Total synthesis, configuration, and biological evaluation of anguinomycin C

Syntheses of Gymnothespirolignans B and C and Non-natural Isomer 9-Epi-gymnothespirolignan B

Syntheses of polycyclic spiro lignans gymnothespirolignans B and C as well as the unnatural isomer 9-epi-gymnothespirolignan B were accomplished using (R)-Roche ester and an appropriately substituted fluorenone. Key features of the convergent syntheses include coupling of the fluorenone and an iodo-alkene intermediate derived from (R)-Roche ester in the presence of the Lewis acid TiCl(OiPr)₃, C9-O bond formation via an S_N2 reaction with retention of stereochemistry, and diastereoselective hydrogenations of a common alkene intermediate guided by accessibility or positioning by the C8-methoxy.

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

Does Size Really Matter? Probing the Efficacy of Structural Reduction in the Optimization of Bioderived Compounds - A Computational "Proof-of-Concept"

Over the years, numerous synthetic approaches have been utilized in drug design to improve the pharmacological properties of naturally derived compounds and most importantly, minimize toxic effects associated with their transition to drugs. The reduction of complex bioderived compounds to simpler bioactive fragments has been identified as a viable strategy to develop lead compounds with improved activities and minimal toxicities. Although this ‘reductive’ strategy has been widely exemplified, underlying biological events remain unresolved, hence the unanswered question remains how does the fragmentation of a natural compound improve its bioactivity and reduce toxicities? Herein, using a combinatorial approach, we initialize a computational “proof-of- concept” to expound the differential pharmacological and antagonistic activities of a natural compound, Anguinomycin D, and its synthetic fragment, SB640 towards Exportin Chromosome Region Maintenance 1 (CRM1). Interestingly, our findings revealed that in comparison with the parent compound, SB640 exhibited improved pharmacological attributes, while toxicities and off-target activities were relatively minimal. Moreover, we observed that the reduced size of SB640 allowed ‘deep access’ at the Nuclear Export Signals (NES) binding groove of CRM1, which favored optimal and proximal positioning towards crucial residues while the presence of the long polyketide tail in Anguinomycin D constrained its burial at the hydrophobic groove. Furthermore, with regards to their antagonistic functions, structural inactivation (rigidity) was more pronounced in CRM1 when bound by SB640 as compared to Anguinomycin D. These findings provide essential insights that portray synthetic fragmentation of natural compounds as a feasible approach towards the discovery of potential leads in disease treatment.

Mild Epoxidation of Allylic Alcohols Catalyzed by Titanium(III) Complexes: Selectivity and Mechanism

A novel methodology for the epoxidation of a broad range of primary, secondary, and tertiary allylic alcohols is described using tert-butyl hydroperoxide as oxidant and Ti(III) species generated by reduction of Ti(IV) complexes, with manganese (0) in 1,4-dioxane under mild reaction conditions. The reaction proceeded with wide substrate scope and high chemo- and diastereoselectivity. A mechanistic pathway for the reaction is also discussed.

Capturing Biological Activity in Natural Product Fragments by Chemical Synthesis

Natural products have had an immense influence on science and have directly led to the introduction of many drugs. Organic chemistry, and its unique ability to tailor natural products through synthesis, provides an extraordinary approach to unlock the full potential of natural products. In this Review, an approach based on natural product derived fragments is presented that can successfully address some of the current challenges in drug discovery. These fragments often display significantly reduced molecular weights, reduced structural complexity, a reduced number of synthetic steps, while retaining or even improving key biological parameters such as potency or selectivity. Examples from various stages of the drug development process up to the clinic are presented. In addition, this process can be leveraged by recent developments such as genome mining, antibody–drug conjugates, and computational approaches. All these concepts have the potential to identify the next generation of drug candidates inspired by natural products.

Structural Basis of Targeting the Exportin CRM1 in Cancer

Recent studies have demonstrated the interference of nucleocytoplasmic trafficking with the establishment and maintenance of various cancers. Nucleocytoplasmic transport is highly regulated and coordinated, involving different nuclear transport factors or receptors, importins and exportins, that mediate cargo transport from the cytoplasm into the nucleus or the other way round, respectively. The exportin CRM1 (Chromosome region maintenance 1) exports a plethora of different protein cargoes and ribonucleoprotein complexes. Structural and biochemical analyses have enabled the deduction of individual steps of the CRM1 transport cycle. In addition, CRM1 turned out to be a valid target for anticancer drugs as it exports numerous proto-oncoproteins and tumor suppressors. Clearly, detailed understanding of the flexibility, regulatory features and cooperative binding properties of CRM1 for Ran and cargo is a prerequisite for the design of highly effective drugs. The first compound found to inhibit CRM1-dependent nuclear export was the natural drug Leptomycin B (LMB), which blocks export by competitively interacting with a highly conserved cleft on CRM1 required for nuclear export signal recognition. Clinical studies revealed serious side effects of LMB, leading to a search for alternative natural and synthetic drugs and hence a multitude of novel therapeutics. The present review examines recent progress in understanding the binding mode of natural and synthetic compounds and their inhibitory effects.

