Novel tetracarboxylatoplatinum(iv) complexes as carboplatin prodrugs

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An international journal of inorganic chemistry

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Volume 41 | Number 47 | 21 December 2012 | Pages 14213–14440

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COVER ARTICLE

Galanski, Keppler et al.

Novel tetracarboxylatoplatinum(^IV) complexes as carboplatin prodrugs

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PAPER

Novel tetracarboxylatoplatinum(IV) complexes as carboplatin prodrugs†

Hristo P. Varbanov, Seied M. Valiahdi, Christian R. Kowol, Michael A. Jakupec, Markus Galanski* and

Bernhard K. Keppler*

Received 26th June 2012, Accepted 26th July 2012

DOI: 10.1039/c2dt31366a

It is widely accepted that platinum(IV) complexes act as prodrugs and have to be activated by reduction to

the respective platinum(^II) analogs. Recently it could be shown that introduction of lipophilic carboxylato

ligands in the axial position leads to diaminedichloridoplatinum(^IV) compounds with exceptionally high

cytotoxicity. With the aim of improving the antiproliferative properties of carboplatin, a series of

twenty-one novel Pt(^IV) complexes, featuring the equatorial ligand sphere of carboplatin as well as

lipophilic axial carboxylato ligands, was synthesized. In depth characterization is based on elemental

analysis, ESI-MS, ATR-IR, and multinuclear (¹H, ¹³C, ¹⁵N, and ¹⁹⁵Pt) NMR spectroscopy. Their

cytotoxic activity in four cell lines (CH1, SK-OV-3, SW480, and A549), lipophilicity, electrochemistry

and additionally the rate of reduction in the presence of ascorbic acid were investigated. In contrast to

analogous diaminedicarboxylatodichloridoplatinum(^IV) compounds, the cytotoxicity of novel

diaminetetracarboxylato counterparts could not substantially be increased by simply enhancing their

lipophilic character. It seems that not only the reduction potential, but also the rate of reduction has a

tremendous inﬂuence on the cytotoxic properties. This has to be taken into account for the development

of novel anticancer platinum(^IV) agents.

(tetraplatin, iproplatin and satraplatin), but none of them gained

Introduction

clinical approval.⁷However, satraplatin currently entered new

After Rosenberg’s serendipitous discovery¹opened up the way for

the introduction of metal complexes in antineoplastic chemo-

therapy, several platinum(^II) complexes (i.e. cisplatin, carboplatin

and oxaliplatin) have become the mainstay of recent cancer treat-

ment.²Nowadays, research in that ﬁeld has focused on platinum(IV)

and nonplatinum metal complexes. It has been expected that

improvement of the pharmacological and toxicological proﬁle in

comparison to platinum(^II) based drugs could be achieved.^3,4

In this context it is worth mentioning that cytotoxic effects of

platinum complexes were ﬁrst found for a Pt(^IV) compound,

namely (OC-6-22)-diamminetetrachloridoplatinum(^IV), a close

analogue of cisplatin.⁵Platinum(IV) complexes are kinetically

more inert than their Pt(^II) counterparts. Reduction under

hypoxic conditions accompanied by the loss of two ligands

occurs more readily than ligand exchange reactions. Their phys-

icochemical characteristics broaden the possibilities for the

design of novel metal-based drugs in various ways, e.g. modu-

lation of pharmacokinetic properties, additional opportunities for

targeted therapy, oral administration, the prodrug concept, etc.⁶

So far, three Pt(IV) complexes were studied in clinical trials

phase I and II clinical trials in combination regimens.⁸Generally,

Pt(^IV) complexes act as prodrugs via activation by reduction in

vivo to the reactive Pt(^II) species.^9–12This mode of action

explains the importance of the rate and mechanism of reduction

as well as the ﬁnal reduction products. An optimal pharmaco-

logical proﬁle can be expected, provided that the prodrug is rela-

tively stable in the blood stream and will preferably be reduced

to the active platinum(^II) species in the tumor cell or tissue. It

has been shown that such redox behavior can be observed for

Pt(^IV) complexes with axial carboxylato ligands.¹³Consequently,

platinum(^IV) complexes were coupled to targeting moieties^14–20

or to pharmacologically relevant molecules ameliorating the

cytotoxic properties of the platinum(^II) fragment.^21–24

A series of cis-diam(m)inebis(carboxylato)dichloridoplatinum(IV)

complexes with high cytotoxic potency has been published

recently.^25–29It was found that increasing the lipophilicity of the

axial ligands yields compounds with IC₅₀values in the nano-

molar range.^27,28Whether the latter concept can be transferred to

kinetically more inert platinum(^II) analogs such as carboplatin

has not yet been investigated. Therefore, we intended to answer

the following question within this project: Is it possible to sig-

niﬁcantly increase the antiproliferative properties of carboplatin

just by synthesizing lipophilic prodrugs only differing in the

axial ligands?

University of Vienna, Institute of Inorganic Chemistry, Waehringer

Strasse 42, A-1090 Vienna, Austria. E-mail: markus.galanski@univie.ac.at,

bernhard.keppler@univie.ac.at; Fax: +43-1-4277-52680;

Tel: +43-1-4277-52601

†Electronic supplementary information (ESI) available. See DOI:

10.1039/c2dt31366a

In line with that task, twenty-one novel Pt(^IV) complexes,

designed as prodrugs for carboplatin, were synthesized and com-

pletely characterized by elemental analysis, ESI-MS, ATR-IR,

14404 | Dalton Trans., 2012, 41, 14404–14415

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Fig. 1 Synthesis of novel tetracarboxylatoplatinum(IV) complexes.

and multinuclear NMR spectroscopy. Their cytotoxic activity in

four human tumor cell lines, originating from ovarian carcinoma

(CH1 and SK-OV-3), colon carcinoma (SW480) and non-small

cell lung cancer (A549), was evaluated by means of the MTT

colorimetric assay. In order to elucidate their biological behavior,

lipophilicity as well as redox properties were determined.

Results and discussion

Synthesis

A general reaction scheme for the synthesis of novel complexes

is shown in Fig. 1. The oxidation of carboplatin to the respective

dihydroxido Pt(^IV) analogue (2) using H₂O₂in aqueous solutions

at ambient temperature was performed successfully with yields

over 90%. Complex 2 was subsequently converted to a series of

tetracarboxylatoplatinum(^IV) complexes using different cyclic

anhydrides in DMF; complexes 3–6 were isolated with yields

over 75%. The latter were used as the starting material for the

synthesis of a plethora of amide and ester derivatives via

activation of the free carboxylic groups with CDI in dry DMF,

followed by reaction with the corresponding amine or alcoholate/

alcohol mixtures. Pure products were isolated with the help of

column chromatography and/or recrystallization with moderate

yields (around 20% for the esters and 30% for the amides).

Fig. 2 ¹⁹⁵Pt NMR spectra of complex 6a processed with line broaden-

ing factors of 100 Hz (a) and 10 Hz (b), respectively. ¹⁹⁵Pt NMR spec-

trum of 6a measured with proton decoupling (c).

shifts of all novel target complexes ranged between 3551 and

3568 ppm.

Spectroscopic characterization

Normally, ¹⁹⁵Pt NMR resonances are broad and unresolved;

half height line widths of several hundred Hertz are common.

Good signal to noise ratios are not easy to get in the case of

¹⁹⁵Pt NMR measurements. Consequently, large line broadening

factors are used for processing of data in order to suppress

unwanted noise (lb = 100 Hz, Fig. 2a). However, a ﬁne structure

of the signal is still visible which can be improved by decreasing

the line broadening factor to 10 Hz (Fig. 2b). When measuring a

¹H decoupled ¹⁹⁵Pt spectrum, a nicely resolved quintet with a

¹J(¹⁴N, ¹⁹⁵Pt) coupling constant of 205 Hz was obtained

(Fig. 2c). The coupling of ¹⁴N and ¹⁹⁵Pt with the protons of the

The novel complexes were fully characterized by elemental

analysis, one and two dimensional multinuclear NMR spec-

troscopy (¹H, ¹³C, ¹⁵N, ¹⁹⁵Pt), ESI-MS and ATR-IR. The oxi-

dation of carboplatin and the following carboxylations are

accompanied by signiﬁcant shifts of the signals in the ¹⁹⁵Pt

NMR spectra. A ¹⁹⁵Pt resonance for the platinum(II) complex (1)

was detected at −88 ppm; upon oxidation to the corresponding

dihydroxidoplatinum(^IV) analogue, the chemical shift was found

at 3268 ppm. Carboxylation of (2) resulted in a downﬁeld shift

of around 290 ppm with signals for complexes 3–6 between

3551 and 3567 ppm. Expectedly, further derivatization of 3–6

had no inﬂuence on the position of the Pt signals; chemical

1

coordinated ammine ligands is visible in the H NMR spectrum

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Table 1 log k₃₀(log k, obtained with a mobile phase, containing 30%

MeOH) and log k_w(values, extrapolated to 0% MeOH) for the new

complexes

Compound

log k_w

log k₃₀

3

4

5

6

1.07

1.35

2.06

2.27

1.61

2.16

2.94

2.71

4.20

2.23

1.68

3.13

3.67

3.32

1.86

2.54

2.10

2.77

2.92

2.97

3.06

−0.15

0.14

0.64

1.13

0.40

0.95

1.53

1.42

2.38

0.80

0.04

1.54

2.03

1.75

0.69

1.22

0.90

1.20

1.30

1.57

1.69

3a

3b

3c

3d

3e

3f

3g

3h

3i

3j

4a

4b

4f

5a

5f

6a

6f

Fig. 3 NH₃signal of complex 6a in the ¹H NMR spectrum.

as exemplarily shown for complex 6a (Fig. 3). The coupling

1

2

1

constants J(¹⁴N, H) = 53 Hz and J(¹⁹⁵Pt, H) = 53 Hz are in

the expected range for platinum(^IV) complexes.³⁰

All signals in the H and ¹³C NMR spectra were found with

1

expected chemical shifts and their correct assignment was per-

formed on the basis of two dimensional ¹H¹H COSY, ¹H¹³C

HSQC and ¹H¹³C HMBC spectra.

extrapolated retention factors log k₃₀and log k_wcan provide

relevant information about the lipophilic properties of the

compounds even without converting them into log P values.³⁸

In previous studies, we have shown that for cis-diam(m)inodi-

chloridobis(carboxylato)platinum(^IV) complexes cytotoxicity,

cellular accumulation as well as DNA platination increase with

increasing log P within a series of analogues.^28,39Herein, we

have measured the lipophilicity of all new compounds with the

help of RP-HPLC, using water/MeOH mobile phases. The

obtained retention factors (log k_wand log k₃₀) are summarized in

Table 1.

All novel complexes were more lipophilic than their Pt(^II)

precursor (carboplatin) and its oxidized form (2); the latter log k

values could not be determined under the conditions used,

because their retention times were lower than that of the

standard, uracil.

