The synthesis of bis-macrolactam 5 as an advanced intermediate for the projected total synthesis of griseoviridin (1) has been achieved via intramolecular Suzuki-Miyaura coupling (>70% yield). Further key steps were the construction of the C(17)-C(18) bond by a SmI2-mediated Barbier-type coupling, the establishment of the C(18) stereocenter by a diastereoselective Narasaka-Prasad reduction, and an electrophilic sulfur transfer to establish the S(1)-C(2) bond. DMP oxidation of 5 and deprotection gave griseoviridin analog 36 in 32% yield.

L-type amino acid transporters LAT1 (SLC7A5) and LAT2 (SLC7A8) facilitate the bidirectional transport of branched and aromatic amino acids (AAs) across the plasma membrane. LAT1 has emerged as a key therapeutic target in cancer due to its upregulation in different tumor types. We generated and characterized LAT1- and LAT2-expressing cells using the human MDST8 cell line lacking these transporters to evaluate the specificity and selectivity of the clinical candidate JPH203 and novel LAT1 inhibitors. Both LAT1 and LAT2 increased the expression of 4F2hc, a heavy chain protein essential for LAT1 functioning and AA transport. We show for the first time that two potent nanomolar LAT1 inhibitors, JPH203 and JX-078, can enter cells through LAT1-independent mechanisms and inhibit LAT1-mediated L-leucine efflux, giving rise to differential cell type and time-dependent effects on the AA metabolome and metabolic activity in cancer cells and human PBMCs. Intriguingly, the antiproliferative effects of JPH203 and JX-078 on MDST8 cells were LAT1-independent, showing also micromolar IC50 values in HT-29 and U937 cells that overexpress LAT1. We found notable differences in the bioavailability of LAT1 inhibitors in mice. Oral administration of JX-078 efficiently penetrated tissues and crossed the blood-brain barrier, leading to increased levels of inhibitory neurotransmitters glycine and GABA in the brain. This study demonstrates the utility of employing targeted metabolomics to interrogate LAT1/2 inhibitor selectivity in different physiological matrices in vitro, ex vivo and in vivo. Overall, our findings reveal LAT1-dependent and previously unrecognized LAT-independent effects of inhibitors believed to act specifically on LAT1.

Isoxeniolide A is highly strained xenicane diterpenoid of marine origin. This natural product is representative for a subfamily of xenicanes incorporating an allylic hydroxy group in the 9-membered ring; members of this xenicane subfamily so far have not been targeted by total synthesis. Here, we describe the first asymmetric total synthesis of isoxeniolide A. Key to forming the challenging E-configured cyclononene ring was a diastereoselective intramolecular Nozaki-Hiyama-Kishi reaction. Other important transformations include an enzymatic desymmetrization for absolute stereocontrol, a diastereoselective cuprate addition and the use of a bifunctional vinyl silane building block. Our strategy also permits access to the enantiomer of the natural product and holds potential to access a multitude of xenicane natural products and of analogs for structure-activity relationship studies.

In the context of a project aiming at the replacement of the 3-substituted β-lactam ring in classical β-lactam antibiotics by an N(3)-acyl-1,3-diazetidinone moiety, we have investigated the reaction of isocyanates with imines derived from allyl glycinate and differently substituted propionaldehydes. Imines of aromatic aldehydes with anilines have been reported to react with acyl isocyanates to give 1,3-azetidinones or 2,3-dihydro-4H-1,3,5-oxadiazin-4-ones, via [2+2] or [4+2] cycloaddition, respectively. However, neither of these products was formed with imines from allyl glycinate and 2-(mono)methyl propionaldehydes. α,α-Dimethylation of the imine enabled the [4+2] cycloaddition pathway, but the desired 1,3-diazetidinone products were not observed. Surprisingly, the imines obtained from thioesters of 2,2-dimethyl 3-oxo propionic acid reacted with aryl isocyanates or with benzyl isocyanate to give 2,2-dimethyl-2,4-dioxo-6-(aryl/akylthio)tetrahydropyrimidines, via thiol displacement and re-addition to a putative six-membered iminium intermediate. Such compounds have not been described in the literature so far. For aryl isocyanates, the reaction was insensitive to the nature of a para-substituent on the aromatic ring. The experimental results obtained for the reactions of acyl isocyanates with imines could be rationalized by DFT calculations. In addition, we have also shown that N(3)-acyl-1,3-azetidinone and 2,3-dihydro-4H-1,3,5-oxadiazin-4-one products can be distinguished based on experimental IR data in combination with theoretical reference spectra employing the IR spectra alignment (IRSA) algorithm. This discrimination was not possible by means of 1H, 13C, or 15N NMR spectroscopy.

