Chemical synthesis of marine saponins.

Covering: 1989-2017 Saponins are characteristic metabolites of starfish and sea cucumbers, and occasionally are also found in sponges, soft coral, and small fish. These steroid or triterpenoid glycosides often show remarkable biological and pharmacological activities, such as antifungal, antifouling, shark repellent, antitumor and anti-inflammatory activities. Over one thousand marine saponins have been characterized; the majority of them can be categorized into three major structural types, i.e., asterosaponins, polyhydroxysteroid glycosides, and holostane glycosides. Thus far, only 12 marine saponins have been synthesized; those representing the major types were successfully synthesized recently. The syntheses involve preparation of the aglycones from the terrestrial steroid or triterpene materials, installation of the carbohydrate units, and manipulation of the protecting groups. Herein, we provide a comprehensive review on these syntheses.

Streamlined Total Synthesis of Trioxacarcins and Its Application to the Design, Synthesis, and Biological Evaluation of Analogues Thereof. Discovery of Simpler Designed and Potent Trioxacarcin Analogues.

A streamlined total synthesis of the naturally occurring antitumor agents trioxacarcins is described, along with its application to the construction of a series of designed analogues of these complex natural products. Biological evaluation of the synthesized compounds revealed a number of highly potent, and yet structurally simpler, compounds that are effective against certain cancer cell lines, including a drug-resistant line. A novel one-step synthesis of anthraquinones and chloro anthraquinones from simple ketone precursors and phenylselenyl chloride is also described. The reported work, featuring novel chemistry and cascade reactions, has potential applications in cancer therapy, including targeted approaches as in antibody-drug conjugates.

Utilization of bench-stable and readily available nickel(II) triflate for access to 1,2-cis-2-aminoglycosides.

The utilization of substoichiometric amounts of commercially available nickel(II) triflate as an activator in the reagent-controlled glycosylation reaction for the stereoselective construction of biologically relevant targets containing 1,2-cis-2-amino glycosidic linkages is reported. This straightforward and accessible methodology is mild, operationally simple and safe through catalytic activation by readily available Ni(OTf)2 in comparison to systems employing our previously in-house prepared Ni(4-F-PhCN)4(OTf)2. We anticipate that the bench-stable and inexpensive Ni(OTf)2, coupled with little to no extra laboratory training to set up the glycosylation reaction and no requirement of specialized equipment, should make this methodology be readily adopted by non-carbohydrate specialists. This report further highlights the efficacy of Ni(OTf)2 to prepare several bioactive motifs, such as blood type A-type V and VI antigens, heparin sulfate disaccharide repeating unit, aminooxy glycosides, and α-GalNAc-Serine conjugate, which cannot be achieved in high yield and α-selectivity utilizing in-house prepared Ni(4-F-PhCN)4(OTf)2 catalyst. The newly-developed protocol eliminates the need for the synthesis of Ni(4-F-PhCN)4(OTf)2 and is scalable and reproducible. Furthermore, computational simulations in combination with 1H NMR studies analyzed the effects of various solvents on the intramolecular hydrogen bonding network of tumor-associated mucin Fmoc-protected GalNAc-threonine amino acid antigen derivative, verifying discrepancies found that were previously unreported.

Synthesis of the 2-deoxy trisaccharide glycal of antitumor antibiotics landomycins A and E.

Synthesis of the 2-deoxy trisaccharide glycal of antitumor antibiotics landomycins A and E has been described. The synthesis involves an anomeric O-alkylation for the synthesis of 2-deoxy ß-linked disaccharide, a tert-butyldimethylsilyl triflate-catalyzed α-selective L-rhodinosylation, and a lithium 4,4'-di-tert-butylbiphenyl-mediated reductive debenzylation and concomitant reductive lithiation-elimination for the production of the 2-deoxy trisaccharide glycal.

Total Synthesis of Trioxacarcins DC-45-A1, A, D, C, and C7″-epi-C and Full Structural Assignment of Trioxacarcin C.

Trioxacarcins DC-45-A2, DC-45-A1, A, D, C7″-epi-C, and C have been synthesized through stereoselective strategies involving BF3·Et2O-catalyzed ketone-epoxide opening and gold-catalyzed glycosylation reactions, and the full structural assignment of trioxacacin C was deciphered via the syntheses of both of its C7″ epimers. The gathered knowledge sets the foundation for the design, synthesis, and biological evalution of analogues of these natural products as potential payloads for antibody-drug conjugates and other delivery systems for targeted and personalized cancer chemotherapy.

