Total synthesis of complex 2,5-diketopiperazine alkaloids

The 2,5-diketopiperazine (DKP) motif is present in many biologically relevant, complex natural products. The cyclodipeptide substructure offers structural rigidity and stability to proteolysis that makes these compounds promising candidates for medical applications. Due to their fascinating molecular architecture, synthetic organic chemists have focused significant effort on the total synthesis of these compounds. This review covers many such efforts on the total synthesis of DKP containing complex alkaloid natural products.

Molecular Dynamics Simulations Guide Chimeragenesis and Engineered Control of Chemoselectivity in Diketopiperazine Dimerases

In the biosynthesis of the tryptophan-linked dimeric diketopiperazines (DKPs), cytochromes P450 selectively couple DKP monomers to generate a variety of intricate and isomeric frameworks. To determine the molecular basis for selectivity of these biocatalysts we obtained a high-resolution crystal structure of selective Csp2 -N bond forming dimerase, AspB. Overlay of the AspB structure onto C-C and C-N bond forming homolog NzeB revealed no significant structural variance to explain their divergent chemoselectivities. Molecular dynamics (MD) simulations identified a region of NzeB with increased conformational flexibility relative to AspB, and interchange of this region along with a single active site mutation led to a variant that catalyzes exclusive C-N bond formation. MD simulations also suggest that intermolecular C-C or C-N bond formation results from a change in mechanism, supported experimentally through use of a substrate mimic.

Lewy Body Pathology and Alzheimer Disease in Down Syndrome

Introduction/Objective

Aging adults with Down syndrome (DS) develop Alzheimer disease neuropathology (AD) by the age of 40 years, primarily due to the overexpression of the amyloid precursor protein on chromosome 21. Lewy bodies (LBs), containing alpha-synuclein protein, are observed in 7-60% of AD patients in the amygdala and in cortex. Prior DS studies (n=20-56 cases) find the frequency of LB pathology to range between 8-50% of cases being affected. We hypothesized that LB pathology would also be present in DS brain with similar locations and prevalence to AD. Thus, we evaluated the frequency of LB in our UCI cohort of DS cases that we have collected over the past 25 years.

Methods/Case Report

Neuropathology reports from 55 cases with DS from the UCI-ADRC were included in this study. Cases were stained for beta-amyloid, phosphor-tau, alpha-synuclein and TDP-43 as per NACC protocols (one case each v7,8,9 and three v11).

Results (if a Case Study enter NA)

We identified 6 cases (10.9%), all male, with a mean age of 57 years (SD=3) that showed LB and/or Lewy neurites. LB pathology was classified as amygdala predominant in 3 cases, brainstem predominant in one, intermediate/transitional in one, and diffuse/neocortical in one. Five cases were BRAAK stage 6 and one was stage 5. Five cases had CERAD neuritic plaque score C and one case had a B score. Two of 3 cases were Thal phase 5, and one was phase 4. The case with diffuse/neocortical LB pathology demonstrated hippocampal sclerosis.

Conclusion

The observation that all our LB positive cases were male may reflect a sample bias. In our study, Lewy pathology was most common in amygdala but other sites of involvement are seen similar to a prior DS study and AD studies. Prior DS studies (n=20-56 cases) find the frequency of LB pathology to range between 8-50% of cases being affected. The prevalence of LB in our DS cohort (10.9%) is in the low end of the range seen in other DS and AD studies.

Total Synthesis of (-)-Voacinol and (-)-Voacandimine C

We describe the first total synthesis of complex aspidosperma alkaloids (-)-voacinol and (-)-voacandimine C via a late-stage C7-methylenation strategy inspired by a biogenetic hypothesis. We envisioned rapid access to these natural alkaloids from a common, symmetrical precursor assembled by methylenation of a D-ring-oxidized variant of the structurally related natural product (-)-deoxoapodine. Chemoselective N9-oxidation of a pentacyclic deoxoapodine precursor enabled the synthesis of the corresponding hexacyclic C8-aminonitrile. Stereocontrolled methylenation of a C8-enamine derivative of deoxoapodine, accessed by ionization of the C8-aminonitrile, afforded a symmetrical dodecacyclic bisaminonitrile as a versatile precursor to these bisindole alkaloids. The final-stage, biosynthesis-inspired, controlled reductive opening of the oxolane substructures of this dodecacyclic intermediate provided a unified approach to (-)-voacinol and (-)-voacandimine C, while direct reduction of the same intermediate afforded the structurally related (-)-methylenebisdeoxoapodine.

Total Synthesis and Stereochemical Assignment of (-)-Psychotridine

We report the first enantioselective total synthesis and stereochemical assignment of (-)-psychotridine. The application of our diazene-directed assembly of enantiomerically enriched cyclotryptamines afforded a highly convergent synthesis of the pentameric alkaloid, allowing its detailed structural assignment. Highlights of the synthesis include the introduction of four quaternary stereocenters with complete stereochemical control in a single step via the photoextrusion of three molecules of dinitrogen from an advanced intermediate and metal-catalyzed C-H amination reactions in challenging settings.

