Syntheses of the Eastern Halves of Ritterazines B, F, G, and H, Leading to Reassignment of the 5,5-Spiroketal Stereochemistry of Ritterazines B and F

Cephalostatins and ritterazines: Distinctive dimeric marine-derived steroidal pyrazine alkaloids with intriguing anticancer activities

Cephalostatins and ritterazines represent fascinating classes of dimeric marine derived steroidal alkaloids with unique chemical structures and promising biological activities. Originally isolated from marine tube worms and the tunicate Ritterella tokioka collected off the coast of Japan, cephalostatins and ritterazines display potent anticancer effects by inducing apoptosis, disrupting cell cycle progression, and targeting multiple molecular pathways. This review covers the chemistry and bioactivities of 45 cephalostatins and ritterazines from 1988 to 2024, highlighting their complex structures and medicinal contributions. With insights into their structure activity relationships (SAR). Key structural elements, such as the pyrazine ring and 5/6 spiroketal moieties, are found crucial for their biological effects, suggesting interactions with lipid membranes or hydrophobic protein domains. Additionally, the formation of oxocarbenium ions from spiroketal cleavage may enhance their potency by covalently modifying DNA. The pharmacokinetics, ADMET and Drug likeness properties of these steroidal alkaloids are thoroughly addressed. Drug likeness analysis shows that these compounds fit well with the Rule of 4 (Ro4) for Protein-Protein Interaction Drugs (PPIDs), underscoring their potential in this area. Ten compounds (20, 27, 33, 34, 39, 40, 41, 42, 43, and 45) have demonstrated favourable pharmacokinetic and ADMET profiles, making them promising candidates for further research. Future efforts should focus on alternative administration routes, structural modifications, and innovative delivery systems, such as prodrugs and nanoparticles, to improve bioavailability and therapeutic effects. Advances in synthetic chemistry, mechanistic insights, and interdisciplinary collaborations will be essential for translating cephalostatins and ritterazines into effective anticancer therapies.

Regioselective Synthesis of Benzannulated [5,6]-Oxaspirolactones via Cu(II)-Catalyzed Cycloisomerization of 2-(5-Hydroxyalkynyl)benzoates

Spiroketals and oxaspirolactones are widely found in biologically active natural products, serving as important structural motifs. In this study, we present a Cu(II)-catalyzed cascade cycloisomerization of 2-(5-hydroxyalkynyl)benzoates, enabling the regioselective synthesis of benzannulated [5,6]-oxaspirolactones containing an isochromen-1-one moiety. This strategy offers a rapid and efficient approach to access a diverse array of benzannulated [5,6]-oxaspirolactones. The methodology presented here showcases a broad substrate scope, delivering good yields and scalability up to gram scale. The structures of the oxaspirolactones were unequivocally confirmed through single-crystal X-ray analysis and by analogy using 1H and 13C{1H} NMR data.

Chemistry and bioactivities of natural steroidal alkaloids

Steroidal alkaloids possess the basic steroidal skeleton with a nitrogen atom in rings or side chains incorporated as an integral part of the molecule. They have demonstrated a wide range of biological activities, and some of them have even been developed as therapeutic drugs, such as abiraterone acetate (Zytiga^®), a blockbuster drug, which has been used for the treatment of prostate cancer. Structurally diverse natural steroidal alkaloids present a wide spectrum of biological activities, which are attractive for natural product chemistry and medicinal chemistry communities. This review comprehensively covers the structural classification, isolation and various biological activities of 697 natural steroidal alkaloids discovered from 1926 to October 2021, with 363 references being cited.

Pyrazine and Phenazine Heterocycles: Platforms for Total Synthesis and Drug Discovery

There are numerous pyrazine and phenazine compounds that demonstrate biological activities relevant to the treatment of disease. In this review, we discuss pyrazine and phenazine agents that have shown potential therapeutic value, including several clinically used agents. In addition, we cover some basic science related to pyrazine and phenazine heterocycles, which possess interesting reactivity profiles that have been on display in numerous cases of innovative total synthesis approaches, synthetic methodologies, drug discovery efforts, and medicinal chemistry programs. The majority of this review is focused on presenting instructive total synthesis and medicinal chemistry efforts of select pyrazine and phenazine compounds, and we believe these incredible heterocycles offer promise in medicine.

Total Synthesis of Ritterazine B

The first total synthesis of the cytotoxic alkaloid ritterazine B is reported. The synthesis features a unified approach to both steroid subunits, employing a titanium-mediated propargylation reaction to achieve divergence from a common precursor. Other key steps include gold-catalyzed cycloisomerizations that install both spiroketals and late stage C-H oxidation to incorporate the C7' alcohol.