ABCG2 is a multidrug transporter that protects tissues from xenobiotics, affects drug pharmacokinetics, and contributes to multidrug resistance of cancer cells. Here, we present tetracyclic fumitremorgin C analog Ko143 derivatives, evaluate their in vitro modulation of purified ABCG2, and report four high-resolution cryo-EM structures and computational analyses to elucidate their interactions with ABCG2. We found that Ko143 derivatives that are based on a ring-opened scaffold no longer inhibit ABCG2-mediated transport activity. In contrast, closed-ring, tetracyclic analogs were highly potent inhibitors. Strikingly, the least potent of these compounds, MZ82, bound deeper into the central ABCG2 cavity than the other inhibitors and it led to partial closure of the transmembrane domains and increased flexibility of the nucleotide-binding domains. Minor structural modifications can thus convert a potent inhibitor into a compound that induces conformational changes in ABCG2 similar to those observed during binding of a substrate. Molecular dynamics simulations and free energy binding calculations further supported the correlation between reduced potency and distinct binding pose of the compounds. We introduce the highly potent inhibitor AZ99 that may exhibit improved in vivo stability.

Ring systems of all sizes are frequent core or substructures in natural products and they are important elements of many drug molecules, as they often confer high binding affinity to and selectivity for disease-relevant biological targets. A uniform key transformation in the synthesis of such structures is the cyclization step. Among the various approaches that have been developed for ring closure, the intramolecular Suzuki-Miyaura reaction has emerged as a powerful option for the construction of normal- and medium-sized rings as well as macrocycles, due to its stereospecificity, the mild reaction conditions, and the non-toxic nature of the boron by-products. In this review, we summarize the state-of-the-art of the application of intramolecular Suzuki-Miyaura cross-coupling reactions in the construction of (macro)cyclic frameworks of natural products and bioactive molecules of synthetic origin, covering (mostly) examples that have been reported since 2015. Target molecules prepared via intramolecular Suzuki-Miyaura cross-coupling as a key step range from natural products/natural product analogs to synthetic drug candidates, featuring ring sizes from 4 to ≫12. We highlight the utility, scope, and limitations of the reaction for different ring sizes and arrays of functional groups. Where possible, comparisons with other methods of cyclization are provided.

AbstractIsoxeniolid A ist ein hochgespanntes marines Diterpenoid aus der Familie der Xenicane. Es steht repräsentativ für eine Unterfamilie von Xenicanen, deren 9‐gliedriger Ring eine allylische Hydroxygruppe aufweist; derartige Xenicane wurden bisher nicht als Zielmoleküle von Totalsynthesen verfolgt. Wir beschreiben hier die erste asymmetrische Totalsynthese eines derartigen Xenicans, nämlich des Isoxeniolid A. Dabei lag der Schlüssel zum Aufbau des besonders herausfordernden E‐konfigurierten Cyclononenrings in einer diastereoselektiven intramolekularen Nozaki–Hiyama—Kishi‐Reaktion. Andere wichtige Transformationen waren eine enzymatische Desymmetrisierung zur Kontrolle der absoluten Stereochemie, eine diastereoselektive Cuprat‐Addition und die Verwendung eines bifunktionalen Vinylsilan‐Bausteins. Unsere Synthesestrategie ermöglicht auch den Zugang zur enantiomeren Form des Naturstoffs sowie prinzipiell zu einer Vielzahl anderer natürlicher Xenicane und synthetischer Analoga für Struktur‐Aktivitäts‐Untersuchungen.

We have prepared a series of analogs of the complex marine macrolide (–)-zampanolide, which incorporate a dioxane-, oxathiane-, or oxathiane-dioxide ring in place of the natural tetrahydropyran moiety and we have determined their microtubule-binding affinity and antiproliferative activity against human cancer cells. The synthesis of these analogs was based on a convergent strategy with a HWE-based macrocyclization and a stereoselective aza-aldol reaction as key steps. The microtubule-binding affinity and cellular potency of the dioxane- and oxathiane-based analogs with a natural (Z,E)-sorbamide-based side chain were essentially indistinguishable from those of natural (–)-zampanolide; changing the configuration of the sorbamide unit from Z,E to E,E resulted in a slight loss in activity. In contrast, the presence of an oxathiane-dioxide ring caused a steep decrease in microtubule-binding and a significant loss in growth inhibitory activity. In addition, a substantial loss in potency was observed against a multidrug-resistant, P-glycoprotein-overexpressing cell line, while no such effect was found for the dioxane- or oxathiane-based analogs. A high-resolution X-ray crystal structure of the complex between beta-tubulin and dioxane-zampanolide was obtained, which showed that this compound, like natural (–)-zampanolide, induces helical structuring of the M-loop.