Stereoselective Synthesis of S-Linked Hexasaccharide of Landomycin A via Umpolung S-Glycosylation.

Stereoselective synthesis of carbohydrate mimics resistant toward acid-mediated or enzymatic hydrolysis is chemically challenging and biologically interesting. In this Letter, the first stereoselective synthesis of a "non-hydrolyzable" S-linked hexasaccharide of antitumor antibiotic landomycin A is described. This synthesis was accomplished through the utilization of our recently developed umpolung reactivity-based S-glycosylation-sulfenylation of stereochemically defined glycosyl lithium species with asymmetric sugar-derived disulfides.

Synthesis and Bioactivity of a Brasilicardin A Analogue Featuring a Simplified Core.

An analogue 2 of Brasilicardin A, 1 (BraA), a potent immunosuppressive and cytotoxic agent, was synthesized in which the natural tricyclic skeleton was replaced with a synthetically more accessible substituted tetrahydronaphthalene core. BraA, this analogue (BraL), and cyclosporine A were tested for their ability to inhibit the proliferation of human T cells upon CD3/CD28 activation. Although BraL did not impact T cell activation over the dose range tested, this study shows the inhibitory activity of BraA on human T cells for the first time.

Total synthesis of trioxacarcin DC-45-A2.

An enantioselective total synthesis of trioxacarcin DC-45-A2 (1) featuring a novel Lewis acid-induced cascade rearrangement of epoxyketone 6 to forge the polyoxygenated 2,7-dioxabicyclo[2.2.1]heptane core of the molecule is described.

Synthetic aminoglycosides efficiently suppress cystic fibrosis transmembrane conductance regulator nonsense mutations and are enhanced by ivacaftor.

New drugs are needed to enhance premature termination codon (PTC) suppression to treat the underlying cause of cystic fibrosis (CF) and other diseases caused by nonsense mutations. We tested new synthetic aminoglycoside derivatives expressly developed for PTC suppression in a series of complementary CF models. Using a dual-luciferase reporter system containing the four most prevalent CF transmembrane conductance regulator (CFTR) nonsense mutations (G542X, R553X, R1162X, and W1282X) within their local sequence contexts (the three codons on either side of the PTC), we found that NB124 promoted the most readthrough of G542X, R1162X, and W1282X PTCs. NB124 also restored full-length CFTR expression and chloride transport in Fischer rat thyroid cells stably transduced with a CFTR-G542XcDNA transgene, and was superior to gentamicin and other aminoglycosides tested. NB124 restored CFTR function to roughly 7% of wild-type activity in primary human bronchial epithelial (HBE) CF cells (G542X/delF508), a highly relevant preclinical model with endogenous CFTR expression. Efficacy was further enhanced by addition of the CFTR potentiator, ivacaftor (VX-770), to airway cells expressing CFTR PTCs. NB124 treatment rescued CFTR function in a CF mouse model expressing a human CFTR-G542X transgene; efficacy was superior to gentamicin and exhibited favorable pharmacokinetic properties, suggesting that in vitro results translated to clinical benefit in vivo. NB124 was also less cytotoxic than gentamicin in a tissue-based model for ototoxicity. These results provide evidence that NB124 and other synthetic aminoglycosides provide a 10-fold improvement in therapeutic index over gentamicin and other first-generation aminoglycosides, providing a promising treatment for a wide array of CFTR nonsense mutations.

Synthesis of the pluramycins 1: two designed anthrones as enabling platforms for flexible bis-C-glycosylation.

Two effective tricyclic platforms are reported for the installation of the two constituent sugars, L-vancosamine and D-angolosamine, in a regio- and stereoselective manner for the synthesis of the pluramycin class of bis-C-glycoside antitumor antibiotics. Two complementary protocols are now available that differ in the order in which the two sugar moieties are installed. Sc(OTf)3 was effective as the Lewis acid.

Synthesis of the pluramycins 2: total synthesis and structure assignment of saptomycin B.

A concise, highly convergent total synthesis of saptomycin B, a member of the pluramycin class of antitumor antibiotics, is reported. The target compound was assembled from four building blocks (a tricyclic platform, two sugars, and an alkynal) in 15% yield through 10 synthetic operations. The key steps included the regioselective installation of two amino sugars (L-vancosamine and D-angolosamine) on the tricycle and the efficient construction of the tetracyclic skeleton by an aldol reaction followed by formation of the pyranone. The unknown configuration at C14 was assigned as R.