Expression, function, and targeting of the nuclear exporter chromosome region maintenance 1 (CRM1) protein

Nucleocytoplasmic trafficking of proteins/RNAs is essential to normal cellular function. Indeed, accumulating evidence suggests that cancer cells escape anti-neoplastic mechanisms and benefit from pro-survival signals via the dysregulation of this system. The nuclear exporter chromosome region maintenance 1 (CRM1) protein is the only protein in the karyopherin-β protein family that contributes to the trafficking of numerous proteins and RNAs from the nucleus. It is considered to be an oncogenic, anti-apoptotic protein in transformed cells, since it reportedly functions as a gatekeeper for cell survival, including affecting p53 function, and ribosomal biogenesis. Furthermore, abnormally high expression of CRM1 is correlated with poor patient prognosis in various malignancies. Therapeutic targeting of CRM1 has emerged as a novel cancer treatment strategy, starting with a clinical trial with leptomycin B, the original specific inhibitor of CRM1, followed by development of several next-generation small molecules. KPT-330, a novel member of the CRM1-selective inhibitors of nuclear export (SINE) class of compounds, is currently undergoing clinical evaluation for the therapy of various malignancies. Results from these trials suggest that SINE compounds may be particularly useful against hematological malignancies, which often become refractory to standard chemotherapeutic agents.

Organocatalytic enantio- and diastereoselective synthesis of highly substituted δ-lactones via a Michael-cyclization cascade

An organocatalyzed Michael-cyclization cascade approach of readily available α,β-unsaturated aldehydes and pyrazoleamides has been developed to get highly substituted δ-lactones in excellent enantioselectivities (up to 97%) and diastereoselectivities. The δ-lactones so obtained could easily be transformed into benzazepine derivatives with excellent enantio- and diastereoselectivities. Furthermore, the pyrazole moiety from the δ-lactones can be simply cleaved without disturbing the stereoselectivity.

An enantioselective cascade reaction between α,β-unsaturated aldehydes and malonic half-thioesters: a rapid access to chiral δ-lactones

We disclose a novel efficient enantioselective organocatalytic cascade reaction for the preparation of δ-lactones in good to excellent yields (69-93%) and with high to excellent enantioselectivities (88-96% ee).

Recent applications of the hetero Diels–Alder reaction in the total synthesis of natural products

The synthetic utility and potential power of the Diels–Alder (D–A) reaction in organic chemistry is evident.

A regio- and stereoselective entry to (Z)-β-halo alkenyl sulfides and their applications to access stereodefined trisubstituted alkenes

A mild and efficient preparation of (Z)-β-halo alkenyl sulfides via the K2CO3-promoted hydrothiolation of haloalkynes has been realized, producing (Z)-β-bromo and (Z)-β-chloro vinylic sulfides in high yields with excellent regio- and stereoselectivity. This approach covers a variety of substrates, including both aryl and alkyl haloalkynes. Meaningfully, it allows a facile access to stereodefined (Z)- or (E)-trisubstituted olefins featuring the iterative cross-coupling of carbon-halide and carbon-sulfur bonds of β-halo alkenyl sulfides.

Total synthesis of aphadilactones A-D

The first total synthesis of aphadilactones A-D, diastereomeric natural products recently isolated from the Meliaceae plant Aphanamixis grandifolia by Yue and co-workers, which possess an unprecedented carbon skeleton, has been achieved. The synthesis features a catalytic asymmetric hetero-Diels-Alder reaction to form the dihydropyran ring, concurrent installation of the lactone and furan moieties via a tandem acid-catalyzed acetal cleavage, oxidation, and cyclization process, and an intermolecular Diels-Alder reaction to forge the target products.

Telomerase inhibitors from cyanobacteria: isolation and synthesis of sulfoquinovosyl diacylglycerols from Microcystis aeruguinosa PCC 7806

By using the Telospot assay, 27 different extracts of cyanobacteria were evaluated for telomerase inhibition. All extracts showed varying, but significant activity. We selected Microcystis aeruguinosa PCC 7806 to identify the active compound and a bioassay guided fractionation led us to isolate mixtures of sulfoquinovosyl diacylglycerols (SQDGs), which were identified by 2D NMR and MS/MS experiments. Pure SQDG derivatives were then synthesized. The IC(50) values of pure synthetic sulfoquinovosyl dipalmitoylglycerol and the monopalmitoylated derivative against telomerase were determined to be 17 and 40 μM, respectively. A structure-activity relationship study allowed the identification of compounds with modified lipophilic acyl groups that display improved activity.

