Synthesis and Biological Evaluation of Ferrocene-Containing

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Organometallics 2010, 29, 4312–4319 DOI: 10.1021/om100614c

Synthesis and Biological Evaluation of Ferrocene-Containing Bioorganometallics Inspired by the Antibiotic Platensimycin Lead Structure Malay Patra,† Gilles Gasser,†,‡ Michaela Wenzel,§ Klaus Merz,† Julia E. Bandow,§ and Nils Metzler-Nolte*,† †

Lehrstuhl f€ ur Anorganische Chemie I, Fakult€ at f€ ur Chemie und Biochemie, Ruhr-Universit€ at Bochum, Geb€ aude NC 3 Nord, Universit€ atsstrasse 150, D-44801 Bochum, Germany, ‡Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland, and §Lehrstuhl f€ ur Biologie der Mikroorganismen, Fakult€ at f€ ur Biologie und Biotechnologie, Ruhr-Universit€ at Bochum, Universit€ atsstrasse 150, D-44801 Bochum, Germany Received June 24, 2010

The recent discovery of the natural product platensimycin (1) as a new antibiotic lead structure has triggered the synthesis of numerous organic derivatives for structure-activity relationships (SAR) in order to improve the poor in vivo efficacy of 1. The synthesis, characterization, and biological evaluation of the first four ferrocene-containing bioorganometallic compounds based on the platensimycin lead structure are reported herein, namely 3-(4,4-diferrocenoylpentanamido)2,4-dihydroxybenzoic acid (2), 3-(4,4-diferrocenoylbutanamido)-2,4-dihydroxybenzoic acid (3), 3-{4-(acetylferrocenoyl)butanamido}-2,4-dihydroxybenzoic acid (4), and 3-(4-ferrocenoylbutanamido)-2,4-dihydroxybenzoic acid (5). All new compounds were unambiguously characterized by all common analytical methods, including 1H and 13C NMR, mass spectrometry, IR spectroscopy, and elemental analysis. Furthermore, the single-crystal X-ray structures of methyl 4,4-diferrocenoylbutanoate (9), methyl 4,4-diferrocenoylpentanoate (10), 4,4-diferrocenoylpentanoic acid (14), 4,4diferrocenoylbutanoic acid (15), and 4-(acetylferrocenoyl)butanoic acid (16) were also determined. Among 2-5 and their intermediate carboxylic acids tested, only 3 was found to inhibit selectively the growth of S. aureus Mu50 strain (VISA) at a minimum inhibitory concentration (MIC) value of 128 μg/mL. Introduction The recent discovery of platensimycin (1; Figure 1) was an important contribution to the search for new treatment options against multidrug resistant pathogens.1 1 displays potent activity against Gram-positive bacterial strains (methicillin-resistant Staphylococcus aureus, vancomycin resistant Enterococcus faecalis) by selectively inhibiting the FabF enzyme in the bacterial fatty acid biosynthesis.2,3 Although 1 is not suited to become a drug, due to its low in vivo efficacy, its impressive antibacterial activity and low intrinsic toxicity toward mammalian cell lines makes 1 an *To whom correspondence should be addressed. Fax: 0234 32-14378. E-mail: [email protected]. (1) Wang, J.; Soisson, S. M.; Young, K.; Shoop, W.; Kodali, S.; Galgoci, A.; Painter, R.; Parthasarathy, G.; Tang, Y. S.; Cummings, R.; Ha, S.; Dorso, K.; Motyl, M.; Jayasuriya, H.; Ondeyka, J.; Herath, K.; Zhang, C.; Hernandez, L.; Allocco, J.; Basilio, A.; Tormo, J. R.; Genilloud, O.; Vicente, F.; Pelaez, F.; Colwell, L.; Lee, S. H.; Michael, B.; Felcetto, T.; Gill, C.; Silver, L. L.; Hermes, J. D.; Bartizal, K.; Barrett, J.; Schmatz, D.; Becker, J. W.; Cully, D.; Singh, S. B. Nature 2006, 441, 358–361. (2) Wright, H. T.; Reynolds, K. A. Curr. Opin. Microbiol. 2007, 10, 447–453. (3) Singh, S. B.; Jayasuriya, H.; Ondeyka, J. G.; Herath, K. B.; Zhang, C.; Zink, D. L.; Tsou, N. N.; Ball, R. G.; Basilio, A.; Genilloud, O.; Diez, M. T.; Vicente, F.; Pelaez, F.; Young, K.; Wang, J. J. Am. Chem. Soc. 2006, 128, 11916–11920. pubs.acs.org/Organometallics

Published on Web 09/09/2010

ideal lead structure in antibiotic research for further drug design and development. Since the discovery of 1, several synthetic routes to 1 and its analogues were developed4-8 and the antibacterial activities of the analogues evaluated.9-12 To date, modifications of the complicated tetracyclic cage were exclusively done by replacing it with different isosteric organic moieties. Notably, Nicolaou et al. reported the two most potent analogues, namely carbaplatensimycin (1b; Figure 1) and adamantaplatensimycin (1c; Figure 1), which showed activity similar to (4) Nicolaou, K. C.; Li, A.; Edmonds, D. J. Angew. Chem., Int. Ed. 2006, 45, 7086–7090. (5) Yao, Y.-S.; Yao, Z.-J. Youji Huaxue 2008, 28, 1553–1560. (6) Nicolaou, K. C.; Li, A.; Edmonds, D. J.; Tria, G. S.; Ellery, S. P. J. Am. Chem. Soc. 2009, 131, 16905–16918. (7) McGrath, N. A.; Bartlett, E. S.; Sittihan, S.; Njardarson, J. T. Angew. Chem., Int. Ed. 2009, 48, 8543–8546. (8) Kaliappan, K. P.; Palanichamy, K. Chem. Asian. J. 2010, 00, 00–00. (9) Nicolaou, K. C.; Lister, T.; Denton, R. M.; Montero, A.; Edmonds, D. J. Angew. Chem., Int. Ed. 2007, 46, 4712–4714. (10) Nicolaou, K. C.; Tang, Y.; Wang, J.; Stepan, A. F.; Li, A.; Montero, A. J. Am. Chem. Soc. 2007, 129, 14850–14851. (11) Nicolaou, K. C.; Stepan, A. F.; Lister, T.; Li, A.; Montero, A.; Tria, G. S.; Turner, C. I.; Tang, Y.; Wang, J.; Denton, R. M.; Edmonds, D. J. J. Am. Chem. Soc. 2008, 130, 13110–13119. (12) Krauss, J.; Knorr, V.; Manhardt, V.; Scheffels, S.; Bracher, F. Arch. Pharm. 2008, 341, 386–392. r 2010 American Chemical Society

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Figure 1. Structures of platensimycin (1), isoplatensimycin (1a), carbaplatensimycin (1b), adamentaplatensimycin (1c), platensimycin A1 (1d), 11-methyl-7-phenyl platensimycin (1e), 7-phenyl platensimycin (1f), and ferrocene-containing bioorganometallics (2-5).

