Facile Access to Bridged Ring Systems via Point-to-Planar Chirality Transfer: Unified Synthesis of Ten Cyclocitrinols

Organophosphates as Versatile Substrates in Organic Synthesis

This review summarizes the applications of organophosphates in organic synthesis. After a brief introduction, it discusses cross-coupling reactions, including both transition-metal-catalyzed and transition-metal-free substitution reactions. Subsequently, oxidation and reduction reactions are described. In addition, this review highlights the applications of organophosphates in the synthesis of natural compounds, demonstrating their versatility and importance in modern synthetic chemistry.

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.

Controllable skeletal reorganizations in natural product synthesis

Covering: 2016 to 2023The synthetic chemistry community is always in pursuit of efficient routes to natural products. Among the many available general strategies, skeletal reorganization, which involves the formation, cleavage, and migration of C-C and C-heteroatom bonds, stands out as a particularly useful approach for the efficient assembly of molecular skeletons. In addition, it allows for late-stage modification of natural products for quick access to other family members or unnatural derivatives. This review summarizes efficient syntheses of steroid, terpenoid, and alkaloid natural products that have been achieved by means of this strategy in the past eight years. Our goal is to illustrate the strategy's potency and reveal the spectacular human ingenuity demonstrated in its use and development.

Bioinspired Skeletal Reorganization Approach for the Synthesis of Steroid Natural Products

ConspectusSteroids, termed "keys to life" by Rupert Witzmann, have a wide variety of biological activities, including anti-inflammatory, antishock, immunosuppressive, stress-response-enhancing, and antifertility activities, and steroid research has made great contributions to drug discovery and development. According to a chart compiled by the Njardarson group at the University of Arizona, 15 of the top 200 small-molecule drugs (by retail sales in 2022) are steroid-related compounds. Therefore, synthetic and medicinal chemists have long pursued the chemical synthesis of steroid natural products (SNPs) with diverse architectures, and vital progress has been achieved, especially in the twentieth century. In fact, several chemists have been rewarded with a Nobel Prize for original contributions to the isolation of steroids, the elucidation of their structures and biosynthetic pathways, and their chemical synthesis. However, in contrast to classical steroids, which have a 6/6/6/5-tetracyclic framework, rearranged steroids (i.e., abeo-steroids and secosteroids), which are derived from classical steroids by reorganization of one or more C-C bonds of the tetracyclic skeleton, have started to gain attention from the synthetic community only in the last two decades. These unique rearranged steroids have complex frameworks with high oxidation states, are rich in stereogenic centers, and have attractive biological activities, rendering them popular yet formidable synthetic targets.Our group has a strong interest in the efficient synthesis of SNPs and, drawing inspiration from nature, we have found that bioinspired skeletal reorganization (BSR) is an efficient strategy for synthesizing challenging rearranged steroids. Using this strategy, we recently achieved concise syntheses of five different kinds of SNPs (cyclocitrinols, propindilactone G, bufospirostenin A, pinnigorgiol B, and sarocladione) with considerably rearranged skeletons; our work also enabled us to reassign the originally proposed structure of sarocladione. In this Account, we summarize the proposed biosyntheses of these SNPs and describe our BSR approach for the rapid construction of their core frameworks. In the work described herein, information gleaned from the proposed biosyntheses allowed us to develop routes for chemical synthesis. However, in several cases, the synthetic precursors that we used for our BSR approach differed substantially from the intermediates in the proposed biosyntheses, indicating the considerable challenges we encountered during this synthetic campaign. It is worth mentioning that during our pursuit of concise and scalable syntheses of these natural products, we developed two methods for accessing synthetically challenging targets: a method for rapid construction of bridged-ring molecules by means of point-to-planar chirality transfer and a method for efficient construction of macrocyclic molecules via a novel ruthenium-catalyzed endoperoxide fragmentation. Our syntheses vividly demonstrate that consideration of natural product biosynthesis can greatly facilitate chemical synthesis, and we expect that the BSR approach will find additional applications in the efficient syntheses of other structurally complex steroid and terpenoid natural products.

