Synthesis of Cardiotonic Steroids Oleandrigenin and Rhodexin B

Intriguing steroid glycosides for cancer therapy by suppressing the DNA damage response and mTOR/S6K signaling pathways

Two rare 8-hydroxysteroid glycosides (6-7), and their downstream metabolites (1-5) with an unprecedented 6/6/5/5/5-pentacyclic scaffold, together with seven known analogues (8-14) were isolated from the twigs and leaves of Strophanthus divaricatus. Their structures were fully assigned by analysis of the spectroscopic and ECD data, NMR calculations, X-ray crystallographic study, and chemical methods. In addition, the inhibitory effects of 1-14 on liver and lung cancer cell lines were evaluated, and preliminary structure-activity relationship was discussed. Data-independent acquisition (DIA)-based quantitative proteomic analysis and biological verification of H1299 cells suggested that this family of compounds may play an anticancer role by suppressing both DNA damage response (DDR) and mTOR/S6K signaling pathways.

Cryptic Cerulenin Rearrangement in Ketosynthase Covalent Inhibition

The antibiotic cerulenin is a fungal natural product identified as a covalent inhibitor of ketosynthases within fatty acid and polyketide biosynthesis. Due to its selective and potent inhibitory activity, cerulenin has found significant utility in multidisciplinary biochemical, biomedical, and clinical studies. Although its covalent inhibition profile has been confirmed, cerulenin's mechanism has not been fully determined at a molecular level, frustrating the drug development of related analogues. Herein, we describe the use of stable isotopic tracking with NMR and MS methods to unravel the covalent mechanism of cerulenin against type II fatty acid ketosynthases. We detail the discovery of a unique C2-C3 retro-aldol bond cleavage and a structural rearrangement upon covalent inhibition of cerulenin at the active cysteine residue in E. coli type II fatty acid ketosynthases FabB and FabF.

Synthesis of Bufadienolide Cinobufagin via Late-Stage Singlet Oxygen Oxidation/Rearrangement Approach

This manuscript describes a concise synthesis of cinobufagin, a natural steroid of the bufadienolide family, from readily available dehydroepiandrosterone (DHEA), as well as its α5-epimer derived from 3-epi-andosterone. This synthesis features expedient installation of the 17β-pyrone moiety with the 14β,15β-epoxide and the 16β-acetoxy group using a photochemical regioselective singlet oxygen [4 + 2] cycloaddition followed by CoTPP-promoted in situ endoperoxide rearrangement to provide a 14β,16β-bis-epoxide in 64% yield with a 1.6:1 d.r. This β,β-bis-epoxide intermediate was subsequently subjected to a regioselective scandium(III) trifluoromethanesulfonate catalyzed House-Meinwald rearrangement to establish the 17β-configuration. The synthesis of cinobufagin is achieved in 12 steps (LLS) and 7.6% overall yield, and we demonstrate that it could be used as a platform for the subsequent medicinal chemistry exploration of cinobufagin analogs such as cinobufagin 5α-epimer.

Total Synthesis of (±)-Rubriflordilactone A

A new and efficient synthesis of rubriflordilactone A has been realized. The key transformations include the following: (1) an intramolecular Prins cyclization to establish the seven-membered ring containing two contiguous stereocenters; (2) a Mukaiyama hydration/oxa-Michael cascade to construct the B-ring; and (3) an unprecedented stereocontrol intermolecular o-QM type [4 + 2]-cycloaddition to rapidly assemble core structure of rubriflordilactone A.

Selective (α)-l-Rhamnosylation and Neuroprotective Activity Exploration of Cardiotonic Steroids

This work describes the studies on the direct C3-glycosylation of the C19-hydroxylated cardiotonic steroids strophanthidol, anhydro-ouabagenin, and ouabagenin using a strategy based on in situ protection of the C5 and C19 hydroxyl groups with boronic acids. While this strategy resulted in a successful one-pot C3-selective glycosylation of strophanthidol and anhydro-ouabegenin, it failed to provide ouabain from ouabagenin. The neuroprotective activity of the synthetic and natural glycosides against LPS-induced neuroinflammation was explored in neonatal mouse primary glia cells. Co-administration of natural and synthetic C3-glycosides at 200 nM concentrations resulted in the significant reduction of the LPS-induced neuroinflammatory markers IL-6, IL-1, TNFα, and IKBKE, with the anhydro-ouabagenin-3-(α)-l-rhamnoside (anhydro-ouabain) showing the most significant effect. At the same time, unglycosylated anhydro-ouabagenin enhanced rather than suppressed LPS-induced neuroinflammation.

Development of α-Selective Glycosylation with l-Oleandral and Its Application to the Total Synthesis of Oleandrin

This letter describes the development of an α-selective glycosylation using l-oleandrose, a 2-deoxysugar that is frequently found in natural products, and its application to the total synthesis of the natural cardiotonic steroids oleandrin and beaumontoside. To improve the reaction diastereoselectivity and to minimize side-product formation, an extensive evaluation and optimization of the conditions leading to α-selective glycosylation of digitoxigenin with l-oleandrose-based donors was conducted. These studies led to the exploration of 8 different phosphine·acid complexes or salts and yielded HBr·PPh₃ as the optimal catalyst, which provided in the cleanest α-glycosylation and produced protected beaumontoside in 67% yield. Subsequent application of these conditions to synthetic oleandrigenin afforded the desired α-product in 69% isolated yield─enabling the completion of the first synthesis of oleandrin in 17 steps (1.2% yield) from testosterone.

New chemical processes to streamline carbohydrate synthesis

Carbohydrates hold potential for the future of therapeutic development due to their important role in essential biological processes. However, it is still challenging to produce homogenous materials, especially for non-mammalian sugars that are considered rare. Recent developments in this field have focused on catalytic methods, including organometallic and organocatalytic approaches to regioselective functionalization. Many approaches to glycosylations also utilize catalysts, increasingly in combination with photoredox conditions, to achieve stereoselectivity. Additionally, there have been significant advancements in the automation of glycosylation to synthesize oligosaccharides in less time and with fewer manually conducted steps by the user.

Chemical synthesis of saponins: An update

Saponins, as secondary metabolites in terrestrial plants and marine invertebrate, constitute one of the largest families of natural products. The long history of folk medicinal applications of saponins makes them attractive candidates for innovative drug design and development. Chemical synthesis has become a practical alternative to the availability of the natural saponins and their modified analogs, so as to facilitate SAR studies and the discovery of optimal structures for clinical applications. The recent achievements in the synthesis of these complex saponins reflect the advancements of both steroid/triterpene chemistry and carbohydrate chemistry. This chapter provides an updated review on the chemical synthesis of natural saponins, covering the literature from 2014 to 2020.

Synthesis of Cardiotonic Steroids Oleandrigenin and Rhodexin B

This article describes a concise synthesis of cardiotonic steroids oleandrigenin (7) and its subsequent elaboration into the natural product rhodexin B (2) from the readily available intermediate (8) that could be derived from the commercially available steroids testosterone or DHEA via three-step sequences. These studies feature an expedient installation of the β16-oxidation based on β14-hydroxyl-directed epoxidation and subsequent epoxide rearrangement. The following singlet oxygen oxidation of the C17 furan moiety provides access to oleandrigenin (7) in 12 steps (LLS) and a 3.1% overall yield from 8. The synthetic oleandrigenin (7) was successfully glycosylated with l-rhamnopyranoside-based donor 28 using a Pd(II)-catalyst, and the subsequent deprotection under acidic conditions provided cytotoxic natural product rhodexin B (2) in a 66% yield (two steps).