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A Journal of the Gesellschaft Deutscher Chemiker

wwwangewandteorgChemie

Accepted Article

Title Chemical syntheses and chemical genetics of carboxyl polyetherionophores Recent highlights

Authors Han Liu Shaoquan Lin Kristian M Jacobsen and ThomasBjoslashrnskov Poulsen

This manuscript has been accepted after peer review and appears as anAccepted Article online prior to editing proofing and formal publicationof the final Version of Record (VoR) This work is currently citable byusing the Digital Object Identifier (DOI) given below The VoR will bepublished online in Early View as soon as possible and may be differentto this Accepted Article as a result of editing Readers should obtainthe VoR from the journal website shown below when it is publishedto ensure accuracy of information The authors are responsible for thecontent of this Accepted Article

To be cited as Angew Chem Int Ed 101002anie201812982Angew Chem 101002ange201812982

Link to VoR httpdxdoiorg101002anie201812982httpdxdoiorg101002ange201812982

Chemical Syntheses and Chemical Genetics of Carboxyl Polyether Ionophores Recent Highlights

Han Liu Shaoquan Lin Kristian M Jacobsen Thomas B Poulsen

Department of Chemistry Aarhus University Langelandsgade 140 8000 Aarhus C Denmark

email thpouchemaudk

Lead-in

A central goal of chemical genetics is to develop molecular probes that enable fundamental studies of cellular systems In the hierarchy of bioactive molecules the so-called ionophore-class occupies an unflattering position in the lower branches with typical labels being ldquonon-specificrdquo and ldquotoxicrdquo In fact the mere possibility that a candidate molecule possesses ldquoionophore-activityrdquo typically prompts its removal from further studies ionophores - from a chemical genetics perspective - are molecular outlaws In stark contrast to this overall poor reputation of ionophores synthetic chemistry owes some of its most amazing achievements to studies of ionophore natural products in particular the carboxyl polyethers renowned for their intricate molecular structures These compounds have for decades been academic battlegrounds where new synthetic methodology is tested and retrosynthetic tactics perfected Herein we review the most exciting recent advances in carboxyl polyether ionophore (CPI) synthesis and in addition discuss the bourgeoning field of CPI-chemical biology

1 Introduction The ability of a small molecule to actively transport metal ions across biological membranes was proposed for the first time in 1964 by Moore and Pressman[1] In the following decade Pressman and others characterized the structure and function of multiple ionophores[23] and classified these molecules as either electrogenic electroneutral or quasi-ionophores[2] The members of the carboxyl polyether ionophore (CPI) class of natural products fall in the electroneutral category which is defined by an ability to exchange metal cations for protons on either side of a membrane - an activity that is governed by pH and metal ion concentration in the local environment and enabled by the acidic functionality (typically a carboxylate group) that is present in all of these compounds (Fig 1) All ionophores owe their ion-transporting activities to the combination of a polar interior with a hydrophobic exterior and although these properties are not directly apparent when viewing the flat chemical structures of prototypical CPIs such as lasalocid (1) or salinomycin (2) cation-complexation induces large structural changes to shield the ion from the hydrophobic environment (Fig 1) By comparison electrogenic ionophores such as the cyclic peptide valinomycin encapsulates the metal ion (potassium in the case of valinomycin) as a positively charged complex and shuttles the ion through the membrane down its concentration gradient[3ndash5]

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Angewandte Chemie International Edition

This article is protected by copyright All rights reserved

Figure 1 Cation-binding of carboxyl polyether ionophores (CPIs) and representative examples

From a socio-economic perspective CPIs are important molecules due to their employment as veterinary antibiotics in several countries mainly to suppress the growth of protozoan parasites (anticoccidial activity) However CPIs also possess antibacterial activity primarily against Gram-positive species and several reports describe the ability to potently target drug-resistant bacterial strains[6] While these effects appear highly promising CPIs have thus far not been subjected to extensive clinical evaluations This situation may change with the emergence of bacteria that are resistant to all known antibiotics in current clinical use Mechanistically ionophores affect the permeability of the bacterial outer membrane to cations and this perturbation is thought to underlie their antibiotic activities In contrast to the relative simplicity of bacterial cells the effect of ionophores on mammalian cells with their multiple membrane enclosed organelles is a much more open question Recent discoveries that will be discussed in the last part of this review concerning the biological activity of selected CPIs in mammalian cells however teach us that we should in fact care deeply about this topic There may be great opportunities for new biological discoveries hidden within this class of compounds Given the recent advancements in synthetic methodology ndash that we will detail in the first part of the review ndash that has enabled unprecedented efficiency in the preparation of these complex targets synthesis may in fact enable much needed systematic studies of how CPIs perturb the biology of mammalian cells 2 Recent progress in the synthesis of polyether ionophores zincophorin and SCH-351448 as examples Due to their intriguing structures[78] and potential therapeutic applications carboxyl polyether ionophores have been targets of chemical synthesis for four decades since the first total synthesis of lasalocid A (1) by Kishi and co-workers published in 1978[9] The activity in this area until 2000 was comprehensively summarized in a former review[10] Since 2000 the introduction of novel synthetic methodologies especially in asymmetric synthesis and transition metal catalysis has significantly expanded the toolbox of synthetic chemists For complex molecule synthesis these advances have contributed to an increased focus on achieving synthetic ideality[11] as well as the design of flexible synthetic routes that aim way beyond achieving the parent target structures[12ndash15] In this part of the review we mainly focus on two CPIs zincophorin (3) and SCH-351448 (5) (Fig 1) to showcase how the introduction of new methodology has shaped the design of the synthetic routes and improved overall efficiency The examples were chosen based on the considerations that several research groups finished the synthesis of these molecules (7 for zincophorin and 9 for SCH-351448) using different strategies and most of them were published after 2000 We have tried to extract the strategic disconnections toward the carbon skeletons and key methods involved in the construction of the structural subunits rather

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than comprehensively detailing every synthetic step from all the different approaches A compilation of synthetic studies on other carboxyl polyether ionophore natural products finished in the time period from 2001 to 2018 are summarized in Table S1 (Supplementary information) with key transformations highlighted[16ndash37] 21 Zincophorin Zincophorin (3) which was formerly named griseochelin and given the current name due to its high affinity to zinc cations was isolated from Streptomyces griseus in 1984 and the structure was unambiguously elucidated via 2D NMR and XRD investigation[38ndash40] Zincophorin exhibits potent activity against Gram-positive bacteria including the highly pathogenic Clostridium coelchii[3840] and the methyl ester derivative shows antiviral activity with reduced toxicity[41] This antibiotic potency makes both zincophorin and its methyl ester (4) attractive synthetic targets Structurally the zincophorin skeleton contains 25 carbons and 13 chiral centers including 8 continuous chiral carbons at C6-C13 bearing alternating methyl and hydroxyl substitutions Based on this intriguing structural feature that encompass part of the 26-trans-tetrahydropyran (THP) ring the stereocontrolled construction of the polypropionate subunit and efficient ligation with the rest of the molecule in the final stage has been the central task of all the published synthetic approaches