Synthesis and bioactivity of bis-steroidal pyrazine 23-deoxy-25-epi ritterostatin GN1N

Cephalostatins, ritterazines and their hybrid bis-steroidal pyrazine analogs such as 25-epi-rittereostatin G_N1_N show unusually high potency against a wide range of cancer cell lines. Herein, we report the synthesis and bioactivity of 23-deoxy-25-epi ritterostatin G_N1_N, which lacks the 23-hydroxyl group of 25-epi rittereostatin G_N1_N. The less oxygenated bis-steroidal pyrazine was ∼50- to 1000-fold less potent than 25-epi ritterostatin G_N1_N, highlighting the importance of the 23-hydroxyl group for the antiproliferative activity of the cephalostatin/ritterazine class of drugs.

Ritterostatin GN 1N , a Cephalostatin-Ritterazine Bis-steroidal Pyrazine Hybrid, Selectively Targets GRP78

Natural products discovered by using agnostic approaches, unlike rationally designed leads or those obtained through high-throughput screening, offer the ability to reveal new biological pathways and, hence, serve as an important vehicle to unveil new avenues in drug discovery. The ritterazine-cephalostatin family of natural products displays robust and potent antitumor activities, with sub-nanomolar growth inhibition against multiple cell lines and potent activity in xenograft models. Herein, we used comparative cellular and molecular biological methods to uncover the ritterazine-cephalostatin cytotoxic mode of action (MOA) in human tumor cells. Our findings indicated that, whereas ritterostatin GN 1N , a cephalostatin-ritterazine hybrid, binds to multiple HSP70s, its cellular trafficking confines activity to the endoplasmic reticulum (ER)-based HSP70 isoform, GRP78. This targeting results in activation of the unfolding protein response (UPR) and subsequent apoptotic cell death.

Synthesis of 23-deoxy-25-epi north unit of cephalostatin 1 via reductive and oxidative modifications of hecogenin acetate

An efficient synthesis of the 23-deoxy-25-epi north unit of cephalostatin 1 has been achieved in 17 steps via reductive and oxidative functionalizations of hecogenin acetate with an overall yield of 3.8%. This synthesis features transetherification-mediated E-ring opening, D-ring oxidation, hemiketalization-mediated E-ring closure, and stereoselective 5/5-spiroketalization.

Total synthesis of cephalosporolide E via a tandem radical/polar crossover reaction. The use of the radical cations under nonoxidative conditions in total synthesis

The present work reports the first example of the use of the chemistry of radical cations under nonoxidative conditions in total synthesis. Using a late-stage tandem radical/polar crossover reaction, a highly stereoselective total synthesis of cephalosporolide E (which is typically obtained admixed with cephalosporolide F) was accomplished. The reaction of a phthalimido derivative with triphenyltin radical in refluxing toluene engenders a contact ion-pair (radical cation) that leads, in the first instance, to the cephalosporolide F, which is transformed into the cephalosporolide E via a stereocontrolled spiroketal isomerization promoted by the diphenylphosphate acid that is formed during the tandem transformation.

Steroid dimers-in vitro cytotoxic and antimicrobial activities

The in vitro cytotoxic activity of previously synthesized steroid dimers with different spacer group (sulfide, trithiolane ring or phosphorotrithioate) and the substituent at C-17 position was tested for their possible effects against following human tumor cell lines: cervical adenocarcinoma (HeLa), chronic myelogenous leukemia (K562) and two human breast cancer cell lines (MDA-MB-361 and MDA-MB-453). These compounds, applied at micromolar concentrations, exhibited cytotoxic activity of different intensity (compared with cisplatin as a control), modality and selectivity in these malignant cell lines. The best activity against all four cell cancer lines was exhibited by dimer-sulfides. All screened compounds exerted concentration-dependent cytotoxic activity against leukemia K562 cells. The compounds which exerted the most pronounced cytotoxic action exhibited notably higher cytotoxic activities against K562, HeLa and MDA-MB-453 cells in comparison to resting and PHA-stimulated PBMC, pointing to a significant selectivity in their antitumor actions. Examination of the mechanisms of cytotoxicity on leukemia K562 cells revealed pro-apoptotic action of each of the investigated compounds applied at concentrations 2IC50. The most prominent pro-apoptotic action was exhibited by dimer-sulfide of cholest-4-en-3-one. Furthermore, almost all of the tested compounds at IC50 concentrations induced G1 phase cell cycle arrest in K562 cells. Antimicrobial activity against Gram-positive, Gram-negative bacteria and fungal cells, and toxicity to brine shrimp Artemia salina, were evaluated. There was no antibacterial activity. The best antifungal activity was exhibited against Saccharomyces cerevisiae by dimers linked with trithiolane ring, indicating a selective activity of investigated compounds.