Cope Rearrangement of 1-Acyl-2-vinylcyclopropanes to Cyclohept-4-Enones

Cycloheptenones are widespread in natural products and bioactive molecules. An efficient and convenient NaH-mediated Cope Rearrangement of doubly activated vinylcyclopropanes is reported for the synthesis of cyclohepten-4-ones. These flexible intramolecular reactions were applicable to a wide range of substrates and could be performed on gram scale. The derivatization of the product leads to short and highly efficient synthesis of some useful functional molecules.

Total Synthesis of abeo-Steroids via Cycloaddition Strategy

ConspectusSteroids continue to play a significant role in organic chemistry, medicinal chemistry, and drug discovery due to their important biological activities and diverse intriguing structures. Although synthetic organic chemists have successfully constructed and elaborated the classical [6-6-6-5] tetracyclic steroid skeleton for nearly a century, synthesis of the unusual rearranged steroids, particularly abeo-steroids with a medium-sized ring, remains a challenge in the synthetic community. Furthermore, the structures of abeo-steroids are complex and diverse, containing a seven-membered ring embedded in the fused or bridged A/B ring system and possessing numerous stereogenic centers. Besides their structural complexity, various abeo-steroids have shown remarkable biological activities. However, the relative scarcity of abeo-steroids in natural sources has impeded the systematic evaluation of their biological activities. In addition, direct strategies to build the core structures of abeo-steroids are very rare, partially because of the high ring-strain energies of their rearranged A/B ring systems. Therefore, the development of direct and efficient synthetic approaches to these complex molecules is highly desired.Our long-standing interest in the total synthesis of abeo-steroids and the development of new cycloaddition reactions for streamlining complex molecule synthesis have led us to develop a series of unique and powerful intramolecular cycloaddition strategies to access a diverse array of highly strained abeo-steroids. These strategies include Ru-catalyzed [5 + 2] cycloaddition, acid-promoted type I [5 + 2] cycloaddition, Rh-catalyzed [2 + 2 + 1] cycloaddition, and type II [5 + 2] cycloaddition. Since 2018, we have accomplished the first total syntheses of five synthetically challenging abeo-steroids, i.e., bufogargarizins A and B, phomarol, bufospirostenin A, and cyclocitrinol, thus facilitating the evaluation of their pharmacological potentials. In this Account, we summarize our laboratory's systematic efforts in the total synthesis of these abeo-steroids via cycloaddition strategies. We highlight the efficiency and versatility of each cycloaddition strategy for constructing structurally complex abeo-steroid cores by forming the A/B ring system. The evolution of each strategy and key lessons learned from the synthetic journey are also discussed. We believe that our unique perspective in this field will promote advances in the total synthesis of abeo- and related steroids.

Bioactive and unusual steroids from Penicillium fungi

Penicillium fungi are represented by various species and can be found worldwide and thrive in a range of environments, such as in the soil, air, and indoors, and in marine environments, as well as food products. Chemical investigation of species of this genus has led to the discovery of compounds from several structural classes with varied bioactivities. As an example, this genus has been a source of bioactive and structurally unusual steroids. The scope of this short review is to cover specialized metabolites of the steroid class and the cytotoxic, antimicrobial, anti-inflammatory as well as phytotoxic activities of these compounds. Other steroids that possess unusual structures, with significant bioactivity yet to determined, will also be discussed to further demonstrate the structural diversity of this compound class from Penicillium fungi, and hopefully inspire the further exploration of such compounds to uncover their activity.

Asymmetric Total Synthesis of (+)-Phainanoid A and Biological Evaluation of the Natural Product and Its Synthetic Analogues

Here, we report our detailed efforts toward the synthesis of phainanoids, a novel class of dammarane-type triterpenoids with potent immunosuppressive activities and unique structural features. Systematic model studies have been carried out, and efficient approaches have been established to construct the benzofuranone-based 4,5-spirocycle, the D/E/F tricyclic core, the [4.3.1] propellane, and the 5,5-oxaspirolactone moieties. The asymmetric synthesis of (+)-phainanoid A has been achieved through kinetic resolution of the tricyclic core followed by diastereoselective installation of the A/B/C and G/H rings and fragment coupling with the enantioenriched I/J rings. In addition, novel estrone-derived phainanoid analogues have been prepared. The immunosuppressive and cell survival assays revealed that (+)-phainanoid A and some of its synthetic analogues can specifically inhibit stimulation-induced lymphocyte proliferation but not cell survival at their effective concentrations. Preliminary structure-activity relationship information has been obtained, which could inspire future design of immunosuppressive phainanoid analogues.