Scheme 1 Danishefskys first total synthesis of zincophorin and key strategy towards the polypropionate structure The first total synthesis of zincophorin was completed by the Danishefsky lab in 1987 and showcased the capability of the hetero-Diels-Alder reaction in the construction of contiguous chiral centers (Scheme 1)[4243] Based on a former study from the same team on the reconstruction of zincophorin from degradation products[44] the Julia olefination was chosen to facilitate late-stage ligation between two major fragments 6 and 7 at C16-C17 Without the assistance from asymmetric C-C bond forming reactions that characterize many current syntheses substrate chiral induction and cyclic stereocontrol was strategically employed in the first construction of zincophorin[4243]

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Scheme 2 Strategic disconnections and corresponding transformations toward zincophorin

Since 2000 six total syntheses of zincophorin have been achieved by five research groups[45ndash51] and several synthetic studies toward fragments of the molecule have also been reported[5253] Here focus will primarily be dedicated to the newest syntheses (after 2010) however earlier studies (before 2009) which were summed up previously[54] will also be discussed to illustrate the evolution of the synthetic strategies The selection of the bonds to be formed in the final stages is critical to the success of any total synthesis project Retrosynthetically these strategic disconnections should divide the target into fragments with comparable size and complexity to maximize the convergence and the reactions to be used to realize these transformations should be highly reliable to minimize risk Traditionally the formation of C-C bonds that generate new chiral centers is not the preferred choice for late stage fragment coupling due to the high sensitivity of the reaction outcome to the substrate structure Following this logic Leighton[49] and Guindon[48] chose the C16-C17 E-configured double bond as the disconnection site (Scheme 2) Compared with Danishefskys pioneering work[4243] Kocienskis modification of the Julia-olefination based on tetrazole derived alkyl sulfone species 13 gave excellent E-selectivity following a simplified one-pot protocol[4955] In Miyashitas synthesis published in 2004 the adjacent C15-C16 bond was formed via palladium-catalyzed sp2-sp3 Suzuki coupling between alkenyl iodide 16 and alkyl borane 15 (Scheme 2)[47] New and robust stereoselective C-C bond forming reactions provide more opportunities for strategic disconnections during synthetic planning and such methods have also appeared in several of the recent routes to zincophorin In Krisches synthesis published in 2015[51] the equivalent final fragment coupling was carried out at the C13-C14 bond via a stereoselective alkyllithium-aldehyde addition reaction The confidence in this choice was based on an expected synergistic 12- and 13-induction effect which strongly supports the formation of the desired configuration at C13 Surprisingly an unselective reaction ensued (11 dr) which was attributed to the polyoxygenated aldehyde fragment To overcome this obstacle a chiral diamine ligand was added to the alkyllithium reactant (generated in situ from iodide 18) resulting in 31 dr preferring the desired configuration in the presence of the matched ligand (Scheme 2) Compared to Krisches disconnection on the edge of the C6-C13 polypropionate unit disconnection within this densely functionalized unit can be a challenging but appealing choice because two chiral centers will be formed during the fragment coupling Thanks to the robustness of modern aldol reactions involving geometrically controlled enolate species disconnections between contiguous chiral centers have been highly successful in some of the zincophorin syntheses In Cossys synthesis published in 2003[4546] the C12-C13 bond was formed via Ti(IV)-mediated aldol reaction of ketone 19 derived Z-enolate and excellent syn-selectivity was achieved (Scheme 2) In Leightons recent synthesis[50] the more deeply buried C10-C11 bond was formed via Cy2BCl mediated anti-selective aldol reaction of the E-enolate derived from ketone 21 Though the selectivity was found to be sensitive to remote substitution on the aldehyde 171 dr was achieved and 53 yield for desired isomer was obtained on 500 mg scale (Scheme 2) Compared with the 1st generation Leighton-synthesis[49] this shift of the strategic disconnection from C16-C17 to C10-C11 helps the authors to avoid the protecting group manipulations on C-9C-11 and simplifies the 3-step global deprotection to a

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one-pot stereoselective catecholborane reductionDDQ oxidation[50] Benefiting from this together with the improved synthesis of the fragments the length of the route was significantly shortened from 21 steps to 9 steps longest linear sequence (LLS)

Scheme 3 Strategies for polypropionate construction

The C6-C13 polypropionate structure in zincophorin an assembly of contiguous chiral centers bearing alternating methyl and hydroxyl substitutions is the central challenge in the synthesis of the molecule In general as illustrated in Scheme 3 the polypropionate is comprised of several 3C units (two carbons in the backbone) which biosynthetically is generated in both syn and anti-configurations from propanoyl CoA by polyketide synthetases From the retrosynthetic point of view the 3C unit can be disconnected into two pairs of synthons AB and AB Aldol reactions and crotylation reactions can be categorized as the AB disconnection while the ring-opening of epoxide by a methyl nucleophile can be categorized as the AB disconnection In the published syntheses of zincophorin most groups focused on the aldol- and crotylation-related approaches which could provide both the syn- and anti-selectivity as will be discussed below On the contrary the Miyashita group developed an iterative Sharpless asymmetric epoxidationring-opening approach to construct the anti 3C unit C9-C10 and C11-C12[47]

Scheme 4 Stereodivergent synthesis of polypropionates via aldolhydrogen transfer

To achieve maximized stereodivergence in the synthesis of the 3C unit from common aldol partners in 2011 Guindon and co-workers developed the Mukaiyama aldoldiastereoselective hydrogen transfer process[56] From the same aldehyde 23 and α-bromopropionate derived silyl enol ether 24 all of the four possible diastereomers can be efficiently prepared with high selectivity in two steps through different combinations between conditions AB (for step 1) and CD (for step 2) (Scheme 4a) Using this strategy in the total synthesis of zincophorin[48] the four chiral centers (C9-C12) in 28 were constructed via two iterative applications of conditions that favor the anti-configuration (Scheme 4b) A similar reaction was also used by the same group to construct the C8 chiral center in a stereodivergent manner[57]

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Scheme 5 Crotylation and related methodologies in zincophorin synthesis