For complexes 5, 5a and 5f a mixture of stereoisomers (RR :

SS : RS = 1 : 1 : 2) was obtained due to the use of prochiral

3-methylglutaric anhydride for esteriﬁcation of 2. Nevertheless,

this cannot be proven in the NMR spectra, as observed in pre-

vious work from our group.²⁷Since the chiral information is

situated on the axial ligands, which will be released during the

reduction, complexes 5, 5a and 5f were investigated without

further chiral separation. Structures of the complexes were also

proved by ESI-MS spectra, measured both in the positive as well

as in the negative ion mode. The highest intensities were

observed for peaks assigned to [M + Na⁺]⁺, [M + H⁺]⁺, [M − H⁺]⁻

and in some cases [M + Cl⁻]⁻. The m/z values as well as the iso-

topic distribution were in accordance with the theoretical values.

Crystal structure of complex 4

X-ray diffraction on a sample of 4 afforded a data set that was

generally rather poor. However, the solution gave the gross struc-

ture of the complex, as shown in Fig. S1 in the ESI.†

Cytotoxicity in cancer cell lines

All novel compounds were tested in comparison to cisplatin and

carboplatin in four human tumor cell lines, originating from

ovarian carcinoma (CH1, SK-OV-3), colon carcinoma (SW480)

and non-small cell lung cancer (A549), by means of the colori-

metric MTT microculture assay. All of these cell lines, except

for CH1 cells, are primarily cisplatin- and carboplatin-resistant

(showing IC₅₀values about one order of magnitude higher than

that in CH1 cells). IC₅₀values are summarized in Table 2, and

complete concentration–effect curves in CH1 cells are depicted

in the ESI (Fig. S2†).

All novel complexes showed cytotoxic activity in the cispla-

tin-sensitive CH1 cell line, with IC₅₀values ranging between 7.7

and 171 μM. However, cytotoxicity is lower in comparison to

the clinically established platinum drugs cisplatin and carbopla-

tin (IC₅₀0.16 and 1.4 μM, respectively). Antiproliferative

activity in cisplatin-resistant A549, SW480 and SK-OV-3 cancer

Lipophilicity

Lipophilicity is one of the most important physicochemical para-

meters, characterizing the pharmacokinetic behavior and dedu-

cing the ability of drugs and drug candidates to pass through cell

membranes. In particular this is true for platinum complexes

(especially for Pt(^IV)), which results in many efforts for estab-

lishing a reliable method for determination and prediction of log

P (octanol–water partition coefﬁcient) values of platinum-based

cytostatics.^31–36Verifying lipophilicity with RP-HPLC, based on

chromatographic retention indexes, has many advantages in

comparison with the classic shake-ﬂask method, like indepen-

dence from the compound’s concentration, faster, robust and

reproducible measurements, etc.³⁷Measuring the isocratic and

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Table 2 Cytotoxicity of the novel complexes in comparison to

cisplatin and carboplatin in four human cancer cell lines

IC₅₀^a(μM)

Compound

CH1

171

32 10

28

24

8.6 1.7

11

44

62 26

15

28

A549

SW480

SK-OV-3

3

3a

3b

3c

3d

3e

3f

3g

3h

3i

3j

4

4a

4b

4f

5

1

>500

>250

>500

91 10

1.3 0.4

>500

350 39

181 44

>500

>250

>500

—

4

5

6

8

—

>500

—

>500

—

5

2

31 13

114 23

33 13

Fig. 4 Comparison of the IC₅₀values of complexes 3a–j and carbopla-

tin in CH1 cells.

7.7 1.4

89

128 48

23

49 13

125 35

22

33

1.4 0.4

7

—

>500

—

>500

—

5a

5f

6

6a

9

>500

85 28

3.5 0.3

8

4

6f

1 (carboplatin)

Cisplatin

67 11

1.9 0.3

0.16 0.03

^a50% Inhibitory concentrations in CH1, A549, SW480 and SK-OV-3

cells in the MTT assay, 96 h exposure. Values are the means standard

deviations obtained from three independent experiments.

cells is generally negligible. In accordance with previous

studies,^26,28the complexes with free carboxylic groups (3, 4, 5

and 6) exert the lowest antiproliferative activity, probably due to

their lower lipophilicity. The respective ester and amide deriva-

tives are more cytotoxic, increasing with the lipophilicity of the

alcoholate, resp. amide moiety. However, a substantial decrease

of IC₅₀values with increasing lipophilicity of the side chains

could not be observed. Contrary to our expectations, differences

in cytotoxicity of, e.g., methyl and propyl ester derivatives (3a

and 3c) are relatively small (IC₅₀values differing by a factor of

1.3). In the case of analogous complexes with two ethylamine

and two chlorido ligands in the equatorial position, the propyl

ester derivative is by more than one order of magnitude more

cytotoxic than the methyl ester counterpart.²⁸

A comparison of cytotoxicity of complex 3 and its ester and

amide derivatives in the CH1 cell line is represented in Fig. 4.

Overall, antiproliferative activity is dependent on lipophilicity.

Esters (3d and 3e) featuring an isopropyl or butyl ester moiety

and cyclopentyl and cyclohexyl amide derivatives (3h and 3i)

show the lowest IC₅₀values. However, it is not possible to reach

the cytotoxic potency of carboplatin by increasing the lipophili-

city of platinum(IV) analogues.

Fig. 5 Comparison of the IC₅₀values of complexes 3–6 and their pro-

pylamide and methyl ester derivatives in CH1 cells.

A semi-logarithmic plot of log k_wvalues (Table 1) versus IC₅₀

values is shown in Fig. 6, demonstrating a rough correlation of

IC₅₀values with lipophilicity, but with poor overall determi-

nation, indicating a considerable inﬂuence of additional factors

on cytotoxicity. Recent studies on diam(m)inebis(carboxylato)

dichloridoplatinum(^IV) complexes have shown that with increas-

ing lipophilicity, compounds with higher cytotoxicity compared

to their Pt(^II) counterparts can be obtained.^26–28Nevertheless,

there seem to be limits to this approach, as was demonstrated by

this series of novel diamminetetracarboxylatoplatinum(^IV) com-

plexes, all of which are less cytotoxic than their precursor carbo-

platin. In order to ﬁnd an explanation for this particular

behavior, additional studies with respect to the redox behavior of

these complexes were performed.

Variation of the spacer (Fig. 5) between the two carbonyl

atoms in the axial ligands of ester or amide derivatives revealed

that derivatization of 2 with a lipophilic anhydride tends to be

most favorable. However, the differences in cytotoxicity are not

tremendous with increasing size of the spacer, which on the

other hand decreases the solubility, and clear-cut structure–

activity relationships with respect to the spacer cannot be formu-

lated unequivocally.

Redox behavior

Electrochemistry. As already mentioned above, the redox

properties of Pt(^IV) complexes have a signiﬁcant inﬂuence on

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Fig. 7 Cyclic voltammogram of complex 3b in DMF containing

0.15 M [n-Bu₄N][BF₄] at a scan rate of 100 mV s⁻¹using a glassy

carbon working electrode.

Fig. 6 Semi-logarithmic plot of lipophilicity (determined as log k_w

with RP-HPLC) vs. cytotoxicity in CH1 cells (IC₅₀) for the new com-

plexes (succ – complex 3 and its derivatives, Glu – complex 4 and its

derivatives, MeGlu – complex 5 and its derivatives, and DiMeGlu –

complex 6 and its derivatives).

Very recently, Gibson and Hambley et al. have shown that, in

the case of platinum(^IV) complexes featuring a dicarboxylato

ligand in the equatorial position, rates of reduction by ascorbate

do not necessarily go parallel to the electrochemical reduction

potentials.⁴⁰

They concluded that the reduction by ascorbate is very slow in

the absence of coordinating hydroxido or chlorido ligands,

which are capable of forming a bridge to the reducing agent and

thereby facilitating the electron transfer. In line with this expla-

nation, the tetracarboxylatoplatinum(^IV) complexes in the present

study (lacking any Cl⁻or OH⁻ligands) should be reduced very

slowly.

Table 3 Summary of the electrochemical data

Compound

E_p/Pt^IV→ Pt^II

3

−0.69

−0.70

−0.68

−0.74

−0.70

3b

3f

4f

5f

Potentials in V 0.02 vs. NHE measured at a scan rate of 100 mV s⁻¹in

0.15 M [n-Bu₄N][BF₄]/DMF.

Incubation with ascorbic acid

In order to judge the inﬂuence of the equatorial dicarboxylato

leaving ligand on the rate of reduction, complex 3f and its

ethylenediaminedichlorido analogue (M1, the structure is shown

in Fig. S3 in the ESI†) were incubated with a 25-fold excess of

ascorbic acid. The fate of reduction was measured with the help

their pharmacological proﬁle. Fast reduction could result in deac-

tivation of the respective platinum(^II) species and high systemic

toxicity, whereas very slow reduction could be responsible for

the lack of anticancer activity. When platinum(^IV) complexes

exhibit axial carboxylato ligands, the reduction potential is situ-

ated in a medium range, which is believed to be a prerequisite

for the platinum(^IV) prodrug strategy.

In order to elucidate the inﬂuence of modiﬁcations in the axial

carboxylato ligands on the redox potential and in last conse-

quence on the cytotoxic properties, complexes 3, 3b, 3f, 4f and

5f were investigated by cyclic voltammetry in DMF solution at a

scan rate of 100 mV s⁻¹using 0.15 M [n-Bu₄N][BF₄] as the sup-

porting electrolyte (Table 3, Fig. 7).

1

of H NMR spectroscopy in a D₂O buffered solution at physio-

logical pH.

Complex M1 was completely reduced after 24 h, with a half

life of ca. 5 h. However, complex 3f showed a very slow

reduction rate; 50% of the complex were reduced after 3 weeks.

Curves of the reduction (Fig. S4†) and comparative NMR

spectra (Fig. S5 and S6) are given in the ESI.† The extremely

slow reduction of 3f might be the reason for its lower cyto-

toxicity in vitro.

All complexes showed an irreversible reduction peak,

common for Pt(^IV) compounds (Fig. 7). The reduction potentials

of the investigated complexes were found to be at ca. −0.7 V vs.

NHE featuring no signiﬁcant deviation in dependency on the

structure of the axial ligand. These values are slightly more

negative compared to those of the dichloridoethylendiamine-

platinum(^IV) analogues which are reduced at around −0.6 V.³⁹

Thus, the novel carboplatin prodrugs are harder to reduce and

should therefore exhibit a slightly lower antiproliferative potency

compared to the dichlorido compounds. However, as mentioned

above the IC₅₀values of novel complexes are much higher than

expected.

Conclusions

A series of novel carboplatin prodrugs was synthesized and

characterized in detail by various analytical techniques. Cyto-

toxicity was dependent on the lipophilicity of the axial ligands

but could not be enhanced down to nanomolar concentrations as

observed recently for analogs featuring chlorido ligands instead

of 1,1-cyclobutanedicarboxylate. The reduction behavior in the

presence of ascorbic acid was investigated and revealed a dra-

matic inﬂuence of the equatorial leaving group on the rate of

reduction and in last consequence on the antiproliferative

14408 | Dalton Trans., 2012, 41, 14404–14415

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Synthesis

potential. Nevertheless, further investigations are needed in order

to get a detailed picture of the reduction process as well as of the

mode of action of platinum(^IV) complexes.