Directed Palladium-Catalyzed Acetoxylation of Indolines. Total Synthesis of N-Benzoylcylindrocarine

We describe a palladium-catalyzed C7-acetoxylation of indolines with a range of amide directing groups. While a variety of substituents are tolerated on the indoline-core and the N1-acyl group, the acetoxylation is most sensitive to the C2- and C6-indoline substituents. The practicality of this indoline C7-acetoxylation is demonstrated using a cinnamamide substrate on a mmol scale. Several N1-acyl groups, including those present in natural alkaloids, guide C7-acetoxylation of indoline substrates over a competitive C5-oxidation. The application of this chemistry allowed for the first synthesis of N-benzoylcylindrocarine by late-stage C17-acetoxylation of N-benzoylfendleridine.

Total synthesis of himastatin

The natural product himastatin has an unusual homodimeric structure that presents a substantial synthetic challenge. We report the concise total synthesis of himastatin from readily accessible precursors, incorporating a final-stage dimerization strategy that was inspired by a detailed consideration of the compound's biogenesis. Combining this approach with a modular synthesis enabled expedient access to more than a dozen designed derivatives of himastatin, including synthetic probes that provide insight into its antibiotic activity.

Total Synthesis of (-)-Kopsifoline A and (+)-Kopsifoline E

We report the first total synthesis of (-)-kopsifoline A and (+)-kopsifoline E. Our synthetic strategy features a biogenetically inspired regioselective C17-functionalization of a versatile intermediate containing the pentacyclic core of aspidosperma alkaloids. The vinylogous urethane substructure of this intermediate affords (-)-kopsifoline D via C3-C21 bond formation under the Mitsunobu reaction conditions, while it enables selective C17-functionalization en route to (-)-kopsifoline A and (+)-kopsifoline E.

Chemoselective α-Sulfidation of Amides Using Sulfoxide Reagents

The direct α-sulfidation of tertiary amides using sulfoxide reagents under electrophilic amide activation conditions is described. Employing convenient and readily available reagents, selective functionalization takes place to generate isolable sulfonium ions en route to α-sulfide amides. Mechanistic studies identified activated sulfoxides as promoters of the desired transformation and enabled the extension of the methodology from benzylic to aliphatic amide substrates.

Structure and Function of NzeB, a Versatile C-C and C-N Bond-Forming Diketopiperazine Dimerase

The dimeric diketopiperazine (DKPs) alkaloids are a diverse family of natural products (NPs) whose unique structural architectures and biological activities have inspired the development of new synthetic methodologies to access these molecules. However, catalyst-controlled methods that enable the selective formation of constitutional and stereoisomeric dimers from a single monomer are lacking. To resolve this long-standing synthetic challenge, we sought to characterize the biosynthetic enzymes that assemble these NPs for application in biocatalytic syntheses. Genome mining enabled identification of the cytochrome P450, NzeB (Streptomyces sp. NRRL F-5053), which catalyzes both intermolecular carbon-carbon (C-C) and carbon-nitrogen (C-N) bond formation. To identify the molecular basis for the flexible site-selectivity, stereoselectivity, and chemoselectivity of NzeB, we obtained high-resolution crystal structures (1.5 Å) of the protein in complex with native and non-native substrates. This, to our knowledge, represents the first crystal structure of an oxidase catalyzing direct, intermolecular C-H amination. Site-directed mutagenesis was utilized to assess the role individual active-site residues play in guiding selective DKP dimerization. Finally, computational approaches were employed to evaluate plausible mechanisms regarding NzeB function and its ability to catalyze both C-C and C-N bond formation. These results provide a structural and computational rationale for the catalytic versatility of NzeB, as well as new insights into variables that control selectivity of CYP450 diketopiperazine dimerases.

n→π* Interactions Modulate the Disulfide Reduction Potential of Epidithiodiketopiperazines

Epithiodiketopiperazines (ETPs) are a structurally complex class of fungal natural products with potent anticancer activity. In ETPs, the diketopiperazine ring is spanned by a disulfide bond that is constrained in a high-energy eclipsed conformation. We employed computational, synthetic, and spectroscopic methods to investigate the physicochemical attributes of this atypical disulfide bond. We find that the disulfide bond is stabilized by two n→π* interactions, each with large energies (3-5 kcal/mol). The n→π* interactions in ETPs make disulfide reduction much more difficult, endowing stability in physiological environments in a manner that could impact their biological activity. These data reveal a previously unappreciated means to stabilize a disulfide bond and highlight the utility of the n→π* interaction in molecular design.

Synthesis of Potent Cytotoxic Epidithiodiketopiperazines Designed for Derivatization

We describe our design, synthesis, and chemical study of a set of functional epidithiodiketopiperazines (ETPs) and evaluation of their activity against five human cancer cell lines. Our structure-activity relationship-guided substitution of ETP alkaloids offers versatile derivatization while maintaining potent anticancer activity, offering exciting opportunity for their use as there are no examples of complex and potently anticancer (nM) ETPs being directly used as conjugatable probes or warheads. Our synthetic solutions to strategically designed ETPs with functional linkers required advances in stereoselective late-stage oxidation and thiolation chemistry in complex settings, including the application of novel reagents for dihydroxylation and cis-sulfidation of diketopiperazines. We demonstrate that complex ETPs equipped with a strategically substituted azide functional group are readily derivatized to the corresponding ETP-triazoles without compromising anticancer activity. Our chemical stability studies of ETPs along with cytotoxic evaluation of our designed ETPs against A549, DU 145, HeLa, HCT 116, and MCF7 human cancer cell lines provide insights into the impact of structural features on potency and chemical stability, informing future utility of ETPs in chemical and biological studies.

