A Concise Account of Various Approaches for Stereoselective Construction of the C-20(H) Stereogenic Center in Steroid Side Chain

Ni-Catalyzed Enantioselective Hydrofunctionalizations of 1,3-Dienes

Ni-catalyzed enantioselective hydrofunctionalizations of conjugated dienes are particularly demanding reactions to devise because they require not only addressing the inherent challenges associated with the development of an enantioselective transformation but also overcoming all other aspects of selective catalysis (chemoselectivity, regioselectivity, diastereoselectivity, etc.). However, the value-added nature of the chiral allylic and homoallylic derivatives obtained by these methods, the lack of efficient alternatives, and the use of an earth-abundant first-row transition metal have led to renewed interest over the past decade. In this Perspective, we give an overview of the developments in this field, from the original findings (often dating back to the last century) to the most recent contributions. Emphasis is placed on the nature of the hydrofunctionalization agent (C(sp), C(sp2), C(sp3), N, P, or O).

Electrochemically Enabled One-Pot Multistep Synthesis of C19 Androgen Steroids

The synthesis of many valuable C19 androgens can be accomplished by removal of the C17 side chain from more abundant corticosteroids, followed by further derivatization of the resulting 17-keto derivative. Conventional chemical reagents pose significant drawbacks for this synthetic strategy, as large amounts of waste are generated, and quenching of the reaction mixture and purification of the 17-ketosteroid intermediate are typically required. Herein, we present mild, safe, and sustainable electrochemical strategies for the preparation of C19 steroids. A reagent and catalyst free protocol for the removal of the C17 side chain of corticosteroids via anodic oxidation has been developed, enabling several one-pot, multistep procedures for the synthesis of androgen steroids. In addition, simultaneous anodic C17 side chain cleavage and cathodic catalytic hydrogenation of a steroid has been demonstrated, rendering a convenient and highly atom economic procedure for the synthesis of saturated androgens.

Diosgenin Derivatives as Potential Antitumor Agents: Synthesis, Cytotoxicity, and Mechanism of Action

Thirty-two new diosgenin derivatives were designed, synthesized, and evaluated for their cytotoxic activities in three human cancer cell lines (A549, MCF-7, and HepG2) and normal human liver cells (L02) using an MTT assay in vitro. Most compounds, especially 8, 18, 26, and 30, were more potent when compared with diosgenin. The structure-activity relationship results suggested that the presence of a succinic acid or glutaric acid linker, a piperazinyl amide terminus, and lipophilic cations are all beneficial for promoting cytotoxic activity. Notably, compound 8 displayed excellent cytotoxic activity against HepG2 cells (IC₅₀ = 1.9 μM) and showed relatively low toxicity against L02 cells (IC₅₀ = 18.6 μM), showing some selectivity between normal and tumor cells. Studies on its cellular mechanism of action showed that compound 8 induces G0/G1 cell cycle arrest and apoptosis in HepG2 cells. Predictive studies indicated that p38α mitogen-activated protein kinase (MAPK) is the optimum target of 8 based on its 3D molecular similarity, and docking studies showed that compound 8 fits well into the active site of p38α-MAPK and forms relatively strong interactions with the surrounding amino acid residues. Accordingly, compound 8 may be used as a promising lead compound for the development of new antitumor agents.

A Dual Role Reductase from Phytosterols Catabolism Enables the Efficient Production of Valuable Steroid Precursors

4-Androstenedione (4-AD) and progesterone (PG) are two of the most important precursors for synthesis of steroid drugs, however their current manufacturing processes suffer from low efficiency and severe environmental issues. In this study, we decipher a dual-role reductase (mnOpccR) in the phytosterols catabolism, which engages in two different metabolic branches to produce the key intermediate 20-hydroxymethyl pregn-4-ene-3-one (4-HBC) through a 4-e reduction of 3-oxo-4-pregnene-20-carboxyl-CoA (3-OPC-CoA) and 2-e reduction of 3-oxo-4-pregnene-20-carboxyl aldehyde (3-OPA), respectively. Inactivation or overexpression of mnOpccR in the Mycobacterium neoaurum can achieve exclusive production of either 4-AD or 4-HBC from phytosterols. By utilizing a two-step synthesis, 4-HBC can be efficiently converted into PG in a scalable manner (100 gram scale). This study deciphers a pivotal biosynthetic mechanism of phytosterol catabolism and provides very efficient production routes of 4-AD and PG.

Dendrodoristerol, a cytotoxic C20 steroid from the Vietnamese nudibranch mollusk Dendrodoris fumata

Using various chromatographic separations, four steroids including one new C₂₀ steroid namely dendrodoristerol (1), were isolated from the Vietnamese nudibranch mollusk Dendrodoris fumata. The structure elucidation was confirmed by combination of spectroscopic experiments including 1D and 2D NMR, HR QTOF MS, and CD. Compound 1 was found to exhibit significant in vitro cytotoxic activity against six human cancer cell lines as HL-60, KB, LU-1, MCF-7, LNCaP, and HepG2. In addition, 1 induced HL-60 cancer cell death by apoptosis and necrosis.

