CHEMMEDCHEM COMMUNICATIONS DOI: 10.1002/cmdc.201402129

The Impact of Cyclopropane Configuration on the Biological Activity of Cyclopropyl-Epothilones Fabienne Z. Gaugaz,[a] Mariano Redondo-Horcajo,[b] Isabel Barasoain,[b] J. Fernando Daz,[b] Amanda Cobos-Correa,[c] Markus Kaufmann,[c] and Karl-Heinz Altmann*[a] Two cis-12,13-cyclopropyl-epothilone B variants have been synthesized, differing only in the configuration of the stereocenters at C12 and C13. The syntheses were based on a common allylic alcohol intermediate that was converted into the corresponding diastereomeric hydroxymethyl-cyclopropanes by means of stereoselective Charette cyclopropanations. Macrocyclizations were accomplished through ring-closing metathesis (RCM). Substantial differences between the two compounds were found with regard to microtubule binding affinity, antiproliferative activity and their effects on the cellular microtubule network. While the analogue with the cyclopropane moiety oriented in a corresponding way to the epoxide configuration in natural epothilones was almost equipotent with epothilone A, the other was significantly less active. Based on these findings, natural epothilone-like activity of cis-fused 12,13-cyclopropyl-epothilone analogues is tightly linked to the natural orientation of the cyclopropane moiety.

Epothilones A and B (Epo A and B; 1 and 2) are natural products of myxobacterial origin with potent in vitro and in vivo antitumor activity.[1] Mechanistically, the inhibition of cancer cell growth by epothilones is based on the suppression of cellular microtubule (MT) dynamics, which leads to cell-cycle arrest in G2/M and the induction of apoptosis.[2] Epo B (2) and several of its analogues have been advanced into clinical trials in humans, and one of these compounds, the Epo B lactam ixabepilone (also known as BMS-247550), was approved by the US Food and Drug Administration (FDA) for breast cancer treatment in 2007 (under the trade name Ixempra).[3] The structure–activity relationships (SAR) of epothilones has been studied extensively, including the effects of changes in the configurations of the various chiral centers.[1] While these studies have firmly established that the epoxide moiety as such is dispensable for potent biological activity, rather surpris-

ingly, a firm assessment of the activity of analogues with inverted cis-epoxide (or -cyclopropane) geometry (i.e., analogues with inverted stereocenters at both C12 and C13) is still lacking.[4] Only limited scattered data on such analogues are available in the literature,[5–8] and these are not fully consistent. For example, Nicolaou and co-workers reported that 12,13-epi-oxazole-Epo B (4) inhibits the growth of ovarian carcinoma cell line 1A9 with only sevenfold lower potency than the corresponding “natural” 12R,13S isomer (IC50 = 1.5 nm and 0.2 nm, respectively); likewise, the tubulin-polymerizing activity of the two isomers appeared to be similar.[5] In contrast, the activity of 12,13-epi-9,10-dehydro-Epo B (5) against the human leukemia cell line CCRF-CEM was found by Danishefsky, Taylor and co-workers to be ca. 60-fold reduced compared with 9,10-dehydro-Epo B (IC50 = 13.4 nm and 0.23 nm, respectively).[6] No data were reported for the tubulin-polymerizing activity of these compounds. Lastly, our own work on side-chain-modified epothilone analogues revealed 12S,13R analogue 6 to be more than 600-fold less potent against human KB-31 cells than its 12R,13S counterpart (IC50 = 82 nm for 6 vs. 0.13 nm for benzothiazole-Epo B); at the same time, compound 6 showed no effect on tubulin polymerization under our standard assay conditions.[7] This latter observation raised the possibility of a tubulin-independent mechanism of cell growth inhibition underlying the still substantial inhibitory activity of 6.

[a] Dr. F. Z. Gaugaz, Prof. K.-H. Altmann ETH Zrich, Department of Chemistry & Applied Biosciences Institute of Pharmaceutical Sciences, HCI H405 Vladimir-Prelog-Weg 4, 8093 Zrich (Switzerland) E-mail: [email protected] [b] Dr. M. Redondo-Horcajo, Dr. I. Barasoain, Dr. J. F. Daz Centro de Investigaciones Biolgicas Consejo Superior de Investigaciones Cientficas Ramiro de Maeztu 9, 28040 Madrid (Spain) [c] Dr. A. Cobos-Correa, M. Kaufmann Novartis Institute for Biomedical Research, 4056 Basel (Switzerland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cmdc.201402129.