Total Synthesis and Anti-Cancer Activity of All Known Communesin Alkaloids and Related Derivatives

A unified enantioselective total synthesis and anticancer evaluation of all known epoxide-containing communesin alkaloids and related derivatives is described. Our synthesis is predicated on the convergent and modular diazene-directed assembly of two complex fragments to secure the critical C3a-C3a' linkage followed by a guided biomimetic aminal reorganization to deliver the heptacyclic core of these alkaloids. Concise enantioselective syntheses of the fragments were devised, with highlights including the application of a rationally designed sulfinamide chiral auxiliary, an efficient calcium trifluoromethanesulfonate promoted intramolecular amination, and a diastereoselective epoxidation that simultaneously converts the new chiral auxiliary to a versatile amine protective group. The modularity of our convergent approach enabled the rapid synthesis of all epoxide-containing members of the communesin family from a single heterodimeric intermediate, including the first total synthesis of communesins C-E, and G-I, and facilitated our stereochemical revision of (-)-communesin I, the most recently isolated communesin alkaloid. Furthermore, the generality of our biogenetically inspired heterodimer rearrangement was demonstrated in a guided synthesis of a communesin derivative with an unnatural topology. Finally, we report the first comparative analysis of the anticancer activities of all naturally occurring communesin alkaloids A-I and eight complex derivatives against five human cancer cell lines. From these data, we have identified (-)-communesin B as the most potent natural communesin and discovered that derivatives with N8'-sulfonamide substitution exhibit up to a 10-fold increase in potency over the natural alkaloids.

Mechanism of Permanganate-Promoted Dihydroxylation of Complex Diketopiperazines: Critical Roles of Counter-cation and Ion-Pairing

The mechanism of permanganate-mediated dual C-H oxidation of complex diketopiperazines has been examined with density functional theory computations. The products of these oxidations are enabling intermediates in the synthesis of structurally diverse ETP natural products. We evaluated, for the first time, the impact of ion-pairing and aggregation states of the permanganate ion and counter-cations, such as bis(pyridine)-silver(I) (Ag⁺) and tetra- n-butylammonium (TBA⁺), on the C-H oxidation mechanism. The C-H abstraction occurs through an open shell singlet species, as noted previously, followed by O-rebound and a competing OH-rebound pathway. The second C-H oxidation proceeds with a second equivalent of oxidant with lower free energy barriers than the first C-H oxidation due to directing effects and the generation of a more reactive oxidant species after the first C-H oxidation. The success and efficiency of the second C-H oxidation are found to be critically dependent on the presence of an ion-paired oxidant. We used the developed mechanistic knowledge to rationalize an experimentally observed oxidation pattern for C³-indole-substituted diketopiperazine (+)-5 under optimal oxidation conditions: namely, the formation of diol (-)-6 as a single diastereomer and lack of the ketone products. We proposed two factors that may impede the ketone formation: (i) the conformational flexibility of the diketopiperazine ring, and (ii) hindrance of this site, making it less accessible to the ion-paired oxidant species.

Abstract A47: Inhibition of histone methyltransferase with novel epipolythiodioxopiperazines (ETP) alkaloids in pediatric high-grade glioma

Pediatric high-grade glioma (HGG), which ranks among the most lethal of cancers among children, contain two recurrent mutations within the histone H3 gene H3F3A (K27M and G34R/V). It is not known whether these mutations directly alter specific epigenetic regulatory mechanisms that may be involved in tumor proliferation and growth, or contribute to their therapeutic resistance and rapid recurrence. Preliminary findings from our lab have identified that Chaetocin (CH), a pan histone methyltransferase (HMT) inhibitor, and two newly synthesized epipolythiodioxopiperazines (ETP) alkaloids that are structurally and functionally distinct HMT inhibitors can effectively kill pediatric HGG (N=7), including three that contain the H3 K27M mutation. In addition, CH and ETPs can also inhibit tumor sphere formation and cell migration at low concentrations (<0.5uM) through in vitro studies. In H3 K27M tumors, these HMTi were shown to reverse hypomethylation of K27me3 transiently (<48 hours). We also found these HMTis to decrease tumor stem cell expression (Sox2, Oct4), and increase tumor-suppressor activity (PTEN, p53) for three pediatric HGG up to 72 hours. These results suggest ETP effectiveness that acts through direct HMT inhibition may also influence other tumorigenic pathways, which may in part contribute to their aggressive nature. Results from this investigation have advanced our understanding of epigenetic regulation of solid brain tumors, and identified drugs that may specifically target pediatric H3 K27M tumor subtypes and could ultimately improve overall survival in pediatric patients with high-grade glioma.