The general reaction scheme is given in Fig. 1. Synthesis of pre-

cursors 1 and 2 is based on literature methods^41,42with some

modiﬁcations.

(SP-4-2)-Diammine(1,1-cyclobutanedicarboxylato)platinum(II)

(1). Carboplatin was prepared, starting from K₂PtCl₄via cis-

Pt(NH₃)₂I₂, its activation with AgNO₃(1.9 equiv.) and reaction

of the formed diamminediaqua complex with the disodium salt

(prepared in situ) of 1,1′-cyclobutanedicarboxylic acid (CBDA)

(0.95 equiv.) in water. After reducing the solvent volume and

cooling (to 4 °C), a white crystalline powder was collected,

washed with small amounts of cold water and dried over P₂O₅

under vacuum. Yield: 56%. M_r= 371.25 g mol⁻¹. Elemental

Experimental

Materials and methods

All reagents and solvents were obtained from commercial suppli-

ers, and were used without further puriﬁcation. Methanol and

ethanol were dried, according to standard procedures. Water was

puriﬁed through reverse osmosis, followed by double distillation.

For column chromatography, silica gel 60 (Fluka) was used.

High purity water used for the HPLC experiments was obtained

from a Millipore Synergy 185 UV Ultrapure Water system (Mol-

sheim, France).

analysis, found:

C 19.25, H 2.91, N 7.49. Calcd for

C₆H₁₂N₂O₄Pt: C 19.41, H 3.26, N 7.55. ¹H NMR: δ = 2.79

(t, ³J_H,H= 7.9 Hz, 4H, H-2), 1.81 (m, 2H, H-1) ppm. ¹⁹⁵Pt NMR:

δ = −88 ppm. IR (ATR): 3258 s, 3190 br (ν_N–H); 2957 w; 1634 s,

¹H, ¹³C, ¹⁵N, ¹⁹⁵Pt and two-dimensional H¹H COSY, H¹³C

and ¹H¹⁵N HSQC, and ¹H¹³C HMBC NMR spectra were

recorded with a Bruker Avance III 500 MHz NMR spectrometer

at 500.32 (¹H), 125.81 (¹³C), 107.55 (¹⁹⁵Pt), and 50.70 MHz

(¹⁵N) in DMF-d₇or D₂O (in the case of carboplatin (1) and its

dihydroxido Pt(^IV) analogue (2)) at ambient temperature, using

the solvent residual peak for ¹H and ¹³C as the internal reference.

The splitting of proton resonances in the ¹H NMR spectra is

deﬁned as s = singlet, bs = broad singlet, d = doublet, t = triplet,

and m = multiplet. ¹⁵N chemical shifts were referenced relative

to external NH₄Cl, whereas ¹⁹⁵Pt chemical shifts were referenced

relative to external K₂[PtCl₄] (see Fig. 8 for the NMR numbering

scheme).

IR spectra were recorded on a Bruker Vertex 70 FT-IR spec-

trometer (4000–400 cm⁻¹) by using an ATR unit. Intensities of

reported IR bands are deﬁned as br = broad, s = strong, m =

medium, and w = weak. Electrospray ionization mass spec-

trometry was carried out with a Bruker Esquire 3000 instrument

using MeOH as the solvent. Elemental analyses were performed

with a Perkin-Elmer 2400 CHN-Elemental Analyser by the

Microanalytical Laboratory of the University of Vienna.

1

1601 s (ν_CvO); 1464 w; 1373 w, 1345 s, 1286 m, 1204 w cm⁻¹

.

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)dihydro-

xidoplatinum(^IV) (2). Carboplatin (1) (1.4862 g, 4.0038 mmol)

was oxidized with 20 ml of 30% H₂O₂in 20 ml of water at room

temperature. The white product formed was ﬁltered off, washed

with cold water and dried over P₂O₅under vacuum. The volume

of the ﬁltrate was reduced, cooled in the fridge for 24 h and the

newly formed precipitate was also collected and dried. Yield:

1

1.5169 g, 93%. M_r= 405.26 g mol⁻¹. H NMR: δ = 2.59 (t,

³J_H,H= 8.1 Hz, 4H, H-2), 1.96 (m, 2H, H-1) ppm. ¹³C NMR:

δ = 180.7 (C-4), 55.8 (C-3), 32.1 (C-2), 15.7 (C-1) ppm. ¹⁹⁵Pt NMR:

δ = 3268 ppm. IR (ATR): 3448 br (ν_PtO–H); 3217 br, 3093 br (ν_N–H);

2963 br, 2727 w; 1615 s, 1570 s (ν_CvO); 1351 s cm⁻¹

.

General procedure for synthesis of compounds 3–6. 4 equiv.

of the corresponding anhydride and (OC-6-33)-diammine(cyclo-

butane-1,1-dicarboxylato)dihydroxidoplatinum(^IV) (2) were sus-

pended in dry DMF and the reaction mixture was stirred at 50 °C

until the solid material dissolved to form a colorless to pale

yellowish solution. DMF was then removed under reduced

pressure (a white foam is formed). The residue was suspended in

acetone, ﬁltered off and dried under vacuum to obtain a white

solid.

(OC-6-33)-Diamminebis(3-carboxypropanoato)(cyclobutane-

1,1-dicarboxylato)platinum(^IV) (3). Succinic anhydride (215 mg,

2.1500 mmol), 218 mg (0.5383 mmol) of 2 in DMF (6 mL),

2 h. Yield: 278 mg, 85%. M_r= 605.4 g mol⁻¹. Elemental analy-

sis, found: C 27.95, H 3.51, N 4.62. Calcd for C₁₄H₂₂N₂O₁₂Pt:

C 27.77, H 3.66, N 4.63. ESI-MS: m/z 627.1 [M + Na⁺]⁺, 605.1

1

[M + H⁺]⁺, 603.9 [M − H⁺]⁻. H NMR: δ = 12.35 (bs, 2H,

3

COOH), 6.73 (m, 6H, NH₃), 2.64 (t, J_H,H= 8.0 Hz, 4H, H-2),

3

2.53 (t, J_H,H= 6.9 Hz, 4H, H-6), 2.47 (t, J_H,H= 6.6 Hz, 4H,

H-7), 1.90 (m, 2H, H-1) ppm. ¹³C NMR: δ = 179.5 (C-5), 176.7

(C-4), 173.8 (C-8), 56.4 (C-3), 31.9 (C-2), 30.3 (³J_C,Pt= 37 Hz,

C-6), 29.8 (C-7), 16.0 (C-1) ppm. ¹⁵N NMR: δ = −53.8 ppm.

¹⁹⁵Pt NMR: δ = 3565 ppm. IR (ATR): 3237 br, 3161 br (ν_N–H);

2949 w; 1767 w, 1719 m 1632 s, 1594 m, 1546 w (ν_CvO);

Fig. 8 NMR numbering scheme for novel tetracarboxylatoplatinum(IV)

complexes.

1407 w, 1349 s, 1328 s, 1286 w, 1178 m cm⁻¹

.

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 14404–14415 | 14409

View Article Online

(OC-6-33)-Diamminebis(4-carboxybutanoato)(cyclobutane-

1,1-dicarboxylato)platinum(^IV) (4). Glutaric anhydride (350 mg,

3.0675 mmol), 306 mg (0.7551 mmol) of 2 in DMF (8 mL),

3 h. The product was recrystallized from methanol, washed with

acetone and dried under vacuum. Yield: 384 mg, 83%. M_r=

633.46 g mol⁻¹. Elemental analysis, found: C 30.24, H 4.00,

N 4.29. Calcd for C₁₆H₂₆N₂O₁₂Pt: C 30.34, H 4.14, N 4.42.

ESI-MS: m/z 654.9 [M + Na⁺]⁺, 633.9 [M + H⁺]⁺, 630.8

temperature, CO₂formed during the activation was removed by

ﬂushing the solution with argon for 10 min. Then sodium alco-

holate in absolute alcohol (prepared in situ by dissolving a cata-

lytic amount of Na in absolute alcohol) was added and the

solution was stirred for 24 h (up to 72 h in some cases). The sol-

vents were removed under reduced pressure and the residues

puriﬁed by recrystallization or column chromatography.

1

[M − H⁺]⁻. H NMR: δ = 12.25 (bs, 2H, COOH), 6.78 (m, 6H,

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((4-methoxy)-4-oxobutanoato)platinum(^IV) (3a). CDI (136 mg,

0.9004 mmol) in DMF (5 mL), 3 (248.5 mg, 0.4105 mmol) in

DMF (6 mL), MeONa/MeOH (10 mL). The crude product was

puriﬁed by suspending in EtOAc, ﬁltration of the yellow powder

and recrystallization of the latter from water. The obtained white

powder was ﬁltered off, washed with a small amount of cold

water and cold diethyl ether and dried in a vacuum desiccator

over P₂O₅. Yield: 60 mg, 23%. M_r= 633.46 g mol⁻¹. Elemental

3

NH₃), 2.62 (t, J_H,H= 8.0 Hz, 4H, H-2), 2.30 (t, J_H,H= 7.6 Hz,

3

4H, H-7), 2.28 (t, J_H,H= 7.6 Hz, 4H, H-6), 1.90 (m, 2H, H-1),

1.73 (m, 4H, H-7′) ppm. ¹³C NMR: δ = 180.1 (C-5), 176.8

(C-4), 174.4 (C-8), 56.3 (C-3), 34.5 (C-6), 33.0 (C-7), 31.9

(C-2), 21.0 (C-7′), 16.0 (C-1) ppm. ¹⁵N NMR: δ = −54.5 ppm.

¹⁹⁵Pt NMR: δ = 3551 ppm. IR (ATR): 3196 br, 3096 br (ν_N–H);

2953 m; 1698 m, 1637 s (ν_CvO); 1411 w, 1332 s, 1234 m, 1087

w cm⁻¹

.

analysis, found:

C 30.18, H 3.90, N 4.43. Calcd for

C₁₆H₂₆N₂O₁₂Pt: C 30.34, H 4.14, N 4.42. ESI-MS: m/z 655.0

[M + Na⁺]⁺, 633.9 [M + H⁺]⁺, 632.5 [M − H⁺]⁻, 669.0 [M +

Cl⁻]⁻. ¹H NMR: δ = 6.74 (m, 6H, NH₃), 3.63 (s, 6H, H-9), 2.63

(OC-6-33)-Diamminebis(4-carboxy-3-methylbutanoato)(cyclo-

butane-1,1-dicarboxylato)platinum(^IV) (5). 3-Methylglutaric

anhydride (528 mg, 4.1208 mmol), 411 mg (1.0141 mmol) of 2

in DMF (10 mL), 2 h. Yield: 520 mg, 78%. M_r= 661.52 g

mol⁻¹. Elemental analysis, found: C 32.62, H 4.48, N 4.22.

