A trimethoxyphenyl substituted ansa-titanocene: A possible anti-cancer drug

www.elsevier.com/locate/poly

Polyhedron 24 (2005) 1250–1255

A trimethoxyphenyl substituted ansa-titanocene: A possibleanti-cancer drug

Franz-Josef K. Rehmann a, Andrew J. Rous a, Oscar Mendoza a, Nigel J. Sweeney a,Katja Strohfeldt a, William M. Gallagher b, Matthias Tacke a,*

a Chemistry Department, Centre for Synthesis and Chemical Biology (CSCB), Conway Institute of Biomolecular and Biomedical Research,

University College Dublin, Belfield, Dublin 4, Irelandb Pharmacology Department, Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland

Received 14 December 2004; accepted 21 February 2005

Available online 14 April 2005

Abstract

Starting from 6-(3 0,4 0,5 0-trimethoxyphenyl) fulvene (1) [1,2-di(cyclopentadienyl)-1,2-di-(3 0,4 0,5 0-trimethoxyphenyl)ethanediyl]

titanium dichloride (2) was synthesised. When titanocene 2 was tested against pig kidney carcinoma cells (LLC-PK), an inhibitory

concentration (IC50) of 9.0 · 10�4 M was observed.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Anti-cancer drug; cis-Platinum; Titanocene; Fulvene; LLC-PK; DFT calculations

1. Introduction

Despite the resounding success of cis-platinum and

closely related platinum antitumor agents, the move-

ment of other transition-metal anti-cancer drugs to-

wards clinical trials has been exceptionally slow [1–3].

Metallocene dichlorides (Cp2MCl2) with M = Ti, V,Nb and Mo show remarkable antitumor activity [4,5].

However, only titanocene dichloride has reached Phase

I clinical trials so far, with a maximum tolerable dose

of 315 mg/m2 per week. The dose limiting effects of

titanocene dichloride include nephrotoxicity and eleva-

tion of creatinine and bilirubin levels [6,7]. Unfortu-

nately, the efficacy of Cp2TiCl2 in Phase II clinical

trials in patients with metastatic renal-cell carcinoma[8] or metastatic breast cancer [9] was too low to be

pursued. Nevertheless, little synthetic effort has been

employed to increase the cytotoxicity of any titanocene

0277-5387/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2005.02.017

* Corresponding author. Tel.: +35317168428; fax: +35317162127.

E-mail address: [email protected] (M. Tacke).

dichloride derivatives [10–12], despite the existence of a

novel synthetic method starting from titanium dichlo-

ride and fulvenes [13–16], which allows direct access

to highly substituted ansa-titanocenes [17–20]. Re-

cently, using this method we have synthesised [1,2-di-

(cyclopentadienyl)-1,2-di-(4-N,N-dimethylaminophenyl)

ethanediyl] titanium dichloride, which has an IC50 va-lue of 2.7 · 10�4 M when tested for cytotoxic effects

on the LLC-PK cell line [21]. It was followed by re-

ports about heteroaryl [22] and methoxyphenyl [23]

substituted ansa-titanocenes. This paper reports the

synthesis of a novel 1,2-diarylsubstituted ethanediyl-

ansa-titanium dichloride, which combines the reactivity

of the titanium dichloride moiety with a trimethoxy-

phenyl substituted Cp ligand leading to an improvedwater solubility.

2. Experimental

Titanium tetrachloride (1 mol solution in toluene),

nBuLi (n-butyl lithium, 2 mol solution in pentane) and

Franz-Josef K. Rehmann et al. / Polyhedron 24 (2005) 1250–1255 1251

3,4,5-trimethoxybenzaldehyde were obtained commer-

cially from Aldrich Chemical Co. THF and toluene were

dried over and distilled from Na/benzophenone prior to

use. Cyclopentadiene was collected under an atmo-

sphere of nitrogen from freshly cracked dicyclopentadi-

ene and pyrrolidine was distilled under argon prior touse. Manipulation of air and moisture sensitive com-

pounds was carried out using standard Schlenk tech-

niques under an argon atmosphere. NMR spectra were

measured on a Varian 300 MHz spectrometer. Chemical

shifts are reported in ppm and are referenced to TMS.

