Platensimycin

By Talitha Pahs, Patrick Müller and Isabella Mladenova

Introduction

Platensimycin is a recently discovered antibiotic with a new mechanism of action, produced by the bacterium Streptomyces platensis which was first isolated out of a South African soil sample by MSD Sharp & Dohme, due to a Substance screening project. They have been searching for a target which is able to inhibit Proteins like FabF being involved in the fatty acid synthesis, as they were successfully detected by an Antisense RNA-Method to be necessary for the biosynthesis of the bacterial cell membrane. The first chemical racemic total synthesis of Platensimycin was completed in 2006 by MSD Sharp & Dohme1.[1]

1. MSD Sharp & Dohme is called Merck & Co., Inc. in Canada and USA [2]

The relevance of Platensimycin

Platensimycin (Fig.1) is a promising antibiotic against several multi resistant gram-positive bacteria without any cross-resistances observed concerning other antibiotics like Vancomycin. It is assumed that fewer bacteria build a resistance against Platensimycin, which provides Platensimycin to be a possible reserve antibiotic. Other points for Platensimycin being a relevant antibiotic, is the selectivity as well as no toxicity being observed in a murine model. Furthermore Platensimycin might be a potent drug against Diabetes and related metabolic diseases. Due to the extrusion mechanism of gram-negative bacteria which is pumping the target out of the cell membrane, Platensimycin is not a suitable antibiotic against gram-negative bacteria as long as the extrusion mechanism is not disturbed. [1]

The Synthesis of Platensimycin

The first total synthesis of racemic Platensimycin was reported in 2006 by Nicolaou and coworkers. Since then different Syntheses of the core substructure were developed by various groups in quite a short time. There are racemic and enantiopure synthesis. [3] Now there are more than 21 Syntheses for the core substructure. [3]

Synthesis in general

(Graphics: Information concerning the Mechanism [7] structures [8])

Synthesis of the aromatic fragment

There are two ways of making the aromatic fragment. Nicolaou and co-worker start with a 2-Nitroresorcin which then gets two times a MOM (Methoxymethyl) protection group. The next step is the reduction to an Amin that afterwards is transformed to N-Boc (tert-Butoxycarbonyl)- Derivate. The carboxylic acid was made after an In-situ Silylieration of the Carbamate through ortho-metallation. The removal of the Boc-protection group made the Amin [8] that is needed for the Synthesis of Platensimycin. [6]

(Graphics: https://publikationen.uni- tuebingen.de/xmlui/bitstream/handle/10900/49592/pdf/Dissertation_Max_Wohland_Endversion.pdf?sequence=1&isAllowed=y)

Giannis has in a not regioselective method Methyl-2,4-dihydroxybenzoat nitrated in a 1:1 assortment. The Isomers are easy to separate and there are two more reactions steps necessary to get the final product. Because of that the Synthesis seems very efficient.

(Graphics: https://publikationen.unituebingen.de/xmlui/bitstream/handle/10900/49592/pdf/Dissertation_Max_Wohland_Endversion.pdf?sequence=1 &isAllowed=y)

The Nicolaou Synthesis

Nicolaou and coworkers started with (R)-carvone, which was converted to branch derivate A. Thus, a

1-2 addition of a Grignard reagent in the present of Ce3Cl2 followed by allylic oxidation of the resulting tertiary alcohol with transposition of the double bond gave [A]. [A] made a oxymercuration- carbocyclization with Hg(OAc)2 and NaBH4 gave the bicyclic product [B] as a mixture of epimers. Then an elimination with Martin’s sulfurane reagent led to the exocyclic olefin [C] which was converted to the aldehyde [D] through the cleavage of the acetal. [D]was treated with SmI2 to give the tricyclic product [E] as a single stereoisomer in a 57% yield. Subsequent occurred an inversion of configuration by a Mitsunobu reaction, followed by treatment with base led to [F] in a 1:1 mixture of diastereoisomers. Stereoselective reduction of the ketone, followed by an acidic work-up and then an oxidation with PCC gave ketone [G]. This substrate was transformed into enone [H] and its regioisomer in a 2:1 ratio. Double alkylation led to [I], which can be used to make Platensimycin. [3] This mechanism led to cyclic ether as an intermediate which has been the pivotal „relay compound for most of the subsequent formal syntheses of Platensimycin. [3]