Calcd for C₁₈H₃₀N₂O₁₂Pt: C 32.68, H 4.57, N 4.23. ESI-MS:

m/z 699.1 [M + K⁺]⁺, 683.3 [M + Na⁺]⁺, 662.2 [M + H⁺]⁺,

3

(t, J_H,H= 8.0 Hz, 4H, H-2), 2.54 (m, 4H, H-6), 2.48 (m, 4H,

H-7), 1.90 (m, 2H, H-1) ppm. ¹³C NMR: δ = 179.2 (C-5), 176.7

(C-4), 172.9 (C-8), 56.3 (C-3), 51.1 (C-9), 31.8 (C-2), 30.2

(C-6), 29.5 (C-7), 15.9 (C-1) ppm. ¹⁵N NMR: δ = −54.0 ppm.

¹⁹⁵Pt NMR: δ = 3566 ppm. IR (ATR): 3256 br, 3204 br (ν_N–H),

3119 br; 2953 w; 1732 m, 1639 s (ν_CvO); 1428 w 1315s,

1

660.1 [M − H⁺]⁻. H NMR: δ = 12.23 (bs, 2H, COOH), 6.80

3

(m, 6H, NH₃), 2.63 (t, J_H,H= 7.9 Hz, 4H, H-2), 2.37 (m, 2H,

1262 w, 1191 w, 1164 m cm⁻¹

.

H-7), 2.32 (m, 2H, H-6), 2.29 (m, 2H, H-7′), 2.12 (m, 2H, H-6),

3

2.08 (m, 2H, H-7), 1.90 (m, 2H, H-1), 0.93 (d, J_H,H= 6.5 Hz,

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((4-ethoxy)-4-oxobutanoato)platinum(^IV) (3b). CDI (223 mg,

1.3753 mmol) in DMF (6 mL), 3 (416 mg, 0.6871 mmol) in

DMF (8 mL), EtONa/EtOH (10 mL). The crude product was

puriﬁed by column chromatography (EtOAc/MeOH, 5 : 1), then

isolated through suspending in EtOAc, ﬁltration and washing

with Et₂O. A white powder was obtained and dried under

vacuum. Yield: 130 mg, 31%. M_r= 661.51 g mol⁻¹. Elemental

6H, H-7′′) ppm. ¹³C NMR: δ = 179.7 (C-5), 176.8 (C-4), 173.8

(C-8), 56.3 (C-3), 42.2 (C-6), 40.6 (C-7), 31.9 (C-2), 27.8

(C-7′), 19.4 (C-7′′), 16.0 (C-1) ppm. ¹⁵N NMR: δ = −55.3 ppm.

¹⁹⁵Pt NMR: δ = 3559 ppm. IR (ATR): 3209 br, 3093 br (ν_N–H);

2963 w; 1704 m, 1637 s (ν_CvO); 1458 w, 1342 s, 1221 m, 1169

w, 1080 w cm⁻¹

.

(OC-6-33)-Diamminebis(4-carboxy-3,3-dimethylbutanoato)-

(cyclobutane-1,1-dicarboxylato)platinum(^IV) (6). 3,3-Dimethyl-

glutaric anhydride (212 mg, 1.4194 mmol), 151 mg

(0.3726 mmol) of 2 in DMF (6 mL), 4 h. Yield: 210 mg, 82%.

M_r= 689.56 g mol⁻¹. Elemental analysis, found: C 34.88,

H 4.88, N 4.08. Calcd for C₂₀H₃₄N₂O₁₂Pt: C 34.84, H 4.97,

N 4.06. ESI-MS: m/z 712.2 [M + Na⁺]⁺, 688.7 [M − H⁺]⁻.

¹H NMR: δ = 12.14 (bs, 2H, COOH), 6.80 (m, 6H, NH₃), 2.63

analysis, found:

C 32.49, H 4.47, N 4.31. Calcd for

C₁₈H₃₀N₂O₁₂Pt: C 32.68, H 4.57, N 4.23. ESI-MS: m/z 699.0

[M + K⁺]⁺, 683.0 [M + Na⁺]⁺, 662.0 [M + H⁺]⁺, 659.7 [M −

H⁺]⁻, 696.7 [M + Cl⁻]⁻. ¹H NMR: δ = 6.73 (m, 6H, NH₃), 4.08

3

(m, 4H, H-9), 2.63 (t, J_H,H= 8.0 Hz, 4H, H-2), 2.53 (m, 4H,

3

H-6), 2.47 (m, 4H, H-7), 1.90 (m, 2H, H-1), 1.20 (t, J_H,H

=

7.1 Hz, 6H, H-10) ppm. ¹³C NMR: δ = 179.3 (C-5), 176.7

(C-4), 172.4 (C-8), 56.3 (C-3), 60.0 (C-9), 31.8 (C-2), 30.2

(C-6), 29.8 (C-7), 15.9 (C-1), 13.8 (C-10) ppm. ¹⁵N NMR: δ =

−54.8 ppm. ¹⁹⁵Pt NMR: δ = 3564 ppm. IR (ATR): 3251 br,

3208 br (ν_N–H); 2980 w; 1727 m, 1640 s (ν_CvO); 1424 w,

3

(t, J_H,H= 7.9 Hz, 4H, H-2), 2.34 (s, 4H, H-7), 2.32 (s, 4H,

H-6), 1.90 (m, 2H, H-1), 1.06 (s, 12H, H-7′′) ppm. ¹³C NMR:

δ = 179.4 (C-5), 176.8 (C-4), 173.2 (C-8), 56.3 (C-3), 46.9

(C-6), 45.1 (C-7), 32.3 (C-7′), 31.9 (C-2), 27.0 (C-7′′), 16.0

(C-1) ppm. ¹⁵N NMR: δ = −55.1 ppm. ¹⁹⁵Pt NMR: δ =

3567 ppm. IR (ATR): 3255 br, 3128 br (ν_N–H); 2964 m, 2879 w;

1712 m, 1697 m, 1638 s, 1611 s (ν_CvO); 1365 s, 1345 s, 1229 s,

1317 s, 1093 m cm⁻¹

.

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((4-propyloxy)-4-oxobutanoato)platinum(^IV) (3c). CDI (0.2143 g,

1.3214 mmol) in DMF (9 mL), 3 (0.4000 g, 0.6607 mmol) in

DMF (10 mL), n-PrONa/n-PrOH (15 mL). The crude product

was puriﬁed by column chromatography (EtOAc/IPA, 3 : 1), then

isolated from an Et₂O suspension, followed by MeOH/EtOAc

recrystallization. The white powder obtained was dried under

vacuum. Yield: 82 mg, 18%. M_r= 689.57 g mol⁻¹. Elemental

1159 m, 1108 w cm⁻¹

.

General procedure for synthesis of ester derivatives 3a–e, 4a–b,

5a and 6a

2 equiv. of CDI (1,1′-carbonyldiimidazole) in dry DMF were

added to a DMF solution of 3, 4, 5 or 6, respectively, and the

mixture was heated at 60°C for 10 min. After cooling to room

analysis, found:

C 34.70, H 4.74, N 4.04. Calcd for

C₂₀H₃₄N₂O₁₂Pt: C 34.84, H 4.97, N 4.06. ESI-MS: m/z 734.0

14410 | Dalton Trans., 2012, 41, 14404–14415

This journal is © The Royal Society of Chemistry 2012

View Article Online

1

[M + 2Na⁺− H⁺]⁺, 711.1 [M + Na⁺]⁺, 689.9 [M + H⁺]⁺, 687.9

[M + Na⁺]⁺, 661.0 [M + H⁺]⁺, 659.9 [M − H⁺]⁻. H NMR: δ =

1

3

[M − H⁺]⁻. H NMR: δ = 6.74 (m, 6H, NH₃), 3.99 (t, J_H,H

=

6.76 (m, 6H, NH₃), 3.63 (s, 6H, H-9), 2.62 (t, J_H,H= 8.0 Hz,

4H, H-2), 2.34 (t, J_H,H= 7.5 Hz, 4H, H-7), 2.27 (t, J_H,H=

3

6.7 Hz, 4H, H-9), 2.63 (t, J_H,H= 8.0 Hz, 4H, H-2), 2.54 (m,

4H, H-6), 2.48 (m, 4H, H-7), 1.90 (m, 2H, H-1), 1.61 (m, 4H,

7.4 Hz, 4H, H-6), 1.90 (m, 2H, H-1), 1.74 (m, 4H, H-7′) ppm.

¹³C NMR: δ = 180.0 (C-5), 176.8 (C-4), 173.4 (C-8), 56.3 (C-3),

50.9 (C-9), 34.4 (C-6), 32.7 (C-7), 31.9 (C-2), 20.9 (C-7′), 16.0

(C-1) ppm. ¹⁵N NMR: δ = −54.4 ppm. ¹⁹⁵Pt NMR: δ =

3555 ppm. IR (ATR): 3240 br, 3210 br (ν_N–H); 2952 w; 1733 m,

3

H-10), 0.91 (t, J_H,H= 7.4 Hz, 6H, H-11) ppm. ¹³C NMR: δ =

179.3 (C-5), 176.7 (C-4), 172.5 (C-8), 65.6 (C-9), 56.3 (C-3),

31.8 (C-2), 30.2 (C-6), 29.8 (C-7), 21.8 (C-10), 15.9 (C-1), 9.9

(C-11) ppm. ¹⁵N NMR: δ = −54.1 ppm. ¹⁹⁵Pt NMR: δ =

3565 ppm. IR (ATR): 3245 br, 3210 br (ν_N–H); 2969 w; 1730 m,

1639 s (ν_CvO); 1436 w, 1355 s, 1241 m, 1202 m, 1148 m cm⁻¹

.

1641 s (ν_CvO); 1318 s, 1259 w, 1090 m cm⁻¹

.

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((5-ethoxy)-5-oxopentanoato)platinum(^IV) (4b). CDI (112.1 mg,

0.6913 mmol) in DMF (7 mL), 4 (219 mg, 0.3457 mmol) in

DMF (5 mL), EtONa/EtOH (8 mL). The crude product was

puriﬁed by column chromatography (EtOAc/IPA, 2 : 1) and iso-

lated via suspending in EtOAc, followed by recrystallization

from MeOH/EtOAc and ﬁnal washing with cold Et₂O. The

white powder obtained was dried under vacuum. Yield: 42 mg,

18%. M_r= 689.57 g mol⁻¹. Elemental analysis, found: C 33.98,

H 4.65, N 3.92. Calcd for C₂₀H₃₄N₂O₁₂Pt·0.5H₂O: C 34.38,

H 5.05, N 4.01. ESI-MS: m/z 712.0 [M + Na⁺]⁺, 690.0 [M +

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis((4-(2-

propyloxy))-4-oxobutanoato)platinum(^IV) (3d). CDI (230 mg,

1.4184 mmol) in DMF (10 mL), 3 (411 mg, 0.6789 mmol) in

DMF (9 mL), i-PrONa/i-PrOH (10 mL). The crude product was

puriﬁed by column chromatography (EtOAc/IPA, 2 : 1), then iso-

lated from an EtOAc suspension, washed with cold Et₂O and

dried under vacuum to obtain a white powder. Yield: 36 mg, 8%.