Truncated militarinone fragments identified by total chemical synthesis induce neurite outgrowth

Short analogs of militarinone D were prepared that omit the configurationally unstable triene side chain bearing stereogenic centers and cut down the number of synthetic steps by 12. These short natural product fragments induce neuritogenesis at markedly lower concentrations.

Visible Light Photocatalysis of Radical Anion Hetero-Diels-Alder Cycloadditions

We have discovered a photocatalytic intramolecular hetero-Diels-Alder reaction of tethered bis(enones). This transformation involves the intermediacy of an enone radical anion and constitutes the formal coupling of an electron-deficient heterodiene with an electronically mismatched enone dienophile. The diastereoselectivity and regioselectivity of the process are high, and the dihydropyran products are amenable to a variety of synthetically useful transformations.

Alkyne elementometalation-Pd-catalyzed cross-coupling. Toward synthesis of all conceivable types of acyclic alkenes in high yields, efficiently, selectively, economically, and safely: "green" way

Palladium-catalyzed cross-coupling reactions, especially those involving Zn, Al, Zr (Negishi coupling), and B (Suzuki coupling), collectively have brought about "revolutionary" changes in organic synthesis. Thus, two regio- and stereodefined carbon groups generated as R(1)M (M = Zn, Al, B, Cu, Zr, etc.) and R(2)X (X = I, Br, OTs, etc.) may now be cross-coupled to give R(1)-R(2) with essentially full retention of all structural features. For alkene syntheses, alkyne elementometalation reactions including hydrometalation (B, Al, Zr, etc.), carbometalation (Cu, Al-Zr, etc.), and haloboration (BX(3) where X is Cl, Br, and I) have proven to be critically important. Some representative examples of highly efficient and selective (>or=98%) syntheses of di-, tri-, and oligoenes containing regio- and stereodefined di- and trisubstituted alkenes of all conceivable types will be discussed with emphasis on those of natural products. Some interesting but undesirable cases involving loss of the initial structural identities of the alkenyl groups are attributable to the formation of allylpalladium species, which must be either tamed or avoided. Some such examples involving the synthesis of 1,3-, 1,4-, and 1,5-dienes will also be discussed.

The cytotoxic styryl lactone goniothalamin is an inhibitor of nucleocytoplasmic transport

An in vivo nuclear export assay (immunostaining of Rio2 in HeLa cells) demonstrated that (R)-goniothalamin is an inhibitor of nucleocytoplasmic transport above 500 nM, which was rationalized also by molecular modeling. The cytotoxic styryl lactone natural product was prepared via an enantioselective Cr(III) catalyzed hetero Diels-Alder reaction and a Sonogashira coupling. A series of analogs was synthesized and only the oxidized goniothalamin derivative featuring an alkyne spacer was found active. Unsaturated lactones of natural origin other than leptomycin (LMB) are thus suggested to operate via a similar mechanism targeting the CRM1 nuclear receptor.

Regioselective Reductive Cross-Coupling Reactions of Unsymmetrical Alkynes

The present microreview summarizes our progress over the last few years in defining regioselective reductive cross-coupling reactions of unsymmetrical alkynes with terminal- and internal alkynes, aldehydes, and imines. We begin with a brief historical perspective of metal-mediated reductive dimerization reactions of aromatic alkynes and discuss the challenges associated with "crossed" versions of this mode of reactivity. Next, a collection of available methods that allow for regioselective reductive cross-coupling of internal alkynes with terminal and internal alkynes, aldehydes, and imines is summarized. After an examination of the requirements for regioselectivity in these cases, the logic behind our design of alkoxide-directed titanium-mediated reductive cross-coupling reactions is presented. A nomenclature is introduced to delineate the presumed mechanistic origin of regioselection associated with each reaction design, and a presentation of alkoxide-directed regioselective reductive cross-coupling reactions of alkynes follows. Throughout, principal issues related to reactivity and selectivity are discussed to assess scope and limitations of available methods and to describe the broad challenges that exist for defining complex fragment union reactions based on reductive cross-coupling chemistry.

