Synthesis of the A,B-ring-truncated OSW saponin analogs and their antitumor activities

Chemical Synthesis of Saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

Effective cytotoxic activity of OSW-1 on colon cancer by inducing apoptosis in vitro and in vivo

As a natural compound, Ornithogalum caudatum Ait is primarily used as an anti-inflammatory and antitumor agent in Chinese folk medicine. In 1992, OSW-1 was isolated from this compound, which is a new member of cholestane saponin family. In numerous recent studies, OSW-1 has been shown to have powerful cytotoxic anticancer effects against various malignant cells. However, the therapeutic efficacy of OSW-1 on colon cancer and the underlying mechanism are not understood. To explore the mechanism underlying OSW-1 in antitumor therapy, a therapeutic function analysis of OSW-1 on colon cancer was performed in vitro and in vivo. It was shown that with low toxicity on normal colonic cells, OSW-1 suppresses colon cancer cells in vitro and this inhibition was via the intrinsic apoptotic pathway, which increased cellular calcium, changed mitochondrial membrane potential, disrupted mitochondrial morphology, and led to the release of cytochrome c and the activation of caspase-3. Furthermore, in a nude mouse model, OSW-1 had a powerful effect on suppressing colon tumor proliferation without significant side effects through the apoptosis pathway. Taken together, these results demonstrate that OSW-1 is a potential drug for colon cancer treatment.

Chemical synthesis of saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Biology-oriented synthesis of a withanolide-inspired compound collection reveals novel modulators of hedgehog signaling

Biology-oriented synthesis employs the structural information encoded in complex natural products to guide the synthesis of compound collections enriched in bioactivity. The trans-hydrindane dehydro-δ-lactone motif defines the characteristic scaffold of the steroid-like withanolides, a plant-derived natural product class with a diverse pattern of bioactivity. A withanolide-inspired compound collection was synthesized by making use of three key intermediates that contain this characteristic framework derivatized with different reactive functional groups. Biological evaluation of the compound collection in cell-based assays that monitored biological signal-transduction processes revealed a novel class of Hedgehog signaling inhibitors that target the protein Smoothened.

Synthesis of three OSW-1 analogs with maltose side chains bearing different protection groups

In order to simplify the synthesis of OSW-1's disaccharide side chain and explore the structure-activity relationship of OSW-1, three 16α-O-maltose OSW-1 analogs carrying three maltose side chains bearing different protections were designed and synthesized.

Synthesis of cholestane saponins as mimics of OSW-1 and their cytotoxic activities

To fulfill the structure-activity relationship (SAR) of OSW-1, and aim at finding the simplest structural part while maintaining most of the biological activities, six cholestane saponins were synthesized by introducing OSW-1 disaccharide (2-O-4-methoxybenzoyl-β-D-xylopyranosyl-(1→3)-2-O-acetyl-α-L-arabinopyranosyl) and its 1→4-linked analogue to the 7-hydroxy or 16-hydroxy of steroidal sapogenins. Cytotoxic activities of the products were tested. Compounds 1 and 3 exhibited potent cytotoxicities against five types of human tumor cells, with minimum IC(50) of 2.0 and 75 nM, respectively. And due to its high activity and easy accessibility compound 1 could be a potential candidate for new anti-tumor agents.

Preferential killing of cancer cells with mitochondrial dysfunction by natural compounds

Mitochondria play essential roles in cellular metabolism, redox homeostasis, and regulation of cell death. Emerging evidences suggest that cancer cells exhibit various degrees of mitochondrial dysfunctions and metabolic alterations, which may serve as a basis to develop therapeutic strategies to preferentially kill the malignant cells. Mitochondria as a therapeutic target for cancer treatment is gaining much attention in the recent years, and agents that impact mitochondria with anticancer activity have been identified and tested in vitro and in vivo using various experimental systems. Anticancer agents that directly target mitochondria or indirectly affect mitochondrial functions are collectively classified as mitocans. This review article focuses on several natural compounds that preferentially kill cancer cells with mitochondrial dysfunction, and discusses the possible underlying mechanisms and their therapeutic implications in cancer treatment. Mitocans that have been comprehensively reviewed recently are not included in this article. Important issues such as therapeutic selectivity and the relevant biochemical basis are discussed in the context of future perspectives.

A total synthesis of OSW-1

A new and practical method was developed to synthesize OSW-1, a natural saponin with potent antitumor activities, from (+)-dehydroisoandrosterone, l-arabinose, and D-xylose on gram scale. The synthesis was achieved in 10 linear steps with an overall yield of 6.4% starting from (+)-dehydroisoandrosterone.