M_r= 689.57 g mol⁻¹. Elemental analysis, found: C 33.58,

H 4.86, N 4.08. Calcd for C₂₀H₃₄N₂O₁₂Pt·H₂O: C 33.95,

H 5.13, N 3.96. ESI-MS: m/z 711.0 [M + Na⁺]⁺, 687.8 [M −

H⁺]⁻. ¹H NMR: δ = 6.74 (m, 6H, NH₃), 4.91 (m, 2H, H-9), 2.63

1

H⁺]⁺, 686.8 [M − H⁺]⁻. H NMR: δ = 6.77 (m, 6H, NH₃), 4.09

3

(t, J_H,H= 8.0 Hz, 4H, H-2), 2.52 (m, 4H, H-6), 2.44 (m, 4H,

(m, 4H, H-9), 2.63 (t, J_H,H= 7.9 Hz, 4H, H-2), 2.33 (t, J_H,H=

3

H-7), 1.90 (m, 2H, H-1), 1.20 (d, J_H,H= 6.3 Hz, 12H, H-10)

7.5 Hz, 4H, H-7), 2.28 (t, J_H,H= 7.4 Hz, 4H, H-6), 1.90 (m,

2H, H-1), 1.74 (m, 4H, H-7′), 1.21 (t, ³J_H,H= 7.1 Hz, 6H, H-10)

ppm. ¹³C NMR: δ = 180.1 (C-5), 176.9 (C-4), 173.0 (C-8), 59.9

(C-9), 56.3 (C-3), 34.4 (C-6), 33.0 (C-7), 31.8 (C-2), 21.0

(C-7′), 16.0 (C-1), 13.9 (C-10) ppm. ¹⁵N NMR: δ = −55.2 ppm.

¹⁹⁵Pt NMR: δ = 3554 ppm. IR (ATR): 3198 br (ν_N–H); 2980 w;

1733 w, 1718 m, 1622 s (ν_CvO); 1416 w, 1358 s, 1327 s,

ppm. ¹³C NMR: δ = 179.3 (C-5), 176.7 (C-4), 171.9 (C-8), 67.4

(C-9), 56.3 (C-3), 31.8 (C-2), 30.3 (C-6), 30.1 (C-7), 21.3 (C-10),

15.9 (C-1) ppm. ¹⁵N NMR: δ = −54.1 ppm. ¹⁹⁵Pt NMR: δ =

3565 ppm. IR (ATR): 3230 br (ν_N–H); 2980 w; 1728 m, 1654 s,

1630 s (ν_CvO); 1339 s, 1255 w, 1205 w, 1172 m, 1105 s cm⁻¹

.

1207 m, 1106 m cm⁻¹

.

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((4-butyloxy)-4-oxobutanoato)platinum(^IV) (3e). CDI (224 mg,

1.3814 mmol) in DMF (9 mL), 3 (400 mg, 0.6607 mmol) in

DMF (10 mL), n-ButONa/n-ButOH (10 mL). The crude product

was puriﬁed by column chromatography (EtOAc/MeOH, 9 : 1),

then recrystallized from MeOH, and dried under vacuum to

obtain a white powder. Yield: 110 mg, 11%. M_r= 717.62 g

mol⁻¹. Elemental analysis, found: C 36.60, H 5.14, N 3.93.

Calcd for C₂₂H₃₈N₂O₁₂Pt: C 36.82, H 5.34, N 3.90. ESI-MS:

m/z 739.2 [M + Na⁺]⁺, 716.1 [M − H⁺]⁻. ¹H NMR: δ = 6.73 (m,

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((5-methoxy)-5-oxo-(3-methyl)pentanoato)platinum(^IV) (5a). CDI

(240 mg, 1.480 mmol) in DMF (10 mL), 5 (481 mg,

0.727 mmol) in DMF (10 mL), MeONa/MeOH (10 mL). The

crude product was puriﬁed by column chromatography (EtOAc/

MeOH, 5 : 1) and isolated via suspending in EtOAc, ﬁltration,

washing with Et₂O and EtOAc and drying under vacuum to

obtain a white powder. Yield: 192 mg, 38%. M_r= 689.57 g

mol⁻¹. Elemental analysis, found: C 34.74, H 4.76, N 4.04.

Calcd for C₂₀H₃₄N₂O₁₂Pt: C 34.84, H 4.97, N 4.06. ESI-MS:

m/z 726.9 [M + K⁺]⁺, 711.1 [M + Na⁺]⁺, 689.9 [M + H⁺]⁺,

687.8 [M − H⁺]⁻, 723.9 [M + Cl⁻]⁻. ¹H NMR: δ = 6.78 (m, 6H,

3

6H, NH₃), 4.04 (t, J_H,H= 6.7 Hz, 4H, H-9), 2.63 (t, J_H,H

=

8.0 Hz, 4H, H-2), 2.54 (m, 4H, H-6), 2.48 (m, 4H, H-7), 1.90

(m, 2H, H-1), 1.58 (m, 4H, H-10), 1.36 (m, 4H, H-11), 0.91 (t,

³J_H,H= 7.4 Hz, 6H, H-12) ppm. ¹³C NMR: δ = 179.3 (C-5),

176.7 (C-4), 172.5 (C-8), 63.9 (C-9), 56.3 (C-3), 31.8 (C-2),

30.6 (C-10), 30.2 (C-6), 29.8 (C-7), 18.9 (C-11), 15.9 (C-1),

13.3 (C-12) ppm. ¹⁵N NMR: δ = −54.1 ppm. ¹⁹⁵Pt NMR: δ =

3565 ppm. IR (ATR): 3305 br, 3236 br (ν_N–H); 2959 w; 1728 m,

3

NH₃), 3.63 (s, 6H, H-9), 2.63 (t, J_H,H= 8.0 Hz, 4H, H-2), 2.44

(m, 2H, H-7), 2.29 (m, 2H, H-6), 2.27 (m, 2H, H-7′), 2.15 (m,

2H, H-6), 2.13 (m, 2H, H-7), 1.90 (m, 2H, H-1), 0.91 (d, ³J_H,H

=

6.5 Hz, 6H, H-7′′) ppm. ¹³C NMR: δ = 179.5 (²J_C,Pt= 28 Hz, C-5),

176.8 (C-4), 172.8 (C-8), 56.3 (C-3), 50.9 (C-9), 42.0 (³J_C,Pt= 36

Hz, C-6), 40.2 (C-7), 31.9 (C-2), 27.8 (C-7′), 19.3 (C-7′′), 16.0

(C-1) ppm. ¹⁵N NMR: δ = −54.6 ppm. ¹⁹⁵Pt NMR: δ = 3560 ppm.

IR (ATR): 3239 br, 3094 br (ν_N–H); 2876 w; 1732 m, 1632 s

(ν_CvO); 1437 w, 1331 s, 1211 m, 1155 m, 1079 m cm⁻¹.

1616 s, 1571 s (ν_CvO); 1424 m, 1399 s, 1169 m cm⁻¹

.

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((5-methoxy)-5-oxopentanoato)platinum(^IV) (4a). CDI (196 mg,

1.209 mmol) in DMF (8 mL), 4 (360 mg, 0.568 mmol) in DMF

(8 mL), MeONa/MeOH (10 mL). The crude product was

puriﬁed by column chromatography (EtOAc/MeOH, 3 : 1) and

isolated via suspending in Et₂O, ﬁltration, washing with Et₂O,

EtOAc and diisopropylester and drying under vacuum to obtain

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((5-methoxy)-5-oxo-(3,3-dimethyl)pentanoato)platinum(^IV) (6a).

CDI (180 mg, 1.110 mmol) in DMF (6 mL), 6 (375 mg,

0.544 mmol) in DMF (10 mL), MeONa/MeOH (8 mL). The

crude product was puriﬁed by column chromatography (EtOAc/

MeOH, 6 : 1) and isolated via suspending in EtOAc, ﬁltration,

a white powder. Yield: 94 mg, 16%. M_r= 661.51 g mol⁻¹

.

Elemental analysis, found: C 32.33, H 4.32, N 4.26. Calcd for

C₁₈H₃₀N₂O₁₂Pt: C 32.68, H 4.57, N 4.23. ESI-MS: m/z 683.1

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 14404–14415 | 14411

View Article Online

washing with Et₂O and drying under vacuum to obtain a white

The ﬁnal product was obtained as a white powder through sus-

pension in EtOAc, ﬁltration, washing with Et₂O and drying

powder. Yield: 125 mg (32%). M_r= 717.62 g mol⁻¹. Elemental

analysis, found:

C

36.51,

H

5.35,

N

3.87. Calcd for

under vacuum. Yield: 72 mg, 21%. M_r= 719.60 g mol⁻¹

.

C₂₂H₃₈N₂O₁₂Pt: C 36.82, H 5.34, N 3.90. ESI-MS: m/z 761.8

Elemental analysis, found: C 32.86, H 4.90, N 7.47. Calcd for

C₂₀H₃₆N₄O₁₂Pt·0.5H₂O: C 32.97, H 5.12, N 7.69. ESI-MS: m/z

741.9 [M + Na⁺]⁺, 720.0 [M + H⁺]⁺, 718.1 [M − H⁺]⁻. ¹H

NMR: δ = 7.86 (bs, 2H, CONH), 6.72 (m, 6H, NH₃), 3.39 (t,

[M + 2Na⁺− H⁺]⁺, 739.9 [M + Na⁺]⁺, 715.9 [M − H⁺]⁻.

¹H NMR: δ = 6.79 (m, 6H, NH₃), 3.61 (s, 6H, H-9), 2.63 (t, ³J_H,H

=

7.9 Hz, 4H, H-2), 2.39 (s, 4H, H-7), 2.29 (s, 4H, H-6), 1.91 (m,

2H, H-1), 1.03 (s, 12H, H-7′′) ppm. ¹³C NMR: δ = 179.2 (C-5),

³J_H,H= 5.9 Hz, 4H, H-10), 3.31 (t, J_H,H= 5.5 Hz, 4H, H-9),

3

176.8 (C-4), 172.2 (C-8), 56.3 (C-3), 50.6 (C-9), 46.8 (³J_C,Pt

=

3.28 (s, 6H, H-11), 2.63 (t, J_H,H= 8.0 Hz, 4H, H-2), 2.49 (t,

3

36 Hz, C-6), 44.6 (C-7), 32.5 (C-7′), 31.9 (C-2), 26.9 (C-7′′),

16.0 (C-1) ppm. ¹⁵N NMR: δ = −55.8 ppm. ¹⁹⁵Pt NMR: δ =

3567 ppm. IR (ATR): 3230 br, 3097 br (ν_N–H); 2956 m, 2876 w;

1735 m, 1618 s, 1570 w (ν_CvO); 1436 w, 1332 s, 1227 m,

³J_H,H= 7.1 Hz, 4H, H-6), 2.38 (t,³J_H,H= 7.5 Hz, 4H, H-7), 1.89

(m, 2H, H-1) ppm. ¹³C NMR: δ = 180.1 (C-5), 176.9 (C-4),

171.8 (C-8), 71.1 (C-10), 57.9 (C-11), 56.4 (C-3), 38.9 (C-9),

31.9 (C-2), 31.6 (C-6), 31.2 (C-7), 16.0 (C-1) ppm. ¹⁵N NMR:

δ = −53.8 (NH₃), 88.2 (NH-amide) ppm. ¹⁹⁵Pt NMR: δ =

3561 ppm. IR (ATR): 3531 br, 3224 br (ν_N–H); 2945 w; 1629 s,

1149 m, 1112 m cm⁻¹

.