IR spectra were recorded on a Perkin–Elmer Paragon

1000 FT-IR Spectrometer employing a KBr disk.

The GCMS spectrum for fulvene 1 was measured ona FINNIGAN TRACE GCMS 2000 Series (70 eV) and

1 · 10�5 M solutions in ethyl acetate were used.

For mass spectrometric analysis of titanocene 2, a

stock solution of the sample was prepared by dissolving

the compound in 0.5 ml dichloromethane. A 10-fold

dilution of these solutions was made in acetonitrile

and electrospray mass spectrometry was performed on

a quadrupole tandem mass spectrometer (Quattro Mi-cro, Micromass/Waters Corp., USA) in a negative ion

mode.

With a view to elucidate the structures, spectroscopic

data, bonding properties and energies of formation, the

application of theoretical methods is advantageous. For

this purpose, the GAUSSIAN 98 Revision A11 [24] running

under Red Hat Linux was used. DFT calculations were

performed at the B3LYP level using the 6-31G* basis setfor the species of interest.

2.1. 6-(3 0,4 0,5 0-Trimethoxyphenyl)fulvene (1)

The synthesis of fulvene 1 was carried out under ar-

gon as outlined in reference [25]. Pyrrolidine (2.5 ml,

30.0 mmol) was added to a solution of 3,4,5-trimeth-

oxybenzaldehyde (3.9 g, 20.0 mmol) and cyclopentadi-ene (4.1 ml, 60.0 mmol) in 30 ml of methanol. After

this addition the solution turned from colourless to

deep red. When TLC analysis (silica/dichloromethane)

showed only one product band after 2 h, acetic acid

(1.8 ml, 32.0 mmol) was added. The reaction mixture

was partitioned between 20 ml of ether and 40 ml water

and extracted with a total of 3 · 20 ml ether. The com-

bined organic extracts were washed with a saturatedaqueous NaCl solution. The organic solution was dried

over magnesium sulfate and the solvent removed under

reduced pressure. The crude product was triturated

with pentane. After solvent removal under reduced

pressure a deep red/orange product was obtained.

3.8 g (85% yield wrt 3,4,5-trimethoxybenzaldehyde);

m.p. 41.0–43.0 �C.1H NMR (dppm CDCl3): 6.75, 6.65, 6.40 (C5H4, 4H

m); 6.95 (C6H2, 2H s); 3.95 (p-OCH3 and o-OCH3, 9H

s); 7.20 (Ph-CH-Cp, 1H s).

13C NMR (dppm CDCl3): 153.3, 144.7, 139.4, 138.3,

135.5, 132.3, 130.6, 127.3, 120.0, 108.0 (C5H4 and

C6H2); 61.0 (o-OCH3); 56.2 (m-OCH3).

IR absorptions (cm�1 KBr): 3001 (m), 2937 (m), 1576

(s), 1503 (m), 1417 (m), 1329 (s), 1242 (m).

GCMS: 244.2 (M+ 70%), 229.1 (M+ � CH3 50%),213.1 (M+ � OCH3 40%), 201.2 (M+ � COCH3 13%),

155.1(M+ � CCH(C5H4) 17%), 115.1 (M+ � COCH3)3100%).

Anal. Calc. for C15H16O3: C, 73.75; H, 6.60; Found:

C, 72.31; H, 6.50%.

2.2. [1,2-Di(cyclopentadienyl)-1,2-di(3 0-4 0-5 0-

trimethoxyphenyl)-ethanediyl] titanium dichloride [1,2-

(3 0,4 0,5 0-(MeO)3-C6H2)2C2H2{g5-C5H4}2]TiCl2 (2)

TiCl4 (6.25 mmol, 1 M in toluene) was added to

90 ml of dry toluene and 10 ml dry THF. The solution

turned immediately from colourless to pale yellow.

