First Total Synthesis of an Exceptionally Potent Antitumor Saponin, OSW-1

Steroid and bioactive molecule conjugates: Improving therapeutic approaches in disease management

Conjugates of steroids and other natural bioactive molecules (such as amino acids or carbohydrates) have proven promising compounds with diverse biological effects. This literature review summarises the importance of steroid conjugates in a broad spectrum of therapeutic applications. Steroid conjugates exhibit improved pharmacokinetic properties, improved target specificity, and reduced side effects compared to the parent compounds. This increases their clinical usefulness. Their versatility extends to drug delivery systems, enabling precise modulation of drug release kinetics and bioavailability. Moreover, steroid conjugates are vital in treating inflammatory and neurodegenerative diseases, hormonal disorders, cancer therapy, and combating microbial infections. The review presents the current state of research on steroid conjugates, highlighting the crucial role of steroid conjugates in modern medicine and their potential to revolutionise therapeutic paradigms and improve patient outcomes. Steroid compounds are excellent for developing agents with better bioavailability and are used as drug carriers or hydrogelators.

A useful strategy for synthesis of the disaccharide of OSW-1

A flexible, efficient, and practical synthesis route was developed to synthesize an OSW-1 disaccharide. The synthesis took 13 steps from l-arabinose and d-xylose derivatives, and the overall yield was 7.2%. The region preferentially protects various d-xylose hydroxides because the TBS group selectively reacts with this hydroxide at low concentrations due to greater activity at the C-4 hydroxyl of d-xylose. Then, high efficiency selectively protects C-2 hydroxyl and C-3 hydroxyl of d-xylose, respectively. The first high yield of glycosylation on an OSW-1 synthesis disaccharide was achieved by taking sulfide donor 4 with β-PMP anomeric l-arabinose acceptor 12. The cytotoxicity reveals that the analogy has a high IC50 for a variety of cell types. This approach should provide a versatile way to modify OSW-1's disaccharide.

A Comprehensive Review on Steroidal Bioconjugates as Promising Leads in Drug Discovery

Ever increasing unmet medical requirements of the human race and the continuous fight for survival against variety of diseases give birth to novel molecules through research. As diseases evolve, different strategies are employed to counter the new challenges and to discover safer, more effective, and target-specific therapeutic agents. Among several novel approaches, bioconjugation, in which two chemical moieties are joined together to achieve noticeable results, has emerged as a simple and convenient technique for a medicinal chemist to obtain potent molecules. The steroid system has been extensively used as a privileged scaffold gifted with significantly diversified medicinal properties in the drug discovery and development process. Steroidal molecules are preferred for their rigidness and good ability to penetrate biological membranes. Slight alteration in the basic ring structure results in the formation of steroidal derivatives with a wide range of therapeutic activities. Steroids are not only active as such, conjugating them with various biologically active moieties results in increased lipophilicity, stability, and target specificity with decreased adverse effects. Thus, the steroid nucleus prominently behaves as a biological carrier for small molecules. The steroid bioconjugates offer several advantages such as synergistic activity with fewer side effects due to reduced dose and selective therapy. The steroidal bioconjugates have been widely explored for their usefulness against various disorders and have shown significant utility as anticancer, anti-inflammatory, anticoagulant, antimicrobial, insecticidal/pesticidal, antioxidant, and antiviral agents along with several other miscellaneous activities. This work provides a comprehensive review on the therapeutic progression of steroidal bioconjugates as medicinally active molecules. The review covers potential biological applications of steroidal bioconjugates and would benefit the wider scientific community in their drug discovery endeavors.

Microwave-enhanced synthesis of 26-amino-22-oxocholestanes and their cytotoxic activity

The synthesis of a series of 26-amino-22-oxocholestanes derived from diosgenin was accomplished via the substitution of an iodine atom at C-26 by primary and secondary amines. The reactions were conducted in refluxing acetonitrile and through microwave-assisted heating. The latter shows significant improvements in terms of reaction times going from hours to a few minutes or even seconds for completion. Only one of the selected amines, 4-aminourazole, did not yield the substitution product and the imine formation pathway was investigated instead, achieving the 26-iminourazole-22-oxocholestane. All the final products have been characterized and the cytotoxic activity of three of them has been evaluated in SiHa, MCF-7 and MDA tumor cell lines by the sulforhodamine B assay.