In the polypropionate synthesis via iterative aldolhydrogen transfer strategy the regeneration of the aldehyde functionality from an ester group can be non-trivial especially in the presence of multiple substitutions As an alternative crotylation and related reactions afford the minimal chiral 3C unit of the polypropionate motif in a single step in a stereodivergent manner and the olefin in the product can be easily cleaved to generate the aldehyde group and thus facilitate the subsequent transformations The absolute configuration of the product can be controlled by the chirality of the reagents and substrates while both syn- and anti-selectivity can be achieved by the ZE configuration of the olefin in the reagent In the syntheses of zincophorin several crotylation and related reactions were used to tackle the polypropionate portion (Scheme 5) In Miyashitas approach[47] the cyclic hemiacetal acetate 29 was activated by Ti(IV) to generate the oxocarbenium cation which was trapped by chiral silane reagent 30 and gave the C7-C8 syn adduct 31 as a single isomer In Leightons approach to construct the C7-C8 anti linkage in 34[49] the non-chiral crotyl trifluoroborate reagent 33 developed by Batey[58] afforded very high (dr gt 201) anti-selectivity In Cossys synthesis[4546] the asymmetric addition of chiral allenylzinc reagent to aldehydes developed by Marshall[59] gave the best anti-selectivity during their survey

Scheme 6 Cascade reactions in zincophorin synthesis

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The (P)-allenylzinc reagent 36 prepared from (R)-alcohol mesylate 35 reacted with the C1-C9 aldehyde 37 to give the C9-C10 anti adduct 38 as single diastereomer[60]

To further improve the efficiency of crotylation-related approaches and to shorten the synthetic routes reaction cascades that combine multiple C-C bond forming events have also appeared in zincophorin syntheses In Leightons 2011 approach[49] a one-pot process comprised of Rh-catalyzed intramolecular siloformylation of alkyne 39 anti-crotylation Tamao-Fleming oxidation and subsequent diastereoselective enol-ketone tautomerization gave the product 40 containing the C8-C10 antianti-stereotriad in good yield and selectivity (Scheme 6a) In their 2nd generation approach published in 2017[50] the Rh-catalyzed asymmetric hydroformylation of terminal alkenes 41 and 46 was combined with asymmetric allylation or diastereoselective crotylation to give the C12-C13 syn-product 45 and C6-C8 antisyn-product 49 respectively (Scheme 6b) Krische has developed the Ir-catalyzed asymmetric crotylation using alcohols as crotyl acceptor and 3-buten-1-yl acetate as crotyl donor[6162] This methodology generates the new C-C bond enantio- and diastereoselectively in redox-neutral manner which avoids the introduction of the aldehyde and therefore can reduce redox-manipulations Employing this methodology in their 2015 synthesis[51] Krische reported the asymmetric bis-crotylation of 13-diol 50 (Scheme 6c) The pseudo-C2 symmetric adduct 52 was treated with molecular iodine which selectively gave the iodoetherification product 53 containing the desired configurations at C6-C10 After 5 additional synthetic steps including olefin cross metathesis Sharpless asymmetric epoxidation regioselective ring-opening by Me2CuLi and regeneration of the alkene the C4-C13 fragment of zincophorin 54 containing 7 new chiral centers was constructed in good overall yield In the above section we surveyed the efforts on the total synthesis of zincophorin in the past three decades with focus on the strategic disconnections and the asymmetric construction of the polypropionate subunit The most impressive recent synthetic routes from the Leighton and Krische groups (9 and 13 LLS respectively) both feature stereoselective C-C bond forming reactions in late stage fragment couplings to maximize convergence Also the use of asymmetric and reagent-controlled methods ensures that the routes could potentially be diverged toward analogues Finally development of one-pot multistep transformations and double-direction elongationdesymmetrization cascade processes further improves efficiency Although it is risky to judge the perfect or the last synthesis the Leighton and Krische routes to zincophorin in our opinion represent the current top-level 22 SCH-351448 SCH-351448 (5) was isolated from the fermentation broth of a Micromonospora microorganism in 2000 via bioassay-guided fractionation and was found to be the first small molecule activator of low-density lipoprotein receptor (LDLR) (ED50 25 microM) [63] Through extensive 2D NMR and XRD analysis the structure of SCH-351448 was elucidated to be a C2-symmetric 28-membered macrodiolide containing four 26-cis substituted tetrahydropyran (THP) units (two THPs in each half) In the crystal structure of the sodium salt the compound forms a pseudo-C2 symmetric cage-like complex with a single sodium cation[63] SCH-351448 thus comprises a promising lead for a novel cholesterol-reducing agent Its intriguing structure and activity have prompted several laboratories to target SCH-351448 as their synthetic goal Till today seven total syntheses[64ndash72] and two formal syntheses[7374] have been reported together with one approach towards the monomeric unit[75] The synthetic efforts published before 2006 have been summarized in a review by Hiersemann[76]

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Scheme 7 Strategic disconnections for SCH-351448 synthesis and attempted dimerization

Within all total syntheses of SCH-351448 the construction of the 28-membered macrodiolide structure via macrocyclization is the pivotal late stage event Three bonds including C13-C14 C21-C22 and the ester bond were chosen as the strategic disconnection sites for the ring-closing step in the seven total syntheses (Scheme 7a) In the approaches reported by De Brabander[67] Crimmins[69] and Panek[71] the lactonization via ester coupling or NaHMDS-mediated intramolecular alcoholysis of the benzo[13]dioxinone was used to give the macrocyclic product in 47-50 yield In the approaches developed by Lee[6465] Rychnovsky[70] and Krische[72] the C13-C14 single bond was generated via ring-closing metathesis (RCM) and subsequent saturation of the alkene (hydrogenation or diazene reduction) In Leightons approach as an alternative the C21-C22 bond was constructed using RCM and hydrogenation It is worth to note that although the most efficient way to construct the C2-symmetric structure of SCH-351448 is by dimerization of two identical linear precursors all attempts of the dimerization from Lee De Brabander Rychnovsky and Panek failed to give the 28-membered macrodiolide structure For example the Ca2+-templated cross metathesis (Rychnovsky)[70] and photoinduced quinoketene generationester formation (De Brabander)[67] only afford the undesired 14-membered lactone product 55 (Scheme 7b)

The construction of two different 26-cis THP fragments (C3-C7 and C15-C19) is the central topics of all the published synthetic efforts toward SCH-351448 In most of the reported routes unique and robust strategies to synthesize the THP fragments were reported and in general the same method was used to construct both THP-rings in each individual route Five different bonds were chosen as ring-closing sites in the 10 totalformal syntheses of SCH-351448 and its monomeric unit toward both THP fragments The key steps for THP construction are summarized in Table 1