1551 m (ν_CvO); 1338 s, 1257 m, 1210 w, 1121 w cm⁻¹

.

General procedure for synthesis of amide derivatives 3f–j, 4f,

5f, 6f

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis((4-

cyclopentylamino)-4-oxobutanoato)platinum(^IV) (3h). CDI

2 equiv. of CDI (1,1′-carbonyldiimidazole) in dry DMF were

added to a DMF solution of 3, 4, 5 or 6, respectively, and the

mixture was heated at 60 °C for 10 min. After cooling to room

temperature, CO₂formed during the activation was removed by

ﬂushing the solution with argon for 10 min. Then, 2.3 equiv. of

the corresponding amine, dissolved in dry DMF, were added and

the solution was stirred for 24 h (up to 48 h in some cases).

Finally, DMF was removed under reduced pressure and the

crude products were puriﬁed by column chromatography.

(210 mg, 1.295 mmol) in DMF (8 mL), 3 (392 mg,

0.648 mmol) in DMF (8 mL), cyclopentylamine (0.14 ml,

1.417 mmol) in DMF (3 mL). The crude product was puriﬁed

by column chromatography (EtOAc/MeOH, 3 : 1). The yellow

powder, obtained via suspension in EtOAc, was recrystallized

from MeOH, precipitated with EtOAc, ﬁltered off, washed with

Et₂O and dried under vacuum. A pale yellow to almost white

powder was obtained. Yield: 184 mg, 38%. M_r= 739.67 g

mol⁻¹. Elemental analysis, found: C 37.71, H 5.19, N 7.40.

Calcd for C₂₄H₄₀N₄O₁₀Pt·H₂O: C 38.04, H 5.59, N 7.39.

ESI-MS: m/z 761.9 [M + Na⁺]⁺, 739.0 [M + H⁺]⁺, 738.1 [M −

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((4-propylamino)-4-oxobutanoato)platinum(^IV) (3f). CDI (218 mg,

1.3444 mmol) in DMF (8 mL), 3 (407 mg, 0.6723 mmol) in

DMF (8 mL), propylamine (112 μl, 1.3567 mmol) in DMF

(2 mL). The crude product was puriﬁed by column chromato-

graphy (EtOAc/MeOH, 2 : 1) and isolated from an EtOAc sus-

pension, followed by recrystallization from MeOH and washing

with a cold mixture of EtOAc and Et₂O. An almost white to pale

yellow powder was obtained and dried under vacuum. Yield:

124 mg, 24%. M_r= 687.20 g mol⁻¹. Elemental analysis, found:

C 34.43, H 5.21, N 8.15. Calcd for C₂₀H₃₆N₄O₁₀Pt·0.5H₂O:

C 34.48, H 5.35, N 8.04. ESI-MS: m/z 709.9 [M + Na⁺]⁺, 685.9

1

3

H⁺]⁻, 774.7 [M + Cl⁻]⁻. H NMR: δ = 7.77 (d, J_H,H= 6.7 Hz,

2H, CONH), 6.73 (m, 6H, NH₃), 4.07 (m, 2H, H-9), 2.62 (t, ³J_H,

= 7.9 Hz, 4H, H-2), 2.46 (t, J_H,H= 7.2 Hz, 4H, H-6), 2.32 (t,

3

H

³J_H,H= 7.4 Hz, 4H, H-7), 1.89 (m, 2H, H-1), 1.82 (m, 4H,

H-10), 1.65 (m, 4H, H-11), 1.52 (m, 4H, H-11), 1.43 (m, 4H,

H-10) ppm. ¹³C NMR: δ = 180.2 (C-5), 176.8 (C-4), 171.1

(C-8), 56.3 (C-3), 50.8 (C-9), 32.5 (C-10), 31.8 (C-2), 31.6

(C-7), 31.3 (C-6), 23.6 (C-11), 16.0 (C-1) ppm. ¹⁵N NMR: δ =

−54.3 (NH₃), 105.8 (NH-amide) ppm. ¹⁹⁵Pt NMR: δ =

3562 ppm. IR (ATR): 3260 br, 3086 br (ν_N–H); 2952 m, 2870 w;

1622 s, 1544 m (ν_CvO); 1332 s, 1252 m, 1207 m, 1097 w cm⁻¹

.

[M − H⁺]⁻. H NMR: δ = 7.78 (bs, 2H, CONH), 6.73 (m, 6H,

1

NH₃), 3.09 (m, 4H, H-9), 2.62 (t, ³J_H,H= 8.0 Hz, 4H, H-2), 2.48

3

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((4-cyclohexylamino)-4-oxobutanoato)platinum(^IV) (3i). CDI

(t, J_H,H= 7.1 Hz, 4H, H-6), 2.35 (t, J_H,H= 7.6 Hz, 4H, H-7),

3

1.89 (m, 2H, H-1), 1.46 (m, 4H, H-10), 0.87 (t, J_H,H= 7.4 Hz,

6H, H-11) ppm. ¹³C NMR: δ = 180.1 (C-5), 176.8 (C-4), 171.5

(C-8), 56.3 (C-3), 40.8 (C-9), 31.8 (C-2), 31.6 (C-7), 31.2 (C-6),

22.7 (C-10), 16.0 (C-1), 11.0 (C-11) ppm. ¹⁵N NMR: δ = −54.3

(NH₃), 94.1 (NH-amide) ppm. ¹⁹⁵Pt NMR: δ = 3562 ppm. IR

(ATR): 3236 br, 3087 br (ν_N–H); 2963 w; 1625 s, 1551 m

(190.7 mg, 1.176 mmol) in DMF (6 mL), 3 (356 mg,

0.588 mmol) in DMF (8 mL), cyclohexylamine (0.15 ml,

1.311 mmol) in DMF (2 mL). The crude product was puriﬁed

by column chromatography (EtOAc/IPA, 2 : 1), then isolated via

suspension in EtOAc, ﬁltration, washing with EtOAc and Et₂O

and drying under vacuum to obtain a white powder. Yield:

162 mg, 36%. M_r= 767.7 g mol⁻¹. Elemental analysis, found:

C 40.29, H 5.50, N 7.22. Calcd for C₂₆H₄₄N₄O₁₀Pt: C 40.68,

H 5.78, N 7.30. ESI-MS: m/z 789.1 [M + Na⁺]⁺, 768.0 [M +

(ν_CvO); 1333 s, 1249 m, 1207 m, 1158 w cm⁻¹

.

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis((4-(2-

methoxy)ethylamino)-4-oxobutanoato)platinum(^IV) (3g). CDI

1

3

(159 mg, 0.9806 mmol) in DMF (5 mL), 3 (283 mg,

0.4675 mmol) in DMF (5 mL), 2-methoxyethylamine (101 μl,

1.1726 mmol) in DMF (4 mL). The crude product was puriﬁed

by column chromatography (EtOAc/MeOH, 1 : 1). A yellow

sticky substance was isolated and puriﬁed again by column

chromatography, using EtOAc/MeOH = 3 : 1 as mobile phase.

H⁺]⁺, 766.2 [M − H⁺]⁻. H NMR: δ = 7.66 (d, J_H,H= 7.6 Hz,

2H, CONH), 6.73 (m, 6H, NH₃), 3.59 (m, 2H, H-9), 2.62 (t, ³J_H,

3

= 7.9 Hz, 4H, H-2), 2.47 (t, J_H,H= 7.7 Hz, 4H, H-6), 2.33 (t,

H

³J_H,H= 7.2 Hz, 4H, H-7), 1.89 (m, 2H, H-1), 1.79 (m, 4H,

H-10), 1.69 (m, 4H, H-11), 1.56 (m, 2H, H-12), 1.27 (m, 4H,

H-11), 1.18 (m, 4H, H-10), 1.16 (m, 2H, H-12) ppm. ¹³C NMR:

14412 | Dalton Trans., 2012, 41, 14404–14415

View Article Online

δ = 180.1 (C-5), 176.8 (C-4), 170.6 (C-8), 56.3 (C-3), 47.9

(C-9), 32.7 (C-10), 31.8 (C-2), 31.7 (C-7), 31.3 (C-6), 25.6

(C-12), 24.9 (C-11), 16.0 (C-1) ppm. ¹⁵N NMR: δ = −54.3

(NH₃), 108.1 (NH-amide) ppm. ¹⁹⁵Pt NMR: δ = 3562 ppm. IR

(ATR): 3319 br, 3272 br (ν_N–H); 2929 m, 2854 w; 1658 s, 1639

by column chromatography (EtOAc/MeOH/IPA, 3 : 1 : 1, fol-

lowed by EtOAc/MeOH, 3 : 1 for the second column), then iso-

lated from an EtOAc suspension and washed with a cold mixture

of EtOAc and Et₂O. The white powder obtained was dried under

vacuum. Yield: 252 mg, 49%. M_r= 743.70 g mol⁻¹. Elemental

s, 1538 m (ν_CvO); 1442 w, 1330 s, 1256 w, 1209 w cm⁻¹

.

analysis, found: C 38.10, H 5.78, N 7.51. Calcd for

C₂₄H₄₄N₄O₁₀Pt·0.5(H₂O): C 38.30, H 6.03, N 7.44. ESI-MS:

m/z 782.0 [M + K⁺]⁺, 766.0 [M + Na⁺]⁺, 743.1 [M + H⁺]⁺,

742.2 [M − H⁺]⁻. ¹H NMR: δ = 7.68 (bs, 2H, CONH), 6.81 (m,

(OC-6-33)-Diamminebis((4-benzylamino)-4-oxobutanoato)-

(cyclobutane-1,1-dicarboxylato)platinum(^IV) (3j). CDI (175 mg,

1.079 mmol) in DMF (7 mL), 3 (322 mg, 0.532 mmol) in DMF

(8 mL), benzylamine (145 μl, 1.329 mmol) in DMF (5 mL). The

crude product was puriﬁed by column chromatography (EtOAc/

MeOH, 4 : 1) and then isolated via suspending in EtOAc. The

obtained substance was recrystallized from MeOH, and precipi-

tated with the help of acetone and ethyl acetate, ﬁltered off,

washed with diisopropyl ether and dried under vacuum to obtain

3

6H, NH₃), 3.10 (m, 4H, H-9), 2.63 (t, J_H,H= 7.9 Hz, 4H, H-2),

2.29 (m, 2H, H-7′), 2.25 (m, 2H, H-6), 2.17 (m, 2H, H-7), 2.10

(m, 2H, H-6), 2.00 (m, 2H, H-7), 1.90 (m, 2H, H-1), 1.47 (m,

3

4H, H-10), 0.90 (d, J_H,H= 4.7 Hz, 6H, H-7′′), 0.88 (t, J_H,H

=

7.4 Hz, 6H, H-11) ppm. ¹³C NMR: δ = 180.0 (C-5), 177.0

(C-4), 171.5 (C-8), 56.4 (C-3), 42.9 (C-7), 42.7 (C-6), 40.7

(C-9), 32.0 (C-2), 28.6 (C-7′), 22.8 (C-10), 19.6 (C-7′′), 16.0

(C-1), 11.2 (C-11) ppm. ¹⁵N NMR: δ = −54.3 (NH₃), 96.7 (NH-

amide) ppm. ¹⁹⁵Pt NMR: δ = 3560 ppm. IR (ATR): 3259 br,

3084 br (ν_N–H); 2962 m, 2875 w; 1624 s, 1551 m(ν_CvO); 1458

a white powder. Yield: 140 mg, 34%. M_r= 783.7 g mol⁻¹

.