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.

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.

Anticancer Effects and Mechanisms of OSW-1 Isolated From Ornithogalum saundersiae: A Review

For centuries, cancer has been a lingering dark cloud floating on people's heads. With rapid population growth and aging worldwide, cancer incidence and mortality are growing rapidly. Despite major advances in oncotherapy including surgery, radiation and chemical therapy, as well as immunotherapy and targeted therapy, cancer is expected be the leading cause of premature death in this century. Nowadays, natural compounds with potential anticancer effects have become an indispensable natural treasure for discovering clinically useful agents and made remarkable achievements in cancer chemotherapy. In this regards, OSW-1, which was isolated from the bulbs of Ornithogalum saundersiae in 1992, has exhibited powerful anticancer activities in various cancers. However, after almost three decades, OSW-1 is still far from becoming a real anticancer agent for its anticancer mechanisms remain unclear. Therefore, in this review we summarize the available evidence on the anticancer effects and mechanisms of OSW-1 in vitro and in vivo, and some insights for researchers who are interested in OSW-1 as a potential anticancer drug. We conclude that OSW-1 is a potential candidate for anticancer drugs and deserves further study.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

OSW-1 inhibits tumor growth and metastasis by NFATc2 on triple-negative breast cancer

OSW‐1 is a natural compound extracted from the bulbs of Ornithogalum saundersiae in 1992. It has been shown strong antitumor activities in various cancer cells. However, the effects of OSW‐1 on tumor growth and metastasis in breast cancer are still poorly understood. In our research, we showed that OSW‐1 had a strong anticancer effect on breast cancer cells, but lower toxicity to normal cells. Accordingly, it also revealed significant inhibition of tumor growth by OSW‐1 in xenograft model. In addition, we performed Annexin V/PI‐labeled flow cytometric assay and TUNEL assay and showed that OSW‐1 inhibited tumor growth by inducing apoptosis. Furthermore, we carried out transwell assays and found that OSW‐1 significantly repressed the migratory and invasive capabilities of triple‐negative breast cancer (TNBC) cells via mediating epithelial‐mesenchymal transition. Besides, OSW‐1 also could inhibit metastasis in an orthotopic model and resulted in a longer survival compared with control group. Finally, we performed RNA‐sequencing and cellular functions to investigate the molecular mechanism of how OSW‐1 inhibits TNBC, and identified NFATc2 may as a pivotal factor for OSW‐1‐mediated effects on cell death, tumor growth, invasion, and migration.

Discovery of SBF1 as an allosteric inhibitor targeting the PIF-pocket of 3-phosphoinositide-dependent protein kinase-1

3-phosphoinositide-dependent protein kinase-1 (PDK1) plays a crucial role in the signal transduction of massive growth-related protein kinases. In this work, a computational study has been performed to investigate the binding pose of the hydrolyzed product of SBF1 (SBF1^-) with PDK1. The binding pose was predicted by Vina and was further refined in a molecular dynamics simulation. For comparison, four published low molecular weight compounds (PS48, PS171, PS182, and PS210) binding with PDK1 were also studied. SBF1^- was anchored in the PIF-pocket of PDK1 with salt bridge interaction using its carboxylate moiety, which is a common feature among the known ligands. Hydrogen bonds to THR148 and vdW interactions with GLN150 also have contributions to the association affinity. The allosteric regulation on PDK1 via the binding of SBF1^- was further addressed. The binding affinity of SBF1^- in complex with PDK1 is comparable to those of PS171 and PS182, with an estimated IC50 in a range from 2.0 to 10.0 μ molar. Comparison between the free energy profiles with the presence or absence of SBF1^- in the binding pocket indicates that the binding of SBF1^- enhances the hinge motion and suppresses the fluctuation of the end-to-end distance in α B of PDK1. These results demonstrate that SBF1^- is a promising allosteric regulator of PDK1 targeting the PIF-binding pocket and can serve as a new scaffold template for the design of new drugs targeting PDK1.

