An expedient synthesis of honokiol and its analogues as potential neuropreventive agents

Neuropharmacological potential of honokiol and its derivatives from Chinese herb Magnolia species: understandings from therapeutic viewpoint

Honokiol is a neolignan biphenol found in aerial parts of the Magnolia plant species. The Magnolia plant species traditionally belong to China and have been used for centuries to treat many pathological conditions. Honokiol mitigates the severity of several pathological conditions and has the potential to work as an anti-inflammatory, anti-angiogenic, anticancer, antioxidant, and neurotherapeutic agent. It has a long history of being employed in the healthcare practices of Southeast Asia, but in recent years, a greater scope of research has been conducted on it. Plenty of experimental evidence suggests it could be beneficial as a neuroprotective bioactive molecule. Honokiol has several pharmacological effects, leading to its exploration as a potential therapy for neurological diseases (NDs), including Alzheimer's disease (AD), Parkinson's disease (PD), cerebral ischemia, anxiety, depression, spinal cord injury, and so on. So, based on the previous experimentation reports, our goal is to discuss the neuroprotective properties of honokiol. Besides, honokiol derivatives have been highlighted recently as possible therapeutic options for NDs. So, this review focuses on honokiol's neurotherapeutic actions and toxicological profile to determine their safety and potential use in neurotherapeutics.

Pharmacological features, health benefits and clinical implications of honokiol

Honokiol (HNK) is a natural polyphenolic compound extracted from the bark and leaves of Magnolia grandiflora. It has been traditionally used as a medicinal compound to treat inflammatory diseases. HNK possesses numerous health benefits with a minimal level of toxicity. It can cross the blood-brain barrier and blood-cerebrospinal fluid, thus having significant bioavailability in the neurological tissues. HNK is a promising bioactive compound possesses neuroprotective, antimicrobial, anti-tumorigenic, anti-spasmodic, antidepressant, analgesic, and antithrombotic features . HNK can prevent the growth of several cancer types and haematological malignancies. Recent studies suggested its role in COVID-19 therapy. It binds effectively with several molecular targets, including apoptotic factors, chemokines, transcription factors, cell surface adhesion molecules, and kinases. HNK has excellent pharmacological features and a wide range of chemotherapeutic effects, and thus, researchers have increased interest in improving the therapeutic implications of HNK to the clinic as a novel agent. This review focused on the therapeutic implications of HNK, highlighting clinical and pharmacological features and the underlying mechanism of action.Communicated by Ramaswamy H. Sarma.

Evaluation of honokiol, magnolol and of a library of new nitrogenated neolignans as pancreatic lipase inhibitors

Obesity is a complex disease defined as an excessive amount of body fat. It is considered a risk factor for several pathologies; therefore, there is an increasing interest in its treatment. Pancreatic lipase (PL) plays a key role in fat digestion, and its inhibition is a preliminary step in the search for anti-obesity agents. For this reason, many natural compounds and their derivatives are studied as new PL inhibitors. This study reports the synthesis of a library of new compounds inspired by two natural neolignans, honokiol (1) and magnolol (2) and bearing amino or nitro groups linked to a biphenyl core. The synthesis of unsymmetrically substituted biphenyls was achieved through an optimisation of the Suzuki-Miyaura cross-coupling reaction followed by the insertion of allyl chains, thus furnishing the O- and/or N-allyl derivatives, and finally, a sigmatropic rearrangement yielding in some cases, the C-allyl analogues. Magnolol, honokiol and the twenty-one synthesised biphenyls were evaluated for their in vitro inhibitory activity toward PL. Three compounds (15b, 16 and 17b) were more effective inhibitors than the natural neolignans (magnolol IC₅₀ = 158.7 µM and honokiol IC₅₀ = 115.5 µM) with IC₅₀ of 41-44 µM. Detailed studies through kinetics suggested better inhibitory activity of the synthetic analogues compared with the natural 1 and 2. Magnolol (K_i = 614.3 µM; K'_i of 140.9 µM) and the synthetic biphenyls 15b (K_i = 286.4 µM; K'_i = 36.6 µM) and 16 (K_i = 176.2 µM; K'_i = 6.4 µM) are mixed-type inhibitors, whereas honokiol (K_i = 674.8 µM) and 17b (K_i = 249 µM) are competitive inhibitors. Docking studies corroborated these findings, showing the best fitting for intermolecular interaction between biphenyl neolignans and PL. The above outcomes highlighted how the proposed structures could be considered interesting candidates for future studies for the development of more effective PL inhibitors.