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We have recently been interested in potent analogues of cyclopropyl-Epo B (3) with functionalized C15 side chains as potential drug payloads for antibody–drug conjugates (ADC).[9] In ChemMedChem 2014, 9, 2227 – 2232

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CHEMMEDCHEM COMMUNICATIONS the context of this work, we have also performed a systematic study on the biological impact of the configuration of the C12 and C13 stereocenters in cis-fused bicylic epothilones. Specifically, we have prepared isomeric epothilone analogues 7 a and 7 b, and we have assessed the in vitro antiproliferative activities of these compounds across a panel of different human cancer cell lines, their microtubule binding affinity, and their effects on the cellular microtubule network.

www.chemmedchem.org with diisobutylaluminumhydride (DIBALH) and subsequent oxidation of the resulting alcohol with manganese dioxide gave aldehyde 10 in 67 % overall yield from 8. Aldehyde 10 underwent asymmetric Brown allylation with ()-allyldiisopinocampheylborane [()-allylB(Ipc)2]—prepared in situ from ()(Ipc)2BCl and allylmagnesium bromide[14]—to furnish homoallylic alcohol 11 in good yield (80 %) and with acceptable ee (83 %). Silylation of 11 followed by oxidative cleavage of the terminal double bond by Sharpless dihydroxylation/sodium periodate cleavage gave aldehyde 12 in 89 % overall yield. The latter was highly prone to elimination, even at lower temperatures, and thus was reacted with the stabilized ylide 13,[15] immediately after purification, to provide ester 14 in excellent yield (97 %). Subsequent reduction of the ester moiety gave the corresponding allylic alcohol; unfortunately, however, this compound did not undergo regioselective cyclopropanation of the trisubstituted double bond. In order to avoid the regioselectivity problem in the cyclopropanation step, 14 was submitted to hydroboration with 9borabicyclo(3.3.1)nonane (9-BBN), the resulting primary hydroxy group was protected as a tert-butyldimethylsilyl (TBS) ether, and the ester moiety was reduced with DIBALH to furnish the desired allylic alcohol 15 (82 % based on 14) as the common precursor for both target structures 7 a and 7 b. Charette cyclopropanation of 15 with ligand 16[12] gave the naturally configured intermediate 18 a in 78 % yield as a 9:1 mixture of diastereoisomers (based on 1H NMR spectroscopy)