that of the parent compound against a number of Grampositive bacteria, including S. aureus.13,14 Interestingly, isoplatensimycin (1a; Figure 1), which has a structure very similar to that of the parent compound (1), exhibited poor activity against different strains of S. aureus but showed significant activity against E. faecium.15 Nonetheless, despite the high degree of structural similarity between 1 and its natural analogue platensimycin A1 (1d; Figure 1), 1d inhibits the S. aureus growth at a minimum inhibitory concentration (MIC) of 16 μg/mL while platensimycin exhibited a MIC value of 0.5 μg/mL against the same bacterial strain.16 The structure-activity relationship (SAR) study by Nicolaou and co-workers showed that the tetracyclic cage of platensimycin is somewhat tolerant to modifications, whilst any small modification on the benzoic acid entity led to complete loss of antibacterial activity.11 Very recently, Jang et al. reported two analogues of platensimycin, 11-methyl-7-phenylplatensimycin (1e; Figure 1) and 7-phenylplatensimycin (1f; Figure 1), which were found to be more potent (MIC = 0.25 μg/mL) than the parent compound platensimycin (MIC = 0.5 μg/mL) against methicillin sensitive and resistant S. aureus strains.17 The poor antibacterial activity of 1a,d and excellent antibacterial activity of 1e,f certainly reveals that very small changes in the tetracyclic core may bring a dramatic change in the antibacterial activity of 1. Inspired by the possibility of replacing the tetracyclic cage of 1 by organometallic cores and studying their influence on (13) Nicolaou, K. C.; Lister, T.; Denton, R. M.; Montero, A.; Edmonds, D. J. Angew. Chem., Int. Ed. 2007, 46, 4712–4714. (14) Nicolaou, K. C.; Tang, Y.; Wang, J.; Stepan, A. F.; Li, A.; Montero, A. J. Am. Chem. Soc. 2007, 129, 14850–14851. (15) Jang, K. P.; Kim, C. H.; Na, S. W.; Kim, H.; Kang, H.; Lee, E. Bioorg. Med. Chem. Lett. 2009, 19, 4601–4602. (16) Singh, S. B.; Jayasuriya, H.; Herath, K. B.; Zhang, C.; Ondeyka, J. G.; Zink, D. L.; Ha, S.; Parthasarathy, G.; Becker, J. W.; Wang, J.; Soisson, S. M. Tetrahedron Lett. 2009, 50, 5182–5185. (17) Jang, K. P.; Kim, C. H.; Na, S. W.; Jang, D. S.; Kim, H.; Kang, H.; Lee, E. Bioorg. Med. Chem. Lett. 2010, DOI: 10.1016/j. bmcl.2010.02.037.

the bioactivity of 1, we have recently reported the synthesis and biological evaluation of Cr-bioorganometallics based on the platensimycin lead structure.18 In this study, we have replaced the synthetically challenging tetracyclic cage by rather simple arene-Cr(CO)3 moieties.18,19 Unfortunately, our most active compound was found to be less potent (MIC= 80 μg/mL against B. subtilis) than platensimycin. Interestingly, however, the mode of action was found to be different from that of the parent compound.20 This unexpected new mode of action is highly encouraging, as the biomedical usefulness of the intrinsic mode of action of platensimycin has been questioned. Indeed, very recently Brinster et al. reported that some Gram-positive pathogens might overcome the drug-induced inhibition of fatty acid biosynthesis in the presence of external fatty acid sources such as human serum,21 although this does not seem to be true for S. aureus.22 In recent years, ferrocene-containing biomolecules have attracted much attention for medicinal applications, mainly in anticancer and antimalarial research.23-28 For example, ferroquine, a ferrocene derivative of the known antimalarial (18) Patra, M.; Gasser, G.; Pinto, A.; Merz, K.; Ott, I.; Bandow, J. E.; Metzler-Nolte, N. ChemMedChem 2009, 4, 1930–1938. (19) For new metal-containing platensimycin analogues, a patent is pending (EP09154125). (20) Patra, M.; Wenzel, M.; Bandow, J. E.; Metzler-Nolte, N. In preparation. (21) Brinster, S.; Lamberet, B.; Staels, B.; Trieu-Cuot, P.; Gruss, A.; Poyart, C. Nature 2009, 458, 83–86. (22) Balemans, W.; Lounis, N.; Gilissen, R.; Guillemont, J.; Simmen, K.; Andries, K.; Koul, A. Nature 2010, 463, E3. (23) Hartinger, C. G.; Dyson, P. J. Chem. Soc. Rev. 2009, 38, 391–401. (24) Jaouen, G. Bioorganometallics: Biomolecules, Labelling, Medicine; Wiley-VCH: Weinheim, Germany, 2006. (25) James, P.; Neud€ orfl, J.; Eissmann, M.; Jesse, P.; Prokop, A.; Schmalz, H.-G. Org. Lett. 2006, 8, 2763–2766. (26) Hillard, E.; Vessieres, A.; Thouin, L.; Jaouen, G.; Amatore, C. Angew. Chem., Int. Ed. 2006, 45, 285–290. (27) van Staveren, D. R.; Metzler-Nolte, N. Chem. Rev. 2004, 104, 5931–5985. (28) Gasser, G.; Ott, I.; Metzler-Nolte, N. J. Med. Chem. 2010, accepted.

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drug chloroquine, was found to have activity similar to that of the parent compound against a chloroquine-sensitive P. falciparum strain (HB3 5CQS), whereas comparison of activity of ferroquine and chloroquine against chloroquineresistant P. falciparum (Dd2) shows that the former is 10 times more active than the latter.29,30 Ferroquine has recently successfully passed the clinical trial phase II. Ferrocifens (i.e., ferrocene-modified tamoxifen derivatives), developed by Jaouen et al., are another successful example of medicinal biorganometallic chemistry. They exhibit activity against hormone-independent breast cancer cell lines, where hydroxytamoxifen and tamoxifens are inactive.26,31 With this in mind, we envisioned replacing the synthetically challenging tetracyclic cage of platensimycin with different bulky ferrocene entities and to study their effects on the biological activity of 1. Herein, we report the synthesis and biological evaluation of four ferrocene-containing bioorganometallics inspired by the antibiotic platensimycin lead structure, namely 3-(4,4diferrocenoylpentanamido)-2,4-dihydroxybenzoic acid (2), 3-(4,4-diferrocenoylbutanamido)-2,4-dihydroxybenzoic acid (3), 3-{4-(acetylferrocenoyl)butanamido}-2,4-dihydroxybenzoic acid (4), and 3-(4-ferrocenoylbutanamido)-2,4dihydroxybenzoic acid (5) (see Figure 1 for their structures).

