A [2 + 2] Cycloaddition Dimer and a Diels–Alder Adduct fromAlpinia katsumadai

Strophiofimbrins A and B: Two Rearranged Norditerpenoids with Novel Tricyclic Carbon Skeletons from Strophioblachia fimbricalyx

Two rearranged norditerpenoids with novel tricyclic carbon skeletons, strophiofimbrin A (1) and strophiofimbrin B (2), were isolated from Strophioblachia fimbricalyx. Their structures were established by 1D/2D NMR spectroscopy, HRESIMS, quantum chemistry calculations, and X-ray diffraction analyses. 1 and 2 represented the first examples of diterpenoids with unprecedented 5/6/7-fused ring systems. In the proposed biosynthetic pathway, they were suspected to derive from cleistanthane norditerpenoids via ring opening, expansion, cyclization, and rearrangement based on the existence of phenanthrenone and cleistanthane diterpenoids from Strophioblachia and Trigonostemon, two closely related genera of the Euphorbiaceae family. Furthermore, compounds 1 and 2 exhibited significant proliferation inhibition and obvious neuroprotective effects.

[2 + 2]-Cycloaddition-derived cyclobutane natural products: structural diversity, sources, bioactivities, and biomimetic syntheses

This review summarizes the structural diversity, bioactivities, and biomimetic synthesis of [2 + 2]-type cyclobutane natural products, along with discussion of their biosynthesis, stereochemical analysis, racemic occurrence, and biomimetic synthesis.

Chalcones, a Privileged Scaffold: Highly Versatile Molecules in [4+2] Cycloadditions

Chalcones are aromatic ketones found in nature as the central core of many biological compounds. They have a wide range of biological activity and are biogenetic precursors of other important molecules such as flavonoids. Their pharmacological relevance makes them a privileged scaffold, advantageous for seeking alternative therapies in medicinal chemistry. Due to their structural diversity and ease of synthesis, they are often employed as building blocks for chemical transformations. Chalcones have a carbonyl conjugated system with two electrophilic centers that are commonly used for nucleophilic additions, as described in numerous articles. They can also participate in Diels-Alder reactions, which are [4+2] cycloadditions between a diene and a dienophile. This microreview presents a chronological survey of studies on chalcones as dienes and dienophiles in Diels-Alder cycloadditions. Although these reactions occur in nature, isolation of chalcones from plants yields very small quantities. Contrarily, synthesis leads to large quantities at a low cost. Hence, novel methodologies have been developed for [4+2] cycloadditions, with chalcones serving as a 2π or 4π electron system.

Substrate-Controlled Regioselective Bromination of 1,2-Disubstituted Cyclobutenes: An Application in the Synthesis of 2,3-Disubstituted Cyclobutenones

Easily available disubstituted cyclobutenes were regioselectively halogenated at the allylic position by means of a reaction with bromine. The regioselectivity of bromination is controlled by the presence of a carbocation-stabilizing group. The prepared disubstituted 3-bromocyclobutenes were converted into the corresponding disubstituted cyclobutenones. On the basis of the performed experiments, the mechanism behind the bromination reaction was also proposed.

Diarylheptanoid-chalcone hybrids with PTP1B and α-glucosidase dual inhibition from Alpinia katsumadai

The EtOH extracts of the dried seeds of Alpinia katsumadai were revealed with hypoglycemic effects on db/db mice at the concentration of 200 mg/kg. In order to clarify the antidiabetic constituents, 16 new diarylheptanoid-chalcone hybrids, katsumadainols A₁-A₁₆ (1-16), together with 13 known analogues (17-29), were isolated from A. katsumadai under the guidance of bioassay. Most of the compounds showed α-glucosidase and PTP1B dual inhibition, among which compounds 1-3, 5-7, 11-14, 21-25, and 27 showed PTP1B/TCPTP selective inhibition with IC₅₀ values ranging from 22.0 to 96.7 μM, which were 2-10 times more active than sodium orthovanadate (IC₅₀, 215.7 μM). All compounds exhibited obvious inhibition against α-glucosidase with IC₅₀ values of 2.9-29.5 μM, indicating 6-59 times more active than acarbose (IC₅₀, 170.9 μM). Study of enzyme kinetics indicated compounds 1, 3, and 12 were PTP1B and α-glucosidase mixed-type inhibitors with K_i values of 13.1, 12.9, 21.6 μM, and 4.9, 7.4, 3.4 μM, respectively.

(+)- and (-)-Alternarilactone A: Enantiomers with a Diepoxy-Cage-like Scaffold from an Endophytic Alternaria sp

The enantiomers (+)- and (-)-alternarilactone A (1), the first examples of dibenzo-α-pyrones bearing a diepoxy-cage-like moiety, were isolated from the endophytic fungus Alternaria sp. hh930. The deficiency in ¹H-¹H COSY and HMBC correlations caused by the highly oxidized caged system of 1 and the deceptive and ambiguous signals such as "W" couplings in NMR data increased the risk of structure misassignment of 1. By performing a quantum chemical calculation of the NMR chemical shifts together with a DP4+ probability analysis and single-crystal X-ray crystallographic experiment, their structures were unambiguously determined, and their absolute configurations were determined by ECD calculations.