While 7 a represents a highly modified variant of Epo B (2), it is important to note that each of the three Epo B analogues that incorporate only one of the three modifications present in 7 a simultaneously (i.e., 9,10-dehydro-Epo B,[6] CP-Epo B (3),[9, 10] and the 12,13-epoxide analogue of 7 a)[11] has been shown to inhibit human cancer cell growth with virtually the same potency as Epo B (2). Thus, we considered it highly likely that any differences between 7 a and 7 b would also be reflective of the differences in biochemical and cellular effects between Epo B (2) and its 12S,13R isomer. The synthesis of epothilone analogues 7 a and 7 b is summarized in Schemes 1 and 2 and was based on the stereoselective establishment of the cyclopropane moieties by asymmetric Charette cyclopropanation[12] of allylic alcohol 15 (Scheme 1 and 2) and macrocyclic ring closure by ring-closing metathesis (RCM)[13] of dienes 21 (Scheme 2) as the key transformations. Allylic alcohol 15 was obtained starting from commercially available 3-nitro-4-fluorobenzoic acid (8), which was first converted into ester 9 by acid-catalyzed esterification with methanol, followed by fluoride displacement with ethanolamine Scheme 1. Reagents and conditions: a) SOCl2, MeOH, 0 8C!RT, 17 h, quant.; b) ethanolamine, NEt3, CH2Cl2, RT, 24 h, and subsequent nitro group requant.; c) Pd/C, H2, MeOH, RT, 20 h, quant.; d) triethyl orthoacetate, sulfamic acid, EtOH, reflux, 72 h, quant.; duction (Scheme 1).[11] Reaction e) TBDPSCl, imidazole, DMF, RT, 24 h, 83 %; f) DIBALH, CH2Cl2, 60 8C!RT, 4.5 h, 82 %; g) MnO2, CHCl3, reflux, 1 h, of 9 with triethyl orthoacetate, 98 %; h) (1) ()-Ipc2Cl, allyl-MgBr, Et2O, 0 8C!10 8C, 1 h; (2) 10, Et2O, 100 8C, 3.5 h, 80 % (ee = 83 %); i) TBSOTf, to form the imidazole ring, fol- imidazole, CH2Cl2, 0 8C!RT, 7 h, 96 %; j) (1) AD-mix b, methanesulfonamide, tBuOH/H2O (1:1), RT, 28 h; (2) NaIO4, CH2Cl2/H2O (4:1), RT, 2 h, 93 %; k) 13, benzene, reflux, 16 h, 97 %; l) (1) 9-BBN, THF, RT, 1.5 h; (2) H2O2, aq. NaOH, lowed by tert-butyldiphenylsilyl RT, 1 h, 93 %; m) TBSCl, imidazole, CH2Cl2, RT, 72 h, 93 %; n) DIBALH, CH2Cl2, 78 8C!RT, 45 min, 95 %. Abbrevia(TBDPS) protection of the free tions: DIBALH = diisobutylaluminumhydride; DMF = dimethylformamide; Ipc2Cl = chlorodiisopinocampheylborane; hydroxy group, ester reduction TBS = tert-butyldimethylsilyl; TBDPS = tert-butyldiphenylsilyl.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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CHEMMEDCHEM COMMUNICATIONS (Scheme 2);[16] the selectivity of the cyclopropanation reaction was somewhat less satisfactory in the presence of ligand 17, which gave a 83:17 mixture of diastereoisomers in 67 % yield.[16] Neither for 18 a nor for 18 b were the isomeric mixtures separable by flash column chromatography, and this was also the case for all subsequent intermediates. Scheme 2 summarizes the elaboration of cyclopropane 18 a into the desired epothilone analogue 7 a. Thus, the hydroxymethyl group in 18 a was transformed into the required methyl group by iodide formation and subsequent reduction with sodium boro-

www.chemmedchem.org hydride to furnish 19 a in 67 % overall yield. This was followed by cleavage of the primary TBS ether moiety and Grieco– Sharpless olefination,[17] which gave olefin 20 a (55 % yield based on 19 a). After treatment of 20 a with camphorsulfonic acid (CSA) in methanol/dichloromethane, the resulting free alcohol was esterified with acid 22[18] under Yamaguchi conditions[19] to provide diene 21 a as the precursor for the crucial RCM-mediated ring closure in good yield (78 %). RCM in the presence of Grubbs II catalyst[13] proceeded smoothly and furnished the desired macrolactone in 57 % yield and essentially as a single double bond isomer.[20] Finally, sequential treatment of the protected macrolactone with tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) and hydrogen fluoride·pyridine produced 7 a. Isomer 7 b was prepared from cyclopropane 18 b by the same sequence of steps as for the synthesis of 7 a from 18 a. Both 7 a and 7 b were obtained as chromatographically pure isomers after HPLC purification of the material produced in the final deprotection step (14 % and 5 % yield, respectively). The microtubule binding affinity of epothilone analogues 7 a and 7 b was determined in a previously established fluorescence-based displacement assay[21] that measures displacement of the fluorescent taxol derivative Flutax-2 from the taxol binding site on b-tubulin (for details, see the Supporting Information). At 37 8C, Ka values of 3.71  108 m1 and 6  105 m1 were determined for 7 a and 7 b, respectively (Figure S1 and Table S1 in the Supporting Information), which makes 7 b a ca. 600-fold weaker microtubule binder than 7 a. At the same time, however, these experiments clearly established specific microtubule binding by 7 b. The critical tubulin concentrations in the presence of an excess of 7 a or 7 b were 0.30 and 1.45 mm, respectively. Thus, in line with the microtubule binding data, 7 a is a stronger inducer of tubulin polymerization than 7 b. In comparison, the critical tubulin concentrations for the negative (DMSO) and positive (docetaxel) controls were 3.30 and 0.33 mm, respectively; the critical tubulin concentrations for Epo A (1) and Epo B (2) under identical conditions have been previously reported as 0.46 mm (1) and 0.21 mm (2).[22]

