Novel transition metal-free synthetic protocols toward the construction of 2,3-dihydrobenzofurans: a recent update

2,3-Dihydrobenzofurans are noteworthy scaffolds in organic and medicinal chemistry, constituting the structural framework of many of the varied medicinally active organic compounds. Moreover, a diverse variety of biologically potent natural products also contain this heterocyclic nucleus. Reflecting on the wide biological substantiality of dihydrobenzofurans, several innovative and facile synthetic developments are evolving to achieve these heterocycles. This review summarizes the transition-metal-free, efficient, and novel synthetic pathways toward constructing the dihydrobenzofuran nucleus established after 2020.

Metabolites from traditional Chinese botanical drugs with anti-hepatitis B virus activity - a review

Hepatitis B virus (HBV)-related liver disease poses a major threat to human health worldwide. Although interferon and nucleoside analogues are commonly administered for treating chronic HBV infection, their use is limited by considerable side effects, drug resistance and incapacity for HBV elimination. Hence, novel HBV therapeutics are urgently required. For numerous years, traditional Chinese botanical drugs have been widely used to treat HBV-related diseases. The natural metabolites derived from these traditional drugs exhibit significant anti-HBV effects and serve as potential novel drugs for treating HBV. For overall understanding the therapeutic potential of these metabolites, the anti-HBV effects and mechanisms of action of 107 natural metabolites are summarized in this article. Mechanistically, these natural metabolites exert their anti-HBV effects by influencing the expression and function of host and/or viral genes, which differs from the mechanism of action of nucleoside analogues. Indeed, combining natural metabolites with nucleoside analogues can exert synergistic effects. Accordingly, natural metabolites or their chemically modified derivatives represent potential novel drugs and adjuvants for anti-HBV treatment.

Anti-hepatitis B virus activity of lithospermic acid, a polyphenol from Salvia miltiorrhiza, in vitro and in vivo by autophagy regulation

ETHNOPHARMACOLOGICAL RELEVANCE

Salvia miltiorrhiza (the roots of S. miltiorrhiza Bunge, Danshen in Chinese), a traditional Chinese medicine, has been clinically used to prevent and treat various diseases, such as cardiovascular and cerebrovascular diseases, diabetes, and hepatitis B, in China and some other Asian countries. Lithospermic acid (LA), a polyphenol derived from S. miltiorrhiza, has been reported to exhibit multiple pharmacological properties, such as anti-inflammatory, anti-HIV, and anti-carbon tetrachloride-induced liver injury activities. However, little is known about the anti-hepatitis B virus (HBV) activity of LA.

AIM OF THE STUDY

The study was projected to investigate the anti-HBV activity of LA in vitro (HepG2.2.15 and pHBV1.3-transfected HepG2 cells) and in vivo (pAAV-HBV1.2 hydrodynamic injection [HBV-HDI] mice) and explore the potential mechanism as well.

MATERIALS AND METHODS

Hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg) contents were detected by ELISA kits. HBV DNA and hepatitis B core antigen (HBcAg) levels were evaluated by quantitative real-time polymerase chain reaction and immunohistochemistry assay, respectively. The proteins in autophagy process, lysosomal acidic function, and autophagy-related signaling pathways were examined by Western blot. Transmission electron microscopy was used to observe the number of autophagosomes and autolysosomes. Confocal microscopy was applied to analyze the autophagic flux and lysosomal acidification, using mCherry-enhanced green fluorescent protein (EGFP)-microtubule-associated protein light chain (LC)3 and lysosomal probes, respectively.

RESULTS

LA exhibited anti-HBV activity by inhibiting HBV DNA replication in HepG2.2.15 and pHBV-transfected HepG2 cells in dose- and time-dependent manners and hampering HBsAg and HBeAg levels in HepG2.2.15 cells to a certain extent. LA reduced HBV DNA, HBsAg/HBeAg, and HBcAg levels in the serum/liver tissues of HBV-HDI C57BL/6 mice during the 3-week treatment and suppressed the withdrawal rebound of HBV DNA and HBsAg in the mice serum. LA increased LC3-II protein expression and the number of autolysosomes/autophagosomes and promoted the degradation of sequestosome 1(p62) protein in vitro and in vivo. LA enhanced the co-localization of LC3 protein with autolysosomes, further confirming the ability of LA to induce a complete autophagy. Knockdown of autophagy-related gene (Atg) 7 or 5 in vitro and administration of 3-methyladenine (an autophagic inhibitor) in vivo disabled the inhibitory efficacy of LA on HBV DNA replication, suggesting that the anti-HBV efficacy of LA depended on its ability of inducing autophagy. LA could enhance lysosomal acidification and improve the function of lysosomes by promoting the protein expression of lysosomal-associated membrane protein (LAMP)-1, LAMP-2, and mature cathepsin D, which may contribute to the autophagic induction of LA. LA inhibited the activation of AKT and mammalian target of rapamycin (mTOR) induced by HBV, which was reversed by IGF-1 (an agonist of the PI3K/AKT/mTOR signaling pathway), indicating that LA elicited autophagy through hampering the PI3K/AKT/mTOR signaling pathway.