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Table 1 Chronological summary of strategies toward 26-cis tetrahydropyran fragments

Entry Key steps in the construction of C3-C7 and C15-C19 THP fragments Ref

1

[6465]

2

[6667]

3

[68]

4

[75]

5

[69]

6

[73]

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7

[70]

8

[71]

9

[74]

10

[72]

In 2004 Lee reported the use of a radical addition to generate the C7-C8 and C18-C19 linkages (entry 1) Under mild initiation the β-alkoxy acrylates 58 and 61 underwent 6-exo-trig cyclization to afford the cis-THPs in excellent yield and diastereoselectivity After further transformations the products 59 and 62 were derived to 60 and 63 respectively ready for one-pot Julia olefination In 2002 an oxa-Michael addition approach was developed by De Brabander to construct the THP-fragments (entry 2) Due to the thermodynamic stability of the bis-equatorial conformation of 26-cis-THP the desired products 65 and 68 with C7-O and C15-O bonds formed were generated under thermodynamically controlled conditions in the presence of potassium tert-butoxide Finally the two THP-fragments were joined together via diastereoselective aldol reaction of boron-enolate 66 and the so-obtained C1-C21 fragment 70 was used in their total synthesis finished in 2005 In Hongs 2011 formal synthesis (entry 9) a related approach was adopted using the more reactive αβ-unsaturated aldehydes as the Michael acceptors In Leightons approach published in 2005 (entry 3) a different strategy based on the diastereoselective hydride reduction of the oxocarbenium cation was developed[77] Towards the C3-C7 THP-ring hemiketal 72 ndash resulting from the addition of lithium enolate to the lactone 71 - was activated by BF3OEt2 to give the oxocarbenium cation which was trapped by Et3SiH from the opposite side of the C7 substituent to establish the desired cis configuration in 73 In the construction of the C15-C19 THP-ring later in the sequence the δ-hydroxyketone 74 gave 67 yield of 75 and absolute cis selectivity which demonstrate the robustness of this strategy A similar diastereoselective ring-closing process of δ-hydroxyketones 76 and 79 was used by Koert et al in their synthesis of SCH-351448 monomeric unit (entry 4) In contrast to the above-mentioned approaches where at least one stereocenter was formed during the THP-ring elaboration Crimmins reported a RCM-based synthesis in 2006 (entry 5) The pentaene substrate 82

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with all stereocenters established via chiral auxiliary based aldol reaction and alkylation was treated with Grubbs 1st generation catalyst and alkene 83 to give the full length C1-C29 fragment 84 containing both the C3-C7 and C15-C19 THP-subunits in an excellent 88 yield The Prins reaction is an established strategy for THP-ring construction During this process the C-O and C-C bonds are sequentially generated via Lewis acid-mediated intermolecular dehydrative formation of oxocarbenium cation and intramolecular trapping with an alkene nucleophile In Lohs 2007 formal synthesis (entry 6) the two THP-units were constructed sequentially via In(III) mediated Prins reactions along with the assembly of the C1-C29 skeleton In the second Prins reaction joining the C1-C15 fragment 88 and C16-C29 fragment 89 42 yield was obtained As an extension of the Prins reaction in 2008 Rychnovsky developed the MAP (Mukaiyama aldolPrins) methodology in their synthesis of the C3-C7 THP-ring (entry 7) In the presence of a TiBr4 catalyst the enol ether 91 reacted with aldehyde 92 in an aldol-like fashion and the oxocarbenium cation thus generated participated in the subsequent Prins cyclization to generate the C6-C7 bond in 93 As an alternative to the typical Prins reaction Panek developed a formal [4+2] cyclization using chiral allyl silane 98 in which the intramolecular allylation of the oxocarbenium cation gave the desired C3-C4 and C18-C19 bonds (entry 8) Compared with Lohs result an improved yield of 70-83 was obtained for 99 and 102 with excellent cis-selectivity and the 246-trimethylsulfonate group in the product was readily transformed to the aldehyde functionality In all total and formal syntheses of SCH-351448 published between 2004 to 2011 the full skeleton was assembled in more than 39 steps (22 to 32 LLS) which could hardly satisfy a potential demand for analogue preparation In order to progress further towards shortening synthetic routes to SCH-351448 the ldquocostsrdquo of non-skeleton-assembly steps clearly had to be significantly reduced eg by avoiding protecting group and oxidation state manipulations In 2016 Krische reported their total synthesis of SCH-351448 with a 22 step (14 LLS) route (entry 10) The C3-C7 THP-ring was synthesized from 5-hexenal 109 via asymmetric Mukaiyama aldol reaction[78] and one-pot cross metathesisintramolecular Michael addition[79] The intermediate 113 was further transformed into Paneks C1-C13 fragment 100 in another 5 steps including Ir-catalyzed asymmetric transfer hydrogenative allylation and crotylation developed by Krisches group In the synthesis of the C15-C19 THP-ring the alcohol 116 generated from 5-hexenol 114 via cross metathesis first underwent Ir-catalyzed asymmetric transfer hydrogenative C-allylation and the THP-ring in 119 was then closed via the Tsuji-Trost reaction with good cis-selectivity After the one-pot regioselective hydroboration and sp2-sp3 Suzuki coupling with 120 Paneks C14-C29 fragment 103 was achieved in 4 steps The high efficiency of the route empowered by robust asymmetric allylationcrotylation methodology and the deliberate modular design affords the opportunity to deviate the structure eg by changing the catalysts configuration This combination makes Krisches approach the currently most appealing choice for future biological studies on the cellular activities of SCH-351448 The impressive achievements in the syntheses of zincophorin and SCH-351448 showcase how new methodology is a driver for increased efficiency Although a measure of lsquoefficiencyrsquo in a given synthesis is necessarily multi-dimensional a low overall step-count high overall yield andor scalability can be considered important guideposts[8081] The examples mentioned here include one or more of these favorable characteristics and at the current level it is fully realistic to target the preparation of several structural derivatives (eg functionalized molecular probes) of these natural products Although no such investigations of zincophorin and SCH-351448 has been published to this date they may be forthcoming At least there is little doubt that more extensive SAR-knowledge will not only facilitate the study of compound-specific biological activities ndash to be discussed below ndash but may also allow more broad conclusions on how carboxyl polyether ionophores perturb mammalian cells 3 Ionophore chemical biology ndash Recent advances Carboxyl polyether ionophores (CPIs) are frequently used to perturb ion concentrations in eukaryotic cells Especially nigericin monensin salinomycin and A23187 has been utilized to equilibrate concentration gradients of potassium sodium and calcium ions respectively[82ndash84] However the subtle details that underlie such use of CPIs eg the organellemembrane specificity the distinct cellular processes affected and especially the time and concentration dependency of all of these parameters is largely unchartered territory In fact the experimental proof that in vitro ion-transport selectivity actually translate to cellular systems is