Results and Discussion Synthesis and Spectroscopic Characterization of Compounds 2-5. The synthesis sequence to obtain the desired ferrocene-containing carboxylic acids 4,4-diferrocenoylpentanoic acid (14), 4,4-diferrocenoyllbutanoic acid (15), and 4-(acetylferrocenoyl)butanoic acid (16) is outlined in Scheme 1. The carboxylic acid 4-ferrocenoylbutanoic acid (17) was prepared by following the literature procedure.32 Methyl ferrocenoate (6) was prepared from ferrocenecarboxylic acid via the ferrocene carboxyl chloride intermediate.33,34 Following literature procedures,35 the Claisen condensation of 6 with acetylferrocene (13) in the presence of lithium diisopropylamide (LDA) afforded 7 in 30-40% yield together with an unknown impurity and the starting materials. However, by changing the base from LDA to potassium tertbutoxide (KO t Bu), we were able to obtain 7 in 85% yield.36 7 was treated with K2CO3 and MeI to afford 8 in 62% yield. The 1H NMR spectrum of 8 shows a doublet at 1.53 ppm and a quartet at 4.31 ppm corresponding to the CH3 and (CO)2CH- groups present. No distinct signal for the enol isomer was observed in either the 1H or 13C NMR spectrum. All our efforts to make 10 by Michael addition of 8 to methyl acrylate using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were unsuccessful. On the other hand, a 1,4-addition of 7 to (29) Delhaes, L.; Biot, C.; Berry, L.; Delcourt, P.; Maciejewski, L. A.; Camus, D.; Brocard, J. S.; Dive, D. ChemBioChem 2002, 3, 418–423. (30) Biot, C.; Glorian, G.; Maciejewski, L. A.; Brocard, J. S.; Domarle, O.; Blampain, G.; Millet, P.; Georges, A. J.; Abessolo, H.; Dive, D.; Lebibi, J. J. Med. Chem. 1997, 40, 3715–3718. (31) Naguen, A.; Vessieres, A.; Hillard, E. A.; Top, S.; Pigeon, P.; Jaouen, G. Chimia 2007, 61, 716–724. (32) Apreutesei, D.; Lisa, G.; Akutsu, H.; Hurduc, N.; Nakatsuji, S.; Scutaru, D. Appl. Organomet. Chem. 2005, 19, 1022–1037. (33) Bonini, B. F.; Comes-Franchini, M.; Fochi, M.; Mazzanti, G.; Ricci, A.; Alberti, A.; Macciantelli, D.; Marcaccio, M.; Roffia, S. Eur. J. Org. Chem. 2002, 543–550. (34) Routaboul, L.; Chiffre, J.; Balavoine, G. G. A.; Daran, J.-C.; Manoury, E. J. Organomet. Chem. 2001, 637, 364–371. (35) (Ina) du Plessis, W. C.; Vosloo, T. G.; Swarts, J. C. J. Chem. Soc., Dalton Trans. 1998, 2507–2514. (36) Nandurkar, N. S.; Bhanushali, M. J.; Patil, D. S.; Bhanage, B. M. Synth. Commun. 2007, 37, 4111–4115.

Patra et al. Scheme 1. Synthesis of Carboxylic Acid Derivatives 14-17, Bearing Ferrocenyl Moietya

a Reagents and conditions: (a) 13, KOtBu, DMF; (b) K2CO3, Bu4NBr, MeI, toluene or acetonitrile; (c) DBU, methyl acrylate, DCM; (d) 10 equiv of aqueous NaOH, THF, or MeOH; (e) Me3SiI, CH3CN; (f) DBU, methyl acrylate, DCM; (g) NaH, methyl 3-bromopropionate, THF; (h) NaH, MeI, THF; (i) 100 equiv of aqueous NaOH, THF, or MeOH; (j) AlCl3, glutaric anhydride, DCM. t

methyl acrylate in the presence of DBU provided 9 in good yield (75%). The difference in reactivity of 7 and 8 arises probably as a consequence of the presence of the methyl group between the two keto groups in 8, which makes this position more crowded and less accessible to DBU. Alternatively, 9 could also be obtained by refluxing the mixture of 7, NaH, and methyl 3-bromopropionate (∼20-30% yield). The 1H NMR spectrum of 9 shows a singlet at 3.75 ppm corresponding to the methyl ester group together with the signals at 4.12-4.95 ppm from the ferrocenyl groups. Only a trace of the enol isomer was observed in the 1H as well as in the 13C NMR spectrum. A clean peak at m/z 549.03 corresponding to [M þ Na]þ was observed in the electrospray ionization mass spectrum (ESI-MS, positive detection mode). The methyl ester 10 was finally prepared from 9 by refluxing a mixture of 9, NaH, and MeI in 78% yield. As expected, a singlet at 1.54 ppm corresponding to the CH3 group was found in the 1H NMR spectrum. The carboxylic acids 14 and 15 were obtained by treating the corresponding methyl esters 10 and 9 with 10 equiv of aqueous NaOH solution (yields of 80-88%), respectively. Use of >30 equiv of NaOH or extending the reaction time to more than 1.5 h produced undesired side products (11 or 12 and ferrocenecarboxylic acid; Scheme 1) together with the desired carboxylic acids 14 and 15, respectively. A complete hydroxide-promoted cleavage of the 1,3ferrocenyl diketone functionality was possible by using >70 equiv of NaOH and stirring the reaction mixture overnight at room temperature (Scheme 1). Alternatively, the carboxylic acids 14 and 15 could also be obtained by treating the corresponding methyl esters with 2.5 equiv of Me3SiI (yield 38-45%). The formation of 14 and 15 was confirmed by the disappearance of the COOCH3 signal in the 1H NMR spectrum of both compounds. Carboxylic acid 16 was prepared in a onestep synthesis, using the Friedel-Crafts reaction of acetylferrocene (13) with glutaric anhydride. 16 showed a sharp

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Scheme 2. Preparation of the Ferrocene Bioorganometallics 2-5a

a Reagents and conditions: (a) HATU, DIPEA, DMF/CH3CN; (b) BBr3, CHCl3; (c) aqueous NaOH, MeOH/THF; (d) HATU, DIPEA, DMF; (e) TASF, DMF; (f) HATU, DIPEA, DMF/CH3CN; (g) aqueous LiOH and THF and then 4 N HCl in dioxane. Abbreviations: TMSE = (trimethylsilyl)ethyl, MOM = methoxymethyl, HATU = 2-(1H-7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate methanaminium, DIPEA = diisopropylethylamine, TASF = tris(dimethylamino)sulfonium difluorotrimethylsilicate.

singlet at 2.38 ppm in the 1H NMR spectrum corresponding to the COCH3 group together with the other expected signals from the ferrocenoyl group. With all the ferrocene-containing carboxylic acids (14-17) in hand, we then focused on coupling them with different aromatic amines relevant for platensimycin chemistry (Scheme 2). The carboxylic acid 15 was first coupled to the protected amine 18 using standard HATU-mediated amide coupling.37 19 was isolated in 89% yield (Scheme 2). Our attempts to carry out the deprotection of 19 using BBr3, as used for the synthesis of sulfonamide analogues of platensimycin, gave only a trace of the desired compound 3 along with 20 and some undesired impurities (confirmed by TLC and ESI-MS spectrometry).37 When 20 was treated with aqueous NaOH, cleavage of the 1,3-diketone functionality was observed exclusively and, instead of 3, a mixture of 21 and ferrocenecarboxylic acid was obtained. However, the synthesis of 2 and 3 was finally achieved by using the amine

22, as utilized by Nicolaou and co-workers in their synthesis of platencin.38 Amide coupling of 22 with the carboxylic acids 14 and 15 afforded 23 and 24 in 60 and 64% yields, respectively. Deprotection of the (trimethylsilyl)ethyl ester of 23 and 24 was achieved using TASF to give the desired bioorganometallics 2 and 3, respectively. In ESI-MS (negative mode), the signals at m/z 675.89 and 661.87, corresponding to the [M - H]- species of 2 and 3, unambiguously confirmed the formation of the expected compounds. The 1H NMR spectrum showed two distinct doublets and a broad singlet for the CONH proton in the aromatic region along with all the signals from the ferrocenoyl entities between 4 and 5 ppm. Coupling of the carboxylic acids 16 and 17 with the methoxymethyl-protected amine 25 gave 26 and 27, respectively, in good yields. 26 and 27 were then deprotected using LiOH and then 4 M HCl in dioxane to obtain the desired compounds 4 and 5 respectively. As expected, the 1H NMR spectra showed

(37) McNulty, J.; Nair, J. J.; Capretta, A. Tetrahedron Lett. 2009, 50, 4087–4091.

(38) Nicolaou, K. C.; Tria, G. S.; Edmonds, D. J. Angew. Chem., Int. Ed. 2008, 47, 1780–1783.

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No inhibition of E. coli W3110 or P. aeruginosa PA01 growth was observed for any of the compounds up to 180 μg/mL. Interestingly, however, among all the compounds tested, only 3 selectively inhibits the Gram-positive S. aureus Mu50 (VISA) growth at a MIC of 128 μg/mL.