Structures and Biological Activities of Plant Glycosides: Cholestane Glycosides from Ornithogalum Saundersiae, O. Thyrsoides and Galtonia Candicans, and Their Cytotoxic and Antitumor Activities

Systematic phytochemical screening of higher pants using a cytotoxicity-guided fractionation procedure resulted in the isolation from the bulbs of Ornithogalum saundersiae (Liliaceae) an acylated cholestane diglycoside, 17α-hydroxy-16β-[(O-(2-O-p-methoxybenzoyl-β-D-xylopyranosyl)-(1→3)-2-O-acetyl-α-L-arabinopyranosyl)oxy]cholest-5-en-22-one, tentatively named OSW-1. In vitro cytotoxic and in vivo antitumor screening of OSW-1 revealed that it is a possible candidate as a novel anticancer agent. Furthermore, more than 20 OSW-1-related compounds were isolated, not only from the bulbs of O. saundersiae, but also from those of O. thyrsoides and Galtonia candicans, which are taxonomically related to O. saundersiae. In vitro cytotoxic evaluation of all the isolated compounds and their semi-synthetic analogues allowed the structure-activity relationships of the OSW-1 derivatives to be established. In addition, these three plants were found to produce a series of novel cholestane glycosides with a new rearranged side-chain moiety, 24(23→22)abeo-cholestane, some of which showed potent cytotoxic activity against HL-60 leukemia cells.

Synthesis and antiproliferative activities of OSW-1 analogues bearing 2-acylamino-xylose residues

We synthesized 38 OSW-1 analogues with 2-acylamino xylose residues and found that the antitumor activities could be greatly enhanced.

The enzymatic biosynthesis of acylated steroidal glycosides and their cytotoxic activity

Herein we describe the discovery and functional characterization of a steroidal glycosyltransferase (SGT) from Ornithogalum saundersiae and a steroidal glycoside acyltransferase (SGA) from Escherichia coli and their application in the biosynthesis of acylated steroidal glycosides (ASGs). Initially, an SGT gene, designated as OsSGT1, was isolated from O. saundersiae. OsSGT1-containing cell free extract was then used as the biocatalyst to react with 49 structurally diverse drug-like compounds. The recombinant OsSGT1 was shown to be active against both 3β- and 17β-hydroxyl steroids. Unexpectedly, in an effort to identify OsSGT1, we found the bacteria lacA gene in lac operon actually encoded an SGA, specifically catalyzing the acetylations of sugar moieties of steroid 17β-glucosides. Finally, a novel enzymatic two-step synthesis of two ASGs, acetylated testosterone-17-O-β-glucosides (AT-17β-Gs) and acetylated estradiol-17-O-β-glucosides (AE-17β-Gs), from the abundantly available free steroids using OsSGT1 and EcSGA1 as the biocatalysts was developed. The two-step process is characterized by EcSGA1-catalyzed regioselective acylations of all hydroxyl groups on the sugar unit of unprotected steroidal glycosides (SGs) in the late stage, thereby significantly streamlining the synthetic route towards ASGs and thus forming four monoacylates. The improved cytotoxic activities of 3′-acetylated testosterone17-O-β-glucoside towards seven human tumor cell lines were thus observable.

Synthesis of a fluorescent photoaffinity probe of OSW-1 by site-selective acylation of an inactive congener and biological evaluation

A novel fluorescent photoaffinity probe of OSW-1 was prepared in two steps from a naturally occurring inactive congener by a sequential site-selective acylation strategy using Me₂SnCl₂. It displayed highly potent anticancer activity and a similar intracellular localization property to that of a fluorescently-tagged OSW-1, thereby demonstrating its potential utility in live cell studies.

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.