Asymmetric Total Synthesis of Twin Bufogargarizins A and B

The first and asymmetric total synthesis of bufogargarizins A and B, two unusual and highly oxygenated twin steroids with rearranged A/B rings, was achieved. The synthetically challenging [7-5-6-5] tetracyclic ring system of bufogargarizin A was efficiently constructed by the first intramolecular Ru-catalyzed [5 + 2] cycloaddition reaction of a vinyl ether cyclopropane-yne. Notably, the interesting [5-7-6-5] tetracyclic skeleton of bufogargarizin B was diastereoselectively reassembled by unique retro-aldol/transannular aldol cascade reactions from the [7-5-6-5] tetracyclic framework.

Cyclopropylcarbinyl cation chemistry in synthetic method development and natural product synthesis: cyclopropane formation and skeletal rearrangement

In this Review, the underrecognized utilities of the cyclopropylcarbinyl cation chemistry are summarized in cyclopropane synthesis and skeletal rearrangements, and their applications in natural product total synthesis are highlighted.

Euphylonoids A and B, Two Highly Modified Jatrophane Diterpenoids with Potent Lipid-Lowering Activity from Euphorbia hylonoma

Euphylonoids A (1) and B (2), two highly modified jatrophane diterpenoids, were isolated from Euphorbia hylonoma. 1 represents a new 9(10→18)-abeo-8,12-cyclojatrophane skeleton containing a cage-like 3,8-dioxatricyclo[5.1.2.0^4,9]decane core, while 2 is a 14(13→20)-abeo-8,12-cyclojatrophane featuring an unusual 17-oxatetracyclo[12.2.1.0^1,5.0^9,13]heptadecane framework. Their structural elucidation was completed by spectroscopic, chemical, computational, and single-crystal X-ray diffraction means. 2 significantly inhibited early adipogenesis in 3T3-L1 adipocytes via activating AMP-activated protein kinase signaling.

Syntheses of Bufospirostenin A and Ophiopogonol A by a Conformation-Controlled Transannular Prins Cyclization

Controlling the conformation of medium-sized rings is challenging because of their flexibility and ring strain effects. Herein, we report non-Curtin-Hammett conditions for the precise control of the conformation of cyclodecenones to effect the first cis-selective transannular Prins cyclization, which enabled concise syntheses of the 5(10→1)abeo-steroids bufospirostenin A and ophiopogonol A in only seven steps from inexpensive starting materials. Computational results indicated that the key cyclization was kinetically controlled and proceeded via either a Prins pathway or a carbonyl-ene pathway, depending on the reaction conditions. Moreover, conformational isomerization played a critical role in determining the stereochemistry of the products.

Natural product total synthesis using rearrangement reactions

The synthetic utility of rearrangement reactions in total synthesis for the rapid construction of core skeletons, the precise control of stereochemistry, and the identification of suitable synthons has been discussed.

Bidirectional Total Synthesis of Phainanoid A via Strategic Use of Ketones

Here we report the total synthesis of phainanoid A, a unique dammarane-type triterpenoid (DTT), using an unusual bidirectional synthetic strategy. It features two transition-metal-mediated highly diastereoselective transformations to access the two challenging strained ring systems that branch toward opposite directions from the tricyclic core. This work also highlights the strategic use of ketones (or enol triflates) as versatile handles for rapid growth of molecular complexity in all key transformations, which paves the way for efficient preparations of complex and biologically significant DTTs.

Gram-Scale Synthesis of Bufospirostenin A by a Biomimetic Skeletal Rearrangement Approach

Bufospirostenin A, which was the first spirostanol to be isolated from an animal, possesses an unprecedented 5/7/6/5/5/6 hexacyclic framework. Herein, we report two biomimetic syntheses of this natural product in just seven or nine steps from a readily available steroidal lactone. Key features of the syntheses include a photosantonin rearrangement and a Wagner-Meerwein rearrangement for rapid construction of the rearranged A/B ring system, as well as a cobalt-mediated olefin hydroselenylation and a selenide E2 reaction to accomplish a challenging olefin transposition. Our syntheses provide experimental support for the biogenetic pathway to 5(10→1)abeo-steroids that we have proposed.