CONCLUSION

We revealed the anti-HBV activity and mechanism of LA in vitro and in vivo. This study facilitates a new understanding of the anti-HBV potent components of S. miltiorrhiza and sheds light on LA for further development as an active constituent or candidate used in the therapy against HBV infection.

Chemical Profiles and Antiobesity Effect of a Mixture of Astragalus membranaceus and Lithospermum erythrorhizon Extract in High Fat Diet Fed Mice

The present study aimed to evaluate the antiobesity potential and synergistic effects of ALM16, a mixture of Astragalus membranaceus (AM) and Lithospermum erythrorhizon (LE) extracts, in HFD-induced obese mice. C57BL/6 mice were fed a normal diet (ND), high-fat diet (HFD), HFD + AM, HFD + LE or HFD + ALM16 (50, 100, and 200 mg/kg) daily for 5 weeks. Compared to the ND group, HFD-fed mice showed significant increases in body weight, food efficiency ratio, weights of white adipose tissues, adipocytes size, liver weight, and hepatic steatosis grade. However, ALM16 significantly reduced those increases induced by HFD. Moreover, as compared to the HFD group, the ALM16 group significantly ameliorated serum levels of lipid profiles (TG, TC, HDL, and LDL), adipokines (leptin and adiponectin), and liver damage markers (AST and ALT levels). Notably, ALM16 was more effective than AM or LE alone and had a similar or more potent effect than Garcinia cambogia extracts, as a positive control, at the same dose. These results demonstrate that ALM16 synergistically exerts anti-obesity effects based on complementary interactions between each component. Also, metabolic profiling between each extract and the ALM16 was confirmed by UPLC-QTOF/MS, and the difference was confirmed by relative quantification.

Repellency of Veratraldehyde (3,4-Dimethoxy Benzaldehyde) against Mosquito Females and Tick Nymphs

Arthropod-borne infectious diseases cause many deaths and a major economic burden worldwide. Repellents play an important role in protecting people from infectious biting arthropods. The repellency of veratraldehyde, a known food additive, and the WJ-1041 formulation containing 10% veratraldehyde was tested against Aedes albopictus and Culex pipiens pallens females and Haemaphysalis longicornis nymphs using arm-in-cage, indoor or filter paper tests. Veratraldehyde exhibited repellency similar to or lower than that of n,n-diethyl-meta-toluamide (DEET) against A. albopictus, but in H. longicornis, the activity of veratraldehyde was better than that of DEET. The repellency of the 10% veratraldehyde solution was comparable to that of 20% DEET against the two mosquitoes. When comparing repellency between the WJ-1041 formulation (10% veratraldehyde) and 10% DEET against C. pipiens pallens, A. Albopictus and H. longicornis, the two showed similar repellency and complete protection time (CPT) values. However, there was a small difference depending on the tested insects. The absorption of veratraldehyde via skin was minimal, if at all. The pharmacokinetic parameters (Cmax and Tmax) of veratraldehyde in blood samples of rats were not different from those of the control group. Based on these results, veratraldehyde has high potential to be commercialized as a repellent agent against infectious disease-borne pests in the near future.

TFA-mediated synthesis of functionalized pyrano[2,3-c]pyrazoles from pyrazol-3-ones, active carbonyl compounds and tert-BuOH

Functionalized pyrano[2,3-c]pyrazoles were synthesized from 2,4-dihydro-3H-pyrazol-3-ones, active carbonyl compounds and tert-BuOH in the presence of trifluoroacetic acid at 65 °C. These reactions are proceeded by acid catalyzed generation of isobutylene from tert-BuOH for LUMO_diene-controlled inverse [4 + 2] cycloaddition reaction with in situ-generated vinylidenepyrazolones.

Metabolomic Elucidation of the Effect of Sucrose on the Secondary Metabolite Profiles in Melissa officinalis by Ultraperformance Liquid Chromatography-Mass Spectrometry

Sucrose induces flavonoid accumulation in plants as a defense mechanism against various stresses. However, the relationship between the biosynthesis of flavonoids as secondary metabolites and sucrose levels remains unknown. To understand the change in flavonoid biosynthesis by sucrose, we conducted secondary metabolite profiling in Melissa officinalis treated with different levels of sucrose using ultraperformance liquid chromatography/quadrupole time-of-flight mass spectrometry. The partial least squares-discriminant and hierarchical clustering analyses showed significant differences in secondary metabolite profiles in M. officinalis at 50, 150, and 300 mM sucrose levels. The levels of 3 flavonoids such as quercetin 3-O-β-d-glucosyl-(1→2)-β-d-glucoside, 6-methoxyaromadendrin 3-O-acetate, and 3-hydroxycoumarin and 19 flavonoids including 6-methoxyaromadendrin 3-O-acetate, aureusidin, iridin, flavonol 3-O-(6-O-malonyl-β-d-glucoside) quercetin 3-O-glucoside, and rutin increased at 150 and 300 mM sucrose, respectively, compared to 50 mM sucrose, indicating that the flavonoids were accumulated in M. officinalis by a higher concentration of sucrose. This is the first investigation of the change in individual flavonoids as secondary metabolites in M. officinalis by varying sucrose levels, and the results demonstrate that the sucrose causes the accumulation of certain flavonoids as a defense mechanism against osmotic stress.