Conclusion

Figure 2. ORTEP plot of methyl ester 10 (ellipsoids drawn at the 50% probability level, hydrogen atoms omitted for clarity). Selected bond lengths (A˚) and angles (deg): Fe(1)-centroid Cp(C1-5)=1.638, Fe(1)-centroid Cp(C6-10)=1.650, Fe(2)centroid Cp(C12-16)=1.635, Fe(2)-centroid Cp(C17-21) = 1.644, average Fe-C Cp’s 2.034(7), Fe(1)-Fe(2) = 6.187; C(2)-C(11)-C(23) = 119.2(5), C(12)-C(22)-C(23) = 119.7(5), C(11)-C(23)-C(22) = 110.8(5).

two distinct doublets and a broad singlet for the CONH proton in the aromatic region together with all the signals from the ferrocenoyl entity. X-ray Crystal Structures of 9, 10, and 14-16. Single crystals of 9, 10, and 14-16 suitable for X-ray diffraction were grown by slow diffusion of pentane into a diethyl ether solution of the respective compounds. The ORTEP plot of 10 is shown in Figure 2 (see Figures S1-S4 in the Supporting Information for the ORTEP plots of 9 and 14-16). Both compounds 10 and 16 crystallized in the monoclinic space group P21/n, while compounds 9 and 14 crystallized in the triclinic space group P1 and compound 15 in the monoclinic space group P21/c. The asymmetric units of 9 and 14 contain two crystallographically independent molecules. As expected for carboxylic acids, 14 forms head-to-head dimers in the solid state through hydrogen bonding interaction between the carboxylic acid groups (Figure S5 in the Supporting Information). Interestingly, compound 15 forms dimeric units through intermolecular hydrogen-bonding interactions between the carboxylic acid and the keto carbonyl group (Figure S5). In the crystal lattice of 16, the molecules self-organize in a two-dimensional assembly through water molecule mediated H-bonding interactions (Figure S6 in the Supporting Information). Selected bond distances and angles, crystallographic data, and parameters for all the five compounds are presented in Tables S1 and S2 in the Supporting Information. The Fe-C distances for all compounds fall in the same range between 1.63 and 1.65 A˚. Antimicrobial Activity. The antimicrobial activities of the new ferrocene-containing bioorganometallics 2-5 and their corresponding carboxylic acid intermediates 14-17 were tested against various bacterial strains, including Grampositive methicilline (MRSA) or vancomycin (VISA) resistant strains (Staphylococcus aureus COL (MRSA), S. aureus Mu50 (VISA), S. aureus ATCC43300 (MRSA), Bacillus subtilis 168) and Gram-negative (E. coli W3110 and Pseudomonas aeruginosa PA01) bacterial strains up to 180 μg/mL.

In summary, we have synthesized and characterized four new ferrocene-containg bioorganometallics (2-5) inspired by the antibiotic platensimycin lead structure. Compounds 2-5 were found to be inactive against Gram-negative E. coli and P. aeruginosa PA01. However, among all the new compounds tested, compound 3, bearing a ferrocenoyl 1,3-diketone functionality, inhibits S. aureus Mu50(VISA) growth selectively at a MIC value of 128 μg/mL. Although the antibacterial activity is moderate, this result is encouraging, as it demonstrates some activity of an organometallic compound against a (partly) resistant strain. Multistep syntheses of novel bioorganometallics based on a natural product lead structure are highly demanding, because of the sometimes limited stability toward various reagents and reaction conditions of organometallic compounds compared to purely organic compounds. In this report, we have discussed the problems related to OH--induced cleavage of the 1,3-ferrocenoyl diketone functionality during the attempted synthesis of 3 from 19 which was resolved using an alternative synthetic route that avoids the basic saponification of the carboxylic ester functionality and the desired compound 3 was obtained in good yield. These efficient multistep synthetic strategies reported herein can potentially be helpful for the synthesis of other 1,3-ferrocenoyl diketone containing organometallics based on natural product lead structures. We are currently planning to use this multistep synthetic route to expand our library of metal-containing bioorganometallics based on the platensimycin lead structure for structure-activity relationship (SAR) studies. These results will be published in due course.

Experimental Section Materials. All chemicals were of reagent grade quality or better, were obtained from commercial suppliers, and were used without further purification. Solvents were used as received or dried over molecular sieves. All preparations were carried out using standard Schlenk techniques. Instrumentation and Methods. 1H and 13C NMR spectra were recorded in deuterated solvents on Bruker DRX 200, 250, 400, and 600 spectrometers at 30 °C. The chemical shifts, δ, are reported in ppm (parts per million). The residual solvent peaks have been used as an internal reference. The abbreviations for the peak multiplicities are as follows: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quartet), m (multiplet), and br (broad). For the compounds that exist as a mixture of keto and enol forms in solution, in the 1H and 13C NMR spectra the signals from the minor and the major isomers are abbreviated as “min” and “maj”, respectively. Infrared spectra were recorded on an ATR unit using a Bruker Tensor 27 FTIR spectrophotometer at 4 cm-1 resolution. The signal intensity is abbreviated br (broad), s (strong), m (medium), and w (weak). ESI mass spectra were recorded on a Bruker Esquire 6000. Melting points were determined with a Buchi apparatus. Elemental microanalyses were performed using a Fisons Carlo Erba EA1108 instrument (CHNS version). High-resolution mass spectra (HRMS) were measured using a LTQ Orbitrap XL hybrid FTMS (ionization, ESI (4 kV at 3 μL/min flow rate); mass accuracy, < 10 ppm; resolution, 60 000). Crystallographic data for 9, 10, and 14-16 were collected using a Bruker-AXS SMART 1000 CCD diffractometer. The structures were solved

