Synthesis of skeletally diverse and stereochemically complex library templates derived from isosteviol and steviol

Recent ring distortion reactions for diversifying complex natural products

Covering: 2013-2022.Chemical diversification of natural products is an efficient way to generate natural product-like compounds for modern drug discovery programs. Utilizing ring-distortion reactions for diversifying natural products would directly alter the core ring systems of small molecules and lead to the production of structurally complex and diverse compounds for high-throughput screening. We review the ring distortion reactions recently used in complexity-to-diversity (CtD) and pseudo natural products (pseudo-NPs) strategies for diversifying complex natural products. The core ring structures of natural products are altered via ring expansion, ring cleavage, ring edge-fusion, ring spiro-fusion, ring rearrangement, and ring contraction. These reactions can rapidly provide natural product-like collections with properties suitable for a wide variety of biological and medicinal applications. The challenges and limitations of current ring distortion reactions are critically assessed, and avenues for future improvements of this rapidly expanding field are discussed. We also provide a toolbox for chemists for the application of ring distortion reactions to access natural product-like molecules.

Preparation of Antiproliferative Terpene-Alkaloid Hybrids by Free Radical-Mediated Modification of ent-Kauranic Derivatives

A convenient strategy for molecular editing of available ent-kauranic natural scaffolds has been developed based on radical mediated C-C bond formation. Iodine atom transfer radical addition (ATRA) followed by rapid ionic elimination and radical azidoalkylation were investigated. Both reactions involve radical addition to the exo-methylenic double bond of the parent substrate. Easy transformations of the obtained adducts lead to extended diterpenes of broad structural diversity and artificial diterpene-alkaloid hybrids possessing lactam and pyrrolidine pharmacophores. The cytotoxicity of selected diterpenic derivatives was examined by in vitro testing on several tumor cell lines. The terpene-alkaloid hybrids containing N-heterocycles with unprecedented spiro-junction have shown relevant cytotoxicity and promising selectivity indexes. These results represent a solid basis for following research on the synthesis of such derivatives based on available natural product templates.

Synthesis of γ-Lactams by Formal Cycloadditions with Ketenes

The synthesis of γ-lactams is reported by a formal (3+2) cycloaddition between readily available ketenes and aziridines or a one-pot formal (2+1+2) cycloaddition using imines as aziridine precursors. The method is practical, is scalable, and affords high yields. It also offers a high level of regio- and diastereoselectivity on a wide range of substrates as well as a high stereoselectivity in the case of enantiopure aziridines.

Discovery of lysosome-targeted covalent anticancer agents based on isosteviol skeleton

Covalent drugs play corresponding bioactivities by forming covalent bonds with the target, which possess many significant pharmacological advantages including high potency, ligand efficiency, and long-lasting effects. However, development of covalent inhibitors is a challenge due to their presumed indiscriminate reactivity. Here, we report the discovery of series of lysosome-targeting covalent anticancer agents by introducing nitrogenous bases to the modified isosteviol skeleton in order to minimize the toxicity and increase the selectivity. By introducing the electrophilic α, β-unsaturated ketones into the A- and D-rings of isosteviol, the cytotoxicity of the obtained compounds were greatly increased. Further nitrogen-containing modifications to the D-ring led to the discovery of novel molecules that targeted lysosomes, and of which, compound 30 was the most potent and selective antiproliferative one to kill A549 cells in vitro and in vivo. Mechanism investigation revealed that compound 30 was trapped into lysosomes and damaged lysosomes to cause cell death.

Divergent synthesis of complex diterpenes through a hybrid oxidative approach

Polycyclic diterpenes exhibit many important biological activities, but de novo synthetic access to these molecules is highly challenging because of their structural complexity. Semisynthetic access has also been limited by the lack of chemical tools for scaffold modifications. We report a chemoenzymatic platform to access highly oxidized diterpenes by a hybrid oxidative approach that strategically combines chemical and enzymatic oxidation methods. This approach allows for selective oxidations of previously inaccessible sites on the parent carbocycles and enables abiotic skeletal rearrangements to additional underlying architectures. We synthesized a total of nine complex natural products with rich oxygenation patterns and skeletal diversity in 10 steps or less from ent-steviol.