Component-based syntheses of trioxacarcin A, DC-45-A1 and structural analogues.

The trioxacarcins are polyoxygenated, structurally complex natural products that potently inhibit the growth of cultured human cancer cells. Here we describe syntheses of trioxacarcin A, DC-45-A1 and structural analogues by late-stage stereoselective glycosylation reactions of fully functionalized, differentially protected aglycon substrates. Key issues addressed in this work include the identification of an appropriate means to activate and protect each of the two 2-deoxysugar components, trioxacarcinose A and trioxacarcinose B, as well as a viable sequencing of the glycosidic couplings. The convergent, component-based sequence we present allows for rapid construction of structurally diverse, synthetic analogues that would be inaccessible by any other means, in amounts required to support biological evaluation. Analogues that arise from the modification of four of five modular components are assembled in 11 steps or fewer. The majority of these are found to be active in antiproliferative assays using cultured human cancer cells.

Synthesis and structure-activity relationship of vicenistatin, a cytotoxic 20-membered macrolactam glycoside.

We have developed two syntheses of vicenistatin and its analogues. Our first-generation strategy included the rapid and sequential assembly of the macrocyclic lactam by using an intermolecular Horner-Wadsworth-Emmons reaction between the C3-C13 fragment and the C1-C2, C14-C19 fragment, followed by an intramolecular Stille coupling reaction. The second-generation strategy utilized a ring-closing metathesis of a hexaene intermediate to generate the desired 20-membered macrolactam. This second-generation strategy made it possible to prepare synthetic analogues of vicenistatin, including the C20- and/or C23-demethyl analogues. Evaluation of the cytotoxic effect of these analogues indicated the importance of the fixed conformation of aglycon for determining the biological activity of the vicenistatins.

Angucyclines: Biosynthesis, mode-of-action, new natural products, and synthesis.

Covering: 1997 to 2010. The angucycline group is the largest group of type II PKS-engineered natural products, rich in biological activities and chemical scaffolds. This stimulated synthetic creativity and biosynthetic inquisitiveness. The synthetic studies used five different strategies, involving Diels-Alder reactions, nucleophilic additions, electrophilic additions, transition-metal mediated cross-couplings and intramolecular cyclizations to generate the angucycline frames. Biosynthetic studies were particularly intriguing when unusual framework rearrangements by post-PKS tailoring oxidoreductases occurred, or when unusual glycosylation reactions were involved in decorating the benz[a]anthracene-derived cores. This review follows our previous reviews, which were published in 1992 and 1997, and covers new angucycline group antibiotics published between 1997 and 2010. However, in contrast to the previous reviews, the main focus of this article is on new synthetic approaches and biosynthetic investigations, most of which were published between 1997 and 2010, but go beyond, e.g. for some biosyntheses all the way back to the 1980s, to provide the necessary context of information.

Total synthesis of landomycin A, a potent antitumor angucycline antibiotic.

The first total synthesis of landomycin A, the longest and most potent antitumor angucycline antibiotic, has been achieved in 63 steps and 0.34% overall yield starting from 2,5-dihydroxybenzoic acid, 3,5-dimethylphenol, triacetyl d-glucal, and d-xylose, with a convergent linear sequence of 21 steps.

A four-enzyme pathway for 3,5-dihydroxy-4-methylanthranilic acid formation and incorporation into the antitumor antibiotic sibiromycin.

The antitumor antibiotic sibiromycin belongs to the class of pyrrolo[1,4]benzodiazepines (PBDs) that are produced by a variety of actinomycetes. PBDs are sequence-specific DNA-alkylating agents and possess significant antitumor properties. Among them, sibiromycin, one of two identified glycosylated PBDs, displays the highest DNA binding affinity and the most potent antitumor activity. In this study, we report the elucidation of the precise reaction sequence leading to the formation and activation of the 3,5-dihydroxy-4-methylanthranilic acid building block found in sibiromycin, starting from the known metabolite 3-hydroxykynurenine (3HK). The investigated pathway consists of four enzymes, which were biochemically characterized in vitro. Starting from 3HK, the SAM-dependent methyltransferase SibL converts the substrate to its 4-methyl derivative, followed by hydrolysis through the action of the PLP-dependent kynureninase SibQ, leading to 3-hydroxy-4-methylanthranilic acid (3H4MAA) formation. Subsequently the NRPS didomain SibE activates 3H4MAA and tethers it to its thiolation domain, where it is hydroxylated at the C5 position by the FAD/NADH-dependent hydroxylase SibG yielding the fully substituted anthranilate moiety found in sibiromycin. These insights about sibiromycin biosynthesis and the substrate specificities of the biosynthetic enzymes involved may guide future attempts to engineer the PBD biosynthetic machinery and help in the production of PBD derivatives.

