α-Hydroxylation at C-15 and C-16 in Cholesterol: Synthesis of (25R)-5α-Cholesta-3β,15α,26-triol and (25R)-5α-Cholesta-3β,16α,26-triol from Diosgenin

Synthesis of steroidal inhibitors for Mycobacterium tuberculosis

Oxidised derivatives of cholesterol have been shown to inhibit the growth of Mycobacterium tuberculosis (Mtb). The bacteriostatic activity of these compounds has been attributed to their inhibition of CYP125A1 and CYP142A1, two metabolically critical cytochromes P450 that initiate degradation of the sterol side chain. Here, we synthesise and characterise an extensive library of 28 cholesterol derivatives to develop a structure-activity relationship for this class of inhibitors. The candidate compounds were evaluated for MIC with virulent Mtb and in binding studies with CYP125A1 and CYP142A1 from Mtb.

Biomimetic Synthesis of the CDE Ring Moiety of Physalins, Complex 13,14-Secosteroids

Physalins are a structurally complex family of 13,14-secosteroids isolated from the genus Physalis. We disclose a two-step construction of the CDE ring moiety of the physalins from a steroidal compound bearing 14-OH, 18-COOMe, and 17, 20-α-epoxide based on our biosynthetic proposal. C13-C14 bond cleavage by an alkoxy radical at C-14 and spontaneous epoxide ring opening gave a compound having a cyclononene and γ-lactone. Diastereoselective dihydroxylation of the resulting alkene with OsO₄ provided the CDE ring moiety of physalin.

A Convenient Synthesis of (16S,20S)-3β-Hydroxy-5α-pregnane-20,16-carbolactam and Its N-alkyl Derivatives

A concise synthesis of (16S,20S)-3β-hydroxy-5α-pregnane-20,16-carbolactam from tigogenin via the corresponding lactone is described. The most efficient synthetic route consisted of the lactone ring-opening with aminoalane reagent followed by PDC or Dess-Martin oxidation. The oxo-amide obtained was subjected to cyclization with Et₃SiH/TFA or Et₃SiH/Bi(TfO)₃. Alternately, the lactone was converted first to the oxo-acid, which was then subjected to the microwave-assisted reductive amination. N-Alkyl derivatives were also obtained in a similar way.

Chemical synthesis of marine saponins

Covering: 1989-2017 Saponins are characteristic metabolites of starfish and sea cucumbers, and occasionally are also found in sponges, soft coral, and small fish. These steroid or triterpenoid glycosides often show remarkable biological and pharmacological activities, such as antifungal, antifouling, shark repellent, antitumor and anti-inflammatory activities. Over one thousand marine saponins have been characterized; the majority of them can be categorized into three major structural types, i.e., asterosaponins, polyhydroxysteroid glycosides, and holostane glycosides. Thus far, only 12 marine saponins have been synthesized; those representing the major types were successfully synthesized recently. The syntheses involve preparation of the aglycones from the terrestrial steroid or triterpene materials, installation of the carbohydrate units, and manipulation of the protecting groups. Herein, we provide a comprehensive review on these syntheses.

New selenosteroids as antiproliferative agents

Starting from natural steroids (diosgenin, hecogenin, smilagenin, estrone), we have prepared a wide panel of selenoderivatives, including benzoselenazolones, selenosemicarbazones, isoselenocyanates, selenoureas, selenocyanates and diselenides, with the aim of developing new families of potential chemotherapeutic agents. The modification of the organoselenium moieties, and their position on the steroid provided valuable information concerning the antiproliferative activities. Among all the families accessed herein, the best profile was achieved for selenoureas on the A ring of estrone, which exhibited GI₅₀ values in the range 2.0-4.1 μM for all the tested tumor cell lines, with increased potency compared with commonly used chemotherapeutic agents, like 5-fluorouracil and cisplatin. Cell cycle analysis revealed that selenoureas induced accumulation of cells in the G₁ phase of the cell cycle in the breast cancer cell lines HBL-100 and T-47D; therefore, a different mechanism than cisplatin, that induces cell cycle accumulation in the S phase as a result of DNA damage, must be involved. In the rest of the tumor cells, a slight increase of the S compartment was observed. Moreover, selenosteoids turned out to be excellent glutathione peroxidase (GPx) mimics for the catalytic removal of deleterious H₂O₂ (t_1/2 8.0-22.5 min) and alkyl peroxides (t_1/2 23.0-38.9 min) when used in substoichiometric amounts (1% molar ratio), thus providing a valuable tool for reducing the intrinsic oxidative stress in tumor progression.

Formation of the steroidal C-25 chiral center via the asymmetric alkylation methodology

A novel approach for the preparation of steroids containing a chiral center at C-25 is reported. The key stereochemistry inducing step was asymmetric alkylation of pseudoephenamine amides of steroidal C-26 acids. The reaction proceeded with high diastereoselectivity (dr > 99 : 1). The developed methodology was successfully applied to the synthesis of (25R)- and (25S)-cholestenoic acids as well as (25R)- and (25S)-26-hydroxy brassinolides.