Citation Format: Stephen Dombrowski, Thamara Abou-Antoun, Anthony Mansour, Rewa Arar, Mohammad Movassaghi, Erin Murphy, Shuvojit Banerjee, Xiao-Nan Li, Peter Houghton, Darell Bigner, Angel Montero Carcaboso, Jeremy Rich, Violette Recinos. Inhibition of histone methyltransferase with novel epipolythiodioxopiperazines (ETP) alkaloids in pediatric high-grade glioma [abstract]. In: Proceedings of the AACR Special Conference: Pediatric Cancer Research: From Basic Science to the Clinic; 2017 Dec 3-6; Atlanta, Georgia. Philadelphia (PA): AACR; Cancer Res 2018;78(19 Suppl):Abstract nr A47.

Enantioselective Synthesis of (-)-Vallesine: Late-Stage C17-Oxidation via Complex Indole Boronation

The first enantioselective total synthesis of (-)-vallesine via a strategy that features a late-stage regioselective C17-oxidation followed by a highly stereoselective transannular cyclization is reported. The versatility of this approach is highlighted by the divergent synthesis of the archetypal alkaloid of this family, (+)-aspidospermidine, and an A-ring-oxygenated derivative, (+)-deacetylaspidospermine, the precursor to (-)-vallesine, from a common intermediate.

Concise total synthesis of (+)-asperazine A and (+)-pestalazine B

The highly convergent total synthesis of dimeric diketopiperazine alkaloids (+)-asperazine A and (+)-pestalazine B is described. A critical aspect of our expedient route was the development of a directed regio- and diastereoselective C3-N1' coupling of complex tetracyclic diketopiperazine components. This late-stage heterodimerization reaction was made possible by design of tetracyclic diketopiperazines that allow C3-carbocation coupling of the electrophilic component to the N1' locus of the nucleophilic fragment. The application of this new coupling reaction to the first total synthesis of (+)-asperazine A led to our revision of the sign and magnitude of the optical rotation for the reported structure.

Concise Synthesis of (-)-Hodgkinsine, (-)-Calycosidine, (-)-Hodgkinsine B, (-)-Quadrigemine C, and (-)-Psycholeine via Convergent and Directed Modular Assembly of Cyclotryptamines

The enantioselective total synthesis of (-)-hodgkinsine, (-)-calycosidine, (-)-hodgkinsine B, (-)-quadrigemine C, and (-)-psycholeine through a diazene-directed assembly of cyclotryptamine fragments is described. Our synthetic strategy enables multiple and directed assembly of intact cyclotryptamine subunits for convergent synthesis of highly complex bis- and tris-diazene intermediates. Photoextrusion of dinitrogen from these intermediates enables completely stereoselective formation of all C3a-C3a' and C3a-C7' carbon-carbon bonds and all the associated quaternary stereogenic centers. In a representative example, photoextrusion of three dinitrogen molecules from an advanced intermediate in a single-step led to completely controlled introduction of four quaternary stereogenic centers and guided the assembly of four cyclotryptamine monomers en route to (-)-quadrigemine C. The synthesis of these complex diazenes was made possible through a new methodology for synthesis of aryl-alkyl diazenes using electronically attenuated hydrazine-nucleophiles for a silver-promoted addition to C3a-bromocyclotryptamines. The application of Rh- and Ir-catalyzed C-H amination reactions in complex settings were used to gain rapid access to C3a- and C7-functionalized cyclotryptamine monomers, respectively, used for diazene synthesis. This convergent and modular assembly of intact cyclotryptamines offers the first solution to access these alkaloids through completely stereoselective union of monomers at challenging linkages and the associated quaternary stereocenters as illustrated in our synthesis of five members of the oligocyclotryptamine family of alkaloids.

Quantitative Modeling of Bis(pyridine)silver(I) Permanganate Oxidation of Hydantoin Derivatives: Guidelines for Predicting the Site of Oxidation in Complex Substrates

The bis(pyridine)silver(I) permanganate promoted hydroxylation of diketopiperazines has served as a pivotal transformation in the synthesis of complex epipolythiodiketopiperazine alkaloids. This late-stage C-H oxidation chemistry is strategically critical to access N-acyl iminium ion intermediates necessary for nucleophilic thiolation of advanced diketopiperazines en route to potent epipolythiodiketopiperazine anticancer compounds. In this study, we develop an informative mathematical model using hydantoin derivatives as a training set of substrates by relating the relative rates of oxidation to various calculated molecular descriptors. The model prioritizes Hammett values and percent buried volume as key contributing factors in the hydantoin series while correctly predicting the experimentally observed oxidation sites in various complex diketopiperazine case studies. Thus, a method is presented by which to use simplified training molecules and resulting correlations to explain and predict reaction behavior for more complex substrates.

Enantioselective Total Synthesis of (-)-Deoxoapodine

The first enantioselective total synthesis of (-)-deoxoapodine is described. Our synthesis of this hexacyclic aspidosperma alkaloid includes an efficient molybdenum-catalyzed enantioselective ring-closing metathesis reaction for the desymmetrization of an advanced intermediate that introduces the C5-quaternary stereocenter. After C21-oxygenation, the pentacyclic core was accessed by electrophilic C19-amide activation and transannular spirocyclization. A biogenetically inspired dehydrative C6-etherification reaction proved highly effective to secure the F-ring and the fourth contiguous stereocenter of (-)-deoxoapodine with complete stereochemical control.