Total synthesis of viridicatumtoxin B and analogues thereof: strategy evolution, structural revision, and biological evaluation

The details of the total synthesis of viridicatumtoxin B (1) are described. Initial synthetic strategies toward this intriguing tetracycline antibiotic resulted in the development of key alkylation and Lewis acid-mediated spirocyclization reactions to form the hindered EF spirojunction, as well as Michael-Dieckmann reactions to set the A and C rings. The use of an aromatic A-ring substrate, however, was found to be unsuitable for the introduction of the requisite hydroxyl groups at carbons 4a and 12a. Applying these previous tactics, we developed stepwise approaches to oxidize carbons 12a and 4a based on enol- and enolate-based oxidations, respectively, the latter of which was accomplished after systematic investigations that revealed critical reactivity patterns. The herein described synthetic strategy resulted in the total synthesis of viridicatumtoxin B (1), which, in turn, formed the basis for the revision of its originally assigned structure. The developed chemistry facilitated the synthesis of a series of viridicatumtoxin analogues, which were evaluated against Gram-positive and Gram-negative bacterial strains, including drug-resistant pathogens, revealing the first structure-activity relationships within this structural type.

Synthesis of the spiroiminal moiety and approaches to the synthesis of marineosins A and B

A short and efficient synthesis of model spiroiminals that have the same stereochemistry as marineosins A and B, but different conformations, was carried out in six or seven steps from 6-methyltetrahydropyran-2-one. These spiroiminals were also prepared biomimetically by reduction of an enol ether. A more highly substituted spiroiminal with the same stereochemistry and conformation as marineosin A was prepared in 11 steps from parasorbic acid. A macrocyclic pyrrole lactone was prepared stereospecifically in 10 steps. A five-step sequence converted the lactone to a late hemi-iminal intermediate that has resisted the methylation and spiroiminal formation that would lead to marineosin A.

Mechanistic insights on the reactivity of furospirostanes with the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain

F-ring opening in spirostanes with the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain produces a Δ(22)-intermediate with an allylic acetoxy group. For this reason the reactivity profile of these compounds deviates from that observed in other naturally occurring or synthetic spirostanes and furospirostanes.

Synthesis of C17-OH-north unit of ritterazine G via "Red-Ox" modifications of hecogenin acetate

The C17-OH-north unit of ritterazine G was prepared in 13 steps from hecogenin acetate. This synthesis features a highly efficient and stereoselective introduction of the C17-OH via E-ring cleavage/F-ring formation, D-ring oxidation, and F-ring cleavage/E-ring formation.

Synthesis of 14',15'-dehydro-ritterazine Y via reductive and oxidative functionalizations of hecogenin acetate

An analog of ritterazine Y was synthesized from hecogenin acetate in 23 steps via functional group manipulations of hecogenin acetate. Preparation of the north G and south Y units and the late stage Guo-Fuchs asymmetric coupling of the both units afforded the ritterazine Y analog.

Cephalostatins and ritterazines

This review article is a tribute to the numerous chemists whose relentless effort for the last quarter of a century resulted in the isolation, identification, and finally the chemical synthesis of a family of bis-steroidal pyrazine alkaloids of marine origin. In the task of defeating cancer, the search for bioactive substances among the naturally occurring compounds is, without any doubt, a preferential approach. The remarkable contribution of Petitt, Fusetani, and their coworkers allowed to discover this family of marine alkaloids that emerge as potential therapeutic anticancer agents, although there is still a long way to go. The challenging and dangerous task of collecting living organisms from deep-waters was followed by a laborious isolation, elucidation of the complicated structures and biological tests. The outcome of this paramount effort was the identification of 45 compounds that stand, to date, as some of the most potent anticancer agents. The intriguing structures of the isolated alkaloids drew the attention of synthetic chemists, valiant enough to undertake the challenging task of synthesizing some of the most active members of the family. Fuchs, Heathcock, Winterfeldt, Suarez, Shair, and their associates pioneered in the establishment of feasible synthetic routes for the preparation of some of the naturally occurring compounds and a large number of synthetic analogs, allowing to establish SAR criteria that have guided the design of new synthetic analogs. Numerous analogs have been prepared to investigate the mechanism of action of bis-steroidal pyrazines, e.g. cephalostatin analogs bearing a strained spiroketal moiety. However, the mechanism of action and the biological target of these compounds remain far from being understood. Therefore, the rational design of simpler, yet highly active analogs seems at the current stage elusive. It is still 1 to clear why these compounds need to be dimeric to show high biological activity. Furthermore, it is not known whether the central pyrazine ring is simply a linker or has some additional function. This could be tested by examining the biological activity of steroidal dimers with other linkers, e.g. with a benzene ring. Such analogs have been actually prepared but without functional groups necessary for biological activity. The clinical trials of cephalostatins have got stuck due to a shortage of material. There is an urgent need to provide highly active, yet not too complex analogs, which could be available in substantial amounts for advanced pharmacological studies.