Elemental analysis, found: C 42.60, H 4.67, N 6.80. Calcd for

C₂₈H₃₆N₄O₁₀Pt: C 42.91, H 4.63, N 7.15. ESI-MS: m/z 805.9

[M + Na⁺]⁺, 783.0 [M + H⁺]⁺, 781.9 [M − H⁺]⁻, 818.6 [M +

w, 1344 m, 1304 m, 1221 w, 1098 w cm⁻¹

.

1

3

Cl⁻]⁻. H NMR: δ = 8.28 (t, J_H,H= 5.5 Hz, 2H, CONH), 7.34

(m, 4H, H-12), 7.31 (m, 4H, H-11), 7.25 (m, 2H, H-13), 6.75

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

(3,3-dimethyl-(5-propylamino)-5-oxopentanoato)platinum(^IV) (6f).

CDI (152 mg, 0.9343 mmol) in DMF (10 mL), 6 (263 mg,

0.3810 mmol) in DMF (8 mL), propylamine (0.1 ml,

1.2112 mmol) in DMF (2 mL). The crude product was puriﬁed

by column chromatography (EtOAc/MeOH/IPA, 4 : 1 : 1), then

isolated from an EtOAc suspension and recrystallized from

MeOH, suspended in EtOAc/Et₂O mixture, ﬁltered off, washed

with Et₂O and dried under vacuum to obtain a white powder.

Yield: 98 mg, 33%. M_r= 771.76 g mol⁻¹. Elemental analysis,

found: C 40.08, H 5.99, N 7.41. Calcd for C₂₆H₄₈N₄O₁₀Pt:

C 40.46, H 6.27, N 7.26. ESI-MS: m/z 810.0 [M + K⁺]⁺, 794.0

[M + Na⁺]⁺, 771.2 [M + H⁺]⁺, 770.0 [M − H⁺]⁻, 806.9 [M +

(m, 6H, NH₃), 4.38 (d, ³J_H,H= 5.9 Hz, 4H, H-9), 2.64 (t, ³J_H,H

=

3

8.0 Hz, 4H, H-2), 2.54 (t, J_H,H= 7.1 Hz, 4H, H-6), 2.44 (t,

³J_H,H= 7.3 Hz, 4H, H-7), 1.89 (m, 2H, H-1) ppm. ¹³C NMR:

δ = 180.0 (C-5), 176.8 (C-4), 171.7 (C-8), 140.0 (C-10), 128.3

(C-12) 127.4 (C-11), 126.8 (C-13), 56.3 (C-3), 42.6 (C-9), 31.8

(C-2), 31.6 (C-7), 31.1 (C-6), 16.0 (C-1) ppm. ¹⁵N NMR: δ =

−54.5 (NH₃), 93.6 (NH-amide) ppm. ¹⁹⁵Pt NMR:

δ =

3562 ppm. IR (ATR): 3258 br, 3076 br (ν_N–H); 2954 w; 1626 s,

1545 m (ν_CvO); 1420 w, 1339 s, 1245 m, 1211 w cm⁻¹

.

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

((5-propylamino)-5-oxopentanoato)platinum(^IV) (4f). CDI (164 mg,

1.011 mmol) in DMF (8 mL), 4 (289 mg, 0.456 mmol) in DMF

(5 mL), propylamine (85 μl, 1.021 mmol) in DMF (3 mL). The

crude product was puriﬁed twice by column chromatography

(EtOAc/MeOH, 1 : 1, followed by EtOAc/MeOH, 3 : 1), then

isolated from an EtOAc suspension and washed with a cold

mixture of EtOAc and Et₂O. The white powder obtained was

1

3

Cl⁻]⁻. H NMR: δ = 7.60 (t, J_H,H= 5.5 Hz, 2H, CONH), 6.83

3

(m, 6H, NH₃), 3.09 (m, 4H, H-9), 2.63 (t, J_H,H= 8.0 Hz, 4H,

H-2), 2.28 (s, 4H, H-6), 2.16 (s, 4H, H-7), 1.90 (m, 2H, H-1),

1.46 (m, 4H, H-10), 1.03 (s, 12H, C-7′′), 0.88 (t, ³J_H,H= 7.4 Hz,

6H, H-11) ppm. ¹³C NMR: δ = 179.7 (C-5), 176.9 (C-4), 170.9

(C-8), 56.3 (C-3), 47.3 (C-6), 47.2 (C-7), 40.6 (C-9), 32.9

(C-7′), 31.9 (C-2), 27.5 (C-7′′), 22.8 (C-10), 16.0 (C-1), 11.2

(C-11) ppm. ¹⁵N NMR: δ = −55.5 (NH₃), 98.7 (NH-amide)

ppm. ¹⁹⁵Pt NMR: δ = 3568 ppm. IR (ATR): 3266 br, 3075 br

(ν_N–H); 2964 m, 2933 m; 1626 s, 1560 m (ν_CvO); 1472 w,

dried under vacuum. Yield: 60 mg, 18%. M_r= 715.65 g mol⁻¹

.

Elemental analysis, found: C 36.58, H 5.34, N 7.58. Calcd for

C₂₂H₄₀N₄O₁₀Pt: C 36.92, H 5.63, N 7.83. ESI-MS: m/z 737.1

1

[M + Na⁺]⁺, 716.1 [M + H⁺]⁺, 715.4 [M − H⁺]⁻. H NMR: δ =

7.65 (bs, 2H, CONH), 6.78 (m, 6H, NH₃), 3.09 (m, 4H, H-9),

1345 m, 1333 m, 1241 w cm⁻¹

.

3

2.62 (t, J_H,H= 8.0 Hz, 4H, H-2), 2.23 (t, J_H,H= 7.5 Hz, 4H,

3

H-6), 2.14 (t, J_H,H= 7.4 Hz, 4H, H-7), 1.89 (m, 2H, H-1), 1.74

(m, 4H, H-7′), 1.46 (m, 4H, H-10), 0.87 (t, J_H,H= 7.4 Hz, 6H,

3

Determination of lipophilicity

H-11) ppm. ¹³C NMR: δ = 180.3 (C-5), 176.9 (C-4), 172.0

(C-8), 56.4 (C-3), 40.7 (C-9), 35.1 (C-7), 35.0 (C-6), 31.8 (C-2),

22.8 (C-10), 22.0 (C-7′), 16.0 (C-1), 11.1 (C-11) ppm. ¹⁵N

NMR: δ = −54.4 (NH₃), 93.6 (NH-amide) ppm. ¹⁹⁵Pt NMR: δ =

3555 ppm. IR (ATR): 3250 br, 3088 br (ν_N–H); 2960 w; 1637 s,

Lipophilicity of the new compounds was determined as chroma-

tographic retention factors (namely log k₃₀and log k_w) using

reversed-phased HPLC.

The analysis was performed on a Dionex Summit system con-

trolled by the Dionex Chromeleon 6.60 software. The samples

were prepared by dissolving (with the help of ultrasonic) around

1.5 mg of each complex in a H₂O/MeOH mixture, followed by

ﬁltration through a 0.2 μm Nylon ﬁlter. The chromatographic

conditions were as follows: an Agilent ZORBAX Bonus-RP

column (4.6 mm × 250 mm); injection volume: 25 μl; ﬂow rate:

1 ml min⁻¹; isocratic elution; temperature of the column: 25 °C;

1629 s, 1560 m (ν_CvO); 1458 w, 1336 s, 1233 m cm⁻¹

.

(OC-6-33)-Diammine(cyclobutane-1,1-dicarboxylato)bis-

(3-methyl-(5-propylamino)-5-oxopentanoato)platinum(^IV) (5f).

CDI (222 mg, 1.3691 mmol) in DMF (8 mL), 5 (453 mg,

0.6848 mmol) in DMF (10 mL), propylamine (0.15 ml,

1.8170 mmol) in DMF (3 mL). The crude product was puriﬁed

Dalton Trans., 2012, 41, 14404–14415 | 14413

View Article Online

Cell lines and culture conditions

UV-vis detection set up at 210 nm; uracil was used as an internal

reference to determine the column dead-time (t₀); mobile phases

containing different percentages of 0.1% TFA water solution and

MeOH (the MeOH fraction ranged from 60% for the most lipo-

philic compounds to 10% for the most hydrophilic compounds);

chromatograms for each complex were run with at least three

different mobile phase compositions and at least two times.

The capacity factors k = (t_R− t₀)/t₀(t_Ris the retention time of

the species analyzed and t₀is the retention time of uracil) of the

investigated compounds were calculated for all eluant compo-

sitions. The partition between the lipophilic stationary phase and

water (0% MeOH) was determined by extrapolation, using the

linear Soczewinski–Snyder relationship^43,44between log k and

the concentration of the organic modiﬁer in the mobile phase:

CH1 (ovarian carcinoma, human) cells were a gift from Lloyd

R. Kelland (CRC Centre for Cancer Therapeutics, Institute of

Cancer Research, Sutton, U.K.). A549 (non-small cell lung

cancer, human) and SW480 (colon carcinoma, human) cells

were kindly provided by Brigitte Marian (Institute of Cancer

Research, Department of Medicine I, Medical University of

Vienna, Austria), and SK-OV-3 (ovarian carcinoma, human)

cells by Evelyn Dittrich (General Hospital, Medical University

of Vienna, Austria). Cells were grown in 75 cm²culture ﬂasks

(Iwaki/Asahi Technoglass) as adherent monolayer cultures in

Minimal Essential Medium (MEM) supplemented with 10%

heat-inactivated fetal bovine serum, 1 mM sodium pyruvate,

4 mM ^L-glutamine and 1% non-essential amino acids (from

100× ready-to-use stock) (all purchased from Sigma-Aldrich)

without antibiotics. Cultures were maintained at 37 °C in a

humidiﬁed atmosphere containing 5% CO₂and 95% air.

log k ¼ log k_wꢀ Sφ;

where log k is the capacity factor in the speciﬁc mobile phase

composition, φ is the volume fraction of MeOH in the eluant,

S is a constant for a given solute and a given HPLC system and

log k_wcorresponds to log k in pure water (buffer).