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typically absent In addition the metal-binding capacity of CPIs may affect cellular metal pools in ways that are not actually due to ion-transport We hypothesize that detailed dissection of the activities of CPIs in mammalian cells using a combination of state-of-the-art analytical methods and CPI-molecular probes may lead to a number of new discoveries about many of these compounds Below we describe in concise manner some of the challenges that needs to be addressed and highlight a notable recent example of the key role of molecular probes in the mechanistic deconvolution of a polyether ionophore with highly specific cellular activity 31 Assessing ionophore activity in vitro and in cells The intricate molecular structures of the polyether carboxyl ionophores bestow these compounds with highly dynamic conformational properties which determine the efficiency and selectivity of ion transport[285] Measurements of these features has traditionally been performed by an in vitro assay where an aqueous solution of an ionophore and a metal ion (aqueous ion source) is separated from a pure water phase (ion-receiving phase) by a bulk organic phase that represent a model membrane (Figure 2a)[2] A more sophisticated method has been developed by the Davis lab for measuring the transmembrane transport into large unilamellar vesicles (LUVs) of chloride ions by synthetic anionophores[86] We suggest that by exchanging the ion sensor this or closely related assays could be used to visualize metal ion transport into vesicles by fluorescence microscopy in real-time (Figure 2b) However due to the unselective nature of most molecular ion-sensors such an assay would not be compatible with complex mixtures of ions A potential solution to this challenge could be to utilize an analytical technique that readily detects multiple metals by their unique emission energies We postulate that an optimal in vitro measurement system could be realized by incubating LUVs or another setup having two membrane-separated compartments with a combination of alkali and transition metal ions in presence of an ionophore to create a situation of competition between the ions For analysis the individual compartments must be directly addressable for sampling or the ion exchange reaction needs to be rapidly terminated before the samples can be processed for identification and quantification of the compartment-entrapped ions eg by inductively-coupled plasma optical emission spectrometry[87] Metal ion sensors and their use in cells have been reviewed in detail[88ndash90] Small-molecule ion sensors although subject to constant improvements are notoriously unselective between ions or can have non-related secondary effects[91] which prompts caution if they are employed for analysis of ion transport in cellular systems Despite of these challenges highly selective ion-sensors for eg Cu+ and Cu2+ have been developed and rigorously characterized in cells and in vivo[92] For detailed analyses of ionophore activities however sub-cellular specificity is required and although small molecules are easily targeted to active mitochondria by incorporation of lipophilic cations[93] other organelles are more difficult to target by small molecules If the ionophore perturbs the membrane potential of the mitochondria this method for mitochondrial targeting might not be suitable Alternatively genetically encoded ion sensors are available for K+ Mg2+ Ca2+ Cu+ Zn2+ Pb2+ and Cd2+ and these can be targeted selectively to organelles by genetically fusing an organelle-specific tag onto the ion sensor[8894ndash96] Genetically encoded ion sensors undergo a conformational change upon complexation with the ion and induce a change in fluorescence intensity or excitation or emission spectra whereas dual fluorescent sensors experience a shift in Foumlrster Resonance Energy Transfer (FRET)[88] The ion chelating domains in the sensors are based upon binding domains of ion binding proteins which makes it possible to fine-tune the binding affinity of the sensor to the ion Thus protein-based ion sensors ndash relative to high-affinity small-molecule chelators ndash have a more appropriate affinity that limits the redistribution of ions already bound in endogenous ion binding proteins

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Figure 2 Methods for ion transport measurements a) To imitate a lipophilic cell membrane two aqueous phases are separated by organic solvents typically CHCl3 The central aqueous phase is spiked with a solution of ionophore and a metal ion If the ionophore and metal ion interact with sufficient affinity the complex can enter the organic phase and subsequently reach the pure water phase (the ion-receiving phase) surrounding the inner aqueous phase (the aqueous ion source) The ion-receiving phase is monitored with an ion-selective electrode that measures the kinetics of ion transport b) Alternatively measurement of transmembrane ion transport is based on ion sensor-loaded LUVs Here ion transport is measured as the increase in fluorescence from LUVs with an ion-specific fluorescent sensor LUVs on a microscope slide is mixed with ionophores and ions and fluorescence is monitored by time-lapse fluorescence microscopy c) An example of the use of an ionophore molecular probe U2OS cells were treated with the salinomycin analogue AM5 that was subsequently visualized by copper-catalysed azide alkyne cycloaddition to AlexaFluor488-N3 Fluorescence microscopy revealed a clear co-localization with the lysosome marker Lysotracker Panel c is adapted from Ref [100] copyright Nature Publishing Group 32 Molecular probes for studying the mode-of-action of salinomycin In a search for small-molecule toxins with selectivity towards cancer stem-like cells (CSCs) Gupta et al identified the carboxyl polyether ionophore salinomycin (Fig 1) as gt100-fold more potent than paclitaxel at reducing subpopulations with stem-like features[97] Early attempts to unveil the mechanistic foundation for this selectivity by Lu and colleagues used a SuperTOPflash reporter cell assay and found that salinomycin and the closely-related ionophore nigericin both inhibited Wnt-signaling at submicromolar concentrations[98] Recently two studies have addressed the mechanism of CSC-selectivity of salinomycin by probing living cells with functionally competent synthetic analogs of salinomycin[99100] Both approaches took their starting point in a chemoselective derivatization of C20 in salinomycin ndash the Strand lab incorporated a

