Synthesis, Characterization and In Vitro Antitumour Activity of Di-n-Butyl, Tri-n-Butyl and Triphenyltin 3,6-Dioxaheptanoates and 3,6,9-Trioxadecanoates

A series of di- and triorganotin 3,6-dioxaheptanoates and 3,6,9-trioxadecanoates were synthesized and characterized by 1H, 13 and 117Sn NMR, electrospray mass and 119mSn Mössbauer spectroscopy, as well as elemental analysis. Their in vitro antitumour activity against seven tumoural cell lines of human origin, two breast cancers (MCF-7, EVSA-T), a colon carcinoma (WiDr), an ovarian cancer (IGROV), a melanoma (M 19 MEL), a renal cancer (A 498) and a non small cell lung cancer (H 226), is reported. They are characterized by similar inhibition doses ID50 as the analogous di- and triorganotin derivatives of 4-carboxybenzo-15-crown-5 and -18-crown-6 and in some cases by much lower ID50 values than clinically used reference compounds such as doxorubicine and methotrexate.


Introduction
Many di-n-butyl, tri-n-butyl and triphenyltin carboxylates display interesting antitumour activities in vitro against tumour cell lines of human origin [1,2]. As early as 1985, Atassi suggested that the usually low water solubility of organotin compounds might be the major drawback to the improvement of their antitumour properties [3]. One possibility to increase water solubility is to replace methylene groups in a polymethylenic chain by oxygen atoms [4]. Accordingly, some new organotin derivatives of 3,6-dioxaheptanoic and 3,6,9trioxadecanoic acid were synthesized. This report presents their synthesis, characterization and in vitro antitumour activity.

M6ssbauer spectroscopy
The Mtssbauer parameters are shown in Table 6. The quadrupole splittings QS are found in the range 3.49-3.90 mm/s. Since M6ssbauer spectroscopy is less sensitive to small variations of the tin environment than tin NMR spectroscopy, the two different tin atoms in 4 and 8 could not be discriminated. The Mtssbauer parameters are in agreement with a polymeric structure for the triorganotin polyoxaalkanoates, in which tin is five-coordinate with a trans-02 configuration [2]. For the diorganotin polyoxaalkanoates, the QS values conform the structures of figure 1.

Electrospray mass spectroscopy
The monoisotopic mass spectra (IH, 12C, 160, 12Sn) in the cationic mode of water/acetonitrile solutions are reported in Table 7. All compounds are easily complexed by normal or hydrolyzed fragments, or solvent molecules. The hydrolyzed species give an indication about stability inside the spectrometer [10]. They are observed for all compounds, but only fragments providing straightforward characterization of M are listed.     8). Compounds 2 and 6 contain water in a (2/1) tributyltin carboxylate/water ratio. The dimeric compounds 4 and 8 are found to contain two molecules of water per dimeric distannoxane unit. The presence of water is confirmed by the proton NMR spectra (see Tables and 2).

In vitro antitumour screening
All compounds were screened against seven tumoural cell lines of human origin.
The IDs0 values are reported in Table 9 and compared to those of some drugs with clinical applications and of organotin carboxylates containing crown ether moieties: di-n-butyl, tri-n-butyl and triphenyltin derivatives of 4-carboxybenzo-18-crown-6 and -15-crown-5 [11] of which the structures are depicted in figure 2   Experimental part Synthesis Triorganotin carboxylates Compounds 1 and 5 are typically prepared by mixing equimolar quantities of triphenyltin hydroxide and the desired polyoxacarboxylic acid in 250 ml benzene in a 500 ml flask equipped with a Dean-Stark funnel. The mixture is refluxed for 8 to 12 hours. The binary azeotrope benzene/water is distilled off up to 50% to the initial solvent volume. The remaining solution is evaporated under vacuum. The synthesis with tri-n-butyltin acetate yielding 2 and 6 is analog.  Diorganotin carboxylates Di-n-butyltin oxide is allowed to react with 1-propanol (typically g of di-butyltin oxide and 4 ml of 1propanol) in benzene (250 ml) in a 500 ml three-necked flask. The reaction mixture is refluxed for 3 hours under elimination of the ternary azeotrope benzene/water/1-propanol (Dean-Stark). After cooling to room temperature, the appropriate amount of polyoxaalkanecarboxylic acid (1/1 molar ratio polyoxaalkanecarboxylic acid / di-n-butyltin oxide for 3 and 7; 2/lmolar ratio for 4 and 8) is added slowly and the mixture is stirred overnight. The solvent is then evaporated and the product treated as indicated in Table 10 in which all experimental synthesis details are summarized.
NMR measurements All 2D NMR spectra and some of the 1D spectra were recorded on a Bruker AMX500 spectrometer interfaced with a X32 computer and operating at 500. 13 [2,3]. 2D gradient enhanced IH-3C HMQC [14] and HMBC [5] correlation spectra were acquired using the pulse sequences of the Bruker program library, adapted to include gradient pulses,[ 619] as described recently [2].

MOssbauer spectroscopy
The M6ssbauer spectra were recorded as described elsewhere [2].

Mass spectrometry
The electrospray mass spectra were recorded in the cationic mode on a Micromass Quattro II instrument coupled with a Masslynx system (ionisation in an electric field of 3,5 kV; source temperature: 80C; source pressure: atm; analyzer pressure: 10 -5 mbar) [ 22,23]. Cone voltages were 30 V for compounds 1 to 3 and 5 to 7, and 70 V for compounds 4 and 8.

Antitumour screening
The protocol followed for the antitumour screenings has already been reported [24,25].