Synthetic Strategies to Diverse Polyquinanes via Olefin Metathesis: Access to the Basic Core of Crinipellin, Presilphiperfolanol, and Cucumin

A rapid and useful synthetic approach to various polyquinane-based natural products was accomplished efficiently by employing ring-rearrangement metathesis and ring-closing metathesis as key steps. Here, we report the synthesis of stereochemically well-defined cis-anti-cis triquinanes (1, 2), tetraquinanes (3, 4), a novel pentaquinane 5, and fused [5-5-5-6] tetracyclic systems (6, 7) that are present in crinipellin, presilphiperfolanol, cucumin, etc. Hence, the current strategy may be suitable for the synthesis of various complex natural and unnatural cyclopentanoid targets. Moreover, our approach to the newly synthesized pentaquinane 5 has paved the way for various complex polyquinanes/molecules having significant applications in theoretical and medicinal chemistry.

Stereoselective synthesis of α-methyl and α-alkyl ketones from esters and alkenes via cyclopropanol intermediates

Alkenes bearing a stereocenter in the allylic position were found to undergo Kulinkovich hydroxycyclopropanation with good diastereoselectivity. For the isomerization of the resulting cyclopropanols to diastereomerically enriched α-methyl ketones, a new mild regioselective method has been developed. A sequence of stereoselective cyclopropanation and cyclopropanol ring opening was successfully employed for the construction of the C20 stereocenter in steroids.

Total synthesis of 1α,25-dihydroxyvitamin D3 analogs modified at the side chain and D-ring

A general approach to generate the natural or unnatural steroid configuration at C20 and unsaturation at the D-ring is exemplified with the total synthesis of 1α,25-dihydroxyvitamin D₃ analogs 3 and 4.

Total synthesis of 1α,25-dihydroxyvitamin D3 (calcitriol) through a Si-assisted allylic substitution

A total synthesis of calcitriol through a Si-assisted substitution to generate the trans-hydrindane core and an asymmetric CuH-catalyzed 1,4-reduction of α,β,γ,δ-unsaturated ester to access the steroid configuration at C20 is described.

Iridium-Catalyzed Selective Isomerization of Primary Allylic Alcohols

This Account presents the development of the iridium-catalyzed isomerization of primary allylic alcohols in our laboratory over the past 8 years. Our initial interest was driven by the long-standing challenge associated with the development of a general catalyst even for the nonasymmetric version of this seemingly simple chemical transformation. The added value of the aldehyde products and the possibility to rapidly generate molecular complexity from readily accessible allylic alcohols upon a redox-economical isomerization reaction were additional sources of motivation. Certainly influenced by the success story of the related isomerization of allylic amines, most catalysts developed for the selective isomerization of allylic alcohols were focused on rhodium as a transition metal of choice. Our approach has been based on the commonly accepted precept that hydrogenation and isomerization are often competing processes, with the latter being usually suppressed in favor of the former. The cationic iridium complexes [(Cy3P)(pyridine)Ir(cod)]X developed by Crabtree (X = PF6) and Pfaltz (X = BArF) are usually considered as the most versatile catalysts for the hydrogenation of allylic alcohols. Using molecular hydrogen to generate controlled amounts of the active form of these complexes but performing the reaction in the absence of molecular hydrogen enabled deviation from the typical hydrogenation manifold and favored exclusively the isomerization of allylic alcohols into aldehydes. Isotopic labeling and crossover experiments revealed the intermolecular nature of the process. Systematic variation of the ligand on the iridium center allowed us to identify the structural features beneficial for catalytic activity. Subsequently, three generations of chiral catalysts have been investigated and enabled us to reach excellent levels of enantioselectivity for a wide range of 3,3-disubstituted aryl/alkyl and alkyl/alkyl primary allylic alcohols leading to β-chiral aldehydes. The combination of the isomerization reaction with enamine catalysis in a sequential process gave access to α,β-chiral aldehydes in high diastereomeric ratio and excellent enantioselectivity. Catalyst-controlled diastereoselective isomerization of stereochemically complex steroid scaffolds has been achieved, giving access indifferently to derivatives with the natural and unnatural C20 configuration, a long-standing challenge in the field. Structural diversification at close proximity of the reactive site and within the polycyclic domain served to further demonstrate the generality and the potential of the method. Models based on quadrant diagrams enabled rationalization of the high levels of enantio- and diastereocontrol obtained in the isomerization of allylic alcohols.

Catalytic cyclometallation in steroid chemistry IV: Efficient method for the synthesis of tetrahydrothiophene, tetrahydroselenophen and cyclopentanone derivatives of (5α)-cholestane

Catalytic cycloalumination of (3β,5α)-3-vinylcholestane and (3α,5α)-3-allylcholestane with Et3Al catalyzed by Cp2ZrCl2 was performed for the first time to give previously unknown aluminacyclopentanes in ∼90% yield; these products were converted in situ to carbo- and heterocyclic (5α)-cholestane derivatives.