Catalytic Oxidative Processes in Steroid Chemistry: Allylic Oxidation, β-Selective Epoxidation, Alcohol Oxidation and Remote Functionalization Reactions

Remote functionalization reactions in steroids: discovery and application

Research published between 2001 and 2022 on the functionalization of remote positions of steroids, as well as the use of this technique in the generation of biologically active compounds has been reviewed. In the first section of the analysis established and novel methods for activation of sites deemed to be remote were reported. A series of manganese- (mainly), rhodium-, ruthenium- and osmium-centered porphyrins as catalysts in the presence of PIDA as oxidant have effected hydroxylation at C-1, -5, -6, -7, -11, -14, -15, -16, -17, -20, -24 and -25. Dioxiranes have been utilized in inserting hydroxyl groups at the 5, 12, 14, 15, 16, 17, 20, 24 and 25 positions (tertiary centers for the most part). Alcohols at C-12 and -16 were oxidized further to ketones. The Schönecker oxidation, discovered and developed during the period, has revolutionized the selective functionalization at C-12 of steroids possessing a 17-keto group. In the presence of iron-centered PDP- and MCP-based catalysts, hydrogen peroxide and acetic acid, substrates tended to be hydroxylated at C-6 and -12, with further oxidation to ketones often accompanying this reaction. The hypohalite reaction, utilizing the more modern Suarez conditions (irradiation in the presence of iodine and PIDA), was reported to facilitate the insertion of a hydroxyl moiety five atoms away from an existing alcohol oxygen. Steroidal-3β-diazoacetates tend to decompose on heating with di-rhodium-centered catalysts while activating carbons four or five atoms away. Chromium- and iron-based acetates were observed to functionalize C-5 and -25. Other reactions involving ring cleavage and halogenation, ketone irradiation and α-hydroxylation of ethers were also covered. The syntheses of compounds with marked biological activity from readily available steroids is described in the second section of the study. Cyclopamine, cephalostatin-1, ritterazine B and three polyhydroxypregnanaes (pergularin, utendin and tomentogenin) were generated in sequences in which a key step required hydroxylation at C-12 using the Schönecker reaction. A crucial stage in the preparation of cortistatin A, the saundersioside core, eurysterol A, 5,6-dihydroglaucogenin C, as well as clinostatins A and B involved the functionalization of C-18 or -19 utilizing hypohalite chemistry. The synthetic route to xestobergsterol A, pavonin-4-aglycone and ouagabagenin included a transformation where ketone irradiation played a part in either producing a Δ14 or a C-19 activated steroid. The radical relay reaction, where a 17α-chloro-steroid was formed, was central in the generation of pythocholic acid. The lead tetraacetate reaction was pivotal in the functionalization of C-19 during the synthesis of cyclocitrinol.

A Predictably Selective Palladium-Catalyzed Aliphatic C-H Oxygenation

Direct oxygenation of nonactivated aliphatic C(sp³)-H groups with peroxycarboxylic acids in the presence of palladium tris(pyridylmethyl)amine complex (0.6 mol %) is reported, providing the corresponding hydroxylated derivatives in up to 94% yields. The oxidation of 3° C-H groups occurs stereospecifically, with the catalyst system demonstrating extremely high sensitivity to electronic effects (adamantane oxidation: 3°:2° up to >300). This suggests potential applications for the 3°-regioselective oxidative functionalization of complex molecules of natural origin.

Synthesis, In Vitro Biological Evaluation of Antiproliferative and Neuroprotective Effects and In Silico Studies of Novel 16E-Arylidene-5α,6α-epoxyepiandrosterone Derivatives

Steroids constitute an important class of pharmacologically active molecules, playing key roles in human physiology. Within this group, 16E-arylideneandrostane derivatives have been reported as potent anti-cancer agents for the treatment of leukemia, breast and prostate cancers, and brain tumors. Additionally, 5α,6α-epoxycholesterol is an oxysterol with several biological activities, including regulation of cell proliferation and cholesterol homeostasis. Interestingly, pregnenolone derivatives combining these two modifications were described as potential neuroprotective agents. In this research, novel 16E-arylidene-5α,6α-epoxyepiandrosterone derivatives were synthesized from dehydroepiandrosterone by aldol condensation with different aldehydes followed by a diastereoselective 5α,6α-epoxidation. Their cytotoxicity was evaluated on tumoral and non-tumoral cell lines by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Furthermore, the assessment of the neuroprotective activity of these derivatives was performed in a dopaminergic neuronal cell line (N27), at basal conditions, and in the presence of the neurotoxin 6-hydroxydopamine (6-OHDA). Interestingly, some of these steroids had selective cytotoxic effects in tumoral cell lines, with an IC50 of 3.47 µM for the 2,3-dichlorophenyl derivative in the breast cancer cell line (MCF-7). The effects of this functionalized epoxide on cell proliferation (Ki67 staining), cell necrosis (propidium iodide staining), as well as the analysis of the nuclear area and near neighbor distance in MCF-7 cells, were analyzed. From this set of biological studies, strong evidence of the activation of apoptosis was found. In contrast, no significant neuroprotection against 6-OHDA-induced neurotoxicity was observed for the less cytotoxic steroids in N27 cells. Lastly, molecular docking simulations were achieved to verify the potential affinity of these compounds against important targets of steroidal drugs (androgen receptor, estrogen receptor α, and 5α-reductase type 2, 17α-hydroxylase-17,20-lyase and aromatase enzymes). This in silico study predicted a strong affinity between most novel steroidal derivatives and 5α-reductase and 17α-hydroxylase-17,20-lyase enzymes.