Small rings in the bigger picture: ring expansion of three- and four-membered rings to access larger all-carbon cyclic systems

The release of the inherent ring strain of cyclobutane and cyclopropane derivatives allows a rapid build-up of molecular complexity. This review highlights the state-of-the-art of the ring expansions of three- and four-membered cycles and is organised by types of reactions with emphasis on the reaction mechanisms. Selected examples are discussed to illustrate the synthetic potential of this elegant synthetic tool.

Atom-Economic Synthesis of Highly Functionalized Bridged Ring Systems Initiated by Ring Expansion of Indene-1,3-dione

An atom economic procedure for the regioselective synthesis of bridged seven-membered-ring compounds from simple reactants such as ynones and indene-1,3-dione has been developed. This process was realized through the one-pot reactions of ring-expansion of indene-1,3-dione with alkynyl ketones and successive formal [4+2] cycloaddition. The Michael addition reaction is the key for the regioselectivity of the formal [4+2] cycloaddition.

Contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids

Many natural products have intriguing biological properties that arise from their fascinating chemical structures. However, the intrinsic complexity of the structural skeleton and the reactive functional groups on natural products pose tremendous challenges to chemical syntheses. Semi-synthesis uses chemical compounds isolated from natural sources as the starting materials to produce other novel compounds with distinct chemical and medicinal properties. In particular, advancements in various types of sp3 C-H bond functionalization reactions and skeletal rearrangement methods have contributed to the re-emergence of semi-synthesis as an efficient approach for the synthesis of structurally complex bioactive natural products. Here, we begin with a brief discussion of several bioactive natural products that were obtained via a semi-synthetic approach between 2008 and 2015 and we then discuss in-depth contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids reported during 2016-2020.

Concise Synthesis of 9,11-Secosteroids Pinnigorgiols B and E

Pinnigorgiols B and E are 9,11-secosteroids with a unique tricyclic γ-diketone framework. Herein, we report the first synthesis of these natural products from inexpensive, commercially available ergosterol. This synthesis features a semipinacol rearrangement and an acyl radical cyclization/hemiketalization cascade; the latter efficiently assembled the tricyclic γ-diketone skeleton, with two rings and three contiguous stereogenic centers being formed in a single step.

Synthetic applications of type II intramolecular cycloadditions

Type II intramolecular cycloadditions ([4+2], [4+3], [4+4] and [5+2]) have emerged recently as an efficient and powerful strategy for the construction of bridged ring systems. In general, type II cycloadditions provide access to a wide range of bridged bicyclo[m.n.1] ring systems with high regio- and diastereoselectivity in an easy and straightforward manner. In each section of this review, an overview of the corresponding type II cycloadditions is presented, which is followed by highlights of method development and synthetic applications in natural product synthesis. The goal of this review is to provide a survey of recent advances in the field covering literature up to 2020. The review will serve as a useful reference for organic chemists engaged in the total synthesis of natural products containing bridged bicyclo[m.n.1] ring systems and provide strong stimulus for invention and further advances in this exciting research field.

Discoveries and Challenges en Route to Swinhoeisterol A

In this work, a full account of the authors' synthetic studies is reported that culminated in the first synthesis of 13(14→8),14(8→7)diabeo-steroid swinhoeisterol A as well as the related dankasterones A and B, 13(14→8)abeo-steroids, and periconiastone A, a 13(14→8)abeo-4,14-cyclo-steroid. Experiments are described in detail that provided further insight into the mechanism of the switchable radical framework reconstruction approach. By discussing failed strategies and tactics towards swinhoeisterol A, the successful route that also allowed an access to structurally closely related analogues, such as Δ22 -24-epi-swinhoeisterol A, is eventually presented.