Table 1. Antiproliferative activity of 7 a, 7 b and Epo A (1).[a]

7a

IC50 [nm] 7b

Epo A

SI[b] (7 b/7 a)

5.41  0.40 6.66  1.51 5.94  0.30 14.74  1.99 2.76  0.71 4.30  0.30 7.88  0.65 5.41  0.40 6.66  1.51 5.61  0.44

179  0.02 594  72 181  12 3195  454 543  40 6083  1031 302  54 179  0.02 594  72 745  51

7.35  0.24 8.58  1.32 8.30  0.28 5.26  0.26 6.54  0.62 11.04  0.68 8.44  0.69 6.88  0.75 9.71 1.08 16.58  2.50

33 89 133 334 30 216 197 1413 38 114

Cell line

Scheme 2. Reagents and conditions: a) (1) Et2Zn, CH2I2, CH2Cl2, 0 8C, 10 min; (2) 15, 16 (or 17), CH2Cl2, 0 8C!RT, 1 h, 18 a: 78 % (de = 80 %)/18 b: 67 % (de = 67 %); b) I2, PPh3, imidazole, CH2Cl2, 0 8C!RT, 2.5 h, 95 % (87 %); c) NaBH4, MeCN, 80 8C, 1 h, 71 % (58 %); d)CSA, CH2Cl2/MeOH (1:1), RT, 75 min, 93 % (82 %); e) (1) o-NO2PhSeCN, PBu3, THF, 30 8C, 3 h; (2) H2O2, NaHCO3, 50 8C, 1.5 h, 59 % (49 %); f) CSA, CH2Cl2/MeOH (1:1), RT, 17 h, 90 % (95 %); g) (1) NEt3, 2,4,6-trichlorobenzoyl chloride, 22, toluene, RT, 1 h; (2) DMAP, 20 a (or 20 b), toluene, RT, 17 h, quant. (72 %); h) Grubbs II, toluene, reflux, 4.5 h, 57 % (72 %); i) (1) TASF, DMF, 0 8C!RT, 48 h; (2) HF·pyridine, MeCN, RT, 16 h , 14 % after RP-HPLC purification (5 %). Yields in parentheses refer to those obtained in the synthesis of 7 b. Abbreviations: CSA = camphorsulfonic acid; DMAP = N,N-dimethylaminopyridine; TASF = tris(dimethylamino)sulfonium difluorotrimethylsilicate; THF = tetrahydrofuran.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A2780 A549 H4 HCT15 HT29 LN18 NCIH460 NCIH1155 RKO SNU119

[a] Data represent the mean  SD of triplicate experiments. IC50 values for 23 additional cell lines are reported in the Supporting Information. [b] Selectivity index (SI): IC50(7 b)/IC50(7 a).

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CHEMMEDCHEM COMMUNICATIONS

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The in vitro antiproliferative activity of 7 a and 7 b was assessed across a panel of 39 human cancer cell lines, and the IC50 values against 10 of these cell lines are summarized in Table 1 (for the complete data set, see Table S2 in the Supporting Information). In general, the naturally configured epothilone analogue 7 a inhibited cancer cell growth with single to double-digit nanomolar IC50 values and with similar potency as Epo A (1). By comparison, the activity of 7 b is 15- to 1400-fold lower; in 20 out of the 39 cell lines investigated, 7 b was more

Figure 2. Nuclear and microtubule effects of 7 a and 7 b in A549 cells. Cells were treated with test compound or DMSO for 20 h. DNA was stained with Hoechst 33342 (blue) and microtubules with a a-tubulin monoclonal antibody (green). The scale bar represents 10 mm. A) untreated control; B) 100 nm Epo A; C) 20.6 nm 7 a; D) 83 nm 7 a; E) 826 nm 7 b; F) 20.6 mm 7 b. Insets show mitotic cells from the same cell preparations: A) upper right: normal spindle; B) lower left: monopolar spindle; C) upper right: multipolar spindle; D) lower left: multipolar spindle; E) upper right: monopolar spindle with lagging chromosomes; F) lower left: monopolar spindle.