Article by direct methods (SHELXS-9739) and refined against F2 with all measured reflections (SHELXL-97,39 Platon-Squeeze40). The crystal structures are deposited in the Cambridge Crystallographic Data Centre (CCDC) and the CCDC numbers are 767283 (compound 9), 767281 (compound 10), 770379 (compound 14), 767280 (compound 15), and 767282 (compound 16). These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif. Minimum Inhibitory Concentration (MIC) Determination. Minimun inhibitory concentrations (MICs) against all bacterial strains were determined in a microtiter plate assay containing 0.2 mL of Luria Broth medium and appropriate compound concentrations up to 180 μg/mL. The tubes were inoculated with 105 cells/mL and incubated at 37 °C for 18 h. The MIC was defined as the lowest concentration that inhibited visible growth. Synthesis. 1, 3-Diferrocenyl-1, 3-propanedione (7).35,36 Potassium tert-butoxide (KOtBu) (614 mg, 5.48 mmol) in 10 mL of N,N-dimethylformamide (DMF) was heated to 50 °C under a nitrogen atmosphere. Acetylferrocene (13; 500 mg, 2.19 mmol) in 2 mL of DMF was added dropwise. After 5 min methyl ferrocenecarboxylate (6; 802 mg, 3.28 mmol) in 2 mL of DMF was added slowly and the progress of the reaction was monitored by TLC. After 1.5 h 100 mL of ethyl acetate was added to the reaction mixture, and this mixture was washed with 1 M HCl (2  50 mL), water (2  50 mL), and brine (1  50 mL). The organic layer was dried over anhydrous Na2SO4 and filtered, and the solvent was evaporated. The residue was chromatographed on silica using a 4:1 hexane and EtOAc mixture to give 820 mg of the desired product 7 as a deep red solid (yield 85%). Compound 8. A mixture of 7 (200 mg, 0.45 mmol), K2CO3 (249 mg, 1.8 mmol), tBu4NBr (725 mg, 2.25 mmol), and MeI (2.6 g, 18 mmol) in 50 mL of toluene or acetonitrile was heated to 50 °C for 25 h. The reaction mixture was then filtered, and 50 mL of Et2O was added to the filtrate. The organic phase was washed with water (2  50 mL) and brine (1  50 mL), dried over anhydrous Na2SO4, and filtered. Solvent removal followed by flash column chromatography on silica using 4/1 hexane/EtOAc yielded 8 as a deep red solid (128 mg, 62%). Mp: 126-129 °C. Rf = 0.50 (silica gel, hexanes/EtOAc 3/1). 1H NMR (200 MHz, CDCl3): δ (ppm) 1.57 (d, 3H, CH3, 3J = 7 Hz), 4.04 (s, 10H, C5H5), 4.31 (q, 1H, (CO)2CH-, 3J = 7 Hz), 4.42-4.49 (m, 4H, C5H4), 4.79-4.88 (m, 4H, C5H4). ESI-MS (positive mode): m/z (%) 453.86 (100) [M]þ, 492.80 (40) [M þ K]þ. HRMS (ESI, positive mode): m/z calcd for C24H23Fe2O2 [M þ H]þ 455.0396, found 455.0392. Anal. Calcd for C24H22Fe2O2: C, 63.48; H, 4.88. Found: C, 63.29; H, 5.12. Methyl Ester 9. To a stirred solution of 1,3-diferrocenyl-1,3propanedione (7; 250 mg, 0.56 mmol) in 50 mL of dichloromethane was added DBU (400 mg, 2.8 mmol) at room temperature under an N2 atmosphere. The mixture was then stirred for 20 min. A solution of methyl acrylate (1.2 g, 14 mmol) in 20 mL of DCM was then added dropwise and the mixture was refluxed for 30 h. The progress of the reaction was followed by TLC (silica gel, 4/1 hexane/EtOAc).The reaction mixture was evaporated, and the residue was chromatographed on silica using 4/1 hexane/EtOAc to give 205 mg of the desired product 9 as a deep red solid (yield 80%). In an alternative procedure, 7 (300 mg, 0.68 mmol) was added to a suspension of NaH (21 mg, 0.88 mmol) in 15 mL of anhydrous THF under an N2 atmosphere. Methyl 3-bromopropionate (568 g, 3.4 mmol) was added, and the mixture was stirred at reflux temperature for 45 h. A 50 mL portion of brine was then added slowly, and the solution was extracted with 50 mL of Et2O. The organic phase was dried over anhydrous Na2SO4 and filtered. Solvent removal followed by flash column (39) Sheldrick, G. M. SHELXL-97; University of G€ottingen, G€ottingen, Germany, 1997. (40) Spek, A. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, C34.

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chromatography (silica gel, hexane/EtOAc 4/1) yielded 9 (∼40%). The compound was recrystallized from an ethyl acetate/hexanes mixture (1/2). Mp: 122-125 °C. Rf = 0.39 (silica gel, hexanes/EtOAc 3/1). 1H NMR (400 MHz, CDCl3): δ (ppm) 2.45-2.50 (m, 4H, 2CH2), 3.71 (min) and 3.73 (maj) (s, 3H, C(O)OCH3, keto and enol), 4.14 (maj) and 4.17 (min) (s, 10H, C5H5, keto and enol), 4.53-4.56 (m, 5H, C5H4 and (CO)2CH-), 4.92-4.95 (maj) and 4.05 (min) (m, 4H, C5H4, keto and enol). ESI-MS (positive mode): m/z (%) 549.03 (100) [M þ Na]þ. HRMS (ESI, positive mode): m/z calcd for C27H27Fe2O4 [M þ H]þ 527.0608, found 527.0608. Anal. Calcd for C27H26Fe2O4: C, 61.63; H, 4.98. Found: C, 61.52; H, 5.02. Methyl Ester 10. To a stirred solution of 9 (800 mg, 1.52 mmol) in 40 mL of anhydrous THF, was added NaH (73 mg, 3.04 mmol) slowly under an N2 atmosphere, and the mixture was stirred for 20 min at room temperature. MeI (10.8 g, 76 mmol) was then added, and the mixture was stirred for 16 h at 50 °C. After completion of the reaction (checked by TLC, silica gel, 3/1 hexane/EtOAc), 60 mL of brine was added slowly and the product was extracted with 2  30 mL of EtOAc. The organic phase was dried over anhydrous Na2SO4 and filtered. Solvent removal followed by flash column chromatography (silica gel, hexane/EtOAc 3.5/1) yielded 10 (640 mg, 78%) as a red solid. The compound was recrystallized from diethyl ether. Mp: 143-146 °C. Rf = 0.5 (silica gel, hexanes/EtOAc 3/1). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.67 (s, 3H, CH3), 2.332.41 (m, 2H, CH2), 2.42-2.50 (m, 2H, CH2), 3.69 (maj) and 3.71 (min) (s, 3H, C(O)OCH3, rotamer), 4.09 (maj) and 4.21, 4.22 (min) (s, 10H, C5H5, rotamer), 4.40 (maj) and 4.51 (min) (s, br, 4H, C5H4, rotamer), 4.64 (maj), 4.77 (maj) and 4.79 (min), 4.80 (min) (s, br, 4H, C5H4, rotamer). ESI-MS (positive mode): m/z (%) 539.92 (100) [M]þ. HRMS (ESI, positive mode): m/z calcd for C28H28Fe2O4 [M]þ 540.0686, found 540.0686. Anal. Calcd for C28H28Fe2O4: C, 62.25; H, 5.22. Found: C, 62.19; H, 5.26. Carboxylic Acid 14. To a stirred solution of 10 (400 mg, 0.74 mmol) in 30 mL of THF was added 7.4 mL of 1 M aqueous NaOH (7.4 mmol), and the mixture was stirred at room temperature. The progress of the reaction was followed by TLC (silica gel, EtOAc). After 1.5 h, 50 mL of water and 9 mL of 1 M HCl was added to this solution. The product was then extracted using EtOAc (2  30 mL). The combined organic phase was washed with 50 mL of water and 30 mL of brine, dried over anhydrous Na2SO4, and filtered. Solvent removal followed by flash column chromatography (silica gel, EtOAc/MeOH 15/1) yielded 14 (319 mg, 82%) as a deep red solid. In an alternative procedure, to a stirred solution of 10 (108 mg, 0.20 mmol) in 7 mL of acetonitrile was added 0.06 mL of Me3SiI (0.40 mmol) under an N2 atmosphere. The mixture was then refluxed (90 °C) for 2 h. The progress of the reaction was monitored by TLC (silica gel, EtOAc). Water was added to the mixture, and the product was extracted using EtOAc (2  15 mL). The combined organic phase was washed with 40 mL of water and 20 mL of brine, dried over anhydrous Na2SO4, and filtered. Solvent removal followed by flash column chromatography (silica gel, EtOAc/MeOH 15/1) yielded 14 (45 mg, 43%). Mp: 164-167 °C. Rf=0.53 (silica gel, EtOAc/MeOH 10/1). 1H NMR (250 MHz, CDCl3): δ (ppm) 1.68 (s, 1H, CH3), 2.43-2.49 (m, 4H, 2CH2), 4.10 (s, 10H, C5H5), 4.42 (m, 4H, C5H4), 4.65 (m, 2H, C5H4), 4.79 (m, 2H, C5H4). ESI-MS (positive mode): m/z (%) 525.97 (100) [M]þ. HRMS (ESI, positive mode): m/z calcd for C27H27Fe2O4 [MþH]þ 527.0608, found 527.0610. Anal. Calcd for C27H26Fe2O4: C, 61.63; H, 4.98. Found: C, 61.98; H, 5.12. Carboxylic Acid 15. To a stirred solution of 9 (350 mg, 0.66 mmol) in 15 mL of MeOH was added 6.6 mL of 1 M aqueous NaOH (6.6 mmol). The mixture was then stirred at room temperature. The progress of the reaction was followed by TLC (silica gel, hexane/EtOAc 3/2). After 1.5 h, 50 mL of water and 8 mL of 1 M HCl were added and the product was extracted using EtOAc. The combined organic phase was washed with 50 mL of water and 30 mL of brine, dried over anhydrous