Cytotoxicity tests in cancer cell lines

Cytotoxicity in the cell lines mentioned above was determined

by the colorimetric MTT assay (MTT = 3-(4,5-dimethyl-2-thia-

zolyl)-2,5-diphenyl-2H-tetrazolium bromide, purchased from

Fluka). Cells were harvested from culture ﬂasks by trypsinization

and seeded in 100 μL aliquots in MEM (see above) into 96-well

microculture plates (Iwaki/Asahi Technoglass) in the following

densities to ensure exponential growth of untreated controls

throughout the experiment: 1.5 × 10³(CH1), 3.5 × 10³

(SK-OV-3), 4.0 × 10³(A549), and 2.5 × 10³(SW480) viable

cells per well. Cells were allowed to settle and resume exponen-

tial growth in a drug-free medium for 24 h, followed by the

addition of dilutions of the test compounds in 100 μL per well of

the same medium. Only for the less water soluble compound 3e,

a stock solution in DMSO/water was prepared (the DMSO

content in the cytotoxicity assay did not exceed 0.5%). After

continuous exposure for 96 h, the medium was replaced by a

100 μL per well RPMI 1640 medium (supplemented with 10%

heat-inactivated fetal bovine serum and 4 mM ^L-glutamine) plus

a 20 μL per well solution of MTT in phosphate-buffered saline

(5 mg mL⁻¹) (all purchased from Sigma-Aldrich). After incu-

bation for 4 h, medium/MTT mixtures were removed, and the

formazan product formed by viable cells was dissolved in

DMSO (150 μL per well). Optical densities at 550 nm were

measured with a microplate reader (Tecan Spectra Classic), using

a reference wavelength of 690 nm to correct for unspeciﬁc

absorption. The quantity of viable cells was expressed as a per-

centage of untreated controls, and 50% inhibitory concentrations

(IC₅₀) were calculated from concentration–effect curves by

interpolation. Evaluation is based on means from three indepen-

dent experiments, each comprising six replicates per concen-

tration level.

Electrochemical experiments

Cyclic voltammograms were measured in a three-electrode cell

using a 2.0 mm diameter glassy carbon disc working electrode, a

platinum auxiliary electrode, and a Ag/Ag⁺reference electrode

containing 0.10 M AgNO₃, the potential of which was corrected

using an internal standard of ferrocenium/ferrocene. Measure-

ments were performed at room temperature using an EG & G

PARC 273A potentiostat/galvanostat. Deaeration of solutions

was accomplished by passing a stream of argon through the solu-

tion for 5 min prior to the measurement and then maintaining a

blanket atmosphere of argon over the solution during the

measurement. The potentials were measured at a scan rate of

100 mV s⁻¹in 0.15 M [n-Bu₄N][BF₄]/DMF, using [Fe(η⁵-

45

1

C₅H₅)₂] (E = +0.72 V vs. NHE) as the internal standard, and

are quoted ²relative to the normal hydrogen electrode (NHE).

Incubation with ascorbic acid

Reduction of complex 3f and its close analogue featuring

ethane-1,2-diamine and two chlorido ligands in the equatorial

position (M1) by ascorbic acid was monitored by ¹H NMR spec-

troscopy at ambient temperature. 1 mM solutions of the com-

pounds were prepared in 50 mM phosphate buffer (in D₂O, pD =

1

7.4) and H NMR spectra were measured from time to time over

a period of 24 h in order to judge the stability of the complexes;

changes in the NMR spectra could not be seen. Then, ascorbic

acid (25 mM) was added and ¹H NMR spectra were recorded for

24 h in the case of M1 and during three weeks for complex 3f.

The reduction was monitored by following the decrease of inten-

sity of the CH₂signal of the ethane-1,2-diamine ligand of M1

resonating at 2.8 ppm. In the case of complex 3f, the increase of

intensity of CH₂protons H2 at 2.8 ppm deriving from the cyclo-

butanedicarboxylato ligand of the reduced platinum(^II) species

was monitored. In both cases, integration of the signals was per-

formed relative to the signals of the terminal methyl groups of

the propylamine moieties.

Acknowledgements

H. V. is thankful for ﬁnancial support from the University of

Vienna within the doctoral program Initiativkolleg Functional

Molecules IKI041-N. The authors are indebted to the FFG –

Austrian Research Promotion Agency, the Austrian Council for

14414 | Dalton Trans., 2012, 41, 14404–14415

View Article Online

23 S. Dhar and S. J. Lippard, Proc. Natl. Acad. Sci. U. S. A., 2009, 106,

22199–22204.

24 K. R. Barnes, A. Kutikov and S. J. Lippard, Chem. Biol., 2004, 11,

557–564.

Research and Technology Development, the FWF (Austrian

Science Fund, P20683-N19) and COST D39.

25 M. Reithofer, M. Galanski, A. Roller and B. K. Keppler, Eur. J. Inorg.

Chem., 2006, 13, 2612–2617.

References

26 M. R. Reithofer, S. M. Valiahdi, M. A. Jakupec, V. B. Arion, A. Egger,

M. Galanski and B. K. Keppler, J. Med. Chem., 2007, 50, 6692–6699.

27 M. R. Reithofer, A. Schwarzinger, S. M. Valiahdi, M. Galanski,

M. A. Jakupec and B. K. Keppler, J. Inorg. Biochem., 2008, 102,

2072–2077.

1 M. Rosenberg, L. VanCamp and T. Krigas, Nature, 1965, 205, 698–699.

2 M. Galanski, Recent Pat. Anti-Cancer Drug Discovery, 2006, 1, 285–295.

3 M. A. Jakupec, M. Galanski, V. B. Arion, C. G. Hartinger and

B. K. Keppler, Dalton Trans., 2008, 183–194.

28 H. Varbanov, S. M. Valiahdi, A. A. Legin, M. A. Jakupec, A. Roller,

M. Galanski and B. K. Keppler, Eur. J. Med. Chem., 2011, 46,

5456–5464.

29 J. J. Wilson and S. J. Lippard, Inorg. Chem., 2011, 50, 3103–3115.

30 U. Bierbach and J. Reedijk, Angew. Chem., 1994, 106, 1701–1703.

31 J. A. Platts, S. P. Oldﬁeld, M. M. Reif, A. Palmucci, E. Gabano and

D. Osella, J. Inorg. Biochem., 2006, 100, 1199–1207.

32 I. V. Tetko, I. Jaroszewicz, J. A. Platts and J. Kuduk-Jaworska, J. Inorg.

Biochem., 2008, 102(7), 1424–1437.

4 M. D. Hall, H. R. Mellor, R. Callaghan and T. W. Hambley, J. Med.

Chem., 2007, 50(15), 3403–3411.

5 B. Rosenberg, Platinum Met. Rev., 1971, 15, 42–51.

6 B. W. Harper, A. M. Krause-Heuer, M. P. Grant, M. Manohar,

K. B. Garbutcheon-Singh and J. R. Aldrich-Wright, Chem.–Eur. J., 2010,

16, 7064–7077.

7 N. J. Wheate, S. Walker, G. E. Craig and R. Oun, Dalton Trans., 2010,

39, 8113–8127.

8 http://www.clinicaltrials.gov, U.S National Library of medicine, Bethesda

(1993).

9 M. D. Hall and T. W. Hambley, Coord. Chem. Rev., 2002, 232, 49–67.

10 H. Choy, Expert Rev. Anti-Infect. Ther., 2006, 6, 973–982.

11 M. D. Hall, H. R. Mellor, R. Callaghan and T. W. Hambley, J. Med.

Chem., 2007, 50(15), 3403–3411.

12 M. Galanski and B. K. Keppler, Inorg. Chim. Acta, 2000, 300–302,

783–789.

13 R. R. Barefoot, J. Chromatogr., Biomed. Appl., 2001, 751, 205–211.

14 S. Mukhopadhyay, C. M. Barnes, A. Haskel, S. M. Short, K. R. Barnes

and S. J. Lippard, Bioconjugate Chem., 2008, 19, 39–49.

15 S. Abramkin, S. M. Valiahdi, M. A. Jakupec, M. Galanski, N. Metzler-

Nolte and B. K. Keppler, Dalton Trans., 2012, 41, 3001–3005.

16 N. Graf, T. E. Mokhtari, A. I. Papayannopoulos and S. J. Lippard,

J. Inorg. Biochem., 2012, 110, 58–63.

17 R. P. Feazell, N. Nakayama-Ratchford, H. Dai and S. J. Lippard, J. Am.

Chem. Soc., 2007, 129, 8438–8439.

18 S. Dhar, Z. Liu, J. Thomale, H. Dai and S. J. Lippard, J. Am. Chem. Soc.,

2008, 130, 11467–11476.

19 S. Dhar, F. X. Gu, R. Langer, O. C. Farokhzad and S. J. Lippard, Proc.

Natl. Acad. Sci. U. S. A., 2008, 105, 17356–17361.

20 S. Dhar, W. L. Daniel, D. A. Giljohann, C. A. Mirkin and S. J. Lippard,

J. Am. Chem. Soc., 2009, 131(41), 14652–14653.

21 W. H. Ang, I. Khalaila, C. S. Allardyce, L. Juillerat-Jeanneret and

P. J. Dyson, J. Am. Chem. Soc., 2005, 127, 1382–1383.

22 M. R. Reithofer, S. M. Valiahdi, M. Galanski, M. A. Jakupec,

V. B. Arion and B. K. Keppler, Chem. Biodivers., 2008, 5, 2160–2170.

33 S. P. Oldﬁeld, M. D. Hall and J. A. Platts, J. Med. Chem., 2007, 50,

5227–5237.

34 G. Caron, M. Ravera and G. Ermondi, Pharm. Res., 2011, 28, 640–646.

35 A. Bytzek, M. Reithofer, M. Galanski, M. Groessl, B. K. Keppler and

C. G. Hartinger, Electrophoresis, 2010, 31, 1144–1150.

36 J. A. Platts, G. Ermondi, G. Caron, M. Ravera, E. Gabano, L. Gaviglio,

G. Pelosi and D. Osella, J. Biol. Inorg. Chem., 2011, 16, 361–372.

37 A. Nasal, D. Siluk and R. Kaliszan, Curr. Med. Chem., 2003, 10,

381–426.

38 K. Valko, J. Chromatogr., A, 2004, 1037, 299–310.

39 M. R. Reithofer, A. K. Bytzek, S. M. Valiahdi, C. R. Kowol, M. Groessl,

C. G. Hartinger, M. A. Jakupec, M. Galanski and B. K. Keppler,

J. Inorg. Biochem., 2011, 105, 46–51.

40 J. Z. Zhang, E. Wexselblatt, T. W. Hambley and D. Gibson, Chem.

Commun., 2012, 48, 847–849.

41 F. D. Rochon and G. Massarweh, Inorg. Chim. Acta, 2006, 359,

4095–4104.

42 F. Kiss, J. Novotny, I. Zavodna and D. Ruzicka, (Czech.). Czech. (1988),

6 pp. CODEN: CZXXA9 CS 255958 B1 19880415 Patent written in

Czech. Application: CS 1985-5536 19850729. Priority: CS 1985-5536

19850729. CAN 111:166201 AN 1989:566201.

43 L. R. Snyder, J. W. Dolan and J. R. Gant, J. Chromatogr., 1979, 165,

3–31.

44 E. Soczewinski and C. A. Wachtmeister, J. Chromatogr., 1962, 7, 311.

45 W. C. Barette Jr., H. W. Johnson Jr. and D. T. Sawyer, Anal. Chem., 1984,

56, 1890–1898.

Dalton Trans., 2012, 41, 14404–14415 | 14415

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Novel tetracarboxylatoplatinum(iv) complexes as carboplatin prodrugs

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