Figure 1. Effects of 7 a and 7 b on the cell cycle of A549 cells. A) 7 a; B) 7 b; C) Epo A (1). Cells in G0-G1 (&); cells in S phase (&); cells in G2/M (&). Cells were treated with test compound for 20 h, fixed with EtOH 70 % for at least 2 h, and then treated with RNAse and propidium iodide 0.01 %. Fluorescence was quantified by flow cytometry.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

than 50-fold less potent than 7 a. Collectively, these data highlight the importance of the natural C12,C13 configuration as a prerequisite for the potent antiproliferative activity of epothilones. Flow cytometry analysis of the effects of 7 a and 7 b on the cell cycle of A549 cells revealed a concentration-dependent increase in the G2/M population after 20 h of treatment with either compound (which at least for 7 a was unsurprising; Figure 1). However, while 7 a (as well as Epo A (1)) led to essentially complete cell cycle arrest in G2/M at a concentration as low as 20 nm, for analogue 7 b, a significant increase in the G2/M population was only observed at concentrations of more than 8 mm. Cells treated with 4 nm 7 a in general had a normal microtubule cytoskeleton as well as nuclear morphology. Very few cells showed small bundles in the cell periphery and multipolar mitotic spindles (Figure S3 A in the Supporting Information); some aberrant bipolar mitosis with lagging chromosomes in one of the spindle poles was also seen. At 20 nm 7 a, cells in interphase had thick and short bundles of microtubules and were mostly micronucleated. Mitotic cells were mainly multipolar (multiple asters) and a few showed aberrant bipolar mitosis (Figure 2 C; see also Figure S3 B in the Supporting Information). Raising the concentration to 80 nm increased the proportion of multiple asters (Figure 2 D; see also Figure S3 D in the Supporting Information). At higher concentrations (0.4–2 mm), mitotic cells had multipolar spindles and the remaining cells in interphase displayed short and very dense microtubule bundles. This pattern was similar to that observed with Epo A (1)treated cells (Figure 2; see also Figure S2 in the Supporting Information), although the proportion of monopolar spindles was higher for 25–100 nm Epo A (1) than 20-80 nm 7 a. At higher concentrations, mostly multiple spindles were observed for both compounds. Interphasic bundles induced by 80 nm– 2 mm 7 a were very short and dense; for Epo A (1) these type of bundles were only visible at concentrations of 1–2.5 mm (Figure S2 E,F in the Supporting Information). ChemMedChem 2014, 9, 2227 – 2232

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CHEMMEDCHEM COMMUNICATIONS Clear changes in microtubule organization in A549 cells were also observed in response to treatment with 7 b, although at significantly higher concentrations than those required for 7 a (Figure 2; see also Figure S4 in the Supporting Information). Thus, micronucleated cells were observed at concentrations of 7 b of 8 mm and above, together with monopolar mitotic spindles and aberrant bipolar spindles; multipolar mitotic spindles appeared at 20 mm and above (Figure 2 F; see also Figure S4 C–E in the Supporting Information). A few microtubule bundles were detected at 7 b concentrations of 20– 40 mm, while in the lower concentration range (0.8–8 mm), the microtubule network appeared to be only slightly disorganized, with a somewhat parallel alignment of microtubules (straighter microtubules) in the absence of microtubule bundles. (The same effect was observed at 5 nm Epo A (1) or 4 nm 7 a). These data suggest that the weak interaction of 7 b with microtubules in biochemical assays at high concentrations translates into cellular effects that would be expected for a microtubule stabilizer. In summary, the data collected here for epothilone analogues 7 a and 7 b clearly demonstrate that the orientation of the cyclopropane ring in cis-fused 12,13-cylclopropyl-epothilones has a profound impact on the biological activity of the respective isomers. Low nanomolar antiproliferative activity against solid tumor cell lines in vitro was only observed for the natural orientation of the three-membered ring. At the same time, the effects of 7 b on cell cycle and microtubule organization together with biochemical data on microtubule binding strongly suggest that the antiproliferative activity of this compound is related to its interactions with the cellular microtubule network, at least to a significant extent. Given the similarities in the profile of 7 a and Epo A (1), it seems likely that the conclusions derived here for 7 a and 7 b also hold true for 12,13-cis-epoxide-based epothilone analogues with opposite configurations at C12/C13.