Synthesis and Evaluation of Agelastatin Derivatives as Potent Modulators for Cancer Invasion and Metastasis

The synthesis of new agelastatin alkaloid derivatives and their anticancer evaluation in the context of the breast cancer microenvironment is described. A variety of N1-alkyl and C5-ether agelastatin derivatives were accessed via application of our strategy for convergent imidazolone synthesis from a common thioester along with appropriately substituted urea and alcohol components. These agelastatin derivatives were evaluated in our three-dimensional coculture assay for the effects of mammary fibroblasts on associated breast cancer cells. We have discovered that agelastatin alkaloids are potent modulators for cancer invasion and metastasis at noncytotoxic doses. Herein, we discuss the increased potency of (-)-agelastatin E as compared to (-)-agelastatin A in this capacity, in addition to identification of new agelastatin derivatives with activity that is statistically equivalent to (-)-agelastatin E. The chemistry described in this report provides a platform for the rapid synthesis of agelastatin derivatives with excellent potency (50-100 nM) as modulators for cancer invasion and metastasis.

Concise Total Syntheses of (+)-Haplocidine and (+)-Haplocine via Late-Stage Oxidation of (+)-Fendleridine Derivatives

We report the first total syntheses of (+)-haplocidine and its N1-amide congener (+)-haplocine. Our concise synthesis of these alkaloids required the development of a late-stage and highly selective C-H oxidation of complex aspidosperma alkaloid derivatives. A versatile, amide-directed ortho-acetoxylation of indoline amides enabled our implementation of a unified strategy for late-stage diversification of hexacyclic C19-hemiaminal ether structures via oxidation of the corresponding pentacyclic C19-iminium ions. An electrophilic amide activation of a readily available C21-oxygenated lactam, followed by transannular cyclization and in situ trapping of a transiently formed C19-iminium ion, expediently provided access to hexacyclic C19-hemiaminal ether alkaloids (+)-fendleridine, (+)-acetylaspidoalbidine, and (+)-propionylaspidoalbidine. A highly effective enzymatic resolution of a non-β-branched primary alcohol (E = 22) allowed rapid preparation of both enantiomeric forms of a C21-oxygenated precursor for synthesis of these aspidosperma alkaloids. Our synthetic strategy provides succinct access to hexacyclic aspidosperma derivatives, including the antiproliferative alkaloid (+)-haplocidine.

Convergent and Biomimetic Enantioselective Total Synthesis of (-)-Communesin F

The first biomimetic enantioselective total synthesis of (-)-communesin F based on a late-stage heterodimerization and aminal exchange is described. Our synthesis features the expedient diazene-directed assembly of two advanced fragments to secure the congested C3a-C3a' linkage in three steps, followed by a highly efficient biogenetically inspired aminal reorganization to access the heptacyclic communesin core in only two additional steps. Enantioselective syntheses of the two fragments were developed, with highlights including the catalytic asymmetric halocyclization and diastereoselective oxyamination reactions of tryptamine derivatives, a stereoselective sulfinimine allylation, and an efficient cyclotryptamine-C3a-sulfamate synthesis by either a new silver-promoted nucleophilic amination or a rhodium-catalyzed C-H amination protocol. The versatile syntheses of the fragments, their stereocontrolled assembly, and the efficient aminal exchange as supported by in situ monitoring experiments, in addition to the final stage N1'-acylation of the communesin core, provide a highly convergent synthesis of (-)-communesin F.

The fibroblast Tiam1-osteopontin pathway modulates breast cancer invasion and metastasis

Background

The tumor microenvironment has complex effects in cancer pathophysiology that are not fully understood. Most cancer therapies are directed against malignant cells specifically, leaving pro-malignant signals from the microenvironment unaddressed. Defining specific mechanisms by which the tumor microenvironment contributes to breast cancer metastasis may lead to new therapeutic approaches against advanced breast cancer.

Methods

We use a novel method for manipulating three-dimensional mixed cell co-cultures, along with studies in mouse xenograft models of human breast cancer and a histologic study of human breast cancer samples, to investigate how breast cancer-associated fibroblasts affect the malignant behaviors of breast cancer cells.

Results

Altering fibroblast Tiam1 expression induces changes in invasion, migration, epithelial-mesenchymal transition, and cancer stem cell characteristics in associated breast cancer cells. These changes are both dependent on fibroblast secretion of osteopontin and also long-lasting even after cancer cell dissociation from the fibroblasts, indicating a novel Tiam1-osteopontin pathway in breast cancer-associated fibroblasts. Notably, inhibition of fibroblast osteopontin with low doses of a novel small molecule prevents lung metastasis in a mouse model of human breast cancer metastasis. Moreover, fibroblast expression patterns of Tiam1 and osteopontin in human breast cancers show converse changes correlating with invasion, supporting the hypothesis that this pathway in tumor-associated fibroblasts regulates breast cancer invasiveness in human disease and is thus clinically relevant.