The solution was stirred and cooled down to

�78 �C, and then was treated dropwise with nBuLi

(7.8 ml, 12.5 mmol). The solution turned from yellowto brown during the addition. After this addition,

the mixture was allowed to warm up slowly to r.t.

and the solution finally turned black. After 20 h stir-

ring, a solution of 1 (3.05 g, 12.5 mmol) in dry toluene

was added to the solution of TiCl2 Æ 2THF at r.t. un-

der argon. It was then stirred under reflux for another

16 h. The solvent was removed under vacuum. The

resulting black solid was extracted with 3 · 20 ml ofchloroform and filtered on celite. The solvent was re-

moved under vacuum and the residue dissolved in

8 ml of chloroform and filtered twice through What-

man No. 1 filter paper. The solvent was removed

again under vacuum and the residue triturated with

a total of 40 ml pentane to give 2.5 g (66% yield) black

solid. The ratio of trans and cis isomers was 58–42%.

The mixture cannot be purified or separated by col-umn chromatography or crystallisation; therefore the

elemental analysis shows some discrepancy between

measured and calculated values and an X-ray crystal

structure is not available.1H NMR (dppm CDCl3): 6.36 (cis-C6H2, 4H s); 6.33

(trans-C6H2, 4H s); 7.23–6.00 (C5H4, 8H m); 5.34 (trans-

PhCH Cp, 2H s); 4.63 (cis-PhC HCp, 2H); 3.80 (cis-m-

CH3, 6H s); 3.78 (cis-m-CH3, 12H s); 3.74 (trans-p-CH3,6H s); 3.67 (trans-m-CH3, 12H s).

13C NMR (dppm CDCl3): 153.3, 153.1, 152.8, 137.5,

137.0, 136.5, 135.8, 133.9, 129.0, 126.6, 120.3, 117.4,

116.5, 115.4, 109.7, 106.5, 105.9, 105.2 (cis and trans-

C6H3 and C5H4); 60.9, 60.8 (cis and trans-o-O(CH3)2),

56.3, 56.2 (cis and trans-m-O(CH3)2); 54.4, 52.2 (cis

and trans-PhCHCp).

IR absorptions (cm�1 KBr): 3104 (m), 2991 (s), 2976(m), 2965 (s), 1583 (m), 1461 (m), 1412 (m), 1120 (m),

1000 (m), 821 (m).

1 2

3

4

5

67

89

10

1112

O

O

O

CH3

CH3

CH3

O3

O2

O1

13

14

15

Fig. 1. Structure of fulvene 1.

1252 Franz-Josef K. Rehmann et al. / Polyhedron 24 (2005) 1250–1255

MS: 641.0 (M + Cl�).

Anal. Calc. for C30H32Cl2O6Ti: C, 58.94; H, 5.31;

Found: C, 57.74; H, 5.19%.

2.3. MTT-based cytotoxicity tests

The pig kidney carcinoma cell line, LLC-PK, was ob-

tained from the American Tissue Culture Collection.

The cytotoxic activity of titanocene 2 was determined

using an MTT-based assay. In more detail, cells were

seeded into a 96-well plate (5000 cells/well) and allowed

to attach for 24 h. Subsequently, the cells were treated

with various concentrations of the cytotoxic agents. In

order to prepare drug solutions, 2 was first dissolvedin DMSO, followed by dilution with medium to the re-

quired maximum concentration of 1.5 · 10�3 M, with a

final concentration of DMSO not exceeding 0.7%. From

this stock solution, solutions with lower concentrations

were prepared by further dilution with medium. Care

was taken that the drug solutions were applied within

1 h on the cells to avoid interference with already hydro-

lysed compounds. After 48 h, the relevant drug was re-moved, the cells were washed twice with PBS and

fresh medium was added for another 24 h for recovery.

Viability of cells was determined by treatment with

MTT in medium (5 mg/11 ml) for 3 h. The purple for-

mazan crystals formed were dissolved in DMSO and

absorbance measured at 540 nm using a VICTOR2 mul-

tilabel plate reader (Wallac). IC50 (inhibitory concentra-

tion 50%) values were determined from the drugconcentrations that induced a 50% reduction in light

absorbance.