Experimental Section For experimental details on the synthesis of 7 a/7 b and their biochemical and biological profiling see the Supporting Information.

Acknowledgements K.H.A. and F.Z.G. are indebted to the Swiss National Science Foundation (SNF) (project 205320-117594) and ETH Zrich for generous financial support. J.F.D. acknowledges financial support by the Ministerio de Economia of Spain and from the Comunidad de Madrid (projects BIO2013-42984-R and S2010/BMD-2457 BIPPED, respectively). Raphael Schiess is gratefully acknowledged for providing acid 22. The authors also thank Kurt Hauenstein for excellent technical assistance and Louis Bertschi and the entire ETHZLOC MS-Service for HRMS spectra acquisition. Finally, the authors are grateful to Dr. Philipp Krastel, Novartis Institute for Biomedical Research (Basel, Switzerland) for establishing the contact with ACC.

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www.chemmedchem.org Keywords: cancer · epothilones · inhibitors · microtubules · structure–activity relationships · stereoselective synthesis [1] a) K.-H. Altmann, B. Pfeiffer, S. Arseniyadis, B. Pratt, K. C. Nicolaou, ChemMedChem 2007, 2, 396 – 423; b) G. Hçfle, H. Reichenbach in Anticancer Agents from Natural Products, (Eds.: G. M. Cragg, D. G. I. Kingston, D. J. Newman), Taylor & Francis, Boca Raton, 2005, pp. 413 – 450; c) K.-H. Altmann, G. Hçfle, R. Mller, J. Mulzer, K. Prantz in Progress in the Chemistry of Organic Natural Products, Vol. 90 (Eds.: A. D. Kinghorn, H. Falk, J. Kobayashi), Springer, Wien, New York, 2009. [2] a) D. M. Bollag, P. A. Mcqueney, J. Zhu, O. Hensens, L. Koupal, J. Liesch, M. Goetz, E. Lazarides, C. M. Woods, Cancer Res. 1995, 55, 2325 – 2333; b) R. J. Kowalski, P. Giannakakou, E. Hamel, J. Biol. Chem. 1997, 272, 2534 – 2541; c) K. Kamath, M. A. Jordan, Cancer Res. 2003, 63, 6026 – 6031. [3] R. J. Lechleider, E. Kaminskas, X. Jiang, R. Aziz, J. Bullock, R. Kasliwal, R. Harapanhalli, S. Pope, R. Sridhara, J. Leighton, B. Booth, R. Dagher, R. Justice, R. Pazdur, Clin. Cancer Res. 2008, 14, 4378 – 4384. [4] For studies on the activity of diastereomeric C12/C13-trans-epothilones and -cyclopropyl-epothilones, see: a) K.-H. Altmann, G. Bold, G. Caravatti, D. Denni, A. Flçrsheimer, A. Schmidt, G. Rihs, M. Wartmann, Helv. Chim. Acta 2002, 85, 4086 – 4110; b) K. C. Nicolaou, K. Namoto, A. Ritzn, T. Ulven, M. Shoji, J. Li, G. D’Amico, D. Liotta, C. T. French, M. Wartmann, K.-H. Altmann, P. Giannakakou, J. Am. Chem. Soc. 2001, 123, 9313 – 9323. [5] K. C. Nicolaou, D. Vourloumis, T. H. Li, J. Pastor, N. Winssinger, Y. He, S. Ninkovic, F. Sarabia, H. Vallberg, F. Roschangar, N. P. King, M. R. V. Finlay, P. Giannakakou, P. Verdier-Pinard, E. Hamel, Angew. Chem. 1997, 109, 2181 – 2187; Angew. Chem. Int. Ed. Engl. 1997, 36, 2097 – 2103. [6] F. Yoshimura, A. Rivkin, A. E. Gabarda, T. C. Chou, H. J. Dong, G. Sukenick, F. F. Morel, R. E. Taylor, S. J. Danishefsky, Angew. Chem. 2003, 115, 2622 – 2625; Angew. Chem. Int. Ed. 2003, 42, 2518 – 2521. [7] M. Wartmann, K.-H. Altmann, Curr. Med. Chem.: Anti-Cancer Agents 2002, 2, 123 – 148. [8] Q.