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Na2SO4, and filtered. Solvent removal followed by flash column chromatography (silica gel, EtOAc/MeOH 10/1) yielded 15 (300 mg, 88%) as a deep red solid. In an alternative procedure, to a stirred solution of 9 (100 mg, 0.19 mmol) in 7 mL of acetonitrile was added 0.09 mL of Me3SiI (0.58 mmol) under an N2 atmosphere and the mixture was refluxed at 90 °C for 2 h. The progress of the reaction was monitored by TLC (silica gel, EtOAc). Water was added to the mixture, and the product was extracted using EtOAc (2  15 mL). The combined organic phases were washed with 40 mL of water and 20 mL of brine, dried over anhydrous Na2SO4, and filtered. Solvent removal followed by flash column chromatography (silica gel, EtOAc/MeOH 15/1) yielded 15 (34 mg, 35%). Mp: 148-151 °C. Rf = 0.51 (silica gel, EtOAc). 1H NMR (400 MHz, CDCl3): δ (ppm) 2.45-2.53 (m, 4H, 2CH2), 4.05 (maj) and 4.09 (min) (keto and enol) (s, 10H, C5H5), 4.38 (m, 1H, (CO)2CH-), 4.46 (m, 4H, C5H4), 4.85 (m, 4H, C5H4). ESI-MS (negative mode): m/z (%) 510.91 (100) [M - H]-, 624.87 (20) [M þ TFA - H]-. HRMS (ESI, positive mode): m/z calcd for C26H25Fe2O4 [M þ H]þ 513.0451, found 513.0450. Anal. Calcd for C26H24Fe2O4 3 0.5H2O: C, 59.88; H, 4.83. Found: C, 60.00; H, 4.85. Carboxylic Acid 16.32 To a stirred solution of acetylferrocene (13; 500 mg, 2.19 mmol) and glutaric anhydride (250 mg, 2.19 mmol) in 50 mL of anhydrous DCM was added AlCl3 (1.4 g, 10.90 mmol) in small portions at 0-5 °C under an N2 atmosphere. The mixture was stirred overnight at room temperature. Then the reaction mixture was poured into 50 mL of ice water containing 50 mL of 1 M HCl. The organic layer was separated, and the aqueous layer was extracted with DCM (2  30 mL). The combined organic layer was washed with water (4  100 mL) and brine (1  50 mL), dried over anhydrous Na2SO4, and filtered. The solvent was evaporated to obtain 16 as a red sticky solid (590 mg, 78%).The crude product was used for the next step without further purification. The compound was recrystallized from diethyl ether. Mp: 81-84 °C. Rf = 0.46 (silica gel, pure EtOAc). 1H NMR (400 MHz, CDCl3): δ (ppm) 2.03-2.1 (m, 2H, CH2), 2.38 (s, 3H, CO-CH3) 2.55 (t, 2H, CH2, 3J = 6.91 Hz), 2.78 (t, 2H, CH2, 3J = 6.91 Hz), 4.53 (s, 4H, C5H4), 4.79 (s, 2H, C5H4), 4.82 (s, 2H, C5H4). ESI-MS (negative mode): m/z (%) 341.03 (100) [M - H]-, 683.03 (40) [2M - H]-. HRMS (ESI, positive mode): m/z calcd for C17H19FeO4 [M þ H]þ 343.0632; found 343.0629. Anal. Calcd for C17H18FeO4: C, 59.67; H, 5.30. Found: C, 59.92; H, 5.79. General Procedure for the Amide Coupling. To a stirred solution of the carboxylic acid in DMF were added HATU and DIPEA, and the mixture was stirred for 30 min under an N2 atmosphere. The amine in a small volume of DMF or acetonitrile was then added, and the mixture was stirred at room temperature until TLC showed the reaction is complete. The volume of the reaction mixture was then reduced under vacuum, brine was added, and this mixture was then extracted with ethyl acetate. The organic layer was washed with 0.5 M HCl, distilled water, and brine.The organic phase was then dried over anhydrous Na2SO4 and filtered. Removal of solvent followed by flash column chromatography afforded the desired products. Amide 19. 15 (600 mg, 1.17 mmol), 18 (495 mg, 2.34 mmol), HATU (751 mg, 2.34 mmol), DIPEA (278 mg, 2.34 mmol), DMF (15 mL), ACN (10 mL) were employed. After 70 h the reaction mixture was worked up following the procedure mentioned above. Column chromatography (silica gel, hexane/ EtOAc 1/1) yielded 19 (740 mg, 89%) as as a red sticky solid. Rf = 0.68 (silica gel, EtOAc). 1H NMR (400 MHz, CDCl3): δ (ppm) 2.54 (m, 4H, 2CH2), 3.79 (s, 3H, -OCH3), 3.81 (s, 3H, -C(O)OCH3), 3.84 (s, 3H, -OCH3), 4.11 (s, 10H, C5H5), 4.51 (m, 4H, C5H4) 4.62 (m, 1H, (CO)2CH-), 4.91-4.95 (m, 4H, C5H4), 6.71 (d, 1H, benzene ring proton), 6.98 (s, br, 1H, NH), 7.81 (d, 1H, benzene ring proton). ESI-MS (positive mode): m/z (%) 704.92 (100) [M]þ, 727.87 (49) [M þ Na]þ, 743.81 (8) [M þ K]þ. HRMS (ESI, positive mode): m/z calcd for C36H36Fe2NO7 [M þ H]þ 706.1190, found 706.1187.