Influence of intramolecular hydrogen bonds on regioselectivity of glycosylation. Synthesis of lupane-type saponins bearing the OSW-1 saponin disaccharide unit and its isomers

A series of lupane-type saponins bearing OSW-1 disaccharide unit as well as its regio- and stereoisomers were prepared and used for the structure-activity relationships (SAR) study. Unexpected preference for 1→4-linked regioisomers and an unusual inversion of the conformation of the sugar rings were noted. Cytotoxic activity of new lupane compounds was evaluated in vitro and revealed that some saponins exhibited an interesting bioactivity profile against human cancer cell lines. Influence of the protecting groups on the cytotoxicity was investigated. These results open the way to the synthesis of various lupane-type triterpene and saponin derivatives as potential anticancer compounds.

Glucosylated cholesterol in mammalian cells and tissues: formation and degradation by multiple cellular β-glucosidases

The membrane lipid glucosylceramide (GlcCer) is continuously formed and degraded. Cells express two GlcCer-degrading β-glucosidases, glucocerebrosidase (GBA) and GBA2, located in and outside the lysosome, respectively. Here we demonstrate that through transglucosylation both GBA and GBA2 are able to catalyze in vitro the transfer of glucosyl-moieties from GlcCer to cholesterol, and vice versa. Furthermore, the natural occurrence of 1-O-cholesteryl-β-D-glucopyranoside (GlcChol) in mouse tissues and human plasma is demonstrated using LC-MS/MS and ¹³C₆-labeled GlcChol as internal standard. In cells, the inhibition of GBA increases GlcChol, whereas inhibition of GBA2 decreases glucosylated sterol. Similarly, in GBA2-deficient mice, GlcChol is reduced. Depletion of GlcCer by inhibition of GlcCer synthase decreases GlcChol in cells and likewise in plasma of inhibitor-treated Gaucher disease patients. In tissues of mice with Niemann-Pick type C disease, a condition characterized by intralysosomal accumulation of cholesterol, marked elevations in GlcChol occur as well. When lysosomal accumulation of cholesterol is induced in cultured cells, GlcChol is formed via lysosomal GBA. This illustrates that reversible transglucosylation reactions are highly dependent on local availability of suitable acceptors. In conclusion, mammalian tissues contain GlcChol formed by transglucosylation through β-glucosidases using GlcCer as donor. Our findings reveal a novel metabolic function for GlcCer.

Synthesis of the aglycon of aspafiliosides E and F based on cascade reactions

Herein, we describe an efficient synthesis of C17α-OH-tigogenin, which is the aglycon of aspafiliosides E and F. The synthesis features three cascade processes, which involve the iodo-lactonization of furostan-26-acid, a cascade hydrolysis/intramolecular S_N2 process, and a cascade intramolecular redox-ketalization process.

Chemistry of xylopyranosides

Xylose is one of the few monosaccharidic building blocks that are used by mammalian cells. In comparison with other monosaccharides, xylose is rather unusual and, so far, only found in two different mammalian structures, i.e. in the Notch receptor and as the linker between protein and glycosaminoglycan (GAG) chains in proteoglycans. Interestingly, simple soluble xylopyranosides can not only initiate the biosynthesis of soluble GAG chains but also function as inhibitors of important enzymes in the biosynthesis of proteoglycans. Furthermore, xylose is a major constituent of hemicellulosic xylans and thus one of the most abundant carbohydrates on Earth. Altogether, this has spurred a strong interest in xylose chemistry. The scope of this review is to describe synthesis of xylopyranosyl donors, as well as protective group chemistry, modifications, and conformational analysis of xylose.

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.

22-Oxocholestanes as plant growth promoters

The spirostanic steroidal side-chain of diosgenin and hecogenin was modified to produce 22-oxocholestane derivatives. This type of side-chain was obtained in good yields through a straightforward four-step pathway. These compounds show potent brassinosteroid-like growth promoting activity evaluated via the rice lamina joint inclination bioassay. This is the first report of steroidal skeletons bearing the 22-oxocholestane side-chain and preserving the basic structure (A-D rings) from their corresponding parent compounds acting as plant growth promoters.