The disorazoles are a family of 26- to 32-membered macrodiolides of mixed non-ribosomal peptide and polyketide origin that show exceptional cytotoxic activity. Of the 39 different disorazoles that have been isolated so far, total syntheses have been described for three family members, namely the non-symmetrical disorazole A1 and the symmetrical disorazoles B1 and C1. With the exception of the first total synthesis of disorazole C1, these syntheses were all reported within the last decade. Different approaches have been followed to meet the challenge of establishing the 30-membered macrodiolide ring in these natural products, including single-step macrolactonization or cyclodimerization via sequential inter-/intramolecular Suzuki coupling, ACM/RCAM, or inter-/intramolecular Sonogashira coupling. In this report, we review the total syntheses of disorazoles that have been developed over the last decade. In addition, we also summarize the synthetic and SAR work that has been performed on non-natural disorazole congeners, which built on the chemistry developed in the course of the total synthesis work.

Several alkaloids of the waltherione family exhibit antitrypanosomal activity in the sub-micromolar or nanomolar range. While the overwhelming majority of waltheriones are based on a quinoline core structure, two structurally simpler pyridone-based congeners, waltheriones S and T, have recently been isolated and found to inhibit Trypanosoma cruzi with single-digit micromolar potency. Here, we report on the synthesis of a series of analogs of waltheriones S and T based on pyridone ring formation via cyclization of an appropriate triketone precursor with ammonia and the assessment of their activity against Trypanosoma cruzi. The data show that the methoxy group at the C(3)-position of the pyridone ring can be removed without significant loss in potency. Further modification of 3-desmethoxy waltherione T through methoxylation at the C(1') position of the C(6)-side chain or double methoxylation at the C(1')-position and the pyridone nitrogen had no significant impact on antitrypanosomal activity. These findings contrast with the activity differences between the corresponding quinoline-based natural waltheriones M, Q, and H, where the methoxy-bearing waltheriones Q and H are one order of magnitude more potent than the unsubstituted parent compound waltherione M. Our data indicate that the SAR for monocyclic waltheriones S and T does not simply parallel that of the quinoline-based congeners and they point to the importance of a rigid quinoline core for potent activity against T. cruzi.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Ko143 is a tetracyclic, synthetic analog of the fungal metabolite fumitremorgin C. Ko143 is a potent and specific inhibitor of the membrane-bound efflux transporter ABCG2, and it reverses ABCG2-mediated drug resistance in cancer cells. Here, we describe an improved synthesis of Ko143 that relies on the highly selective, substrate-controlled reduction of an imine that is formed in a Bischler-Napieralski reaction with the amide derived from 6-methoxy-l-tryptophan methyl ester and isovaleric acid as a key step. We have also developed a new route to 6-methoxy-l-tryptophan methyl ester from Cbz-l-aspartic acid methyl ester, m-anisidine and differently substituted benzaldehydes. With p-nitrobenzaldehyde as one of the starting materials, this route gave access to 6-methoxy-l-tryptophan methyl ester in five steps and 20 % overall yield; however, it is less efficient than a previously reported synthesis of 6-methoxy-l-tryptophan methyl ester from 6-methoxy indole.