Goniothalamin-Related Styryl Lactones: Isolation, Synthesis, Biological Activity and Mode of Action

This review covers the chemistry and biological aspects of goniothalamin-related styryl lactones isolated from natural sources. This family of secondary metabolites has been reported to display diverse uses in folk medicine, but only a limited number of these compounds have been throughly investigated regarding their biological profile. Herein, we cover the goniothalamin-related styryl lactones having a C6-C3-C4 framework which appeared in the literature for the first time in the period 2000-2017, and the reports on the synthesis, biological activity and mechanism of action which were published from 2007-2017.

Three new kavalactone dimers from Piper methysticum (kava)

Three new dimeric kavalactones, designated as diyangonins A-C (1-3), along with two known analogs were isolated from the roots of Piper methysticum. Their structures were elucidated by means of extensive analysis of their 1D, 2D NMR, and mass spectroscopic data. All these dimers possess a skeleton featuring a cyclobutane ring connecting two kavalactone units in head-to-tail or head-to-head mode. Compounds 1-5 were evaluated for their cytotoxic activities against human tumor cell lines.

Multiflorumisides A-G, Dimeric Stilbene Glucosides with Rare Coupling Patterns from the Roots of Polygonum multiflorum

Multiflorumisides A-G (1-7), seven new dimeric stilbene glucosides with two rare coupling patterns, were isolated from the roots of Polygonum multiflorum. The structures of these new dimeric stilbene glucosides were elucidated through comprehensive spectroscopic and chemical analyses. The absolute configurations of 3 and 5-7 were established by comparing their experimental and quantum-chemical ECD data. Putative biosynthetic pathways toward the dimers and their suppressive effects against nitric oxide production in lipopolysaccharide-stimulated RAW264.7 cells are also discussed.

Calyxin Y sensitizes cisplatin-sensitive and resistant hepatocellular carcinoma cells to cisplatin through apoptotic and autophagic cell death via SCF βTrCP-mediated eEF2K degradation

The down-regulation of eukaryotic elongation factor-2 kinase (eEF2K) is associated with an enhancement in the sensitivity of malignant cells to chemotherapeutic agents. In this study, we found that the silencing of eEF2K enhanced cisplatin (CDDP)-induced cytotoxicity in CDDP-sensitive (HepG2) and resistant (HepG2/CDDP) cells. Calyxin Y, a unique chalcone diarylheptanoid adduct, down-regulated eEF2K by promoting Skp1-Cul1-F-box protein (SCF) β-transducin repeat-containing protein (βTrCP)-mediated protein degradation and synergistically enhanced the cytotoxicity of CDDP. Subsequently, we identified a potential mechanism of this cooperative interaction by showing that the combination of calyxin Y and CDDP enhanced apoptotic cell death via mitochondrial dysfunction. In addition, the combination induced autophagy, which contributed to the synergistic cytotoxic effect. Further research revealed that calyxin Y synergistically sensitized HepG2 and HepG2/CDDP cells to CDDP through enhanced apoptotic and autophagic cell death via the SCF βTrCP-eEF2K pathway. Finally, in vivo studies demonstrated that calyxin Y could enhance the response of HepG2/CDDP cells to CDDP in xenograft models with low systemic toxicity. Thus, the combination of calyxin Y and CDDP might represent an attractive therapeutic strategy for the treatment of chemotherapy-sensitive and resistant hepatocellular carcinoma cells.

α-Pyrone derivatives with cytotoxic activities, from the endophytic fungus Phoma sp. YN02-P-3

Four new α-pyrone derivatives phomones C-F (1-4) together with four known compounds (5-8) were isolated from the endophytic fungus Phoma sp. YN02-P-3. Compound 1 is the first example of 6-α,β-unsaturated ester-2-pyrone dimers via intermolecular symmetrical [2+2] cycloaddition. The chemical structures of these compounds were determined from spectroscopic data (1D/2D NMR, MS and IR). The acetylated product (9) of 1 along with compounds 1-8 were then tested for their cytotoxicity against HL-60, PC-3 and HCT-116 cell lines. Compounds 2, 3, 5 and 9 with acetyl groups showed significant inhibitory activities against the three cell lines with IC₅₀ values in the range 0.52-9.85μM. while compounds 1, 4 and 6-8 that possess no acetyl group showed no inhibitory activity (IC₅₀>50μM), indicating that the acetyl group at 10- or 12- are essential for their cytotoxic activities. The structure-activity relationships of these phomones were also reported.