-H. Chen, T. Ganesh, Y. Jiang, A. Banerjee, S. Sharma, S. Bane, J. P. Snyder, D. G. I. Kingston, Chem. Commun. 2010, 46, 2019 – 2021. [9] R. Schiess, J. Gertsch, W. B. Schweizer, K.-H. Altmann, Org. Lett. 2011, 13, 1436 – 1439. [10] J. Johnson, S. H. Kim, M. Bifano, J. DiMarco, C. Fairchild, J. Gougoutas, F. Lee, B. Long, J. Tokarski, G. Vite, Org. Lett. 2000, 2, 1537 – 1540. [11] S. A. Dietrich, L. Riediker, J. Gertsch, K.-H. Altmann, Chimia 2010, 64, 136 – 139. [12] A. B. Charette, H. Juteau, H. Lebel, C. Molinaro, J. Am. Chem. Soc. 1998, 120, 11943 – 11952. [13] For a recent general review on the metathesis reaction see: A. H. Hoveyda, A. R. Zhugralin, Nature 2007, 450, 243 – 251. [14] H. C. Brown, P. K. Jadhav, J. Am. Chem. Soc. 1983, 105, 2092 – 2093. [15] a) C. P. Amonkar, S. G. Tilve, P. S. Parameswaran, Synthesis 2005, 14, 2341 – 2344; b) H. J. Bestmann, Angew. Chem. 1965, 77, 651 – 666; Angew. Chem. Int. Ed. Engl. 1965, 4, 645 – 660. [16] Based on an ee of 83 % for 15, the dr of 9:1 for the cyclopropanation product(s) 18 a translates into a product ratio of ca. 90:8:2:0.15 for the corresponding (12R,13S,15S), (12R,13S,15R), (12S,13R,15S), and (12S,13R,15R) isomers; for 18 b (67 % de) the product ratio for the (12S,13R,15S), (12S,13R,15R), (12R,13S,15S), and (12R,13S,15R) isomers can be calculated as ca. 82:8:9:1. The absolute configuration (at C12,C13) of the different products has not been strictly proven but is inferred from the known and well-established stereochemical preferences of the catalysts 16 and 17. The stereochemical assignments are also supported by the potent biological activity of 7 a. [17] a) P. Grieco, S. Gilman, M. Nishizawa, J. Org. Chem. 1976, 41, 1485 – 1486; b) K. B. Sharpless, M. W. Young, J. Org. Chem. 1975, 40, 947 – 949. [18] F. Feyen, A. Jantsch, K. Hauenstein, B. Pfeiffer, K.-H. Altmann, Tetrahedron 2008, 64, 7920 – 7928. [19] J. Inanaga, K. Hirata, H. Saeki, T. Katsuki, M. Yamaguchi, Bull. Chem. Soc. Jpn. 1979, 52, 1989 – 1993. [20] No sign of a cis-isomer was detectable in the NMR spectra of the purified product. However, we cannot exclude that minor amounts of the cis-isomer had been removed in the purification process.

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CHEMMEDCHEM COMMUNICATIONS [21] a) J. F. Daz, R. Strobe, Y. Engelborghs, A. A. Souto, J. M. Andreu, J. Biol. Chem. 2000, 275, 26265 – 26276; b) R. M. Buey, J. F. Daz, J. M. Andreu, A. O’Brate, P. Giannakakou, K. C. Nicolaou, P. K. Sasmal, A. Ritzn, K. Namoto, Chem. Biol. 2004, 11, 225 – 236.

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www.chemmedchem.org [22] S. A. Dietrich, R. Lindauer, C. Stierlin, J. Gertsch, R. Matesanz, S. Notararigo, J. F. Daz, K.-H. Altmann, Chem. Eur. J. 2009, 15, 10144 – 10157. Received: April 17, 2014 Published online on July 8, 2014

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The impact of cyclopropane configuration on the biological activity of cyclopropyl-epothilones.

Two cis-12,13-cyclopropyl-epothilone B variants have been synthesized, differing only in the configuration of the stereocenters at C12 and C13. The sy...
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