Small Molecule NF-κB Inhibitors as Immune Potentiators for Enhancement of Vaccine Adjuvants

Adjuvants are added to vaccines to enhance the immune response and provide increased protection against disease. In the last decade, hundreds of synthetic immune adjuvants have been created, but many induce undesirable levels of proinflammatory cytokines including TNF-α and IL-6. Here we present small molecule NF-κB inhibitors that can be used in combination with an immune adjuvant to both decrease markers associated with poor tolerability and improve the protective response of vaccination. Additionally, we synthesize a library of honokiol derivatives identifying several promising candidates for use in vaccine formulations.

Synthesis of Bisphenol Neolignans Inspired by Honokiol as Antiproliferative Agents

Honokiol (2) is a natural bisphenol neolignan showing a variety of biological properties, including antitumor activity. Some studies pointed out 2 as a potential anticancer agent in view of its antiproliferative and pro-apoptotic activity towards tumor cells. As a further contribution to these studies, we report here the synthesis of a small library of bisphenol neolignans inspired by honokiol and the evaluation of their antiproliferative activity. The natural lead was hence subjected to simple chemical modifications to obtain the derivatives 3–9; further neolignans (12a-c, 13a-c, 14a-c, and 15a) were synthesized employing the Suzuki–Miyaura reaction, thus obtaining bisphenols with a substitution pattern different from honokiol. These compounds and the natural lead were subjected to antiproliferative assay towards HCT-116, HT-29, and PC3 tumor cell lines. Six of the neolignans show GI₅₀ values lower than those of 2 towards all cell lines. Compounds 14a, 14c, and 15a are the most effective antiproliferative agents, with GI₅₀ in the range of 3.6–19.1 µM, in some cases it is lower than those of the anticancer drug 5-fluorouracil. Flow cytometry experiments performed on these neolignans showed that the inhibition of proliferation is mainly due to an apoptotic process. These results indicate that the structural modification of honokiol may open the way to obtaining antitumor neolignans more potent than the natural lead.

Total Synthesis of Tedarene A

Tedarene A is a macrocyclic diaryl ether heptanoid isolated from the marine sponge Tedania ignis showing an inhibitory effect against nitric oxide production. The first total synthesis of tedarene A was achieved starting from the commercially available 3-(4-methoxyphenyl)propan-1-ol in nine steps and 15.3% overall yield. The synthetic sequence featured an E,Z-dienic bond introduction and a macrocyclization under Ullman conditions. During the synthesis, the E,E-isomer of tedarene A was also obtained and fully characterized.

Semisynthesis and insecticidal activity of arylmethylamine derivatives of the neolignan honokiol against Mythimna separata Walker

A series of novel arylmethylamine derivatives of honokiol (5a-m) was prepared. Their insecticidal activity was tested against the pre-third-instar larvae of the oriental armyworm (Mythimna separata Walker), a typical lepidopteran pest. Compounds 5a, 5b, 5e, 5h, and 5k exhibited insecticidal activity equal to, or higher than, that of the positive control toosendanin.