Conclusions

These findings suggest a new therapeutic paradigm for preventing breast cancer metastasis. Pro-malignant signals from the tumor microenvironment with long-lasting effects on associated cancer cells may perpetuate the metastatic potential of developing cancers. Inhibition of these microenvironment signals represents a new therapeutic strategy against cancer metastasis that enables targeting of stromal cells with less genetic plasticity than associated cancer cells and opens new avenues for investigation of novel therapeutic targets and agents.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-016-0674-8) contains supplementary material, which is available to authorized users.

Concise Total Synthesis of (+)-Asperazine, (+)-Pestalazine A, and (+)-iso-Pestalazine A. Structure Revision of (+)-Pestalazine A

The concise, enantioselective total syntheses of (+)-asperazine (1), (+)-iso-pestalazine A (2), and (+)-pestalazine A (3) have been achieved by the development of a late-stage C3-C8' Friedel-Crafts union of polycyclic diketopiperazines. Our modular strategy enables the union of complex polycyclic diketopiperazines in virtually their final forms, thus providing rapid and highly convergent assembly at the challenging quaternary stereocenter of these dimeric alkaloids. The significance of this carbon-carbon bond formation can be gauged by the manifold constraints that were efficiently overcome, namely the substantial steric crowding at both reactive sites, the nucleophilic addition of C8' over N1' to the C3 carbocation, and the multitude of reactivity posed by the use of complex diketopiperazine fragments in the coupling event. The success of the indoline π-nucleophile that evolved through our studies is notable given the paucity of competing reaction pathways observed in the presence of the highly reactive C3 carbocation generated. This first total synthesis of (+)-pestalazine A also allowed us to revise the molecular structure for this natural alkaloid.

Concise Total Synthesis of (+)-Luteoalbusins A and B

The first total synthesis of (+)-luteoalbusins A and B is described. Highly regio- and diastereoselective chemical transformations in our syntheses include a Friedel-Crafts C3-indole addition to a cyclotryptophan-derived diketopiperazine, a late-stage diketopiperazine dihydroxylation, and a C11-sulfidation sequence, in addition to congener-specific polysulfane synthesis and cyclization to the corresponding epipolythiodiketopiperazine. We also report the cytoxicity of both alkaloids, and closely related derivatives, against A549, HeLa, HCT116, and MCF7 human cancer cell lines.

Direct Observation of Intermediates Involved in the Interruption of the Bischler-Napieralski Reaction

The first mechanistic investigation of electrophilic amide activation of α,α-disubstituted tertiary lactams and the direct observation of key intermediates by in situ FTIR, (1)H, (13)C, and (19)F NMR in our interrupted Bischler-Napieralski-based synthetic strategy to the aspidosperma alkaloids, including a complex tetracyclic diiminium ion, is discussed. The reactivity of a wide range of pyridines with trifluoromethanesulfonic anhydride was systematically examined, and characteristic IR absorption bands for the corresponding N-trifluoromethanesulfonylated pyridinium trifluoromethanesulfonates were assigned. The reversible formation of diiminium ether intermediates was studied, providing insight into divergent mechanistic pathways as a function of the steric environment of the amide substrate and stoichiometry of reagents. Importantly, when considering base additives during electrophilic amide activation, more hindered α-quaternary tertiary lactams require the use of non-nucleophilic pyridine additives in order to avoid deactivation via a competing desulfonylation reaction. The isolation and full characterization of a tetracyclic iminium trifluoromethanesulfonate provided additional correlation between in situ characterization of sensitive intermediates and isolable compounds involved in this synthetic transformation.

Electrophilic Carbonyl Activation: Competing Condensative Cyclizations of Tryptamine Derivatives

A series of tryptamine derived bisindole substrates were subject to electrophilic activation of the functional grouping at their alpha-nitrogen in the form of iminium ions to enable cyclization onto the sterically hindered indole substructure. Our observations regarding divergent cyclization outcomes using electronically distinct bisindole substrates are described. Surprising preference for Friedel-Crafts alkylation reaction and evidence for an intriguing reversible spirocyclization are discussed.

Biogenetically-inspired total synthesis of epidithiodiketopiperazines and related alkaloids