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nitrobenzoxadiazole (NBD) fluorophore via a carbonate linker whereas the Rodriguez lab installed a propargyl amine at C20 by reductive amination to yield the analog AM5 (Fig 2c)[99100] Salinomycin and the NBD analog display equipotent inhibition of Wnt signaling while AM5 harbored improved potency and selectivity towards CSCs The two studies both utilized their respective analogues to visualize the subcellular distribution of salinomycin The Strand lab found that the NBD analog primarily was contained in the endoplasmic reticulum (ER) and lipid droplets Subsequently the compound induced an increase in the cytosolic Ca2+ concentration that coincided with an upregulation of ER stress-related proteins such as ATF6 and CHOP Consequently the Strand lab concluded that salinomycin must function in the ER membrane to facilitate K+Ca2+ exchange and subsequent cell death via deleterious ER stress Oppositely the Rodriguez lab found a strong co-localization of AM5 with lysosomes (Fig 2c)[100101] Furthermore western blots and microscopy experiments revealed features of lysosomal degradation of iron binding proteins that led to lysosomal and lipid ROS and lysosomal membrane permeabilization The involvement of iron and lipid ROS in the cell death led the authors to challenge the toxicity of salinomycin and AM5 with inhibitors of ferroptosis Ferroptosis is a nonapoptotic form of regulated cell death that is dependent on iron and is executed via lipid ROS[102103] Interestingly the Rodriguez lab found that cell death by salinomycin could be blocked by the ferroptosis inhibitors N-acetylcysteine ferrostatin-1 and deferoxamine which made the team to conclude that salinomycin induced ferroptosis to selectively target CSCs Collectively these studies demonstrate that chemical derivatization of ionophores can enable the discovery of critical mechanistic details Very recently nucleolin was identified as a direct and functionally-relevant binding protein of salinomycin in neuroblastoma cell lines This interesting observation may shed additional light on how salinomycin exerts its inhibitory effects against CSCs and it furthermore underscores that cellular activities not involving ion-transport are entirely possible for the carboxyl polyether ionophores[104] The in vitro studies performed in the 1970rsquos on which we base the use of ionophores as cell biological tools do not capture the complexity of cellular systems having a plethora of ion-gradients that are often interrelated The combination of cutting-edge methods that can reveal effects on multiple cellular pathways with the preparation of molecular probes promises novel insight into the biology of carboxyl polyether ionophore natural products Such studies ndash strongly enabled by advances in synthesis ndash are just visible on the horizon 4 Acknowledgements We thank the Carlsberg foundation (grant CF17-0800 to TBP) and the Independent Research Fund Denmark (Sapere Aude 2 grant 6110-00600A to TBP) for financial support 5 References [1] C Moore B C Pressman Biochem Biophys Res Commun 1964 15 562ndash567 [2] B C Pressman Annu Rev Biochem 1976 45 501ndash530 [3] B C Pressman Ann N Y Acad Sci 1969 147 829ndash841 [4] V T Ivanov I A Laine N D Abdulaev L B Senyavina E M Popov Y A Ovchinnikov M M

Shemyakin Biochem Biophys Res Commun 1969 34 803ndash811 [5] M Pinkerton L Steinrauf P Dawkins Biochem Biophys Res Commun 1969 35 512ndash518 [6] D A Kevin II D A Meujo M T Hamann Expert Opin Drug Discov 2009 4 109ndash146 [7] C J Dutton B J Banks C B Cooper Nat Prod Rep 1995 12 165ndash181 [8] J Rutkowski B Brzezinski Biomed Res Int 2013 2013 1ndash31 [9] T Nakata G Schmid B Vranesic M Okigawa T Smith-Palmer Y Kishi J Am Chem Soc 1978

100 2933ndash2935 [10] M M Faul B E Huff Chem Rev 2000 100 2407ndash2473 [11] T Gaich P S Baran J Org Chem 2010 75 4657ndash4673 [12] A Fuumlrstner D Kirk M D B Fenster C Aiumlssa D De Souza O Muumlller Proc Natl Acad Sci U S

A 2005 102 8103ndash8108 [13] A Fuumlrstner Isr J Chem 2011 51 329ndash345 [14] T Oskarsson P Nagorny I J Krauss L Perez M Mandal G Yang O Ouerfelli D Xiao M A

S Moore J Massague et al 2009 3772ndash3776

101002anie201812982

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[15] L M Suen M A Tekle-Smith K S Williamson J R Infantine S K Reznik P S Tanis T D Casselman D L Sackett J L Leighton Nat Commun 2018 9 4710

[16] R Isaka L Yu M Sasaki Y Igarashi H Fuwa J Org Chem 2016 81 3638ndash3647 [17] S-G Li Y Wu Chem - An Asian J 2013 8 2792ndash2800 [18] H Fuwa K Sakamoto T Muto M Sasaki Org Lett 2015 17 366ndash369 [19] H Fuwa K Sakamoto T Muto M Sasaki Tetrahedron 2015 71 6369ndash6383 [20] Y Pang C Fang M J Twiner C O Miles C J Forsyth Angew Chem Int Ed 2011 50 7631ndash

7635 Angew Chem 2011 123 7773ndash7777 [21] K Matsumotoa S A Kozmina Adv Synth Catal 2008 350 557ndash560 [22] J McCabe A J Phillips Tetrahedron 2013 69 5710ndash5714 [23] I Larrosa P Romea F Urpiacute Org Lett 2006 8 527ndash530 [24] J-F Brazeau A-A Guilbault J Kochuparampil P Mochirian Y Guindon Org Lett 2010 12 36ndash

39 [25] J S Yadav V K Singh P Srihari Org Lett 2014 16 836ndash839 [26] L C Dias L S A Jardim A A Ferreira H U Soarez J Braz Chem Soc 2001 12 463ndash466 [27] L C Dias G Z Melgar L S A Jardim Tetrahedron Lett 2005 46 4427ndash4431 [28] M Lautens J T Colucci S Hiebert N D Smith G Bouchain Org Lett 2002 4 1879ndash1882 [29] M Gaumlrtner G Satyanarayana S Foumlrster G Helmchen Chem - A Eur J 2013 19 400ndash405 [30] E Bourque P J Kocienski M Stocks J Yuen Synthesis (Stuttg) 2005 3219ndash3224 [31] J P Cooksey P J Kocienski Y F Li S Schunk T N Snaddon Org Biomol Chem 2006 4

3325ndash3336 [32] J A Marshall A M Mikowski Org Lett 2006 8 4375ndash4378 [33] Y Li J Cooksey Z Gao P Kocieński S McAteer T Snaddon Synthesis (Stuttg) 2011 2011

104ndash108 [34] J S Yadav N N Yadav B V Subba Reddy Tetrahedron 2015 71 7539ndash7549 [35] S Kang W Lee B Jung H-S Lee S H Kang Asian J Org Chem 2015 4 567ndash572 [36] W Lee S Kang B Jung H-S Lee S H Kang Chem Commun 2016 52 3536ndash3539 [37] C Fang Y Pang C J Forsyth Org Lett 2010 12 4528ndash4531 [38] U Graumlfe W Schade M Roth L Radics M Incze K Ujszaacuteszy J Antibiot (Tokyo) 1984 37 836ndash

846 [39] L Radics J Chem Soc Chem Commun 1984 599 [40] H A Brooks D Gardner J P Poyser T J King J Antibiot (Tokyo) 1984 37 1501ndash1504 [41] E Tonew M Tonew U Grafe P Zopel Pharmazie 1988 43 717ndash719 [42] S J Danishefsky H G Selnick M P DeNinno R E Zelle M P DeNinno J Am Chem Soc

