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Synlett 2006(9): 1384-1388  
DOI: 10.1055/s-2006-939727
LETTER
© Georg Thieme Verlag Stuttgart · New York

Total Synthesis and Biological Assessment of Cyclopropane-Based Epothilone Analogues - Modulation of Drug Efflux through Polarity Adjustments

Fréderic Cachouxa,, Thomas Isarnoa,, Markus Wartmannb, Karl-Heinz Altmann*a
a ETH Zürich, Department of Chemistry and Applied Biosciences, Institute of Pharmaceutical Sciences, ETH Hönggerberg, HCI H 405, Wolfgang-Pauli-Str. 10, 8093 Zürich, Switzerland
b Oncology DA, Novartis Institute for Biomedical Research, Basel, Switzerland
Fax: +41(44)6331360; e-Mail: karl-heinz.altmann@pharma.ethz.ch;
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Publikationsverlauf

Received 24 February 2006
Publikationsdatum:
22. Mai 2006 (online)

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Abstract

Benzimidazole-based analogues of epothilones containing a cyclopropane ring in place of the natural epoxide moiety have been prepared based on the selective cyclopropanation of homo­allylic alcohol intermediates as one of the key steps. In contrast to the epoxide-containing parent compounds the cyclopropane analogues are not or only minimally susceptible to P-gp170-mediated drug efflux.

Key words

antitumor agents - epothilones - total synthesis - natural products - stereoselectivity

1

Current address: Prestwick Chemical, Illkirch, France.

2

Current address: AMCIS AG, Bubendorf, Switzerland.

8

The clogP values for the most relevant compounds are as follows: Epo A, 3.256; Epo B, 3.703; 3, 2.871; 1, 4.154 (calculated with the Molinspiration Desktop Property Calculator from Molinspiration Cheminformatics: http://www.molinspiration.com/docu/mipc/index.html).

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The isomer ratio was determined by 400 MHz 1H NMR spectroscopy in benzene-d 6 and is based on the ratio of integrals observed for the signals of the isolated aromatic proton of the benzimidazole moiety (i.e., the proton ortho to the unsubstituted nitrogen of the imidazole ring). No signal splitting was observed for any other well-resolved signal.

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Preparation of 11.
To a solution of Et2Zn (3.38 mmol) in 5 mL of CH2Cl2 and 3.38 mL of hexane (obtained through addition of 3.38 mL of 1 M Et2Zn in hexane to 5 mL of CH2Cl2), CF3CO2H (0.259 mL, 3.38 mmol) in 2 mL of CH2Cl2 was added dropwise at -13 °C over a period of 10 min. The reaction mixture was stirred at -13 °C for 15 min followed by dropwise addition of a solution of CH2I2 (0.273 mL, 3.38 mmol) in 2 mL of CH2Cl2. After additional 30 min at -13 °C, a solution of 10 (0.28 g, 0.375 mmol) in 2 mL of CH2Cl2 was added dropwise and the reaction mixture was stirred for 20 min at -13 °C. The reaction was then quenched with sat. aq NH4Cl, the organic layer was separated, and the aqueous solution was extracted with three 20 mL portions of CH2Cl2. The combined organic extracts were dried (MgSO4) and concentrated in vacuo. Purification by flash column chromatography (hexane-acetone 1:1) afforded 220 mg (77%) of the title compound.
1H NMR (400 MHz, CD3OD): δ = 7.58 (s, 1 H, Harom), 7.42 (d, 1 H, Harom), 7.30 (d, 1 H, Harom), 4.67 (m, 1 H, CHOH), 4.38 (m, 1 H, CHOTBS), 3.81 (s, 3 H, CO2CH 3), 3.77 (m, 1 H, CHOTBS), 3.64 (s, 3 H, NCH 3), 3.30 (m, 1 H), 3.20 (m, 1 H, CH3CH), 2.70 [s, 3 H, CH 3 (benzimidazole)], 2.45 (m, 1 H, CH 2CO2Me), 2.25 (m, 1 H, CH 2CO2Me), 1.60 (m, 2 H), 1.40 (m, 4 H), 1.22 (s, 3 H, gem-CH 3), 1.07 (s, 3 H, gem-CH 3), 1.05 (d, 3 H, CH 3CHCO), 0.92 (d, 3 H, CH 3CH), 0.91 (s, 18 H, t-BuSi), 0.32 (m, 2 H, H-cyclopropane), 0.15 (s, 3 H, CH 3Si), 0.08 (s, 3 H, CH 3Si), 0.05 (s, 3 H, CH 3Si), 0.02 (s, 3 H, CH 3Si). ESI-MS: m/z = 759.2 [M + H]+. HRMS (MALDI): m/z calcd for C42H74N2O6Si2: 759.5188; found: 759.5142 [M + H]+.
Only one set of signals was observed in the NMR spectrum of 11 either in CD3OD or C6D6. It is not clear, however, whether the presence of a mixture of isomers would have been reflected in the doubling of at least some of the NMR peaks at this stage. Nevertheless, 11 can be inferred to contain at most trace amounts of the undesired cyclopropane isomer based on the NMR analysis of the derived cyclization products 2 as well as its protected precursor [2-(TBS)2], for both of which the occurrence of separate sets of signals for the two cyclopropane isomers had been established in the cyclopropanation experiments with 5 (Scheme [1] ). Only one set of signals was observed for either 2 or 2-(TBS)2.