Natural products chemistry has historically been the prime arena for the discovery of new chemical transformations and the fountain of insights into key biological processes. It remains a fervent incubator of progress in the fields of chemistry and biology and an exchange mediating the flow of ideas between these allied fields of science. It is with this ethos that our group has taken an interest in and pursued the synthesis of a complex family of natural products termed the dimeric epipolythiodiketopiperazine (ETP) alkaloids. We present here an Account of the highly complex target molecules to which we pegged our ambitions, our systematic and relentless efforts toward those goals, the chemistry we developed in their pursuit, and the insight we have gained for their translational potential as potent anticancer molecules. The dimeric ETP alkaloids are fungal metabolites that feature a highly complex molecular architecture comprising a densely functionalized core structure with many stereogenic centers, six of which are fully substituted, and a pair of vicinal quaternary carbon stereocenters, decorated on polycyclic architectures in addition to the unique ETP motif that has been recognized as acid-, base-, and redox-sensitive. A cyclo-dipeptide consisting of an essential tryptophan residue and a highly variable ancillary amino acid lies at the core of these structures; investigation of the transformations that take this simplistic core to the complex alkaloids lies at the heart of our research program. The dimeric epidithiodiketopiperazine alkaloids have largely resisted synthesis on account of their complexity since the 1970s when the founding members of this class, chaetocin A ( Hauser , D. et al. Helv. Chim. Acta 1970 , 53 , 1061 ) and verticillin A ( Katagiri , K. et al. J. Antibiot. 1970 , 23 , 420 ), were first isolated. This was despite their potent cytotoxic and bacteriostatic activities, which were well appreciated at the time of their discovery. In the past decade, an increasing number of studies have uncovered powerful new biological processes that these molecules can uniquely effect, such as the inhibition of histone methyltransferases by chaetocin A ( Greiner , D. et al. Nat. Chem. Biol. 2005 , 1 , 143 ). In fact, the complete collection of hexahydropyrroloindoline alkaloids features a diverse range of potent biological properties including cytotoxic, antitumor, antileukemic, antiviral, antibiotic, and antinematodal activities ( Jiang , C.-S. ; Guo , Y.-W. Mini-Rev. Med. Chem. 2011 , 11 , 728 ). This mélange of activities is reflective of their structural diversity. Under the precepts of retrobiosynthetic analysis, we have accomplished the syntheses of more than a dozen natural products, including members of the bionectin, calycanthaceous, chaetocin, gliocladin, naseseazine, and verticillin alkaloids. More importantly, these molecules have acted as venerable venues for the development of new strategies to address structural challenges including, but not limited to, C3-C3' vicinal quaternary centers, heterodimeric linkages, C3-Csp(2) linkages, diketopiperazine oxidation, stereoselective thiolation, homologue-specific polysulfidation, and C12-hydroxyl incorporation. Synthesis of these natural products has resulted in the structural confirmation, and sometimes revision such as the case of (+)-naseseazines A and B, as well as access to many plausible biogenetically relevant intermediates and new synthetic ETP derivatives. Furthermore, our studies have paved the way for the formulation of a comprehensive SAR profile and the identification of lead compounds with in vitro subnanomolar IC50's against a broad range of cancer types.

EASTERN DODDER (CUSCUTA MONOGYNA VAHL.) SEED GERMINATION AFFECTED BY SOME HERBACEOUS DISTILLATES

Eastern dodder (Cuscuta monogyna Vahl.) is one of the noxious parasitic weeds that infected many ornamental trees in green spaces and gardens. Our purpose is to find natural inhibitors for prevention of its seed germination. In order to reach this aim, laboratory studies were conducted by using of herbaceous distillates of Dracocephalum moldavica, Nasturtium officinalis, Malva neglecta, Mentha piperita, Mentha pulegium, Rosa damascene, Ziziphora tenuior, and Urtica dioica on seed germination of C. monogyna. Z. tenuior distillate stimulated C. monogyna seed germination, whereas others reduced this parasitic weed's seed germination. D. moldavica caused maximum inhibition on weed seed germination. Seedling growth of C. monogyna was more affected than its seed germination. All of these herbaceous distillates reduced C. monogyna seedling length so that the latter decreased from 28.2 mm in distilled water to 4.5, 3.97, 3.85, 3.67, 3.1, 2.87, 2.57, 1.9, and 1.17 in M. pulegium, M. piperita, F. officinalis, Z. tenuior, N. officinalis, M. neglecta, R. damascene, U. dioica and D. moldavica, respectively. By using these medicinal plants distillates instead of herbicides, the parasitic weed seedling length and host plant infection will reduce.

C7-derivatization of C3-alkylindoles including tryptophans and tryptamines

A versatile strategy for C7-selective boronation of tryptophans, tryptamines, and 3-alkylindoles by way of a single-pot C2/C7-diboronation-C2-protodeboronation sequence is described. The combination of a mild iridium-catalyzed C2/C7-diboronation followed by an in situ palladium-catalyzed C2-protodeboronation allows efficient entry to valuable C7-boroindoles that enable further C7-derivatization. The versatility of the chemistry is highlighted by the gram-scale synthesis of C7-boronated N-Boc-L-tryptophan methyl ester and the rapid synthesis of C7-halo, C7-hydroxy, and C7-aryl tryptophan derivatives.

Concise and enantioselective total synthesis of (-)-mehranine, (-)-methylenebismehranine, and related Aspidosperma alkaloids

We report an efficient and highly stereoselective strategy for the synthesis of Aspidosperma alkaloids based on the transannular cyclization of a chiral lactam precursor. Three new stereocenters are formed in this key step with excellent diastereoselectivity due to the conformational bias of the cyclization precursor, leading to a versatile pentacyclic intermediate. A subsequent stereoselective epoxidation followed by a mild formamide reduction enabled the first total synthesis of the Aspidosperma alkaloids (-)-mehranine and (+)-(6S,7S)-dihydroxy-N-methylaspidospermidine. A late-stage dimerization of (-)-mehranine mediated by scandium trifluoromethanesulfonate completed the first total synthesis of (-)-methylenebismehranine.