(25R)-16β-Acet-oxy-3β-bromo-23',26-ep-oxy-23',25-dimethyl-5α-cholest-23,23'-en-6-one dichloro-methane monosolvate

The crystal structure of the title compound, C31H45BrO5·CH2Cl2, prepared in six steps from diosgenin, confirmed that the configurations of the stereogenic centers, positions 20S and 25R, remain unchanged during the reaction. The six-membered A, B and C rings have chair conformations. The five-membered ring D has an envelope conformation (with the methyl-substituted C atom fused to ring C as the flap) and the six-membered dihydro-pyran ring E adopts a twist-boat conformation. In the crystal, mol-ecules are linked via C-H⋯O and C-H⋯Cl hydrogen bonds, the latter involving the dichloro-methane solvent mol-ecule, forming a three-dimensional supra-molecular network.

On the proposed structures and stereocontrolled synthesis of the cephalosporolides

The synthesis of four candidate stereoisomers of cephalosporolide H is described, made possible by a zinc-chelation strategy for controlling the stereochemistry of oxygenated 5,5-spiroketals. The same strategy likewise enables the first stereocontrolled synthesis of cephalosporolide E, which is typically isolated and prepared admixed with its spiroketal epimer, cephalosporolide F.

Survey of marine natural product structure revisions: a synergy of spectroscopy and chemical synthesis

The structural assignment of new natural product molecules supports research in a multitude of disciplines that may lead to new therapeutic agents and or new understanding of disease biology. However, reports of numerous structural revisions, even of recently elucidated natural products, inspired the present survey of techniques used in structural misassignments and subsequent revisions in the context of constitutional or configurational errors. Given the comparatively recent development of marine natural products chemistry, coincident with modern spectroscopy, it is of interest to consider the relative roles of spectroscopy and chemical synthesis in the structure elucidation and revision of those marine natural products that were initially misassigned. Thus, a tabulated review of all marine natural product structural revisions from 2005 to 2010 is organized according to structural motif revised. Misassignments of constitution are more frequent than perhaps anticipated by reliance on HMBC and other advanced NMR experiments, especially when considering the full complement of all natural products. However, these techniques also feature prominently in structural revisions, specifically of marine natural products. Nevertheless, as is the case for revision of relative and absolute configuration, total synthesis is a proven partner for marine, as well as terrestrial, natural products structure elucidation. It also becomes apparent that considerable 'detective work' remains in structure elucidation, in spite of the spectacular advances in spectroscopic techniques.

Synthesis and evaluation of a fluorescent ritterazine-cephalostatin hybrid

The cephalostatin and ritterazine natural products comprise a potent family of bis-steroidal pyrazines that display potent single-digit nanomolar inhibition of tumor cell growth. An active fluorescent ritterazine-cephalostatin hybrid probe was developed using detailed SAR data derived through total synthetic efforts. A combination of time course and confocal imaging studies indicate that this natural product family is rapidly taken up in tumor cells and localizes subcellularly within ER and surrounding the nuclear-ER interface.

Synthesis of an A'B' Precursor to Angelmicin B: Product Diversification in the Suárez Lactol Fragmentation

We describe a synthetic strategy for the angelimicin family of anthraquinoid natural products that involves converting a central highly oxygenated decalin intermediate to the AB and A'B' subunits. Herein, we report the synthesis of the bicyclic A'B' subunit that complements our earlier route to the tricyclic AB framework. The differentiating tact in the two syntheses focused on controlling the Suárez radical fragmentation of lactol precursors by modulating the substrate's structural rigidity. A more flexible lactol gave the tricyclic AB framework, whereas a more rigid substrate led to the bicyclic A'B' precursor, presumably through divergent pathways from the radical produced in the initial fragmentation step. These results establish a versatile advanced synthetic precursor for the angelimicins, and on a more general note, illustrate strategies for applying the Suárez fragmentation to diverse and complex molecular frameworks.

