L.-M. Rečnik, et al.
Bioorganic&MedicinalChemistryLetters30(2020)127527
ketone 4 using pyridinium chlorochromate. Furthermore, alcohol 3 was
also transformed to tosylate 5 using tosyl chloride, triethylamine and
DMAP. The resulting tosylate 5 was then treated with sodium azide in
the presence of Bu4NHSO4 in DMF to yield the azide-containing com-
pound 6 which was further reduced to amine 7 using Pd/C under H2
atmosphere. Tosylate 5 was also transformed to iodide 8 with NaI in
acetone. The functionalised cbdc diethyl esters 3, 4 and 6 were hy-
drolysed with LiOH in THF/H2O to yield the corresponding free di-
carboxylic acids 9–11. Hydrolysis of amino-cbdc diethyl ester 7 to the
amino-cbdc ligand 12 was not successful under these conditions most
likely due to the formation of the hydrochloride salt which could not be
isolated. However, hydrogenation of azido-cbdc 11 with Pd/C under
H2-atmosphere resulted in the formation of compound 12. Iodo-cbdc
ethyl ester 8 could not be hydrolysed to compound 13 under these
conditions either, as the iodide was not stable under basic conditions.
Therefore di-t-butyl ester 8 was synthesised using the same synthetic
pathway but using di-t-butyl malonate instead of diethyl malonate.
Ester 8 was then hydrolysed under acidic conditions using TFA in DCM
to yield iodo-cbdc 13.
Fig. 1. Structures of cisplatin and carboplatin.
based drugs in an in vitro model.14 Both examples involved the at-
tachment of the cell-targeting molecule early in the synthesis via an
alkyl spacer. Brunner et al. reported a porphyrin-containing carboplatin
complex with the porphyrin unit bound to carboplatin via an ether
containing bifunctional linker via an ester bond.16 All examples with
conjugated drug delivery systems increased the antitumour activity of
carboplatin.
Our interest focused on the synthesis of carboplatin derivatives
containing new functional handles at position 3 of the cyclobutane
moiety. These functional groups can be used for attachment to various
drug delivery systems such as peptides, monoclonal antibodies, nano-
particles or small molecules via a bifunctional linker. The functionalised
analogues we chose to synthesise were the previously described hy-
droxy and keto derivatives9,17 as well as the novel iodo, azido and
amino analogues as they allow for quick and reliable methods of at-
tachment via ester or ether bond formation (hydroxy group), metal-
catalysed cross-coupling reactions (iodo group), Cu-catalysed alkyne-
azide cycloaddition (CuAAC, azide group), amide or oxime bond for-
mation (amino and keto group, respectively). In this work, five different
carboplatin derivatives were synthesised and tested in vitro for their
antitumour activity on ovarian cancer and colorectal cancer cell lines
including a platinum-based drug-resistant cell line. To demonstrate the
interest of functionalisation, one complex coupled to a bifunctional
maleimide-containing linker via an amide bond was synthesised and
evaluated in vitro.
For the synthesis of the platinum complexes (Scheme 2), potassium
droxide to give cis-diamminediiodoplatinum (II) 14. The sulfato com-
plex was prepared in situ by addition of AgSO4 to the diiodo complex
14. Further treatment with the barium salts of the cbdc ligands 9–13
resulted in the formation of complexes 15–19 bearing a hydroxy, oxo,
iodo, azido or amino group at position 3 of the cbdc ligand.
One example of carboplatin derivatives coupled to a bifunctional
linker was synthesised (Scheme 3). Aminocarboplatin 19 was coupled
antibody-drug conjugates.20 The coupling reaction of amino-cbdc with
the linker was carried out prior to the complexation reaction. For this
purpose, di-t-butyl 3-aminocyclobutyldicarboxylate 20 was synthesised
according to the above procedure in five steps starting from t-butyl
malonate and compound 1 and coupled to 6-maleimidocaproic acid.
The t-butyl group was chosen as a protecting group for the carboxylic
acid as the harsh basic deprotection conditions necessary for the hy-
drolysis of ethyl esters are not tolerated by maleimides. After hydrolysis
of the t-butyl ester 21 with TFA the resulting dicarboxylic acid 22 was
used for complexation with the platinum salt as described above to give
compound 23.
In order to establish a library of carboplatin analogues, differently
substituted cbdc ligands were synthesised (Scheme 1). According to a
tained in 3 steps.18,19 Commercially available epichlorohydrin and
benzyl bromide were heated overnight in the presence of HgCl2 to give
2-benzyloxy-1-bromo-3-chloropropane 1 which was reacted with die-
thyl malonate and sodium hydride over 3 days to result in the formation
of the benzyl-protected alcohol 2. Deprotection of the alcohol with Pd
(OH)2 and H2 at 50 psi gave alcohol 3 which was further oxidised to
Compounds 15–19 and 23 were tested in vitro for their cytotoxicity
in an MTS assay using SK-OV-3 (ovarian cancer cell line), HCT116
Scheme 1. Reagents and conditions: (i) BnBr, HgCl2, 115 °C, 65%; (ii) diethyl malonate, NaH, 1,4-dioxane, 0 °C to r.t. to rf, 67% (2a), 63% (2b); (iii) Pd(OH)2, H2 (50
psi), EtOH, r.t., 97% (3a), 67% (3b); (iv) PCC, DCM, r.t., 91%; (v) TsCl, Et3N, DMAP, DCM, r.t., 99% (5a), 71% (5b); (vi) NaN3, Bu4NHSO4, DMF, 80 °C, 94%; (vii) Pd/
C, H2, EtOAc, r.t., 76%; (viii) NaI, acetone, rf, 34%; (ix) LiOH, THF, H2O, r.t., 58% (9), 66% (10), 67% (11); (x) Pd/C, EtOAC, r.t., 74%; (xi) TFA, DCM, r.t., 68%.
2