypersampsones S–W, new polycyclic polyprenylated acylphloroglucinols from Hypericum sampsonii

Five new polycyclic polyprenylated acylphloroglucinols (1–5) with a bicyclo[3.3.1]nonane skeleton, along with five known analogues (6–10), were isolated from the aerial parts of Hypericum sampsonii.

Cambogin Induces Caspase-Independent Apoptosis through the ROS/JNK Pathway and Epigenetic Regulation in Breast Cancer Cells

Cambogin is a polycyclic polyprenylated acylphoroglucinol (PPAP) from the Garcinia genus, which has been used traditionally for cancer treatment across Southeastern Asia. In this study, we found that cambogin inhibited breast cancer cell proliferation and induced cell apoptosis in vitro. Cambogin induced the activation of the caspase-independent mitochondrial apoptotic pathway, as indicated by an increase in the ratio of Bax/Bcl-2 and the nuclear translocation of apoptosis inducing factor (AIF). Two-dimensional gel electrophoresis and mass spectrometry revealed that the expression of proteins involving in the radical oxygen species (ROS) pathway was among the most affected upon cambogin treatment. Cambogin enhanced cellular ROS production, and induced the activation of the ASK1-MKK4/MKK7-JNK/SAPK signaling pathway. Pretreatment with ROS scavenger N-acetylcysteine (NAC), an antioxidant, or the JNK inhibitor SP600125 was able to restore cell viability in the presence of cambogin. Importantly, cambogin treatment led to the activation of activating transcription factor-2 (ATF-2) and the trimethylation of histone H3K9 in the activator protein 1 (AP-1) binding region of the Bcl-2 gene promoter. Finally, cambogin exhibited a potential antitumor effect in MCF-7 breast cancer xenografts without apparent toxicity. Taken in conjunction, the present study indicates that cambogin can induce breast adenocarcinoma cell apoptosis and therefore represents therapeutic potential for cancer treatment.

Asymmetric, stereodivergent synthesis of (-)-clusianone utilizing a biomimetic cationic cyclization

We report a stereodivergent, asymmetric total synthesis of (-)-clusianone in six steps from commercial materials. We implement a challenging cationic cyclization forging a bond between two sterically encumbered quaternary carbon atoms. Mechanistic studies point to the unique ability of formic acid to mediate the cyclization forming the clusianone framework.

The natural compound oblongifolin C inhibits autophagic flux and enhances antitumor efficacy of nutrient deprivation

Metabolic stress induces autophagy as an alternative source of energy and metabolites. Insufficient autophagy in nutrient-deprived cancer cells would be beneficial for cancer therapy. Here, we performed a functional screen in search of novel autophagy regulators from natural products. We showed that oblongifolin C (OC), a natural small molecule compound extracted from Garcinia yunnanensis Hu, is a potent autophagic flux inhibitor. Exposure to OC results in an increased number of autophagosomes and impaired degradation of SQSTM1/p62. Costaining of GFP-LC3B with LysoTracker Red or LAMP1 antibody demonstrates that autophagosome-lysosome fusion is blocked by OC treatment. Furthermore, OC inhibits lysosomal proteolytic activity by altering lysosomal acidification and downregulating the expression of lysosomal cathepsins. Importantly, OC can eliminate the tolerance of cancer cells to nutrient starvation. Starvation dramatically increases the susceptibility of cancer cells to OC-induced CASP3-dependent apoptosis in vitro. Subsequent studies in xenograft mouse model showed that OC has anticancer potency as revealed by increased staining of cleaved CASP3, LC3 puncta, and SQSTM1, as well as reduced expression of lysosomal cathepsins. Combined treatment with OC and caloric restriction potentiates anticancer efficacy of OC in vivo. Collectively, these data demonstrated that OC is a novel autophagic flux inhibitor and might be useful in anticancer therapy.

A P212121 polymorph of (+)-clusianone

The title compound, C33H42O4 [systematic name: (1S,5S,7R)-3-benzoyl-4-hy-droxy-8,8-dimethyl-1,5,7-tris-(3-methyl-but-2--enyl)bi-cyclo-[3.3.1]nona-3-ene-2,9-dione], has a central bi-cyclo-[3.3.1]nonane-2,4,9-trione surrounded by tetra-prenyl-ated and benzoyl groups. The compound was recrystallized several times in methanol using both a slow evaporation method and with a crystal-seeding technique. This subsequently produced diffraction-quality crystals which crystallize in the ortho-rhom-bic space group P212121, in contrast to a previous report of a structure determination in the Pna21 space group [McCandlish et al. (1976 ▶). Acta Cryst. B32, 1793-1801]. The title compound has a melting point of 365-366 K, and a specific rotation [α](20) value of +51.94°. A strong intra-molecular O-H⋯O hydrogen bond is noted. In the crystal, mol-ecules are assembled in the ab plane by weak C-H⋯O inter-actions.