A gold-catalyzed alkyne-diol cycloisomerization for the synthesis of oxygenated 5,5-spiroketals

A highly efficient synthesis of oxygenated 5,5-spiroketals was performed towards the synthesis of the cephalosporolides. Gold(I) chloride in methanol induced the cycloisomerization of a protected alkyne triol with concomitant deprotection to give a strategically hydroxylated 5,5-spiroketal, despite the potential for regiochemical complications and elimination to furan. Other late transition metal Lewis acids were less effective. The use of methanol as solvent helped suppress the formation of the undesired furan by-product. This study provides yet another example of the advantages of gold catalysis in the activation of alkyne π-systems.

Terpenoid-Alkaloids: Their Biosynthetic Twist of Fate and Total Synthesis

Terpenes and alkaloids are ever-growing classes of natural products that provide new molecular structures which inspire chemists and possess a broad range of biological activity. Terpenoid-alkaloids originate from the same prenyl units that construct terpene skeletons. However, during biosynthesis, a nitrogen atom (or atoms) is introduced in the form of β-aminoethanol, ethylamine, or methylamine. Nitrogen incorporation can occur either before, during, or after the cyclase phase. The outcome of this unique biosynthesis is the formation of natural products containing unprecedented structures. These complex structural motifs expose current limitations in organic chemistry, thus providing opportunities for invention. This review focuses on total syntheses of terpenoid-alkaloids and unique issues presented by this class of natural products. More specifically, it examines how these syntheses relate to the way terpenoid-alkaloids are made in Nature. Developments in chemistry that have facilitated these syntheses are emphasized, as well as chemical technology needed to conquer those that evade synthesis.

Synthesis of the antiproliferative agent hippuristanol and its analogues via Suárez cyclizations and Hg(II)-catalyzed spiroketalizations

A full account of the synthesis of hippuristanol and its analogues is described. Hecogenin acetate was identified as a suitable and economical starting material for this work, and substrate-controlled stereoselection was obtained throughout the construction of the key spiroketal unit. Suárez cyclization was first used, but Hg(II)-catalyzed spiroketalization of the 3-alkyne-1,7-diol motif was finally identified as the most convenient strategy.

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.

Marine natural products

This review covers the literature published in 2007 for marine natural products, with 948 citations(627 for the period January to December 2007) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidarians,bryozoans, molluscs, tunicates, echinoderms and true mangrove plants. The emphasis is on new compounds (961 for 2007), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.1 Introduction, 2 Reviews, 3 Marine microorganisms and phytoplankton, 4 Green algae, 5 Brown algae, 6 Red algae, 7 Sponges, 8 Cnidarians, 9 Bryozoans, 10 Molluscs, 11 Tunicates (ascidians),12 Echinoderms, 13 Miscellaneous, 14 Conclusion, 15 References.

Synthesis of C14,15-dihydro-C22,25-epi north unit of cephalostatin 1 via "red-ox" modifications of hecogenin acetate

The C14,15-dihydro-C22,25-epi north unit of cephalostatin 1 has been synthesized in 11 operations from commercially available hecogenin acetate via multiple reductions and oxidations. The key transformations include (i) Cr(VI)-catalyzed E-ring opening, (ii) C17 hydroxylation, and (iii) a base-triggered cyclization cascade.

Review of cytotoxic cephalostatins and ritterazines: isolation and synthesis

The cephalostatins and ritterazines comprise a family of structurally related natural products reported by Professors G. R. Pettit and N. Fusetani from 1988 -1998. Isolated from the invertebrate marine chordates Cephalodiscus gilchristi and Ritterella tokioka, the cephalostatins and ritterazines exhibit potent cytotoxicity toward the murine P388 lymphocytic leukemia cell line. In fact, cephalostatin 1 ( 1, ED 50 0.1-0.001 pM) proved to be one of the most powerful cancer cell growth inhibitors ever tested by the U.S. National Cancer Institute. The ritterazines and cephalostatins share many common structural features in which two highly oxygenated steroidal units with side chains forming either 5/5 or 5/6 spiroketals are fused via a pyrazine core. Professor P. L. Fuchs and colleagues reported the total syntheses of 1, cephalostatins 7 ( 7), and 12 ( 12), ritterazines K ( 30) and M ( 32), and cytotoxic analogues. The synthesis of 1, described in 1998, required 65 synthetic operations to complete.

Toward the total synthesis of ritterazine N

Zr-mediated equilibrating cyclocarbonylation of a designed triene led with high diastereocontrol to the ABC 6-6-5 tricyclic core of ritterazine N. The 5-5 EF spiroketal side chain of ritterazine N was prepared by equilibrating cyclization of an acyclic keto diol. The two components were coupled, and the D ring was assembled by intramolecular aldol condensation.