Isolation and synthesis of cryptosanguinolentine (isocryptolepine), a naturally-occurring bioactive indoloquinoline alkaloid

Cryptosanguinolentine (isocryptolepine) is one of the minor naturally-occurring monomeric indoloquinoline alkaloids, isolated from the West African climbing shrub Cryptolepis sanguinolenta. The natural product displays such a simple and unique skeleton, which chemists became interested in well before it was found in Nature. Because of its structure and biological activity, the natural product has been targeted for synthesis on numerous occasions, employing a wide range of different strategies. Hence, discussed here are aspects related to the isolation of isocryptolepine, as well as the various approaches toward its total synthesis.

Diversity-oriented synthesis of 17-spirosteroids

A diversity-oriented synthesis (DOS) approach has been used to functionalize 17-ethynyl-17-hydroxysteroids through a one-pot procedure involving a ring-closing enyne metathesis (RCEYM) and a Diels–Alder reaction on the resulting diene, under microwave irradiations. Taking advantage of the propargyl alcohol moiety present on commercially available steroids, this classical strategy was applied to mestranol and lynestrenol, giving a collection of new complex 17-spirosteroids.

Diversity-Oriented Library Synthesis from Steviol and Isosteviol-Derived Scaffolds

The readily available natural product stevioside provides a unique diterpene core structure that can be explored for small molecule library development by diversity-oriented synthesis and functional group transformations. Validation arrays were prepared from steviol, isosteviol, and related analogues, derived from stevioside, to produce over 90 compounds. These compounds were submitted to the NIH Molecular Libraries Small Molecule Repository for screening in the Molecular Libraries Screening Center Network. Micromolar hits were identified in multiple high-throughput assays for several library members. A cheminformatics analysis of the compounds was performed that verified the expected diversity and complexity of this set of compounds. The screening results indicate that scaffolds-derived natural products can provide screening hits against multiple target proteins.

Antileishmanial activity and cytotoxicity of ent-beyerene diterpenoids

We describe the in vitro activity of two natural isomeric ent-beyerene diterpenes, several derivatives and synthetic intermediates. Beyerenols 1 and 2 showed EC₅₀ of 4.6 ± 9.4 and 5.3 ± 9.4 μg/mL against amastigotes of L. (V) brazilensis, with SI of 5.1 and 7.7, respectively. Beyerenol 1 was synthesized from stevioside. In vivo experiments with bereyenols showed cure in 50% of hamsters infected with L. (V) brazilensis topically applied as Cream I (beyerenol 1, 0.81%, w/w) and Cream III (beyerenol 2, 1.96%, w/w). These results suggest that beyerenols are potential candidates for cutaneous leishmaniasis chemotherapy by topical application. In vitro assays of amastigotes of L. (V) brazilensis showed EC₅₀ of 1.1 ± 0.1 and 1.3 ± 0.04 μg/mL, with SI of 3.1 and 3.5 for hydrazone intermediates 10 and 11, respectively.

Syntheses of (-)-Tripterifordin and (-)-Neotripterifordin from Stevioside

We report short syntheses of (-)-tripterifordin and (-)-neotripterifordin, potent inhibitors of HIV replication, from stevioside, a natural sweetener used worldwide. The key transformations are reduction at C13 through the formation of a tertiary chloride and subsequent three-step lactonization including a selective iodination at C20 by the photoreaction of the C19-alcohol. The title compounds were reliably obtained from stevioside in 9 and 11 steps (with 5-7 isolation steps), respectively. Additionally, the related lactone-containing ent-kaurenes, doianoterpenes A and B, and two more natural products were synthesized.

Diversifying Complexity by Domino Benzannulation of Polycyclic Natural Products

Herein we describe a salicylaldehyde-annulation reaction as a plug and play toolkit to diversify the complexity of naturally occurring ketones. The protocol entails the transformation of the polycyclic natural ketone into its propargyl vinyl ether derivative (two synthetic steps) and its microwave-assisted imidazole-catalyzed domino rearrangement to generate the salicylaldehyde ring. This annexed unit allows further synthetic transformations: e.g., the installation of a pharmacophore module to generate natural product-pharmacophore hybrids endowed with unknown biological (pharmaceutical) annotations.

