Cascade Transformation of the Ansamycin Benzoquinone Core into Benzoxazole Influencing Anticancer Activity and Selectivity

High-yielding total synthesis of embelin, rapanone, and irisoquin A, D, F

Simple and sustainable three- and four-step sequences of di-OH-protection/mono-OMe-deprotection/OrgRC and di-OH-protection/mono-OMe-deprotection/OrgRC/OMe-deprotection protocols were developed to construct biologically active natural products of irisoquin, irisoquin A, irisoquin D, irisoquin F, sorgoleone-364, embelin, rapanone, 5-O-methylembelin, 5-O-methylrapanone and their analogues from the commercially available 2,5-dihydroxy-1,4-benzoquinone, aliphatic aldehydes and Hantzsch ester (1,4-DHP) in very good to excellent yields by using organocatalytic reductive coupling (OrgRC) as key reaction. Many of these natural compounds exhibited a broad spectrum of biological activities including antioxidant, anti-inflammatory, anticonvulsant, anxiolytic, analgesic, anthelmintic, antitumor, antibacterial, and antifertility properties. At the same time, simple and readily available 2,5-dihydroxy-1,4-benzoquinone was transformed into a functionally rich library of 2,5-dihydroxy-3,6-dialkyl-1,4-benzoquinones in very good yields by using sequential OrgRC followed by deprotection reactions and resulting natural/unnatural products would be excellent targets for investigation to show their biological activities compared to known natural products. Further, we tried to synthesize another mero-sesquiterpenoid of (+)-cyclospongiaquinone-1 from a chiral bicyclic aldehyde and hydroxy-1,4-benzoquinone by using OrgRC followed by etherification reactions, but we ended up with the synthesis of their ring-open analogues in good yields. The high-yielding gram-scale chemoselective synthesis of natural products irisoquin and demethylated-irisoquin demonstrated the potential to conduct these processes on larger scales.

Geldanamycin, a Naturally Occurring Inhibitor of Hsp90 and a Lead Compound for Medicinal Chemistry

Geldanamycin remains a driver in the medicinal chemistry of heat shock protein 90 (Hsp90) inhibition, even half a century after its original isolation from nature. This Perspective focuses on the properties of the benzoquinone ring of the natural product that enable a range of functionalization reactions to take place. Therefore, inherent reactivity at C-17, where the methoxy group serves as a vinylogous ester, and at C-19 that demonstrates nucleophilic, enamide-type character toward electrophiles, and also as a conjugate acceptor to react with nucleophiles, has facilitated the synthesis of semisynthetic derivatives. Thus, a range of C-17-substituted amine derivatives has been investigated in oncology applications, with a number of compounds in this series reaching clinical trials. In contrast, the 19-position of geldanamycin has received less attention, although 19-substituted derivatives offer promise with markedly reduced toxicity compared to geldanamycin itself, while retaining Hsp90 inhibitory activity albeit with diminished potency in cellular studies.

The Benzoxazole Heterocycle: A Comprehensive Review of the Most Recent Medicinal Chemistry Developments of Antiproliferative, Brain-Penetrant, and Anti-inflammatory Agents

The benzoxazole is one of the most widely exploited heterocycles in drug discovery. Natural occurring and synthetic benzoxazoles show a broad range of biological activities. Many benzoxazoles are available for treating several diseases, and, to date, a few are in clinical trials. Moreover, an ever-increasing number of benzoxazole derivatives are under investigation in the early drug discovery phase and as potential hit or lead compounds. This perspective is an attempt to thoroughly review the rational design, the structure-activity relationship, and the biological activity of the most notable benzoxazoles developed during the past 5 years (period 2019-to date) in cancers, neurological disorders, and inflammation. We also briefly overviewed each target and its role in the disease. The huge amount of work examined suggests the great potential of the scaffold and the high interest of the scientific community in novel biologically active compounds containing the benzoxazole core.

Geldanamycins: Potent Hsp90 Inhibitors with Significant Potential in Cancer Therapy

Geldanamycin, an ansa-macrolide composed of a rigid benzoquinone ring and an aliphatic ansa-bridge, was isolated from Streptomyces hygroscopicus. Geldanamycin is a potent heat shock protein inhibitor with remarkable antiproliferative activity. However, it shows pronounced hepatotoxicity in animal models and unfavorable pharmacokinetic properties. Four geldanamycin analogs have progressed through various phases of clinical trials, but none have yet completed clinical evaluation or received FDA approval. To enhance the efficacy of these Hsp90 inhibitors, strategies such as prodrug approaches or nanocarrier delivery systems could be employed to minimize systemic and organ toxicity. Furthermore, exploring new drug combinations may help overcome resistance, potentially improving therapeutic outcomes. This review discusses the mechanism of action of geldanamycin, its pharmacokinetic properties, and the various approaches employed to alleviate its toxicity and maximize its clinical efficacy. The main focus is on those derivatives that have progressed to clinical trials or that have shown important in vivo activity in preclinical models.

Diverse ansamycin derivatives from the marine-derived Streptomyces sp. ZYX-F-97 and their antibacterial activities

Four new ansamycin derivatives, named 1,19-epithio-geldanamycin A (1), 17-demethoxylherbimycin H (2), herbimycin M (3), and seco-geldanamycin B (4), together with eight known ansamycin analogues (5-12) were isolated from the solid fermentation of marine-derived actinomycete Streptomyces sp. ZYX-F-97. The structures of new compounds were elucidated by extensive spectroscopic analysis as well as nuclear magnetic resonance (NMR) and electronic circular dichroism (ECD) calculations. All the compounds were assayed for their antibacterial activity. Among them, compounds 4, 8, and 12 exhibited remarkable inhibition against Listeria monocytogenes with minimum inhibitory concentrations (MIC) values ranging from 8 μg·mL-1 to 64 μg·mL-1, and displayed moderate inhibition against methicillin-resistant Staphylococcus aureus (MRSA) with MIC value of 64 μg·mL-1. Compounds 4, 8, 9, and 12 showed moderate inhibition activities against both Staphylococcus aureus and Bacillus subtilis with MIC values ranging from 32 μg·mL-1 to 128 μg·mL-1.