1987 109 4368ndash4378 [43] S J Danishefsky H G Selnick R E Zelle M P DeNinno J Am Chem Soc 1988 110 4368ndash

4378 [44] R E Zelle M P DeNinno H G Selnick S J Danishefsky J Org Chem 1986 51 5032ndash5036 [45] M Defosseux N Blanchard C Meyer J Cossy Org Lett 2003 5 4037ndash4040 [46] M Defosseux N Blanchard C Meyer J Cossy J Org Chem 2004 69 4626ndash4647 [47] K Komatsu K Tanino M Miyashita Angew Chem Int Ed 2004 43 4341ndash4345 Angew Chem

2004 116 4441ndash4445 [48] F Godin P Mochirian G St-Pierre Y Guindon Tetrahedron 2015 71 709ndash726 [49] T J Harrison S Ho J L Leighton J Am Chem Soc 2011 133 7308ndash7311 [50] L-A Chen M A Ashley J L Leighton J Am Chem Soc 2017 139 4568ndash4573 [51] Z A Kasun X Gao R M Lipinski M J Krische J Am Chem Soc 2015 137 8900ndash8903 [52] J P Cooksey Org Biomol Chem 2013 11 5117 [53] G Sabitha R Srinivas J Yadav Synthesis (Stuttg) 2011 2011 1484ndash1488 [54] Z Song A G Lohse R P Hsung Nat Prod Rep 2009 26 560ndash571 [55] F Godin P Mochirian G St-Pierre Y Guindon Tetrahedron 2015 71 709ndash726 [56] P Mochirian F Godin I Katsoulis I Fontaine J-F Brazeau Y Guindon J Org Chem 2011 76

7654ndash7676 [57] F Godin I Katsoulis Eacute Fiola-Masson S Dhambri P Mochirian Y Guindon Synthesis (Stuttg)

101002anie201812982

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2012 44 474ndash488 [58] A N Thadani R A Batey Org Lett 2002 4 3827ndash3830 [59] J A Marshall N D Adams J Org Chem 1999 64 5201ndash5204 [60] J Cossy N Blanchard M Defosseux C Meyer Angew Chem Int Ed 2002 41 2144ndash2246

Angew Chem 2002 114 2248ndash2250 [61] S W Kim W Zhang M J Krische Acc Chem Res 2017 50 2371ndash2380 [62] J M Ketcham I Shin T P Montgomery M J Krische Angew Chem Int Ed 2014 53 9142ndash

9150 Angew Chem 2014 126 9294ndash9302 [63] V R Hegde M S Puar P Dai M Patel V P Gullo P R Das R W Bond A T McPhail

Tetrahedron Lett 2000 41 1351ndash1354 [64] E J Kang E J Cho Y E Lee M K Ji D M Shin Y K Chung E Lee J Am Chem Soc 2004

126 2680ndash2681 [65] E J Kang E J Cho M K Ji Y E Lee D M Shin S Y Choi Y K Chung J-S S Kim H-J J

Kim S-G G Lee et al J Org Chem 2005 70 6321ndash6329 [66] A Bhattacharjee O Soltani J K De Brabander Org Lett 2002 4 481ndash484 [67] O Soltani J K De Brabander Org Lett 2005 7 2791ndash2793 [68] S Bolshakov J L Leighton Org Lett 2005 7 3809ndash3812 [69] M T Crimmins G S Vanier Org Lett 2006 8 2887ndash2890 [70] L L Cheung S Marumoto C D Anderson S D Rychnovsky Org Lett 2008 10 3101ndash3104 [71] K Zhu J S Panek Org Lett 2011 13 4652ndash4655 [72] G Wang M J Krische J Am Chem Soc 2016 138 8088ndash8091 [73] K-P Chan Y H Ling T-P Loh Chem Commun 2007 939 [74] H Park H Kim J Hong Org Lett 2011 13 3742ndash3745 [75] J R Backes U Koert European J Org Chem 2006 2006 2777ndash2785 [76] M Hiersemann Nachrichten aus der Chemie 2006 54 867ndash872 [77] M D Lewis J K Cha Y Kishi J Am Chem Soc 1982 104 4976ndash4978 [78] S Kiyooka Y Kaneko M Komura H Matsuo M Nakano J Org Chem 1991 56 2276ndash2278 [79] H Fuwa K Noto M Sasaki Org Lett 2010 12 1636ndash1639 [80] T Gaich P S Baran J Org Chem 2010 75 4657ndash4673 [81] J Schwan M Christmann Chem Soc Rev 2018 47 7985ndash7995 [82] W K Lutz H-K Wipf W Simon Helv Chim Acta 1970 53 1741ndash1746 [83] D R Pfeiffer P W Reed H A Lardy Biochemistry 1974 13 4007ndash4014 [84] M Mitani T Yamanishi Y Miyazaki Biochem Biophys Res Commun 1975 66 1231ndash1236 [85] F G Riddell Chirality 2002 14 121ndash125 [86] C M Dias H Valkenier A P Davis Chem - A Eur J 2018 24 6262ndash6268 [87] Y Hiraga M Kurokawa J-R Guo T Suga Biosci Biotechnol Biochem 1999 63 958ndash960 [88] K P Carter A M Young A E Palmer Chem Rev 2014 114 4564ndash4601 [89] C M Ackerman S Lee C J Chang Anal Chem 2017 89 22ndash41 [90] D J Hare E J New M D De Jonge G McColl Chem Soc Rev 2015 44 5941ndash5958 [91] N A Smith B T Kress Y Lu D Chandler-Militello A Benraiss M Nedergaard Sci Signal

2018 11 eaal2039 [92] M C Heffern H M Park H Y Au-Yeung G C Van de Bittner C M Ackerman A Stahl C J

Chang Proc Natl Acad Sci 2016 113 14219ndash14224 [93] J Zielonka J Joseph A Sikora M Hardy O Ouari J Vasquez-Vivar G Cheng M Lopez B

Kalyanaraman Chem Rev 2017 117 10043ndash10120 [94] H Bischof M Rehberg S Stryeck K Artinger E Eroglu M Waldeck-Weiermair B Gottschalk

R Rost A T Deak T Niedrist et al Nat Commun 2017 8 1422 [95] V Peacuterez Koldenkova T Nagai Biochim Biophys Acta - Mol Cell Res 2013 1833 1787ndash1797 [96] Y Shen S-Y Wu V Rancic Y Qian S-I Miyashita K Ballanyi R E Campbell M Dong

bioRxiv 2018 [97] P B Gupta T T Onder G Jiang K Tao C Kuperwasser R a Weinberg E S Lander Cell 2009