Fig. 2. Gaussview plot of the optimised structure of fulvene 1.

3. Results and discussion

3.1. Synthesis

Fulvene 1 (Scheme 1) was synthesised according to

reference [23], by reacting the corresponding benzalde-

hyde with cyclopentadiene in the presence of pyrrolidine

1

re

-78oC,toluene,5 % THF

+ 2TiCl2 . 2 THF

TiCl4 + 2 nBuLi

OMe

H

MeO

OM

Scheme 1. Synthesis

as a base. Afterwards titanocene 2 (Fig. 2) could be

obtained by a reductive dimerisation of fulvene 1 with

titanium dichloride (Scheme 1). TiCl2 was synthesised

by reduction of TiCl4 with nBuLi as described in the

literature [17,18] (Scheme 1). The determined cis–trans

ratio at the bridge is 58:42 for 2.

3.2. Theoretical studies

Density functional theory calculations were carried

out for fulvene 1 and titanocene 2 at the B3LYP level

using the 6-31G** basis set.

Selected bond lengths of the optimised structure of

fulvene 1 (Fig. 2, for the atom numbering scheme see

6 h

fluxTi

Cl

ClH

H

MeO

MeO

MeO

MeO

MeO

OMe

e

of titanocene 2.

Table 1

Selected bond lengths of the DFT-calculated structure of 1

1: Bond length [pm]

C(1)–C(2) 148.1

C(2)–C(3) 136.4

C(3)–C(4) 147.5

C(1)–C(6) 136.4

C(6)–C(7) 146.4

C(7)–C(8) 141.2

C(8)–C(9) 139.6

C(9)–C(10) 140.8

C(9)–O(1) 139.9

C(10)–O(2) 140.0

O(1)–C(13) 147.5

O(2)–C(14) 148.0

Fig. 3. Gaussview plot of the optimised structure of titanocene 2.

TiCl'

ClH

H

R

R

1

23

4

5

1'

2'

3'

4'5'

6

6'

Scheme 2. Numbering scheme for structural discussion of titanoc-

enes 2.

Franz-Josef K. Rehmann et al. / Polyhedron 24 (2005) 1250–1255 1253

Fig. 1) can be found in Table 1. As expected, the car-

bon–carbon bond lengths in the cyclopentadiene system

of this fulvene vary significantly, demonstrating the flex-

ibility of the resonance system within the five-membered

ring. This is confirmed by the length of the regular exo-

cyclic double bond C(1)–C(6), calculated as 136.4 pm,

and the single bond carrying the aryl substituent C(6)–

C(7) is calculated as 146.4 pm (Table 1). For fulvene 1,a distortion from planarity is seen with a dihedral angle

between the phenyl ring and the five-membered dienyl

ring of 25.5�. In the calculated structure of 1, the three

methyl groups are oriented alternating up and down rel-

ative to the plane of the phenyl ring.

Optimised structures were also calculated for titano-

cene 2 (Fig. 3, for the atom numbering scheme see

Scheme 2) at the B3LYP level using the 6-31G** basisset. Selected bond lengths of this structure are listed in

Table 2.

The length of the bonds between the metal centre and

the carbon atoms of the cyclopentadienyl rings bound to

the metal ion vary between 236.4 and 245.3 pm and are

in the range of bond lengths found in previous calcula-

tions for comparable ansa-titanocenes (236.4–

245.4 pm) [20–23]. The carbon–carbon bonds of thecyclopentadienyl rings have values of 140.3–143.0 pm.