(Graphics: http://ccc.chem.pitt.edu/wipf/courses/2320_08_files/HO/Nicolaou%20(Platensimycin,%202008).pdf structure of Platensimycin [8] )

The Gosh Synthesis

The Gosh Synthesis started with an (S)-carvone [A]and was first reported in 2007 by Gosh and Xi. Their strategy was based on the conversion of (S)-carvone into the known oxabicycle[3.3.0]octane ketone [B]. [3] [B] was made of [A] with an oxidation under Bayer-Villiger conditions in excellent yield. Then further oxidation of the methyl ketone appendage gave the corresponding alcohol [C]. In the following step a protection group as TBS ether was made and then an olefination according to Petasis, followed by hydroboration which gave product [D] as a 2:1 mixture of isomers [6]. Deprotection of the TBS ether with DDQ gave the corresponding secondary alcohol which following oxidized to the ketone. The ketone was treated with chiral phosphonoacetate and gave product [E] in a seperavle 3.2:1 mixture of E-/Z- esters. The reduction of product [E] led to the alcohol [F], which was prepared for the Diels- Alder reaction. First a transformation led to the methoxydiene [H] and a following thermal intramolceluar Diels-Alder reaction afforded the core substructure [I] as the O-methyl ether. The core substructure is closely related to Nikolaou’s key intermediate, but does not make a formal synthesis yet. [6]

(Graphics: http://onlinelibrary.wiley.com/doi/10.1002/anie.200705303/full)

Comparison with Nicolaou Synthesis

 Both synthesises started with carvone but they use different enantiomers  Crucial for the conception of the synthesis plan was a similar radical carbon cycle closure  In the step of bicyclic ketone , the synthesis pathways differ : The product A was isolated in the Gosh synthesis of its epimer [3]

Nikolaou’s two asymmetric syntheses

The first total synthesis of racemic Platensimycin by Nicolaou and coworkers was soon changed into an asymmetric synthesis due to an enantioselective cycloisomerization or diastereoselective alkylation. The enantioselective cycloisomerization reaction started with substrate [A]which react to the spirocyclic product [B] (in a 91% yield and high enantiomeric excess) due to a Ru(II) catalyst according to Zhang. The residual carboxylate function needed to be removed, which was made using the Barton’s method. During this reaction there happened a 1,3 Hydrid shift. Follwed by a hydrolysis of the acetal group and the treatment of SmI2 led to product [E]. [E] was treated with TFA and react to the core substructure. [3] (Graphics [3],[8])

In the second asymmetric synthesis was an oxidative cycloaromatization of the key amid [B] due to an acylation of (S , S) -pseudoephedrine with the carboxylic acid [A]. Following by an asymmetric Myer- alkylation with benzyl bromide and LDA led to product [C]. In 4 steps the substrate [ D ] was made. This is needed for the oxidative cyclodearomatzation, which is the key reaction. Substrate [E] reacts in three steps to the optic active core substructure Platensimycin. [3]

( Graphics: http://www.unc.edu/depts/mtcgroup/litmeetings/platensimycin.pdf) Yamamoto’s asymmetric synthesis

A profitable Synthesis of the core substructure of Platensimycin was developed in 2007 by Yamamoto and coworkers. Substrate [B]was made by an asymmetric Diels-Alder reaction of methylcyclopentadiene and methyl acrylate in a 92% yield and 99% ee. An N-nitrose aldol addition- decarboxylation sequences led to the enantiomerically pure ketone [C]. Afterwards a Bayer-Villiger oxidation with H2O2 under basic conditions, followed by hydrolysis and dehydrative lactonization led to the bycicle lactone [D]. A following SN2 addition of a vinyl group and a lactonization of the intermediate cyclopentane with triflourmethansulfonimide as organic solvent. These rections gave product [E] that is further elaborated to F and F’. Substrate [F] is necessary for the following reaction. [F‘] can be recycelt. The enone [F] was extended through a critical L-proline- mediated Robinson annulation. The resulting product is furnished a stoichiometric amount of L-proline in DMF which effected an intramolecular Michael addition and led to product [G]. The treatment of [G] with NaOh led to an aldol condensation and the formation of the core substructure with a good diastereoselctivity (dr = 5:1) [3].