Synthesis of the 8,19-Epoxysteroid Eurysterol A

We report the first chemical synthesis of eurysterol A, a cytotoxic and antifungal marine steroidal sulfate with a unique C8−C19 oxy‐bridged cholestane skeleton. After C19 hydroxylation of cholesteryl acetate, used as an inexpensive commercial starting material, the challenging oxidative functionalization of ring B was achieved by two different routes to set up a 5α‐hydroxy‐7‐en‐6‐one moiety. As a key step, an intramolecular oxa‐Michael addition was exploited to close the oxy‐bridge (8β,19‐epoxy unit). DFT calculations show this reversible transformation being exergonic by about −30 kJ mol⁻¹. Along the optimized (scalable) synthetic sequence, the target natural product was obtained in only 11 steps in 5 % overall yield. In addition, an access to (isomeric) 7β,19‐epoxy steroids with a previously unknown pentacyclic ring system was discovered.

Total Synthesis of Natural Products with Bridged Bicyclo[m.n.1] Ring Systems via Type II [5 + 2] Cycloaddition

Natural products containing bridged ring systems are widely identified and show significant biological activity. The development of efficient synthesis reactions and strategies to construct bridged ring systems is a long-standing but very significant challenge in organic chemistry. In 2014, our group developed a unique type II [5 + 2] cycloaddition reaction that provides a facile and direct methodology for constructing highly functionalized bridged bicyclo[4.3.1], bicyclo[4.4.1], bicyclo[5.4.1], bicyclo[6.4.1], and other bicyclo[m.n.1] systems containing a strained bridgehead double bond. In this Account, we summarize the methodology development and report the results of application of our unique strategy for the total synthesis of several natural products with bridged ring systems (i.e., cyclocitrinol, cerorubenic acid-III, and vinigrol) during the past 5 years in our laboratory. In the first part, we introduce the logic behind the design and discovery of type II [5 + 2] cycloadditions. The substrates can be easily synthesized by a modular approach, followed by base-promoted group elimination under heat to form an oxidopyrylium ylide, which can undergo cycloaddition under relatively mild conditions with a variety of double bonds to generate bridged bicyclo[m.n.1] frameworks in high yield. The diastereocontrol and unique endo selectivity of this methodology are favorable for further application to the synthesis of complex natural products. In the second part, we highlight our endeavors in the total synthesis of several different types of molecules bearing bridged ring systems using our methodology. The bridged bicyclo[4.4.1] system is the core structure of two different types of natural products, cyclocitrinol and cerorubenic acid-III, that can be efficiently constructed by type II [5 + 2] cycloadditions. The development of suitable strategies and methods for site-selective cleavage of the C-O bond of the oxa-[3.2.1] ring system in the products of type II [5 + 2] cycloadditions is also discussed and highlighted during the syntheses. Moreover, the bridged bicyclo[5.3.1] system is the core structure of vinigrol, which can be constructed through a novel ring contraction sequence of the bicyclo[5.4.1] system formed by a type II [5 + 2] cycloaddition. By combining with a ring contraction cascade, we believe that type II [5 + 2] cycloadditions have the potential to be used as a unified approach to constructing natural products containing bridged bicyclo[m.n.1] frameworks.

Intercepted Retro-Nazarov Reaction: Syntheses of Amidino-Rocaglate Derivatives and Their Biological Evaluation as eIF4A Inhibitors

Rocaglates are a family of natural products isolated from the genus Aglaia which possess a highly substituted cyclopenta[b]benzofuran skeleton and inhibit cap-dependent protein synthesis. Rocaglates are attractive compounds due to their potential for inhibiting tumor cell maintenance in vivo by specifically targeting eukaryotic initiation factor 4A (eIF4A) and interfering with recruitment of ribosomes to mRNA. In this paper, we describe an intercepted retro-Nazarov reaction utilizing intramolecular tosyl migration to generate a reactive oxyallyl cation on the rocaglate skeleton. Trapping of the oxyallyl cation with a diverse range of nucleophiles has been used to generate over 50 novel amidino-rocaglate (ADR) and amino-rocaglate derivatives. Subsequently, these derivatives were evaluated for their ability to inhibit cap-dependent protein synthesis where they were found to outperform previous lead compounds including the rocaglate hydroxamate CR-1-31-B.