We have prepared a series of partially reduced or demethylated analogs of the natural microtubule stabilizer (−)-zampanolide and we have assessed their antiproliferative activity, their microtubule-binding affinity and their effects on the cellular microtubule network and on cell cycle progression. For reasons of synthetic efficiency, these analogs were derived from 13-desmethylene-(-)-zampanolide, which we had previously shown to be an equally potent cancer cell growth inhibitor as the natural product. The synthesis of all compounds was based on a unified strategy that included final formation of the macrobicyclic core by an intramolecular HWE reaction and a stereoselective aza-aldol reaction to establish the C(20) stereocenter as the key steps. For the 5-desmethyl macrocycle, ring-closure relied on macrolactonization; however, elaboration of the macrocyclic aldehyde into the corresponding zampanolide analog was unsuccessful.All structural modifications investigated led to reduced cellular activity and lower microtubule-binding affinity compared to the parent 13-desmethylene-(–)-zampanolide, which may be ascribed to increased conformational flexibility due to the formal reduction of double bonds or the removal of the C(17)-methyl group. Notwithstanding this general trend, the cellular potency of 2,3-dihydro-13-desmethylene zampanolide as the most potent analog identified remained within a 9-fold range of that of 13-desmethylene-(–)-zampanolide (for 5 out of 6 cell lines). Notably, while the formal reduction of the C=C double bond of the enone system that is required for the covalent attachment of (−)-zampanolide to beta-tubulin caused a drop in antiproliferative activity of several hundred fold, the compound does bind to microtubules and shows the typical cellular hallmarks of a microtubule-stabilizing agent.

Paclitaxel (Taxol) is a taxane and a chemotherapeutic drug that stabilizes microtubules. While the interaction of paclitaxel with microtubules is well described, the lack of high-resolution structural information on a tubulin-taxane complex precludes a comprehensive description of the binding determinants that affect its mechanism of action. Here, we solved the crystal structure of baccatin III the core moiety of paclitaxel-tubulin complex at 1.9 Å resolution. Based on this information, we engineered taxanes with modified C13 side chains, solved their crystal structures in complex with tubulin, and analyzed their effects on microtubules (X-ray fiber diffraction), along with those of paclitaxel, docetaxel, and baccatin III. Further comparison of high-resolution structures and microtubules' diffractions with the apo forms and molecular dynamics approaches allowed us to understand the consequences of taxane binding to tubulin in solution and under assembled conditions. The results sheds light on three main mechanistic questions: (1) taxanes bind better to microtubules than to tubulin because tubulin assembly is linked to a βM-loopconformational reorganization (otherwise occludes the access to the taxane site) and, bulky C13 side chains preferentially recognize the assembled conformational state; (2) the occupancy of the taxane site has no influence on the straightness of tubulin protofilaments and; (3) longitudinal expansion of the microtubule lattices arises from the accommodation of the taxane core within the site, a process that is no related to the microtubule stabilization (baccatin III is biochemically inactive). In conclusion, our combined experimental and computational approach allowed us to describe the tubulin-taxane interaction in atomic detail and assess the structural determinants for binding.

The sponge-derived, highly unsaturated marine macrolide (−)-zampanolide is a potent antimitotic agent and cancer cell growth inhibitor. By using a convergent synthetic approach, the shown series of partially reduced and/or demethylated analogs were obtained. Depending on the site of modification, these compounds show a range of microtubule-binding affinities and cellular activities, thus providing new insights into the relative importance of specific double bonds and methyl groups for biological activity. More information can be found in the Research Article by K.-H. Altmann and co-workers (DOI: 10.1002/chem.202300703).

Breast cancer resistance protein (BCRP, ABCG2) is an efflux transporter that plays a crucial role in multidrug resistance to antineoplastic drugs. Ko143, an analogue of the natural product fumitremorgin C, is a potent inhibitor of ABCG2 but is rapidly hydrolyzed to an inactive metabolite in vivo. To identify ABCG2 inhibitors with improved metabolic stability, we have assessed a series of Ko143 analogues for their ability to inhibit ABCG2-mediated transport in ABCG2-transduced MDCK II cells and determined the stability of the most potent compounds in liver microsomes. The most promising analogues were evaluated in vivo by positron emission tomography. In vitro, three of the tested analogues were potent ABCG2 inhibitors and stable in microsomes. In vivo, they increased the distribution of the ABCG2/ABCB1 substrate [11C]tariquidar to the brain both in wild-type (with Abcb1a/b transport blocked by tariquidar) and Abcb1a/b(-/-) mice. One analogue was more potent than Ko143 in both animal models.

Maytansinol is a valuable precursor for the preparation of maytansine derivatives (known as maytansinoids). Inspired by the intriguing structure of the macrocycle and the success in targeted cancer therapy of the derivatives, we explored the maytansinol acylation reaction. As a result, we were able to obtain a series of derivatives with novel modifications of the maytansine scaffold. We characterized these molecules by docking studies, by a comprehensive biochemical evaluation, and by determination of their crystal structures in complex with tubulin. The results shed further light on the intriguing chemical behavior of maytansinoids and confirm the relevance of this peculiar scaffold in the scenario of tubulin binders.