Patra et al. Amide 23.38 14 (200 mg, 0.38 mmol), 22 (306 mg, 1.14 mmol), HATU (366 mg, 1.14 mmol), DIPEA (196 mg, 1.52 mmol), and DMF (6 mL) were employed. After 34 h the reaction mixture was worked up following the procedure mentioned above. Column chromatography (silica gel, hexane/EtOAc 4/1 f 3/1) yielded 23 (177 mg, 60%) as a red sticky solid. Rf = 0.44 (silica gel, Hex/EtOAc 3/1). 1H NMR (400 MHz, CDCl3): δ (ppm) 0.03 (s, 9H, Si(CH3)3), 1.04-1.08 (m, 2H, CH2CH2Si), 1.65 (s, 3H, CH3), 2.44-2.50 (m, 4H, 2CH2), 4.01 (s, 10H, 2C5H5), 4.30-4.36 (m, 6H, CH2CH2Si and C5H4), 4.55-4.59 (m, 2H, C5H4), 4.71-4.75 (m, 2H, C5H4), 6.41 (d, 1H, benzene ring proton), 7.49 (d, 1H, benzene ring proton), 8.02 (s, 1H, NH), 10.94 (s, br, 1H, OH), 11.73 (s, 1H, OH). ESI-MS (positive mode): m/z (%) 799.91 (50) [M þ Na]þ, 776.95 (100) [M]þ. HRMS (ESI, positive mode): m/z calcd for C39H43Fe2NO7Si [M]þ 777.1507, found 777.1505. Anal. Calcd for C39H43Fe2NO7Si: C, 60.24; H, 5.57; N, 1.80. Found: C, 60.28; H, 5.90; N, 1.53. Amide 24.38 15 (100 mg, 0.19 mmol), 22 (157 mg, 0.58 mmol), HATU (188 mg, 0.58 mmol), DIPEA (100 mg, 0.78 mmol), and DMF (3 mL) were employed. After 34 h the reaction mixture was worked up following the procedure mentioned above. Column chromatography (silica gel, hexane/EtOAc 5/1 f 4/1) yielded 24 (95 mg, 64%) as a red sticky solid. Rf = 0.31 (silica gel, Hex/EtOAc 2/1). 1H NMR (400 MHz, CDCl3): δ (ppm) (keto and enol isomeric mixture) 0.09 (keto and enol) (s, 9H, Si(CH3)3), 0.80-0.95 (min) and 1.08-1.20 (maj) (keto and enol) (m, 2H, CH2CH2Si), 1.27 (enol) (s, 1H, OH), 2.11-2.27 (min) and 2.53-2.76 (maj) and 2.82-2.95 (min) (keto and enol) (m, 4H, 2CH2), 4.15 (maj) and 4.20 (min) (keto and enol) (s, 10H, 2C5H5), 4.35-4.47 (keto and enol) (m, 2H, CH2CH2Si), 4.54.62 (keto and enol) (m, 5H, C5H4 and (CO)2CH), 4.80-4.84 (min) and 4.92-4.99 (maj) (keto and enol) (m, 4H, C5H4), 6.54 (keto and enol) (d, 1H, benzene ring proton), 7.57 (keto and enol) (d, 1H, benzene ring proton), 8.01 (maj) and 8.10 (min) (keto and enol) (s, 1H, NH), 11.08 (keto and enol) (s, br, 1H, OH), 11.82 (keto and enol) (s, 1H, OH). ESI-MS (positive mode): m/z (%) 762.99 (10) [M]þ, 346.14 (25) [M - (SiMe3) þ 2H]2þ, 310.18 (100) [M - (Me3SiCH2CH2CO2) þ 2H]2þ. HRMS (ESI, positive mode): m/z calcd for C38H42Fe2NO7Si [M þ H]þ 664.1429; found 664.1430. Anal. Calcd for C38H41Fe2NO7Si: C, 59.78; H, 5.41; N, 1.83. Found: C, 60.00; H, 5.91; N, 2.32. Amide 26. 16 (2 g, 5.84 mmol), 25 (2.35 g, 8.7 mmol), HATU (4.4 g, 11.6 mmol), DIPEA (1.5 g, 11.6 mmol), DMF (15 mL), and ACN (20 mL) were employed. After 60 h the reaction mixture was worked up following the procedure mentioned above. Column chromatography (silica gel, hexane/EtOAc 1/1.5 f 0/1) yielded 26 (1.67 g, 65%) as as a red sticky solid. Rf = 0.29 (silica gel, pure EtOAc). 1H NMR (400 MHz, CDCl3): δ (ppm) 2.0-2.18 (m, 2H, CH2), 2.35 (s, 3H, COCH3), 2.432.66 (m, 2H, CH2), 2.73-2.89 (m, 2H, CH2), 3.51 (s, 3H, OCH3), 3.58 (s, 3H, OCH3), 3.87 (s, 3H, COCH3), 4.49-4.51 (m, 4H, C5H4), 4.77 (s, br, 2H, C5H4), 4.83 (s, 2H, br, C5H4), 5.09 (s, 2H, OCH2), 5.25 (s, 2H, OCH2), 7.03 (d, 1H, benzene ring proton), 7.45-7.61 (s, br, 1H, NH), 7.80 (d, 1H, benzene ring proton). ESI-MS (positive mode): m/z (%) 618.14 (100) [M þ Na]þ. HRMS (ESI, positive mode): m/z calcd for C29H34FeNO9 [M þ H]þ 596.1583, found 596.1583. Amide 27. 17 (1.3 g, 4.33 mmol), 25 (2.35 g, 8.66 mmol), HATU (3.3 g, 8.66 mmol), DIPEA (1.12 g, 8.66 mmol), DMF (15 mL), and ACN (20 mL) were employed. After 60 h the reaction mixture was worked up following the procedure mentioned above. Column chromatography (silica gel, hexane/ EtOAc 1/1.5 f 0/1) yielded 27 (1.71 g, 73%) as as a red sticky solid. Rf = 0.24 (silica gel, hexanes/EtOAc 1/1). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.94-2.10 (m, 2H, CH2), 2.33-2.57 (m, 2H, CH2), 2.70-2.89 (m, 2H, CH2), 3.37 (min) and 3.38 (maj) (rotamers, s, 3H, OCH3), 3.46 (s, 3H, OCH3), 3.76 (maj) and 3.81 (min) (rotamers, 3H, COCH3), 4.10 (s, 5H, C5H5), 4.39 (s, br, 2H, C5H4), 4.71 (s, 2H, br, C5H4), 4.98 (s, 2H, OCH2), 5.12 (s, 2H, OCH2), 6.62 (min) and 6.92 (maj) (rotamers, d, 1H,