Total synthesis, stereochemical assignment, and biological activity of all known (-)-trigonoliimines

A full account of our concise and enantioselective total syntheses of all known (-)-trigonoliimine alkaloids is described. Our retrobiosynthetic analysis of these natural products enabled identification of a single bistryptamine precursor as a precursor to all known trigonoliimines through a sequence of transformations involving asymmetric oxidation and reorganization. Our enantioselective syntheses of these alkaloids enabled the revision of the absolute stereochemistry of (-)-trigonoliimines A, B, and C. We report that trigonoliimines A, B, C and structurally related compounds showed weak anticancer activities against HeLa and U-937 cells.

Application of diazene-directed fragment assembly to the total synthesis and stereochemical assignment of (+)-desmethyl-meso-chimonanthine and related heterodimeric alkaloids

We describe the first application of our methodology for heterodimerization via diazene fragmentation towards the total synthesis of (-)-calycanthidine, meso-chimonanthine, and (+)-desmethyl-meso-chimonanthine. Our syntheses of these alkaloids feature an improved route to C3a-aminocyclotryptamines, an enhanced method for sulfamide synthesis and oxidation, in addition to a late-stage diversification leading to the first enantioselective total synthesis of (+)-desmethyl-meso-chimonanthine and its unambiguous stereochemical assignment. This versatile strategy for directed assembly of heterodimeric cyclotryptamine alkaloids has broad implications for the controlled synthesis of higher order derivatives with related substructures.

Synthesis and anticancer activity of all known (-)-agelastatin alkaloids

The full details of our enantioselective total syntheses of (-)-agelastatins A-F (1-6), the evolution of a new methodology for synthesis of substituted azaheterocycles, and the first side-by-side evaluation of all known (-)-agelastatin alkaloids against nine human cancer cell lines are described. Our concise synthesis of these alkaloids exploits the intrinsic chemistry of plausible biosynthetic precursors and capitalizes on a late-stage synthesis of the C-ring. The critical copper-mediated cross-coupling reaction was expanded to include guanidine-based systems, offering a versatile preparation of substituted imidazoles. The direct comparison of the anticancer activity of all naturally occurring (-)-agelastatins in addition to eight advanced synthetic intermediates enabled a systematic analysis of the structure-activity relationship within the natural series. Significantly, (-)-agelastatin A (1) is highly potent against six blood cancer cell lines (20-190 nM) without affecting normal red blood cells (>333 μM). (-)-Agelastatin A (1) and (-)-agelastatin D (4), the two most potent members of this family, induce dose-dependent apoptosis and arrest cells in the G2/M-phase of the cell cycle; however, using confocal microscopy, we have determined that neither alkaloid affects tubulin dynamics within cells.

Concise Total Synthesis of (+)-Bionectins A and C

The concise and efficient total synthesis of (+)-bionectins A and C is described. Our approach to these natural products features a new and scalable method for erythro-β-hydroxytryptophan amino acid synthesis, an intramolecular Friedel-Crafts reaction of a silyl-tethered indole, and a new mercaptan reagent for epipolythiodiketopiperazine (ETP) synthesis that can be unravelled under very mild conditions. In evaluating the impact of C12-hydroxylation, we have identified a unique need for an intramolecular variant of our Friedel-Crafts indolylation chemistry. Several key discoveries including the first example of permanganate-mediated stereoinvertive hydroxylation of the α-stereocenters of diketopiperazines as well as the first example of a direct triketopiperazine synthesis from a parent cyclo-dipeptide are discussed. Finally, the synthesis of (+)-bionectin A and its unambiguous structural assignment through X-ray analysis provides motivation for the reevaluation of its original characterization data and assignment.

Synthesis of spirocyclic indolines by interruption of the Bischler-Napieralski reaction

The development of a versatile method for the synthesis of spirocyclic pyrrolidinoindolines is discussed. Treatment of N-acyltryptamines with trifluoromethanesulfonic anhydride-2-chloropyridine reagent combination affords highly persistent spiroindoleninium ions that are subject to intra- and intermolecular addition at C2 by nucleophiles.

Synthesis and Anticancer Activity of Epipolythiodiketopiperazine Alkaloids

The epipolythiodiketopiperazine (ETP) alkaloids are a highly complex class of natural products with potent anticancer activity. Herein, we report the application of a flexible and scalable synthesis, allowing the construction of dozens of ETP derivatives. The evaluation of these compounds against cancer cell lines in culture allows for the first expansive structure-activity relationship (SAR) to be defined for monomeric and dimeric ETP-containing natural products and their synthetic cognates. Many ETP derivatives demonstrate potent anticancer activity across a broad range of cancer cell lines, and kill cancer cellsviainduction of apoptosis. Several traits thatbode well for the translational potential of the ETP class of natural products includeconcise and efficient synthetic access, potent induction of apoptotic cell death, activity against a wide range of cancer types, and a broad tolerance for modifications at multiple sitesthat should facilitate small-molecule drug development, mechanistic studies, and evaluation in vivo.