Cholestane glycosides from the bulbs of Galtonia candicans and their cytotoxicity

Steroidal glycosides from Ornithogalum thyrsoides bulbs and their cytotoxicity toward HL-60 human promyelocytic leukemia cells and SBC-3 human small-cell lung cancer cells

Ornithogalum thyrsoides Jacq belongs to the Asparagaceae family and is cultivated for ornamental purposes. The authors have previously reported several cholestane- and spirostan-type steroidal glycosides from O. thyrsoides. Conventional TLC analysis of the methanolic bulb extract of O. thyrsoides suggested the presence of unprecedented compounds; therefore, a detailed phytochemical investigation of the extract was performed and 35 steroidal glycosides (1-35), including 21 previously undescribed ones (1-21) were collected. The structures of 1-21 were determined mainly by analyses of their 1H and 13C NMR spectra with the aid of two-dimensional NMR spectroscopy. The isolated compounds were classified into three distinct groups: furostan-type (1, 2, 8-12, and 22), spirostan-type (3-7 and 23-26), and cholestane-type (13-21 and 27-35). Although the C/D-ring junction of the steroidal skeleton is typically trans-oriented, except for some cardiotonic and pregnane-type steroidal derivatives, 7 possess a cis C/D-ring junction. This is the first reported instance of such a configuration in spirostan-type steroidal derivatives, marking it as a finding of significant interest. Compounds 1-35 were evaluated for cytotoxicity against HL-60 human promyelocytic leukemia cells and SBC-3 human small-cell lung cancer cells. Compounds 3-6, 9, 17-21, 23-25, and 30-35 demonstrated cytotoxicity in a dose-dependent manner with IC50 values ranging from 0.000086 to 18 μM and from 0.00014 to 37 μM toward HL-60 and SBC-3 cells, respectively. Compound 19, which is obtained in a good yield and shows relatively potent cytotoxicity among the undescribed compounds, induces apoptosis in HL-60 cells, accompanied by arresting the cell cycle of HL-60 cells at the G2/M phase. In contrast, 19 causes oxidative stress-associated necrosis in SBC-3 cells. The cytotoxic mechanism of 19 is different between HL-60 and SBC-3 cells.

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.

Galtonosides A-E: Antiproliferative and Antiplasmodial Cholestane Glycosides from Galtonia regalis

An extract of Galtonia regalis from the Natural Products Discovery Institute showed moderate antiplasmodial activity, with an IC50 value less than 1.25 μg/mL. The two known cholestane glycosides 1 and 2 and the five new cholestane glycosides galtonosides A-E (3-7) were isolated after bioassay-directed fractionation. The structures of the new compounds were determined by interpretation of their NMR and mass spectra. Among these compounds, galtonoside B (4) displayed the most potent antiplasmodial activity, with an IC50 value of 0.214 μM against the drug-resistant Dd2 strain of Plasmodium falciparum.

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.

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.

The chemistry and biological activity of the Hyacinthaceae

The Hyacinthaceae (sensu APGII), with approximately 900 species in about 70 genera, can be divided into three main subfamilies, the Hyacinthoideae, the Urgineoideae and the Ornithogaloideae, with a small fourth subfamily the Oziroëoideae, restricted to South America. The plants included in this family have long been used in traditional medicine for a wide range of medicinal applications. This, together with some significant toxicity to livestock has led to the chemical composition of many of the species being investigated. The compounds found are, for the most part, subfamily-restricted, with homoisoflavanones and spirocyclic nortriterpenoids characterising the Hyacinthoideae, bufadienolides characterising the Urgineoideae, and cardenolides and steroidal glycosides characterising the Ornithogaloideae. The phytochemical profiles of 38 genera of the Hyacinthaceae will be discussed as well as any biological activity associated with both crude extracts and compounds isolated. The Hyacinthaceae of southern Africa were last reviewed in 2000 (T. S. Pohl, N. R. Crouch and D. A. Mulholland, Curr. Org. Chem., 2000, 4, 1287-1324; ref. 1); the current contribution considers the family at a global level.

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.

Carbohydrate-steroid conjugation by Ugi reaction: one-pot synthesis of triple sugar/pseudo-peptide/spirostane hybrids

The one-pot synthesis of novel molecular chimeras incorporating sugar, pseudo-peptide, and steroidal moieties is described. For this, a new carbohydrate-steroid conjugation approach based on the Ugi four-component reaction was implemented for the ligation of glucose and chacotriose to spirostanic steroids. The approach proved wide substrate scope, as both mono and oligosaccharides functionalized with amino, carboxy, and isocyano groups were conjugated to steroidal substrates in an efficient, multicomponent manner. Two alternative strategies based on the hydrazoic acid variant of the Ugi reaction were employed for the synthesis of tetrazole-based chacotriose-diosgenin conjugates resembling naturally occurring spirostan saponins. This is the first time that triple sugar/pseudo-peptide/steroid hybrids are produced, thus opening up an avenue of opportunities for applications in drug discovery and biological chemistry.