Electrochemical synthesis of glycoconjugates from activated sterol derivatives

Several derivatives of cholesterol and other 3β-hydroxy-Δ(5)-steroids were prepared and tested as sterol donors in electrochemical reactions with sugar alcohols. The reactions afforded glycoconjugates with sugar linked to a steroid moiety by an ether bond. Readily available sterol diphenylphosphates yielding up to 54% of the desired glycoconjugate were found to be the best sterol donors.

Fluorescent analog of OSW-1 and its cellular localization

OSW-1 is a steroidal saponin, which has emerged as an attractive anticancer agent with highly cancer cell selective activity. A fluorescent analog was prepared from the natural product to analyze its cellular uptake and localization. We found that the fluorescent analog is rapidly internalized into cells and is primarily distributed in endoplasmic reticulum and Golgi apparatus.

Transcriptome-wide identification and characterization of Ornithogalum saundersiae phenylalanine ammonia lyase gene family

Transcriptome-wide identification and characterization ofOrnithogalum saundersiaephenylalanine ammonia lyase gene family.

Open-chain steroidal glycosides, a diverse class of plant saponins

Saponins are an important class of plant natural products that consist of a triterpenoid or steroidal skeleton that is glycosylated by varying numbers of sugar units attached at different positions. Steroidal saponins are usually divided into two broad structural classes, namely spirostanol and furostanol saponins. A third, previously unrecognized structural class of plant saponins, the open-chain steroidal saponins, is introduced in this review; these possess an acyclic sidechain in place of the heterocyclic ring/s present in spirostanols and furostanols. Open-chain steroidal saponins are numerous and structurally diverse, with over 150 unique representatives reported from terrestrial plants. Despite this, they have to date been largely overlooked in reviews of plant natural products. This review catalogs the structural diversity of open-chain steroidal saponins isolated from terrestrial plants and discusses aspects of their structure elucidation, biological activities, biosynthesis, and distribution in the plant kingdom. It is intended that this review will provide a point of reference for those working with open-chain steroidal saponins and result in their recognition and inclusion in future reviews of plant saponins.

Cephalostatins and ritterazines

This review article is a tribute to the numerous chemists whose relentless effort for the last quarter of a century resulted in the isolation, identification, and finally the chemical synthesis of a family of bis-steroidal pyrazine alkaloids of marine origin. In the task of defeating cancer, the search for bioactive substances among the naturally occurring compounds is, without any doubt, a preferential approach. The remarkable contribution of Petitt, Fusetani, and their coworkers allowed to discover this family of marine alkaloids that emerge as potential therapeutic anticancer agents, although there is still a long way to go. The challenging and dangerous task of collecting living organisms from deep-waters was followed by a laborious isolation, elucidation of the complicated structures and biological tests. The outcome of this paramount effort was the identification of 45 compounds that stand, to date, as some of the most potent anticancer agents. The intriguing structures of the isolated alkaloids drew the attention of synthetic chemists, valiant enough to undertake the challenging task of synthesizing some of the most active members of the family. Fuchs, Heathcock, Winterfeldt, Suarez, Shair, and their associates pioneered in the establishment of feasible synthetic routes for the preparation of some of the naturally occurring compounds and a large number of synthetic analogs, allowing to establish SAR criteria that have guided the design of new synthetic analogs. Numerous analogs have been prepared to investigate the mechanism of action of bis-steroidal pyrazines, e.g. cephalostatin analogs bearing a strained spiroketal moiety. However, the mechanism of action and the biological target of these compounds remain far from being understood. Therefore, the rational design of simpler, yet highly active analogs seems at the current stage elusive. It is still 1 to clear why these compounds need to be dimeric to show high biological activity. Furthermore, it is not known whether the central pyrazine ring is simply a linker or has some additional function. This could be tested by examining the biological activity of steroidal dimers with other linkers, e.g. with a benzene ring. Such analogs have been actually prepared but without functional groups necessary for biological activity. The clinical trials of cephalostatins have got stuck due to a shortage of material. There is an urgent need to provide highly active, yet not too complex analogs, which could be available in substantial amounts for advanced pharmacological studies.