Catalytic Allylic Oxidation of Cyclic Enamides and 3,4-Dihydro-2H-Pyrans by TBHP

Allylic oxidation of heteroatom substituted cyclic alkenes by tert-butyl hydroperoxide (70% TBHP in water) using catalytic dirhodium caprolactamate [Rh₂(cap)₄] forms enone products with a variety of 2-substituted cyclic enamides and 3,4-dihyro-2H-pyrans. These reactions occur under mild reaction conditions, are operationally convenient to execute, and are effective for product formation with as low as 0.25 mol% catalyst loading. With heteroatom stabilization of the intermediate allylic free radical two sites for oxidative product formation are possible, and the selectivity of the oxidative process varies with the heteroatom when R = H. Cyclic enamides produce 4-piperidones in good yields when R = alkyl or aryl, but oxidation of 2H-pyrans also gives alkyl cleavage products. Alternative catalysts for TBHP oxidations show comparable selectivities but give lower product yields.

Synthesis and structural characterization of three unique Helicobacter pylori α-cholesteryl phosphatidyl glucosides

Steryl glycosides produced by bacteria play important biological roles in the evasion and modulation of host immunity. Step-economical syntheses of three cholesteryl-6-O-phosphatidyl-α-D-glucopyranosides (αCPG) unique to Helicobacter pylori have been achieved. The approach relies upon regioselective deprotection of per-O-trimethylsilyl-α-D-cholesterylglucoside at C6 followed by phosphoramidite coupling. Global TMS ether deprotection in the presence of oxygen and subsequent deprotection of the cyano ethyl phosphoester afforded the target compounds in 16-21 % overall yield starting from D-glucose. The structures of these natural products were determined using a combination of 2D NMR methods and mass spectrometry. These robust synthesis and characterization protocols provide analogues to facilitate glycolipidomic profiling and biological studies.

Elucidation of the regio- and chemoselectivity of enzymatic allylic oxidations with Pleurotus sapidus - conversion of selected spirocyclic terpenoids and computational analysis

Allylic oxidations of olefins to enones allow the efficient synthesis of value-added products from simple olefinic precursors like terpenes or terpenoids. Biocatalytic variants have a large potential for industrial applications, particularly in the pharmaceutical and food industry. Herein we report efficient biocatalytic allylic oxidations of spirocyclic terpenoids by a lyophilisate of the edible fungus Pleurotus sapidus. This ''mushroom catalysis'' is operationally simple and allows the conversion of various unsaturated spirocyclic terpenoids. A number of new spirocyclic enones have thus been obtained with good regio- and chemoselectivity and chiral separation protocols for enantiomeric mixtures have been developed. The oxidations follow a radical mechanism and the regioselectivity of the reaction is mainly determined by bond-dissociation energies of the available allylic CH-bonds and steric accessibility of the oxidation site.

Bismuth compounds in medicinal chemistry

In recent years, the chemical potential of bismuth and bismuth compounds has been actively exploited. Bismuth salts are known for their low toxicity, making them potential valuable reagents for large-scale synthesis, which becomes more obvious when dealing with products such as active pharmaceutical ingredients or synthetic intermediates. Conversely, bismuth compounds have been widely used in medicine. After extensive use in the treatments of syphilis and other bacterial infections before the advent of modern antibiotics, bismuth compounds remain important for the treatment of several gastrointestinal disorders and also exhibit antimicrobial properties and cytotoxic activity, among others. This review updates relevant advances in the past few years, concerning the application of bismuth reagents and catalysts in innovative synthetic processes for the preparation of compounds of medicinal interest, as well as the preparation, biological evaluation and potential medicinal uses of bismuth compounds.