A multiply convergent platform for the synthesis of trioxacarcins.

Many first-line cancer drugs are natural products or are derived from them by chemical modification. The trioxacarcins are an emerging class of molecules of microbial origin with potent antiproliferative effects, which may derive from their ability to covalently modify duplex DNA. All trioxacarcins appear to be derivatives of a nonglycosylated natural product known as DC-45-A2. To explore the potential of the trioxacarcins for the development of small-molecule drugs and probes, we have designed a synthetic strategy toward the trioxacarcin scaffold that enables access to both the natural trioxacarcins and nonnatural structural variants. Here, we report a synthetic route to DC-45-A2 from a differentially protected precursor, which in turn is assembled in just six steps from three components of similar structural complexity. The brevity of the sequence arises from strict adherence to a plan in which strategic bond-pair constructions are staged at or near the end of the synthetic route.

Optimization of the assembly efficiency for lidamycin chromophore bound to its apoprotein: a case study using orthogonal array.

OBJECTIVE: Lidamycin (LDM) can be dissociated to an apoprotein (LDP) and an active enediyne chromophore (AE). The detached AE can reassemble with its LDP-containing fusion protein to endow the latter with potent antitumor activity. However, the reassembly of AE with LDP is affected by several factors. Our aim was to optimize the assembly efficiency of the AE with a LDP-containing fusion protein and investigate the influence of several factors on the assembly efficacy. METHODS: A method based on RP-HPLC was developed to analyze the assembly rate, and an orthogonal experimental design L(9) (3(4)) was used to investigate the effects of temperature, assembly time, pH and molecular ratio of LDP-containing fusion protein to AE on the assembly rate. Furthermore, the determined optimum conditions for the assembly rate of the LDP-containing fusion protein with AE were applied and evaluated. RESULTS: A calibration curve based on the LDM micromolar concentration against the peak-area of AE by HPLC was obtained. The order in which individual factors in the orthogonal experiment affected the assembly rate were temperature>time>pH>molar ratio of AE to protein and all were statistically significant (P<0.01). The optimal assembly conditions were temperature at 10°C, time of 12 h, pH 7.0, and the molar ratio of AE: protein of 5:1. The assembly rate of AE with a LDP-containing fusion protein was improved by 23% after condition optimization. CONCLUSION: The assembly rate of chromophore of lidamycin with its LDP-containing fusion protein was improved after condition optimization by orthogonal design, and the optimal conditions described herein should prove useful for the development of this type of LDP-containing fusion protein.

Repairing faulty genes by aminoglycosides: development of new derivatives of geneticin (G418) with enhanced suppression of diseases-causing nonsense mutations.

New pseudo-di- and pseudo-trisaccharide derivatives of the aminoglycoside drug G418 were designed, synthesized and their ability to readthrough nonsense mutations was examined in both in vitro and ex vivo systems, along with the toxicity tests. Two novel lead structures, NB74 and NB84, exhibiting significantly reduced cell toxicity and superior readthrough efficiency than those of gentamicin, were discovered. The superiority of new leads was demonstrated in six different nonsense DNA-constructs underling the genetic diseases cystic fibrosis, Duchenne muscular dystrophy, Usher syndrome and Hurler syndrome.

Nickel-catalyzed stereoselective formation of alpha-2-deoxy-2-amino glycosides.

The development of a new method for the stereoselective synthesis of alpha-2-deoxy-2-amino glycosides is described. This methodology relies on the nature of the cationic nickel catalyst, generated in situ from L(n)NiCl(2) and AgOTf, to direct the anomeric stereoselectivity. The new glycosylation reaction is highly alpha-selective and proceeds under mild conditions with 5-10 mol % of the nickel catalyst loading at ambient temperature. This new method has been applied to both D-glucosamine and galactosamine trichloroacetimidate donors as well as an array of primary, secondary, and tertiary alcohol nucleophiles to provide the desired glycoconjugates in good yields with excellent alpha-selectivity. Mechanistic studies of the present reaction are underway and will be reported in due course.