(Graphics: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2553032/figure/F3/)

Corey’s entioselective synthesis

Lalic and Corey found a new synthesis in 2007. The synthesis started with 6-methoxy – α-naphtol [A]. In a one-step reaction 6-methoxy – α-naphtol [A]was converted to 1,4-naphthoqinone [B] by oxidative ketalization with bis-trifouroacetoxyiodobenzene. For the following reactions trimethylamine is an essential compound. [B] react in a enantioselective conjugate addition to [C] in the presence of Ru-BINAP catalyst and trimethylamine. In five stepts [B] react to [C]. [C] got a protection group to protect the reactive phenolic hydroxyl group. Then a treatment with bromine in

CH2Cl2 led to tricyclic bromide [D]. A following treatment with TBAF led to quinone [E]. [E] was hydrogenated in a stereoselective manner in the presence of chiral rhodium phosphine catalyst to build ketone [F]. The last step is the formation of the TMS silyl enol ether, followed by oxidation with IBX. The product is the core substructure. [3]

(Graphics: http://repositories.lib.utexas.edu/bitstream/handle/2152/17858/heckere.pdf?sequence=2)

Snider’s formal synthesis of (±)-Platensimycin

The synthesis pathway of Snider’s and co-workers synthesis started with a four step conversion of the 5-methoxytetraloe [A] to the enone [B] in a mixture of cis- and trans- decalines. The mixure of decalines were separated and equilibrated to a 6:4 ratio. For the further synthesis the necessarily cis- decalin [B] was treated with tributyltin hydride and AIBN what led to a reductively cyclization which formed tricycle [C]. This dione was treated with L-Selectride which gave the diol [D] in a mixture of axial and equatorial isomers. [4] The axial isomer could react directly in an elimination reaction to alkene [E] while the equatorial isomer needed to be treated with 1M HCl or silicia gel to form the alkene. The last steps included an allylic oxidation with SeO2 under microwave irradiation followed by an oxidation with MnO2 to form the core substructure, which can react to Platensimycin. [6]

(Graphics:http://repositories.lib.utexas.edu/bitstream/handle/2152/17858/heckere.pdf?sequence=2) A Bismuth(III)-Catalyzed Friedel−Crafts Cyclization and stereocontrolled Organocatalytic Approach to (−)-Platensimycin

Stanley T.-C. Eey and Martin J. Lear have developed this synthetic pathway for the core substructure 2010. The synthesis pathway started with the allylic alcohol [A] which was treated with L-(+) DIPT to form the epoxy alcohol [B] in 98% yield and 91% ee. The desired stereocenter at C12 were made by the opening of regioselective epoxide with allylic magnesium chloride. The following treatment with Martinelli’s regeoselctive catalytic monotosylation led to product [C] which reacted to the cis-tosyl lactol [D] in a 85% yield through oxidative double bond cleavage. By a new catalytic combination of 5 mol % Bi(OTf3) with 3 equiv of LiClO4 as a cocatalyst a Friedel Crafts intramolecular cyclization led to

[E] in a 94% yield within 3,5h. The Reduction with H2/Pd/C led to the benzyl deprotected tosyl-phenol [F]. The treatment of [F] with TBAF in xylene at 130°C gave the desired dieone caged core [G] in 86% yield over two steps. A chemo- and stereocontrolled conjugated reduction led to the enone [H1] which gave [J] after a treatment with H2/Pd/C in a 3:1 diastereomirc ratio. The treatment with of [J] with AlCl3 / TBAl gave [K]. By mesylation of [K] and heating with LiBr/Li2CO3 in DMSO gave the targeted core substructure. [5]

(Graphics: http://pubs.acs.org/doi/full/10.1021/ol102390t)

Mechanism of Action

Platensimycin shows antibacterial activity against gram-positive bacteria, for example S. aureus (MRSA). The characteristic difference is the inhibition of the fatty acid synthesis II (FAS II) of bacteria. [1][9]

FAS II carries out the bacterial fatty acid synthesis. The ketoacyl-ACP carrier Protein (elongation of the fatty acid chain) is inhibited by Platensimycin by catalysis of the FabF- enzyme. The FAS II is necessary for the biosynthesis of the bacterial cell membrane. [1][9][10]

Platensimycin did not affect the DNA, RNA, protein or cell wall biosynthesis of the mammals. [9] (Graphics: http://dx.doi.org/10.1016/j.chembiol.2014.01.005)

This Graphic does show the most important interactions between Platensimycin and FabF.