Effective killing of leukemia cells by the natural product OSW-1 through disruption of cellular calcium homeostasis

3β,16β,17α-Trihydroxycholest-5-en-22-one 16-O-(2-O-4-methoxybenzoyl-β-D-xylopyranosyl)-(1→3)-2-O-acetyl-α-L-arabinopyranoside (OSW-1) is a natural product with potent antitumor activity against various types of cancer cells, but the exact mechanisms of action remain to be defined. In this study, we showed that OSW-1 effectively killed leukemia cells at subnanomolar concentrations through a unique mechanism by causing a time-dependent elevation of cytosolic Ca(2+) prior to induction of apoptosis. A mechanistic study revealed that this compound inhibited the sodium-calcium exchanger 1 on the plasma membrane, leading to an increase in cytosolic Ca(2+) and a decrease in cytosolic Na(+). The elevated cytosolic Ca(2+) caused mitochondrial calcium overload and resulted in a loss of mitochondrial membrane potential, release of cytochrome c, and activation of caspase-3. Furthermore, OSW-1 also caused a Ca(2+)-dependent cleavage of the survival factor GRP78. Inhibition of Ca(2+) entry into the mitochondria by the uniporter inhibitor RU360 or by cyclosporin A significantly prevented the OSW-1-induced cell death, indicating the important role of mitochondria in mediating the cytotoxic activity. The extremely potent activity of OSW-1 against leukemia cells and its unique mechanism of action suggest that this compound may be potentially useful in the treatment of leukemia.

Assembly of naturally occurring glycosides, evolved tactics, and glycosylation methods

Glycosylation of proteins and lipids is critical to many life processes. Secondary metabolites (or natural products), such as flavonoids, steroids, triterpenes, and antibiotics, are also frequently modified with saccharides. The resulting glycosides include diverse structures and functions, and some of them have pharmacological significance. The saccharide portions of the glycosides often have specific structural characteristics that depend on the aglycones. These molecules also form heterogeneous "glycoform" mixtures where molecules have similar glycosidic linkages but the saccharides vary in the length and type of monosaccharide unit. Thus, it is difficult to purify homogeneous glycosides in appreciable amounts from natural sources. Chemical synthesis provides a feasible access to the homogeneous glycosides and their congeners. Synthesis of a glycoside involves the synthesis of the aglycone, the saccharide, the connection of these two parts, and the overall manipulation of protecting groups. However, most synthetic efforts to date have focused on the aglycones, treating the attachment of saccharides onto the aglycones as a dispensable topic. The synthesis of the aglycone and the synthesis of the saccharide belong to two independent categories of chemistry, and different types of the aglycones and saccharides pose as specific synthetic subjects in their own disciplines. The only reaction that integrates the broad chemistry of glycoside synthesis is the glycosidic bond formation between the saccharide and the aglycone. Focusing on this glycosylation reaction in this Account, we string together our experience with the synthesis of the naturally occurring glycosides. We briefly describe the synthesis of 18 glycosides, including glycolipids, phenolic glycosides, steroid glycosides, and triterpene glycosides. Each molecule represents a prototypical structure of a family of the natural glycosides with interesting biological activities, and we emphasize the general tactics for the synthesis of these diverse structures. We provide a rationale for four tactics for the synthesis of glycosides, based on the stage at which the glycosidic bond is formed between the saccharide and the aglycone. This choice of tactic determines the success or failure of a synthesis, and the flexibility and the overall efficiency of the synthesis as well. Toward the synthesis of heterogeneous glycoform mixtures, we discuss successive and random glycosylation reactions. Finally, we have developed two new glycosylation protocols that address the challenges in the glycosylation of aglycones that are poorly nucleophilic, extremely acid labile, or extremely electrophilic. One of these new protocols takes advantage of glycosyl trifluoroacetimidate donors, and a second protocol uses gold(I)-catalyzed glycosylation with glycosyl ortho-alkynylbenzoate donors.