Stereoselective synthesis of trisubstituted (E,E)-1,3-dienes by the site-selective reductive cross-coupling of internal alkynes with terminal alkynes: a fragment coupling reaction for natural product synthesis

A highly selective convergent coupling reaction is described between alkynes for the synthesis of stereodefined trisubstituted (E,E)-1,3-dienes-structural motifs commonly found embedded in the skeletons of bioactive polyketide-derived natural products. While numerous multistep processes for the synthesis of this stereodefined functional group exist, the current method represents a significant advance as it does not require stereodefined olefinic coupling partners (vinyl halide or vinyl organometallic); it proceeds by a single convergent C-C bond-forming event (avoiding multistep methods based on carbonyl olefination) and is tolerant of a diverse array of functional groups including free hydroxyls. Through a systematic study of titanium-mediated reductive cross-coupling reactions of internal alkynes with terminal alkynes, a fragment coupling reaction of great utility in natural product synthesis has emerged. Here, use of a proximal hydroxy group to control regioselection in the functionalization of a preformed titanacyclopropene has led to the establishment of a highly selective bimolecular coupling process, where C-C bond formation occurs in concert with the establishment of two stereodefined alkenes. Compared to the body of literature known for related metal-mediated coupling reactions, the current work defines a powerful advance, achieving site-selective bimolecular C-C bond formation without the need for using TMS-alkynes or conjugated alkynes. Overall, complex 1,3-dienes relevant for the synthesis of polyketide-derived natural products of varying stereochemistry were prepared with typically >or=20:1 selectivity, defining the important role of an alkoxide directing group located delta to preformed titanacyclopropenes.

Application of tandem ring-closing enyne metathesis: formal total synthesis of (-)-cochleamycin A

A tandem ring-closing metathesis of a silaketal-based dienyne substrate proceeded efficiently to provide a bicyclic siloxane, which upon removal of the silicon tether afforded an (E,Z)-1,3-dienediol. Further manipulation of this key functional motif rendered synthesis of the entire C1-C19 linear skeleton of (-)-cochleamycin A, a late-stage intermediate employed in the previous total synthesis of (+)-cochleamycin A by Roush and co-workers.

Distinct cytoplasmic maturation steps of 40S ribosomal subunit precursors require hRio2

During their biogenesis, 40S ribosomal subunit precursors are exported from the nucleus to the cytoplasm, where final maturation occurs. In this study, we show that the protein kinase human Rio2 (hRio2) is part of a late 40S preribosomal particle in human cells. Using a novel 40S biogenesis and export assay, we analyzed the contribution of hRio2 to late 40S maturation. Although hRio2 is not absolutely required for pre-40S export, deletion of its binding site for the export receptor CRM1 decelerated the kinetics of this process. Moreover, in the absence of hRio2, final cytoplasmic 40S maturation is blocked because the recycling of several trans-acting factors and cytoplasmic 18S-E precursor ribosomal RNA (rRNA [pre-rRNA]) processing are defective. Intriguingly, the physical presence of hRio2 but not its kinase activity is necessary for the release of hEnp1 from cytoplasmic 40S precursors. In contrast, hRio2 kinase activity is essential for the recycling of hDim2, hLtv1, and hNob1 as well as for 18S-E pre-rRNA processing. Thus, hRio2 is involved in late 40S maturation at several distinct steps.

(+)- and (-)-mutisianthol: first total synthesis, absolute configuration, and antitumor activity

The first synthesis of the natural product (+)-mutisianthol was accomplished in 11 steps and in 21% overall yield from 2-methylanisole. The synthesis of its enantiomer was also performed in a similar overall yield. The absolute configuration of the sesquiterpene (+)-mutisianthol was assigned as (1S,3R). Key steps in the route are the asymmetric hydrogenation of a nonfunctionalized olefin using chiral iridium catalysts and the ring contraction of 1,2-dihydronaphthalenes using thallium(III) or iodine(III). The target molecules show moderate activity against the human tumor cell lines SF-295, HCT-8, and MDA-MB-435.

Surface modifications based on the cyanobacterial siderophore anachelin: from structure to functional biomaterials design

This review describes the design, synthesis and evaluation of novel catechol based anchors for surface modification. The anachelin chromophore, the catecholate fragment of the siderophore anachelin from the cyanobacterium Anabaena cylindrica, allows for the immobilization of polyethylene glycol (PEG) on titania and glass surfaces thus rendering them protein resistant and antifouling. It is proposed that catecholate siderophores constitute a class of natural products useful for surface modification similar to dihydroxyphenylalanine and dopamine derived compounds found in mussel adhesive proteins. Second-generation dopamine derivatives featuring a quaternary ammonium group were found to be equally efficient in generating antifouling surfaces. The anachelin chromophore, merged via a PEG linker to the glycopeptide antibiotic vancomycin, allowed for the generation of antimicrobial surfaces through an operationally simple dip-and-rinse procedure. This approach offers an option for the prevention of nosocomial infections through antimicrobial implants, catheters and stents. Consequences for the mild generation of functional biomaterials are discussed and novel strategies for the immobilization of complex natural products, proteins and DNA on surfaces are presented.