Steroidal glycosides from the rhizomes of Ruscus hypophyllum

Seven steroidal glycosides, along with one known glycoside, were isolated from the rhizomes of Ruscus hypophyllum (Liliaceae). Comprehensive spectroscopic analysis, including 2D NMR spectroscopy, and the results of acid hydrolysis allowed the chemical structures of the compounds to be assigned as (23S,25R)-23-hydroxyspirost-5-en-3beta-yl O-alpha-l-rhamnopyranosyl-(1-->4)-beta-d-glucopyranoside (1), 1beta-hydroxyspirosta-5,25(27)-dien-3beta-yl O-alpha-l-rhamnopyranosyl-(1-->4)-beta-d-glucopyranoside (2), (22S)-16beta,22-dihydroxycholest-5-en-3beta-yl O-alpha-l-rhamnopyranosyl-(1-->4)-beta-d-glucopyranoside (3), (22S)-16beta-[(beta-d-glucopyranosyl)oxy]-22-hydroxycholest-5-en-3beta-yl O-alpha-l-rhamnopyranosyl-(1-->4)-beta-d-glucopyranoside (4), (22S)-16beta-[(beta-d-glucopyranosyl)oxy]-22-hydroxycholest-5-en-3beta-yl beta-d-glucopyranoside (5), (22S)-16beta-[(beta-d-glucopyranosyl)oxy]-3beta,22-dihydroxycholest-5-en-1beta-yl O-alpha-l-rhamnopyranosyl-(1-->2)-(3,4-di-O-acetyl-beta-d-xylopyranoside) (6), and (22S)-16beta-[(beta-d-glucopyranosyl)oxy]-3beta,22-dihydroxycholest-5-en-1beta-yl O-alpha-l-rhamnopyranosyl-(1-->2)-O-[beta-d-xylopyranosyl-(1-->3)]-beta-d-xylopyranoside (7), respectively. This is the first isolation of a series of cholestane glycosides from a Ruscus species.

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

The A,B-ring-truncated OSW saponin analogs (1, 18a, and 18b) were synthesized. These greatly simplified trans-hydrindane disaccharides retained considerable inhibitory activity against the growth of HeLa and Jurkat T cells (IC(50)=0.8-21.1 microM).

Synthesis of OSW saponin analogs with modified sugar residues and their antiproliferative activities

Eight monosaccharide analogs of the potent antitumor OSW saponins (2-9) were synthesized. One analog, 2-O-acetyl-alpha-l-arabinopyranoside 3, showed antiproliferative activity against the Jurkat cells (IC(50)=0.078microM) comparable to that of the disaccharide derivative (1).

OSW saponins: facile synthesis toward a new type of structures with potent antitumor activities

[reaction: see text] OSW saponins, featuring a 16beta,17alpha-dihydroxycholest-22-one aglycon and an acylated beta-D-xylopyranosyl-(1-->3)-alpha-L-arabinopyranosyl residue attached to the 16-hydroxyl group, have recently been discovered from a group of lily plants, which show potent antitumor activities with a novel mechanism of action. This paper describes an aldol approach to the stereoselective construction of the 16alpha,17alpha-dihydroxycholest-22-one structure from 16alpha-hydroxy-5-androsten-17-ones and propionates. Elaboration of the aldol adducts toward OSW-1, involving installation of the isoamyl ketone side chain, inversion of the 16-hydroxyl configuration, and selective protection of the C22-oxy function, has been explored and accomplished. In particular, the present route was found convenient for the synthesis of OSW saponin analogues with a C22-ester side chain. Thus, the 23-oxa-analogue of OSW-1 (40) was prepared starting from the industrial dehydroisoandrosterone (1) in a linear eight-step sequence and in 26% overall yield. Analogues with a variety of modified side chains were prepared, via aldol condensation with propionates of varying length, thiopropionate, and acetate (for preparation of 68-75) or via aminolysis of the 22,16-lactone 26 (for preparation of the 23-N-analogues). Cross metathesis (CM) reaction was also found feasible for modification at the final stage from C22-allyl ester 70. Valuable structure-activity relationships (SAR), together with the practical synthetic approach, have thus been provided to set a new stage for further studies on this new type of antitumor structures.

Synthesis of analogues of a potent antitumor saponin OSW-1

A series of side chain analogues (5a-e), a 22-glycosylated isomer (10), and 16beta-O-l-arabinosyl (13a) or 16beta-O-d-xylosyl (13b) analogues of OSW-1 were synthesized. All analogues were found to be less cytotoxic against breast and endometrial cancer cell lines than the natural product.

Synthesis of OSW-1 analogs with modified side chains and their antitumor activities

Four analogs of OSW-1 (1-4) with modified side chains on the steroidal skeleton were synthesized following modification of our previous route for the total synthesis of OSW-1. Testing of the analogs against growth of tumor cells demonstrated that the 22-one function and the full length of the side chain of OSW-1 were not required for the antitumor action of OSW-1.