These values suggest that the titanocene has no plane

of symmetry bisecting the Cl–Ti–Cl plane and that the

calculated structures exhibit C2 symmetry. The tita-

nium–chlorine bond length is 234.5 pm. These values

are in agreement with the corresponding values calcu-

lated previously for (1,2-diphenyl-1,2-dicyclopentadie-

nyl)ethanediyl} titanium dichloride [20]. The bridgelength [C(6)–C(6 0)] is 157.1 pm and is slightly elongated

in comparison to (1,2-diphenyl-1,2-dicyclopentadienyl)-

ethanediyl} titanium dichloride (153.6 pm) [20]. The ste-

ric bulk of the subsituted phenyl rings attached to the

bridging carbon centres causes a lengthening of the

bond, in order to relieve the resultant steric strain.

The TiCl2 angle was calculated for 2 to be 97.7�. Thedihedral angle between the aryl rings was calculated as

58.1�, whereas the dihedral angle between the cyclopen-

tadienyl rings is �46.4�. The angle formed by the bonds

between C(1), C(6) and C(6 0) takes a value of 107.4�, be-tween C(7), C(6), and C(6 0) a value of 113.6�, and be-

tween C(7), C(6) and C(1) a value of 114.5�. All these

angles are in agreement with the corresponding values

calculated previously for comparable ansa-titanocenes

prepared by our group [20–23].

3.3. Cytotoxicity studies

The in vitro cytotoxicity of compound 2 was deter-

mined by an MTT-based assay [26] involving a 48 h

drug exposure period, followed by 24 h of recovery time.

The compound was tested for its activity on pig kidney

carcinoma (LLC-PK) cells. The IC50 value found for 2 is9.0 · 10�4 M [Fig. 4] and this is slightly worse with re-

spect to the inhibition value found previously for [1,2-

di(cyclopentadienyl)-1,2-di(4-N,N-dimethylaminophe-

Table 2

Selected bond lengths from the DFT-calculated structure of complex 2

2: Bond length [pm]

Ti–C(1) 242.3

Ti–C(2) 236.7

Ti–C(3) 243.2

Ti–C(4) 245.1

Ti–C(5) 241.4

Ti–C(1 0) 241.5

Ti–C(2 0) 236.4

Ti–C(3 0) 243.7

Ti–C(4 0) 245.3

Ti–C(5 0) 241.9

C(1)–C(2) 142.6

C(2)–C(3) 142.3

C(3)–C(4) 140.3

C(4)–C(5) 142.4

C(5)–C(1) 141.7

C(10)–C(20) 143.0

C(20)–C(30) 142.1

C(30)–C(40) 140.4

C(40)–C(50) 142.3

C(50)–C(10) 141.5

Ti–Cl 234.5

C(6)–C(60) 157.1

C(1)–C(6) 251.1

1E-8 1E-7 1E-6 1E-5 1E-4 1E-30.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

cel

l via

bilit

y

log10 drug concentration

2, IC50: (9.0+/-1.7)E-4

Fig. 4. Cytotoxicity curve from typical MTT assays showing the effect

of compound 2 on the viability of pig kidney carcinoma (LLC-PK)

cells.

1254 Franz-Josef K. Rehmann et al. / Polyhedron 24 (2005) 1250–1255

nyl)ethanediyl] titanium dichloride, which has an IC50

value of 2.7 · 10�4 M [21]. Under identical conditions,

cis-platin showed an IC50 value of 3.3 · 10�6 M,

whereas Cp2TiCl2 is found at 2.0 · 10�3 M [21].

4. Conclusion and outlook

Compound 2 has an IC50 value against LLC-PK cells

in the upper 10�4 M range, which is slightly more cyto-

toxic than unsubstituted titanocene dichloride, for

which Phase I/II clinical trials have been performed.

The use of three methoxy substituents has improved

the water solubility quite well, but the observed cytotox-

icity is not impressing. It is intended to perform further

in vitro cellular tests with the compound to evaluate itspotential for testing in animal models and additionally

to search for differently substituted titanocenes also de-

rived from fulvenes.

Acknowledgements

The authors thank the Higher Education Authority(HEA) and the Centre for Synthesis and Chemical Biol-

ogy (CSCB) for funding through the HEA PRTLI cycle

3. In addition funding from Science Foundation Ireland

(SFI/04/BRG/C0682) and COST D20 (WG 0001) was

granted.

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