Synthesis of 5(6)-dihydro-OSW-1 analogs bearing three kinds of disaccharides linking at 15-hydroxy and their antitumor activities

In order to study the SAR of 5(6)-dihydro-OSW-1, eight 15(α)β-O-glycosyl analogs (26-33) carrying three kinds of disaccharides including [β-d-Xylp-(1-3)-α-l-Arap], [β-d-Xylp-(1-4)-α-l-Arap] and [α-l-Rhap-(1-2)-(α)β-d-Glcp] were designed and synthesized. Their in vitro antitumor activities were tested by the standard MTT assay which disclosed that compound 33 (IC(50)=0.28-0.52 μM) showed potential antitumor activities.

Novel steroidal penta- and hexacyclic compounds derived from 12-oxospirostan sapogenins

The E ring regioselective acid-catalyzed opening of spirostanic sapogenins possessing a carbonyl group at C-12, such as botogenin and hecogenin, provided the new 12,23-cyclo-22,26-epoxycholesta-11,22-diene skeleton, in addition to new compounds of the already known 12,23-cyclocholest-12(23)-en-22-one frameworks. This transformation proceeds in a single step, under slightly acidic conditions. Both, penta- and hexacyclic steroids were obtained with retention of configuration of all asymmetric centers.

Synthesis, aggregation behavior and cholesterol solubilization studies of 16-epi-pythocholic acid (3 alpha,12 alpha,16 beta-trihydroxy-5 beta-cholan-24-oic acid)

Synthesis, aggregation behavior and in vitro cholesterol solubilization studies of 16-epi-pythocholic acid (3 alpha,12 alpha,16 beta-trihydroxy-5 beta-cholan-24-oic acid, EPCA) are reported. The synthesis of this unnatural epimer of pythocholic acid (3 alpha,12 alpha,16 alpha-trihydroxy-5 beta-cholan-24-oic acid, PCA) involves a series of simple and selective chemical transformations with an overall yield of 21% starting from readily available cholic acid (CA). The critical micellar concentration (CMC) of 16-epi-pythocholate in aqueous media was determined using pyrene as a fluorescent probe. In vitro cholesterol solubilization ability was evaluated using anhydrous cholesterol and results were compared with those of other natural di- and trihydroxy bile acids. These studies showed that 16-epi-pythocholic acid (16 beta-hydroxy-deoxycholic acid) behaves similar to cholic acid (CA) and avicholic acid (3 alpha,7 alpha,16 alpha-trihydroxy-5 beta-cholan-24-oic acid, ACA) in its aggregation behavior and cholesterol dissolution properties.

Biological activities and syntheses of steroidal saponins: the shark-repelling pavoninins

Steroidal saponins are complex compounds that have a steroid attached to a carbohydrate moiety. They are natural surfactants and detergents and exhibit a number of biological effects. Steroidal saponins have shown membrane-permeabilizing, hypocholesterolemic, immunostimulant, and anticancer properties. They have also been found to affect the growth, food intake and reproductive capabilities of animals. Furthermore, they have been shown to act as antiviral and antifungal agents. They have been isolated from many plants and some animals, especially sea cucumbers and starfish. Fish belonging to the species Pardachirus pavoninus excrete a mixture of six steroidal N-acetylglucosaminides, pavoninins 1-6, with shark-repelling properties. We report syntheses of the C-15alpha pavoninin-4 by both direct synthesis from diosgenin and by remote functionalization. A general solution for the glycosylation of hindered alcohols was developed using glycosyl fluorides as good glycosyl donors. The syntheses of two C-16beta structural analogs of OSW-1 are described.

Synthesis of the Aglycone of the Shark Repellent Pavoninin-4 Using Remote Functionalization

[reaction: see text] The aglycone of shark repellent pavoninin-4, (25R)-5alpha-cholestan-3alpha,15alpha,26-triol 26-acetate 1a, was synthesized from (25R)-cholest-5-en-3beta,26-diol 4 (26-hydroxycholesterol) in eight steps in 18% overall yield. Breslow's remote functionalization strategy was used as a key step to introduce the C-15alpha alcohol on a steroid D ring. An efficient synthesis of the 26-hydroxycholesterol from the 16beta hydroxyl steroid, (25R)-cholest-5-ene-3beta,16beta,26-triol (3a), is also reported.

Synthesis of the Shark Repellent Pavoninin-4

[reaction: see text] The first synthesis of the shark repellent pavoninin-4, 3, was achieved in 12 steps with 21% overall yield from diosgenin, 8. Key reactions involve an efficient synthesis of the C-15alpha hydroxyl steroid from a C-16beta hydroxyl steroid by an unexpected 1,2-transposition strategy, a stereospecific glycosylation of a hindered C-15alpha alcohol using glycosyl fluoride as a glycosyl donor and a highly chemoselective acetylation of the C-26 primary alcohol by catalytic transesterification.