Evaluation of anti-HIF and anti-angiogenic properties of honokiol for the treatment of ocular neovascular diseases

Pathological activation of the hypoxia-inducible-factor (HIF) pathway leading to expression of pro-angiogenic genes, such as vascular endothelial growth factor (VEGF), is the fundamental cause of neovascularization in ocular ischemic diseases and cancers. We have shown that pure honokiol inhibits the HIF pathway and hypoxia-mediated expression of pro-angiogenic genes in a number of cancer and retinal pigment epithelial (RPE) cell lines. The crude extracts, containing honokiol, from Magnolia plants have been used for thousands of years in the traditional oriental medicine for a number of health benefits. We have recently demonstrated that daily intraperitoneal injection of honokiol starting at postnatal day (P) 12 in an oxygen induced retinopathy mouse model significantly reduced retinal neovascularization at P17. Here, we evaluate the mechanism of HIF inhibition by honokiol in RPE cells. Using chromatin immunoprecipitation experiments, we demonstrate that honokiol inhibits binding of HIF to hypoxia-response elements present on VEGF promoter. We further show using a number of in vitro angiogenesis assays that, in addition to anti-HIF effect, honokiol manifests potent anti-angiogenic effect on human retinal micro vascular endothelial cells. Our results suggest that honokiol possesses potent anti-HIF and anti-angiogenic properties. These properties of honokiol make it an ideal therapeutic agent for the treatment of ocular neovascular diseases and solid tumors.

Metal-assisted synthesis of unsymmetrical magnolol and honokiol analogs and their biological assessment as GABAA receptor ligands

We present the synthesis of new derivatives of natural products magnolol (1) and honokiol (2) and their evaluation as allosteric ligands for modulation of GABA_A receptor activity. New derivatives were prepared via metal assisted cross-coupling reactions in two consecutive steps. Compounds were tested by means of two-electrode voltage clamp electrophysiology at the α1β2γ2 receptor subtype at low GABA concentrations. We have identified several compounds enhancing GABA induced current (I_GABA) in the range similar or even higher than the lead structures. At 3 μM, compound 8g enhanced I_GABA by factor of 443, compared to 162 and 338 of honokiol and magnolol, respectively. Furthermore, 8g at EC_10–20 features a much bigger window of separation between the α1β2γ2 and the α1β1γ2 subtypes compared to honokiol, and thus improved subtype selectivity.

Suzuki-Miyaura coupling of halophenols and phenol boronic acids: systematic investigation of positional isomer effects and conclusions for the synthesis of phytoalexins from pyrinae

The Suzuki-Miyaura couplings of o-, m-, and p-halophenols with o-, m-, and p-phenol boronic acids were investigated for all combinations under standardized conditions, using Pd/C as a heterogeneous catalyst and water as a solvent. In the case of iodophenols, conventional heating was used, while for bromophenols significantly better results could be obtained using microwave irradiation. This systematic study revealed that 2,4'-biphenol is particularly difficult to access, irrespective of the starting materials used, but that these difficulties can be overcome by using different additives. The conclusions drawn from this investigation allowed us to identify conditions for the protecting group-free or minimized total synthesis of biaryl-type phytoalexins. These compounds possess antibacterial activity and are produced by fruit trees as a response to microbial infection.

Honokiol analogs: a novel class of anticancer agents targeting cell signaling pathways and other bioactivities

Honokiol (3,5-di-(2-propenyl)-1,1-biphenyl-2,2-diol) is a natural bioactive neolignan isolated from the genus Magnolia. In recent studies, honokiol has been observed to have anti-angiogenic, anticancer, anti-inflammatory, neuroprotective and GABA-modulating properties in vitro and in preclinical models. Honokiol and its analogs target multiple signaling pathways including NF-κB, STAT3, EGFR, mTOR and caspase-mediated common pathway, which regulate cancer initiation and progression. Honokiol and its targets of action may be helpful in the development of effective analogs and targeted cancer therapy. In this review, recent data describing the molecular targets of honokiol and its analogs with anticancer and some other bioactivities are discussed.

A strategy for the synthesis of the fargenone/fargenin family of natural products: synthesis of the tricyclic core

A synthesis of the core ring structure of the fargenin/fargenone family of natural products is presented. The general strategy is based upon biosynthetic speculation and exploits a cascade reaction, which transforms a spirocyclic dienone into the core ring system via a deprotonation-oxy-Michael-Wittig olefination sequence. This study represents the first synthesis work towards this family of natural products.