138 645ndash659 [98] D Lu M Y Choi J Yu J E Castro T J Kipps D a Carson Proc Natl Acad Sci 2011 108

101002anie201812982

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13253ndash13257 [99] X Huang B Borgstroumlm J Stegmayr Y Abassi M Kruszyk H Leffler L Persson S Albinsson

R Massoumi I G Scheblykin et al ACS Cent Sci 2018 4 760ndash767 [100] T T Mai A Hamaiuml A Hienzsch T Cantildeeque S Muumlller J Wicinski O Cabaud C Leroy A

David V Acevedo et al Nat Chem 2017 9 1025ndash1033 [101] T Cantildeeque S Muumlller R Rodriguez Nat Rev Chem 2018 2 202-215 [102] B R Stockwell J P Friedmann Angeli H Bayir A I Bush M Conrad S J Dixon S Fulda S

Gascoacuten S K Hatzios V E Kagan et al Cell 2017 171 273ndash285 [103] J Lewerenz G Ates A Methner M Conrad P Maher Front Neurosci 2018 12 214 [104] F Wang S Zhou D Qi S-H Xiang E T Wong X Wang E Fonkem T Hsieh J Yang B

Kirmani et al J Am Chem Soc 2019 jacs8b12872 Author Biographies

From left to right Han Liu Kristian M Jacobsen Shaoquan Lin Thomas B Poulsen Han Liu got his BSc (2005) and PhD (2010) degrees in organic chemistry from Peking University P R China From 2010 to 2018 he worked as a postdoctoral fellow in the University of Hong Kong P R China with Prof Xuechen Li on carbohydrate and peptide chemistry In May 2018 he moved to the Department of Chemistry Aarhus University Denmark Currently he is working as a postdoctoral fellow with Prof Thomas B Poulsen His research interest spans from the total synthesis of bioactive natural productsbiomolecules methodology development and chemical biology Kristian M Jacobsen is currently working as a postdoctoral researcher with Prof Thomas B Poulsen at Aarhus University Denmark He obtained his BSc MSC and PhD degrees in the Poulsen lab from 2013 to 2018 where his focus was on elucidating the mechanism of action of hypoxia-selective toxins His research interests span mechanistic studies of bioactive small molecules cell death and treatment-resistant cancer cells Shaoquan Lin received his BSc (2009) in Applied Chemistry at Southwest University of Science and Technology and his MSc (2013) in Organic Chemistry at Shanghai Institute of Organic Chemistry under the supervision of Professors Xiuli Sun and Yong Tang He did his PhD studies at the Institute of Microbial Chemistry under the supervision of Professors Naoya Kumagai and Masakatsu Shibasaki He received his PhD (2017) in Pharmaceutical Sciences from the University of Tokyo He is currently doing postdoctoral

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research in the laboratory of Professor Thomas B Poulsen at Aarhus University His research interests include the development of new methodologies and synthesis of bioactive compounds Thomas B Poulsen (born 1979) is an Associate professor at the Department of Chemistry Aarhus University in Denmark Thomas completed his PhD with Prof Karl Anker Joslashrgensen in 2008 working on the development of a series of new asymmetric organocatalytic transformations He then moved to Harvard University as a post-doctoral fellow with Prof Matthew D Shair at the Dept of Chemistry and Chemical Biology where he worked on target-identification studies of complex anti-cancer natural products From 2010-11 Thomas was an independent researcher at Aarhus University supported by an elite junior scientist programme (Sapere Aude) from the Danish Research Council and from 2012-17 he was an assistant professor at the same institution Thomasrsquo research interests span complex molecule synthesis chemical biology and mechanistic cell biology

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Chemical Syntheses and Chemical Genetics of Carboxyl Polyether Ionophores Recent Highlights

Han Liu Shaoquan Lin Kristian M Jacobsen Thomas B Poulsen

Department of Chemistry Aarhus University Langelandsgade 140 8000 Aarhus C Denmark

email thpouchemaudk

Lead-in

A central goal of chemical genetics is to develop molecular probes that enable fundamental studies of cellular systems In the hierarchy of bioactive molecules the so-called ionophore-class occupies an unflattering position in the lower branches with typical labels being ldquonon-specificrdquo and ldquotoxicrdquo In fact the mere possibility that a candidate molecule possesses ldquoionophore-activityrdquo typically prompts its removal from further studies ionophores - from a chemical genetics perspective - are molecular outlaws In stark contrast to this overall poor reputation of ionophores synthetic chemistry owes some of its most amazing achievements to studies of ionophore natural products in particular the carboxyl polyethers renowned for their intricate molecular structures These compounds have for decades been academic battlegrounds where new synthetic methodology is tested and retrosynthetic tactics perfected Herein we review the most exciting recent advances in carboxyl polyether ionophore (CPI) synthesis and in addition discuss the bourgeoning field of CPI-chemical biology

1 Introduction The ability of a small molecule to actively transport metal ions across biological membranes was proposed for the first time in 1964 by Moore and Pressman[1] In the following decade Pressman and others characterized the structure and function of multiple ionophores[23] and classified these molecules as either electrogenic electroneutral or quasi-ionophores[2] The members of the carboxyl polyether ionophore (CPI) class of natural products fall in the electroneutral category which is defined by an ability to exchange metal cations for protons on either side of a membrane - an activity that is governed by pH and metal ion concentration in the local environment and enabled by the acidic functionality (typically a carboxylate group) that is present in all of these compounds (Fig 1) All ionophores owe their ion-transporting activities to the combination of a polar interior with a hydrophobic exterior and although these properties are not directly apparent when viewing the flat chemical structures of prototypical CPIs such as lasalocid (1) or salinomycin (2) cation-complexation induces large structural changes to shield the ion from the hydrophobic environment (Fig 1) By comparison electrogenic ionophores such as the cyclic peptide valinomycin encapsulates the metal ion (potassium in the case of valinomycin) as a positively charged complex and shuttles the ion through the membrane down its concentration gradient[3ndash5]

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1

[6465]

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[6667]

3

[68]

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[75]

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[69]

6

[73]

101002anie201812982

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7

[70]

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[71]

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1

[6465]

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[68]

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[73]

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101002anie201812982

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101002anie201812982

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101002anie201812982

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Link to VoR: Angewandte Angew. Chem. Angew. Chem. Int. Ed...

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Transcript of Link to VoR: Angewandte Angew. Chem. Angew. Chem. Int. Ed...