Synthetic approaches to natural products containing 2,3-dihydrobenzofuran skeleton

This review describes the synthetic approaches to natural products containing the 2,3-dihydrobenzofuran (2,3-DHB) skeleton.

Stereoselective α-Hydroxylation of Amides Using Oppolzer's Sultam as Chiral Auxiliary

An Oppolzer's sultam-based highly stereoselective α-hydroxylation of amides was developed to deliver the desired products in good yield and excellent diastereoselectivity (>20/1). The generally crystalline products and the recyclability of the chiral auxiliary illustrate the practicability and scalability of the current approach.

Bioinspired syntheses of dimeric hydroxycinnamic acids (lignans) and hybrids, using phenol oxidative coupling as key reaction, and medicinal significance thereof

Lignans are mainly dimers of 4-hydroxycinnamic acids (HCAs) and reduced analogs thereof which are produced in Nature through phenol oxidative coupling (POC) as the primary C-C or C-O bond-forming reaction under the action of the enzymes peroxidases and laccases. They present a large structural variety and particularly interesting biological activities, therefore, significant efforts has been devoted to the development of efficient methodologies for the synthesis of lignans isolated from natural sources, analogs and hybrids with other biologically interesting small molecules. We summarize in the present review those methods which mimic Nature for the assembly of the most common lignan skeleta by using either enzymes or one-electron inorganic oxidants to effect POC of HCAs and derivatives, such as esters and amides, or cross-POC of pairs of HCAs or HCAs with 4-hydrocycinnamyl alcohols. We, furthermore, provide outlines of mechanistic schemes accounting for the formation of the coupled products and, where applicable, indicate their potential application in medicine.

Efficient synthesis strategies by application of transition metal-catalyzed carbene/nitrene insertions into C-H bonds

Transition metal-catalyzed insertion of carbenes and nitrenes into C-H bonds has become a powerful tool for the construction of C-C and C-N bonds in the synthesis of complex natural products. In this Highlight, a selection of syntheses are detailed involving the implementation of C-H insertion reactions leading to strategies marked by improved efficiency.

Explorations of caffeic acid derivatives: total syntheses of rufescenolide, yunnaneic acids C and D, and studies toward yunnaneic acids A and B

Yunnaneic acids A-D, isolated from the roots of Salvia yunnanensis , are hexameric (A and B) and trimeric (C and D) assemblies of caffeic acid that feature an array of synthetically challenging and structurally interesting domains. In addition to being caffeic acid oligomers, yunnaneic acids A and B are formally dimeric and heterodimeric adducts of yunnaneic acids C and D. Herein we report the first total syntheses of yunnaneic acids C and D featuring the formation of their bicyclo[2.2.2]octene cores in a single step from simple precursors via an oxidative dearomatization/Diels-Alder cascade that may have biogenetic relevance. In addition, exploitation of the key intermediate resulting from this cascade reaction has enabled rapid access to the structurally related caffeic acid metabolite rufescenolide through an unexpected Lewis acid-mediated reduction. Finally, we report the results of extensive model studies toward forming the dimeric yunnaneic acids A and B. These explorations indicate that the innate reactivities of the monomeric fragments do not favor spontaneous formation of the desired dimeric linkages. Consequently, enzymatic involvement may be required for the biosynthesis of these more complex family members.

Total synthesis of (+)-pentamethylsalvianolic acid C

The total synthesis of a methylated analogue of (+)-Salvianolic acid C has been achieved. Key aspects of the synthetic route include an economical Cu(I) acetylide coupling, unique carboxyl activation conditions via microwave irradiation and a novel lipase catalysed kinetic resolution of a racemic mixture of secondary alcohol Danshensu. The preparation of this methylated analogue will not only improve the bioavailability, but also enable access to new and wider bioactivity applications for (+)-Salvianolic acid C.

Enantioselective total synthesis of (+)-lithospermic acid

An enantioselective synthesis of (+)-lithospermic acid, a potent anti-HIV agent, has been accomplished in a convergent manner in nine steps. The synthesis features an enantioselective intramolecular oxa-Michael addition catalyzed by a quinidine derivative, a hypervalent iodine-mediated rearrangement of chromanone to dihydrobenzofuran, an enantioselective α-oxyamination, and an intermolecular C-H olefination.