(Graphics: http://www.scipharm.at/download.asp?id=1506)

Relations – Platensimycin and Analogs

1.) Platencin

Platencin (Fig. 2) has a similar structure then Platensimycin, which provides a similar mechanism of action. The only component missing is the ether-ring. Two advantages of Platencin are inhibiting FabH as well, which is thought to be responsible for a lower risk of bacteria building resistance and Platencin being effective against gram-negative bacteria. But there is a conflict between the inhabitation of FabH and FabF. Platencins effect against FabF is six times less effective than Platensimycin and for times better against FabH. But together it provides a good antibiotic [1].

2.) Structural diversifications

Scientists tried to modify Platensimycin to increase its antibiotic effect and to promote the synthesis. There have been found two suitable analogs, 7-Phenylplatensimycin (Fig. 3) and 11–Methyl 7- Phenylplatensimycin (Fig. 4). Both do promote the cyclic addition and show higher activity against gram-positive bacteria then Platensimycin does [1].

3.) Inspiration for Bioorganic Metals

Platensimycin has been an inspiration for a few bioorganic Metals where 3-(4- [acetylferrocenoyl]butanamido)-2,4-dihydroxybenzoic acid (Fig. 5) established itself for being effective against Aureus selective [1].

References

[1] Adil M Allahverdiyev (2013). The use of platensimycin and platencin to fight antibiotic resistance. Infection and Drug Resistance 2013:6 99–114 [Access: 15. June 2015]

[2] http://www.msd.de/msd/unsere-geschichte/ [Access: 17.06.2015]

[3]Hanessian, S. (2013). Design and Strategy in Organic Synthesis . Wiley-VCH. 484-499

[4]Hecker, E. (2008): "Studies toward the Total Synthesis of (±)-Chartelline and (–)-Platensimycin" http://repositories.lib.utexas.edu/bitstream/handle/2152/17858/heckere.pdf?sequence=2 [Access: 20. May 2015]

[5]Lear, S. T.-C. (2010, Oktober 29). A Bismuth(III)-Catalyzed Friedel−Crafts Cyclization and Stereocontrolled Organocatalytic Approach to (−)-Platensimycin. Organic Letters. http://pubs.acs.org/doi/full/10.1021/ol102390t [Access: 01. June 2015]

[6]Tiefenacher, K. Mulzer, J. (2008, March 2007). Synthesis of Platensimycin. Angewandte Chemie . http://onlinelibrary.wiley.com/doi/10.1002/anie.200705303/full [Access: 20. Mai 2015]

[7] Stanley Eey(2013)Total Synthesis of the Potent Antibiotic Platensimycin http://www.scs.illinois.edu/denmark/presentations/2013/gm-2013-9-10.pdf [Access: 18.06.2015]

[8] ACD/Chemsketch, version 14.01, Advanced Chemistry Development, Inc., Toronto, On, Canada, www.acdlabs.com, 2012.

[9] Jun Wang, (18.May 2006) Platensimycin is a selective FabF Inhibitor with potent antibiotic properties, Nature 441 358 – 361 http://www.nature.com/nature/journal/v441/n7091/pdf/nature04784.pdf [Access: 19.06.2015]

[10] Peterson, M.R., (20. March 2014) Mechanisms of Self-Resistance in the Platensimycin and Platencin-Producing Streptomyces platensis MA7327 and MA7339 Strains, Chemistry & Biology 21, 389-397,March 20, 2014 – http://dx.doi.org/10.1016/j.chembiol.2014.01.005 [Access: 19.06.2015]