Synthesis, Structure and Antitumor Activity of Dibutyltin Oxide Complexes with 5-Fluorouracil Derivatives. Crystal Structure of [(5-Fluorouracil)-1-CH2CH2COOSn(n-Bu) 2]4O2

Dibutyltin (IV) oxide complex reacts with the fluorouracil compounds 5-fluorouracil-1-propanonic or 5-fluorouracil-1-acetic acid (Fu) to give the complexes [(5-Fu)-1-(CH2)nCOOSn(n-Bu)2]4O2 (I, n=2; II, n=1) which were characterized by IR and 1H-NMR. The crystal structure of complex I shows that the molecular is a dimer, in which two [(5-Fu)-1-CH2CH2COOSn(n-Bu)2]2O units are linked by a bridging oxygen atom, and the tin atoms adopt distorted trigonal bipyramids via two carbons from a dibutyl moiety and three oxygen atoms from 5-Fu and bridging oxygen. These complexes have potential anti-tumour activity: in vitro tests showed that complexes I and II exhibit high cytotoxicity against OVCAR-3 and PC-14.


Introduction
Many groups have reported the chemotherapeutic properties of tin compounds possessing antitumor activity [1][2][3][4][5]. Since then intense interest has developed in this field and a large number of organotin compounds have been synthesized and tested [6][7]. Among these compounds dibutyltin derivatives have displayed both higher activity and relatively low toxicity [8]. On the other hand, the cycle-specific schedule dependent antimetabolite 5-fluorouracil (5-Fu) has been in clinical use for 40 years and has evolved as an important agent in the treatment of a large spectrum of tumors, including all gastrointestinal cancers and breast cancer. While attempting to study organotin (IV) complexes as possible candidates for anti-tumor agents and the structure-activity relationships of these complexes, we successfully prepared two kinds of new 5-Fu dibutylorganotin (IV) derivatives and determined the X-ray crystal structure of one of them. At the same time, their anticancer activity was also tested. We report these results herein.

Synthesis
The [(5-fluorouracil)-1-(CH 2 ) n COOSn(n-Bu) 2 ] 4 O 2 compounds I (n=2) and II (n=1) were synthesized by the reaction of (Bu) 2 SnO with (5-fluorouracil)-1-(CH 2 ) n COOH (n = 2,1) in the appropriate molar ratio of 1:1. A possible mechanism for the formation of compounds I and II is shown in Scheme 1 The (5-fluorouracil)-1-(CH 2 ) n COOH (n = 2, 1) staring materials were obtained by the reaction of 5-fluorouracil with CH 2 =CHCN (n = 2) or ClCH 2 COOH (n = 1) in NaOH solution. 4Å molecular sieves were used as the catalyst for the dehydration reaction. If the reaction temperature were raised to 80˚C, the 2:1 molar ratio (5-Fu:Sn) complexes would be formed. The resulting complexes I and II are stable in the air and are difficult to dissolve in aromatic or ether solvents, but can be easily dissolved in methanol or H 2 O when heated.

Molecular structure
The IR and 1 H-NMR spectroscopic spectra supported the fact that the complexes I and II contain the expected 5-Fu and butyl moieties. In order to confirm their molecular structure, the X-ray crystal structure of I was also determined. Its molecular structure and the unit cell are shown in Figures 1 and 2, and selected interatomic bond distances (Å) and angles () are listed in Table 1.   (10) Figure 3 below [12]. This indicates that the coordination of two of the circular-linked 5-Fu ligand oxygen atoms to tin atoms is weaker than that of the other oxygen atom of the linearly-linked 5-Fu ligand.

Biological activity
The MTT method was used for a preliminary estimation of the in vitro tumor-inhibiting activity of complexes I and II. The data summarized in Table 2 shows that complexes I and II possess rather high cytotoxicity towards tumor cells of OVCAR-3 and PC-14 and the dibutyltin 5-fluorouracil-1propanonic acid derivative is more active than the dibutyltin 5-fluorouracil-1-propanonic acid derivative.

General Methods
IR spectra were recorded using KBr pellets on a Nicolet NEXUS 470 FT-IR. 1 H-NMR spectra were recorded at 298K for CDCl 3 solutions with TMS as the internal reference using a Unity-400MHz-NMR spectrometer. The elemental analyses were determined on a GmhH Varin EL analyzer. Tin was determined using complexometric titration with EDTA and lead nitrate as the volumetric solution. Toluene was dried over Na and distilled prior to use under N 2. Dibutyltin oxide was synthesized following the literature method [9]. 5-Fluorouracil-1-acetic acid and 5-fluorouracil-1-propanoic acids were prepared according to the literature methods [10][11]. 5-Fluorouracil was a gift from Rudong Pharmaceutical Factory of Jiangsu.
The tumor inhibiting effect of complex 1 and 2 was tested in vitro using the human ovarian carcinoma cell line OVCAR-3 and lung carcinoma cell line PC-14 by the MTT method.