Experimental and theoretical calculation studies on the structure elucidation and absolute configuration of calyxins from Alpinia katsumadai

Six novel calyxins, named calyxin T-W, ent-calyxin T and ent-calyxin U were isolated from the seeds of Alpinia katsumadai Hayata. Their relative configurations were elucidated by means of detailed UV, IR, NMR and MS spectroscopic data. Their absolute configurations were assigned by collaborative studies on single crystal X-ray diffraction analysis, Mosher's method, electronic circular dichroism (ECD), optical rotation and theoretical calculations. These compounds are Friedel-Cranft alkylation adducts composed of coexisted diarylheptanoids and flavanone from the seeds of Alpinia katsumadai. The antiproliferative activity of the six compounds against NCI-H460, HeLa, SMMC-7721 and HCT-116 cell lines was also reported, and most of them showed moderate to strong activities.

Two pairs of enantiomeric α-pyrone dimers from the endophytic fungus Phoma sp. YN02-P-3

(±) Phomones A (1) and B (2), two pairs of novel enantiomeric α-pyrone dimers from the endophytic fungus Phoma sp. YN02-P-3 are reported.

Diasteltoxins A-C, Asteltoxin-Based Dimers from a Mutant of the Sponge-Associated Emericella variecolor Fungus

Three novel asteltoxin-bearing dimers namely diasteltoxins A-C (1-3) along with asteltoxin were isolated from a mutated strain of a sponge-derived fungus Emericella variecolor XSA-07-2. Their structures were determined by extensive spectroscopic analyses including the computed electronic circular dichroism (ECD) data for the configurational assignment. The biogenetic formation of the dimers through [2 + 2] cycloaddition of asteltoxin was postulated. Diasteltoxins 1-3 exerted inhibitory effects against the tumor cell lines H1299 and MCF7 and exhibited significant inhibition against thioredoxin reductase (TrxR).

Involucratusins A-H: Unusual Cadinane Dimers from Stahlianthus involucratus with Multidrug Resistance Reversal Activity

Three novel cadinane dimers, involucratusins A–C (1–3), five unique nor-cadinane-dimers, involucratusins D–H (4–8), together with a known compound (9) were isolated from the rhizomes of Stahlianthus involucratus. Their challenging structures and absolute configurations were determined by spectroscopic data, CD experimentation, chemical conversions and single-crystal X-ray diffraction. Compounds 1–3 are unusual cadinane dimers with new connection and novel cores. Compound 4 is a unique nor-cadinane-dimer, and 5 and 6 are two pairs of hemiketal racemates with novel dinor-cadinane-dimer backbone. Compounds 7 and 8 represent unusual dodecanor-cadinane-dimer and tetradecanor-cadinane-dimer carbon skeletons, respectively. The possible biogenetic pathways of 1–8 were proposed, involving nucleophilic addition, S_N2 nucleophilic displacement, [3 + 3] benzannulation, oxidative cleavage, decarboxylation, and oxidative phenol coupling reactions. Multidrug resistance (MDR) reversal activity assay of the isolates were evaluated in doxorubicin-resistant human breast cancer cells (MCF-7/DOX). The combined use of these novel cadinane dimers at a concentration of 10 μM increased the cytotoxicity of doxorubicin by 2.2–5.8-fold. It is the first report about the MDR reversal activity of cadinane dimers.

Involucratustones A-C: Unprecedented Sesquiterpene Dimers Containing Multiple Contiguous Quaternary Carbons from Stahlianthus involucratus

Involucratustones A-C (1-3), three cadinane dimers containing multiple contiguous quaternary carbons, were isolated from the rhizomes of Stahlianthus involucratus. Their structures were determined by a combination of NMR spectroscopy, chemical conversion, and X-ray diffraction analysis. Compounds 1 and 2 are rearranged homodimers of cadinane sesquiterpene fused with a unique fully substituted 1-oxaspiro[4.4]nonane core observed for the first time in natural products, and 3 is a novel 3',4'-seco-cadinane-dimer. Compounds 1 and 2 exhibited potent cytotoxic activities, and 3 showed notable anti-inflammatory effect.

Bioinspired total synthesis of katsumadain A by organocatalytic enantioselective 1,4-conjugate addition

Katsumadain A, a naturally occurring influenza virus neuraminidase (NA) inhibitor, was synthesized by using a bioinspired, organocatalytic enantioselective 1,4-conjugate addition of styryl-2-pyranone with cinnamaldehyde, followed by a tandem Horner–Wadsworth–Emmons/oxa Michael addition.

Tetracyclic diterpenoids with isomerized isospongian skeleton and labdane diterpenoids from the fruits of Amomum kravanh

Four novel diterpenoids, including three tetracyclic diterpenes with isomerized isospongian skeletons, kravanhins A-C (1-3), and kravanhin D (4), and three new labdane diterpenes (5-7) were isolated from the fruits of Amomum kravanh. Compounds 1-4 had unprecedented isospongian diterpene skeletons with a trans-anti-cis fused tricyclic ring system. The structures of compounds 1-7 were established on the basis of extensive analysis of NMR spectra, CD, and X-ray crystallography. Compound 2 showed inhibitory activity on nitric oxide production in lipopolysaccharide-induced RAW264.7 macrophages with an IC(50) value of 36.2 μM.