Achieving regio- and enantioselectivity of P450-catalyzed oxidative CH activation of small functionalized molecules by structure-guided directed evolution

Directed evolution of the monooxygenase P450-BM3 utilizing iterative saturation mutagenesis at and near the binding site enables a high degree of both regio- and enantioselectivity in the oxidative hydroxylation of cyclohexene-1-carboxylic acid methyl ester. Wild-type P450-BM3 is 84% regioselective for the allylic 3-position with 34% enantioselectivity in favor of the R alcohol. Mutants enabling R selectivity (>95% ee) or S selectivity (>95% ee) were evolved, while reducing other oxidation products and thus maximizing regioselectivity to >93%. Control of the substrate-to-enzyme ratio is necessary for obtaining optimal and reproducible enantioselectivities, an observation which is important in future protein engineering of these mono-oxygenases. An E. coli strain capable of NADPH regeneration was also engineered, simplifying directed evolution of P450 enzymes in general. These synthetic results set the stage for subsequent stereoselective and stereospecific chemical transformations to form more complex compounds, thereby illustrating the viability of combining genetically altered enzymes as catalysts in organic chemistry with traditional chemical methods.

Synthesis of antifungal glucan synthase inhibitors from enfumafungin

An efficient, new, and scalable semisynthesis of glucan synthase inhibitors 1 and 2 from the fermentation product enfumafungin 3 is described. The highlights of the synthesis include a high-yielding ether bond-forming reaction between a bulky sulfamidate 17 and alcohol 4 and a remarkably chemoselective, improved palladium(II)-mediated Corey-Yu allylic oxidation at the highly congested C-12 position of the enfumafungin core. Multi-hundred gram quantities of the target drug candidates 1 and 2 were prepared, in 12 linear steps with 25% isolated yield and 13 linear steps with 22% isolated yield, respectively.

Regio- and stereoselectivity of P450-catalysed hydroxylation of steroids controlled by laboratory evolution

A current challenge in synthetic organic chemistry is the development of methods that allow the regio- and stereoselective oxidative C-H activation of natural or synthetic compounds with formation of the corresponding alcohols. Cytochrome P450 enzymes enable C-H activation at non-activated positions, but the simultaneous control of both regio- and stereoselectivity is problematic. Here, we demonstrate that directed evolution using iterative saturation mutagenesis provides a means to solve synthetic problems of this kind. Using P450 BM3(F87A) as the starting enzyme and testosterone as the substrate, which results in a 1:1 mixture of the 2β- and 15β-alcohols, mutants were obtained that are 96-97% selective for either of the two regioisomers, each with complete diastereoselectivity. The mutants can be used for selective oxidative hydroxylation of other steroids without performing additional mutagenesis experiments. Molecular dynamics simulations and docking experiments shed light on the origin of regio- and stereoselectivity.

Bismuth(III) reagents in steroid and terpene chemistry

Steroid and terpene chemistry still have a great impact on medicinal chemistry. Therefore, the development of new reactions or “greener” processes in this field is a contemporaneous issue. In this review, the use of bismuth(III) salts, as “ecofriendly” reagents/catalysts, on new chemical processes involving steroids and terpenes as substrates will be focused. Special attention will be given to some mechanistic considerations concerning selected reactions.

Copper-catalyzed remote sp3 C-H chlorination of alkyl hydroperoxides

A copper-catalyzed methodology to functionalize remote sp(3) C-H bonds in alkyl hydroperoxides is presented. The atom-transfer chlorination utilizes simple ammonium chloride salts as the chlorine source, and the internal redox process requires no external redox reagents.

An effective method for allylic oxidation of Delta5-steroids using tert-butyl hydroperoxide

An allylic oxidation method for Delta(5)-steroids using TBHP as oxidant with a 2-quinoxalinol salen Cu(II) complex as catalyst is reported. A variety of Delta(5)-steroidal substrates are selectively oxidized to the corresponding enones. Excellent yields are achieved (up to 99% under optimized conditions) while significantly reducing reaction times required as compared to other current methods.

16α,17α-Ep-oxy-5α-hydr-oxy-6β-nitrooxy-20-oxopregnan-3β-yl acetate

The title steroid, C(23)H(33)NO(8), is a pregnane derivative obtained regio-, stereo- and chemoselectively from the ring opening of the corresponding 5α,6α;16α,17α-diepoxide with bis-muth(III) nitrate. There are two symmetry-independent mol-ecules in the asymmetric unit that show no significant differences concerning bond lengths and angles. All rings are trans-fused. The conformations of the six-membered rings are close to chair forms, while the five-membered ring adopts an envelope conformation. The mol-ecules are held together by an extensive O-H⋯O hydrogen-bonding network of chains runnning along the a axis.

6β-Acetamido-5α-hydroxy-cholestan-3β-yl acetate

The title steroid, C₃₁H₅₃NO₄, was prepared from the corresponding 5α,6α-epoxy­cholestane. The conformation of the six-membered rings is close to a chair form, while the five-membered ring adopts a twist conformation. The hydroxyl and acetamide groups are in axial positions. The nucleophilic species bound to the steroid nucleus at position 6 by the β-face, whereas the hydroxyl group at position 5 has α-orientation. All rings are trans-fused. The crystal packing shows that the mol­ecules related by twofold symmetry exist as O—H⋯O hydrogen-bonded dimers.