Article benzene ring proton), 7.45-7.58 (s, br, 1H, NH), 7.62 (min) and 7.70 (maj) (rotamers, d, 1H, benzene ring proton). ESI-MS (positive mode): m/z (%) 553.20 (100) [M]þ, 576.18 (25) [M þ Na]þ. HRMS (ESI, positive mode): m/z calcd for C27H31FeNO8 [M]þ 553.1399, found 553.1386. Compound 2.38 To a stirred solution of amide 23 (150 mg, 0.19 mmol) in DMF (3 mL) was added tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF; 107 mg, 0.39 mmol) at room temperature and the mixture was heated at 40 °C for 40 min. The reaction mixture was then cooled to room temperature and brine was added to it with EtOAc (3  40 mL). The combined organic phase was dried over Na2SO4, filtered, and concentrated. Flash column chromatography of the resulting residue (EtOAc/MeOH/AcOH 20/1/0.05) gave compound 2 (126 mg, 96%) as an orange powder. Mp: 180-183 °C. Rf = 0.30 (silica gel, EtOAc/MeOH/AcOH 20/1/0.05). 1H NMR (400 MHz, DMSO-d6): δ (ppm) 1.65 (s, 3H, CH3) 2.37-2.42 (m, 4H, 2CH2), 4.15 (s, 10H, C5H5), 4.46-4.49 (m, 4H, C5H4), 4.504.52 (m, 2H, C5H4), 4.60-4.62 (m, 2H, C5H4), 6.42 (d, 1H, benzene ring proton), 7.57 (d, 1H, benzene ring proton), 9.02 (s, 1H, NH), 10.14 (s, br, 1H, OH). ESI-MS (negative mode): m/z (%) 675.89 (100) [M - H]-. HRMS (ESI, positive mode): m/z calcd for C34H31Fe2NO7 [M]þ 677.0798, found 677.0798. Anal. Calcd for C34H31Fe2NO7 3 1.5H2O: C, 57.93; H, 4.87; N, 1.99. Found: C, 58.09; H, 5.18; N, 1.94. Compound 3.38 To a stirred solution of amide 24 (80 mg, 0.10 mmol) in DMF (1.5 mL) was added tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF; 55 mg, 0.21 mmol) at room temperature, and the mixture was heated at 40 °C for 40 min. The solution was then cooled to room temperature and brine was added to it. The mixture was extracted with EtOAc (3  25 mL), and the combined organic phase was dried over Na2SO4, filtered, and concentrated. Flash column chromatography of the resulting residue (EtOAc/MeOH/AcOH 20/1/0.05) gave compound 3 (61 mg, 89%) as an orange powder. Mp: 134-137 °C. Rf = 0.30 (silica gel, EtOAc/MeOH/AcOH 20/ 1/0.0.1). 1H NMR (400 MHz, DMSO-d6): δ (ppm) 2.30-2.42 (m, 2H, CH2), 2.48-2.58 (m, 2H, CH2) (overlaps with DMSOd6), 4.22 (s, 10H, C5H5), 4.56-4.58 (min) and 4.62-4.68 (maj) (m, 5H, C5H4 and (CO)2CH-, keto and enol), 4.80-4.83 (min) and 4.86-4.92 (maj) (m, 4H, C5H4, keto and enol), 6.42 (d, 1H, benzene ring proton), 7.56 (d, 1H, benzene ring proton), 9.09 (min) and 9.15 (maj) (s, 1H, NH, keto and enol), 10.27 (s, br, 1H, OH). ESI-MS (negative mode): m/z (%) 661.87 (100) [M - H]-. HRMS (ESI, positive mode) m/z calcd for C33H30Fe2NO7 [M þ H]þ 664.0727, found 664.0721. Anal. Calcd for C33H29Fe2NO7 3 H2O: C, 58.18; H, 4.59; N, 2.06. Found: C, 58.28; H, 5.12; N 2.22. Compound 4. To a stirred solution of 8 (1 g, 1.68 mmol) in 40 mL of a THF/H2O mixture (THF/H2O 4/1) was added LiOH 3 H2O (3.78 g, 92.43 mmol) under an argon atmosphere. The mixture was heated at 45 °C for 20 h (TLC shows complete ester hydrolysis). The reaction mixture was then evaporated to dryness under vacuum. Degassed 4 N HCl in dioxane was then added slowly to adjust the pH of the reaction mixture to near zero. The mixture was then stirred at room temperature. After 15 min the reaction was complete (checked by TLC) and 50 mL of brine was added to the mixture, which was then extracted with EtOAc (3  30 mL). The combined organic phases were washed with distilled water (5  50 mL) and brine (2  25 mL), dried over anhydrous Na2SO4, and filtered. Removal of the solvent gave a red sticky solid. Flash column chromatography (silica gel, EtOAc/MeOH/AcOH 1/0/0 f 20/1/0.2) was done. The volume of the solvent was reduced to about 5 mL and pentane was added to it; the compound precipitated, and the pentane was drained off to yield compound 4 as an orange powder (500 mg, 60%). Mp: 57-60 °C. Rf =0.28 (silica gel, EtOAc/MeOH/AcOH 20/1/0.2). 1 H NMR (400 MHz, CDCl3): δ (ppm) 2.10 (s, br, 2H, CH2), 2.29 (s, 3H, CH3), 2.65 (s, br, 2H, CH2), 2.75 (s, br, 2H, CH2), 4.46 (s, br, 4H, C5H4), 4.72 (s, br, 2H, C5H4), 4.76 (s, br, 2H, C5H4), 6.48

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(d, 1H, benzene ring proton), 7.57 (d, 1H, benzene ring proton), 8.15 (s, 1H, NH), 11.12 (s, br, 1H, OH), 11.57 (s, br, 1H, OH). ESI-MS (negative mode): m/z (%) 492.02 (100) [M - H]-. HRMS (ESI, positive mode): m/z calcd for C24H24FeNO7 [M þ H]þ 494.0904, found 494.0904. Anal. Calcd for C24H23FeNO7 3 2H2O: C, 54.46; H, 5.14; N, 2.65. Found: C, 53.97; H, 5.22; N, 2.46. Compound 5. To a stirred solution of 9 (200 mg, 0.36 mmol) in 20 mL of a THF/H2O mixture (THF/H2O 4/1) was added LiOH 3 H2O (811 mg, 19.8 mmol) under an argon atmosphere. The mixture was heated at 45 °C for 20 h (TLC shows complete ester hydrolysis). Then the reaction mixture was evaporated to dryness under vacuum and degassed 4 N HCl in dioxane was added to make the pH of the reaction mixture near zero and this mixture was stirred at room temperature. After 20 min the reaction was complete (checked by TLC). A 50 mL portion of brine was then added, and the mixture was extracted with EtOAc (3  30 mL). The combined organic phase was washed with distilled water (5  50 mL) and brine (2  25 mL), dried on anhydrous Na2SO4, and filtered. Removal of the solvent gave a red sticky solid. Flash column chromatography (silica gel, EtOAc/MeOH/AcOH 1/0/0 f 20/1/0.1) was done. The volume of the solvent was reduced to about 3 mL, and pentane was added to it; the compound precipitated, and the pentane was drained off to yield compound 5 as an orange powder (138.6 mg, 85%). Mp: 143-146 °C. Rf = 0.37 (silica gel, EtOAc/MeOH/ AcOH 20/1/0.1). 1H NMR (400 MHz, pyridine-d5): δ (ppm) 2.35-2.48 (m, 2H, CH2), 2.85-2.98 (m, 2H, CH2), 3.02-3.15 (m, 2H, CH2), 4.18 (s, 5H, C5H5), 4.45 (s, br, 2H, C5H4), 4.95 (s, 2H, br, C5H4), 6.94 (d, 1H, benzene ring proton), 8.12 (d, 1H, benzene ring proton), 10.61 (s, 1H, OH), 10.75-11.34 (s, br, 1H, NH). ESI-MS (negative mode): m/z (%) 449.98 (100) [M - H]-. HRMS (ESI, positive mode): m/z calcd for C22H21FeNO6 [M]þ 451.0718, found 451.0717. Anal. Calcd for C22H21FeNO6: C, 58.56; H, 4.69; N, 3.10. Found: C, 58.33; H, 5.17; N, 3.04.

Abbreviations TMSE,(trimethylsilyl)ethyl; MOM, methoxymethyl; HATU, 2-(1H-7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate methanaminium; DIPEA, diisopropylethylamine, TASF, tris(dimethylamino)sulfonium difluorotrimethylsilicate; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; Fc, ferrocene; MIC, minimum inhibitory concentration; Cp, cyclopentadienyl.

Acknowledgment. We wish to acknowledge the International Max Planck Research School for Chemical Biology (fellowship to M.P.), the Alexander von Humboldt Foundation (fellowship to G.G.), the Swiss National Science Foundation (Ambizione Grant PZ00P2_126404 to G.G.), and the DFG-Deutsche Forschungsgemeinschaft (FOR 630) for financial support of this project. Support from the Research Department Interfacial System Chemistry (J.E.B. and N.M.-N.) is also gratefully acknowledged. G.G. thanks Prof. Roger Alberto for generous access to all the facilities of the Institute of Inorganic Chemistry of the University of Zurich. We thank Dr. Dirk Wolters and Benjamin Fraenzel from Ruhr-University Bochum for the HRMS measurements. Supporting Information Available: ORTEP plots of 9 and 14-16 (Figures S1-S6), selected bond distances, angles, crystallographic data, and parameters for 9, 10, and 14-16 (Tables S1 and S2), text and figures giving 13C NMR and IR data and 1H and 13C NMR spectra of all new compounds, and CIF files giving X-ray crystallographic data for compounds 9, 10, and 14-16. This material is available free of charge via the Internet at http://pubs.acs.org.