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Preparation of 2.
To a solution of 2-(TBS)2 (70 mg, 0.096 mmol) in 4 mL of MeCN in a Teflon tube was added 1 mL of HF·pyridine (70:30) at r.t. and the reaction mixture was stirred at r.t. for 2 h. It was then washed with 5% aq NaHCO3 (pH 5), followed by extraction with three 15 mL portions of EtOAc. The combined organic extracts were dried (MgSO4) and the solvent was evaporated to give 40 mg (83%) of crude 2. This material was further purified by preparative HPLC to afford 20 mg (42%) of pure title compound.
1H NMR (400 MHz, CD3OD): δ = 7.60 (s, 1 H, Harom), 7.38 (m, 1 H, Harom), 7.28 (m, 1 H, Harom), 5.97 (m, 1 H, PhCHO), 4.30 (m, 1 H, CHOH), 3.92 (m, 1 H, CHOH), 3.70 (s, 3 H, NCH 3), 3.30 (m, 1 H, CH3CH), 2.60 [s, 3 H, CH 3 (benzimidazole)], 2.42 (m, 1 H, CH 2CO2), 2.10 (m, 1 H, CH 2CO2H), 1.60 (m, 2 H), 1.40 (m, 1 H), 1.37 (s, 3H, gem-CH 3), 1.20 (d, 3 H, CH 3CHCO), 1.01 (s, 3 H, gem-CH 3), 0.99 (d, 3 H, CH 3CH), 0.80 (m, 1 H), 0.60 (m, 1 H), 0.22 (m, 2 H, H-cyclopropane). ESI-MS: m/z = 498.6 [M + H]+. HRMS (MALDI): m/z calcd for C29H42N2O5: 499.3167; found: 499.3159 [M + H]+.

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The IC50 values ± SD (three independent experiments) for compounds 1 and 2 are as follows: KB-31 cells: 1, 0.48 ± 0.03 nM; 2, 0.17 ± 0.04 nM. KB-8511 cells: 1, 0.96 ± 0.06 nM; 2, 0.13 ± 0.02 nM. The IC50-values for 3 and 4 against KB-31/KB-8511 cells are 0.59/6.62 nM and 0.21/1.02 nM, respectively.7a The corresponding values for 1a are 26.9 ± 5.3 nM (KB-31) and 35.9 ± 6.9 nM (KB-8511).