Late Stage Azidation of Complex Molecules

Selective functionalization of complex scaffolds is a promising approach to alter the pharmacological profiles of natural products and their derivatives. We report the site-selective azidation of benzylic and aliphatic C-H bonds in complex molecules catalyzed by the combination of Fe(OAc)₂ and a PyBox ligand. The same system also catalyzes the trifluoromethyl azidation of olefins to form derivatives of natural products containing both fluorine atoms and azides. In general, both reactions tolerate a wide range of functional groups and occur with predictable regioselectivity. Azides obtained by functionalization of C-H and C=C bonds were converted to the corresponding amines, amides, and triazoles, thus providing a wide variety of nitrogen-containing complex molecules.

Synthesis and anticancer evaluation of complex unsaturated isosteviol-derived triazole conjugates

BACKGROUND

For the last two decades, diterpenoid isosteviol and its derivatives have gained significant attention for novel chemical transformation in the drug discovery field.

RESULTS

An efficient way towards the synthesis of structurally diverse isosteviol derivatives was described here employing unsaturated functionalities as attractive templates for further transformation such as epoxidation. These structurally diverse compounds exhibited promising cytotoxic activities on different types of cancer cell lines, leading to drug discovery derived from natural products for the treatment of cancer.

CONCLUSION

In this work, novel isosteviol derivatives with Michael acceptors were synthesized to expand the diversity and complexity of a class of isosteviol-derived triazole conjugates to facilitate the development of potential antitumor agents.

Diterpenoids of terrestrial origin

This review covers the isolation and chemistry of diterpenoids from terrestrial as opposed to marine sources and includes labdanes, clerodanes, pimaranes, abietanes, kauranes, gibberellins, cembranes and their cyclization products. The literature from January to December 2013 is reviewed.

Design and synthesis of isosteviol triazole conjugates for cancer therapy

One of the keys for successfully developing drugs against the broad spectrum of cancer cell types is structural diversity. In the current study, we focused on a family of isosteviol derivatives as potential novel antitumor agents. Isosteviol is a tetracyclic diterpenoid obtained by acid hydrolysis of steviol glycoside extracts isolated from abundant Stevia rebaudiana plants. In this work, we have designed and synthesized a panel of isosteviol triazole conjugates using "click" chemistry methodology. Evaluation of these compounds against a series of cancer cell lines derived from primary and metastatic tumors demonstrated that these conjugates exhibit cytotoxic activities with IC50 in the low μM range. In addition, their anti-proliferative activities are cancer cell type specific. Taken together, our studies underscore the importance of structural diversity in achieving cancer cell type specific drug development.

Natural products as starting points for the synthesis of complex and diverse compounds

Covering: up to 2013. Natural products and their derivatives are used as treatments for numerous diseases. Many of these compounds are structurally complex, possessing a high percentage of sp(3) hybridized carbons and multiple stereogenic centers. Due to the difficulties associated with the isolation of large numbers of novel natural products, lead discovery efforts over the last two decades have shifted toward the screening of less structurally complex synthetic compounds. While there have been many success stories from these campaigns, the modulation of certain biological targets (e.g. protein-protein interactions) and disease areas (e.g. antibacterials) often require complex molecules. Thus, there is considerable interest in the development of strategies to construct large collections of compounds that mimic the complexity of natural products. Several of these strategies focus on the conversion of simple starting materials to value-added products and have been reviewed elsewhere. Herein we review the use of natural products as starting points for the generation of complex compounds, discussing both early ad hoc efforts and a more recent systematization of this approach.

Remodeling of fumagillol: discovery of an oxygen-directed oxidative Mannich reaction

An efficient, two-step construction of highly complex alkaloid-like compounds from the natural product fumagillol is described. This approach, which mimics a biosynthetic cyclase/oxidase sequence, allows for rapid and efficient structure elaboration of the basic fumagillol scaffold with a variety of readily available coupling partners. Mechanistic experiments leading to the discovery of an oxygen-directed oxidative Mannich reaction are also described.