Synthesis of Ferrocenyl Oxindole Compounds with Potential Anticancer Activity

The ferrocenyl (Fc) group has been incorporated to the structure of a number of biologically active molecules resulting in increased anticancer and antimalarial activity, amongst others. Addition, alkylation, acylation, condensation-dehydration, coupling and nucleophilic substitution reactions have been employed to link this group to a variety of compounds. The mechanisms of the cytotoxicity of ferrocene derivatives have been investigated. Osella et al. proposed that the cytotoxic activity of ferrocenium salts is due to their ability to generate oxygen active species that induce oxidative DNA damage. Kondapi et al. have shown that ferrocene derivatives inhibit topoisomerase II, a major molecular target for a number of DNA-binding anticancer drugs. Indolin-2-ones have been shown to exhibit antitumor activity by inhibiting receptor tyrosine kinases VEGF-R, PDGF-R or CDK. The indolin-2-one 1 (sunitinib) has been approved by the U.S. Food and Drug Administration for the treatment of metastatic renal cell cancer and gastrointestinal stromal tumors (Figure 1). Raghunathan et al. reported recently the synthesis of the ferrocenyl oxindole 2a with (E)-configuration from the reaction of oxindole and ferrocenecarboxyaldehyde (FcC(O)H) in ethanol, in the presence of catalytic amounts of piperidine.


Introduction
The ferrocenyl (Fc) group has been incorporated to the structure of a number of biologically active molecules resulting in increased anticancer 1,2 and antimalarial [3][4][5] activity, amongst others.]6,7 The mechanisms of the cytotoxicity of ferrocene derivatives have been investigated. 8Osella et al. 9 proposed that the cytotoxic activity of ferrocenium salts is due to their ability to generate oxygen active species that induce oxidative DNA damage.Kondapi et al. 10,11 have shown that ferrocene derivatives inhibit topoisomerase II, a major molecular target for a number of DNA-binding anticancer drugs.
Indolin-2-ones have been shown to exhibit antitumor activity by inhibiting receptor tyrosine kinases VEGF-R, PDGF-R or CDK. 12 The indolin-2-one 1 (sunitinib) has been approved by the U.S. Food and Drug Administration for the treatment of metastatic renal cell cancer and gastrointestinal stromal tumors 13,14 (Figure 1).Raghunathan et al. 15 reported recently the synthesis of the ferrocenyl oxindole 2a with (E)-configuration from the reaction of oxindole and ferrocenecarboxyaldehyde (FcC(O)H) in ethanol, in the presence of catalytic amounts of piperidine.
We describe herein the condensation reactions of several oxindole derivatives with FcC(O)H in the presence of KOH in EtOH from which good to excellent yields of the products with E and Z configurations were obtained (Scheme 1). 16he products were characterized by mass spectrometry and infrared, 1 H and 13 C NMR spectroscopies, including nOe and NOESY experiments for the determination of the double-bond geometry.Preliminary studies of the reaction of oxindole with FcC(O)H (Table 1) were carried out to assess the catalytic potential of various bases.
Except for Et 3 N, all the bases employed led to good conversions.However, formation of products with both E and Z configurations depend on the base employed.The use of KOH in EtOH gave the best yield and selectivity for the E-isomer 2a (entry 1).In the presence of piperidine (entry 3) we were able to isolate the E-isomer 2a in 67% yield together with a 30% isolated yield for the corresponding Z-isomer 2b.The geometries of isomers 2a and 2b were determined by nOe experiments and NOESY interactions.The NOESY experiment for 2a shows a correlation between H4 and H2' of the cyclopentadienyl ring, thus revealing an (E)-configuration for this isomer.In the case of 2b, we have observed a 2.15% intensity enhancement of the peak corresponding to H4 upon irradiation of H8, which confirms the (Z)-configuration (Figure 2).
The condensation reactions of a series of substituted oxindoles with FcC(O)H were then investigated in the presence of KOH in EtOH.Products and yields are summarized in Table 2.
The reactions of monosubstituted oxindoles 3, 4 and 10 (entries 1, 2 and 8, respectively) gave good conversions and the same product distribution as that of oxindole, with E-products being formed as the major isomers in yields above 90%.Product yields decreased with further ring substitution, and were also sensitive to the relative position of the substituents: whereas the reactions of 5,7-dichloro and 5,7-dibromo oxindoles (5 and 7, entries 3 and 5, respectively) gave both isomers albeit in lower yields than oxindole, those of the 4,7-dichloro and 4,6-dibromo oxindoles (6 and 8, entries 4 and 6, respectively) resulted only in the formation of the E-isomer in around 45% yields.
As shown in Table 3, in all cases, the olefinic hydrogen H8 of E-isomer (2a-10a) appear at higher frequency than for the Z-isomer (2b-10b), due to the diamagnetic anisotropy effect of the carbonyl oxygen (C2) on H8 of E-isomers 2a-10a.The same effect was observed for H3´and H5´ of the Z-isomers 2b-10b.H8 chemical shifts for compounds 6a and 8a at 8.40 and 8.34 pm, respectively, confirm the (E)-configuration.

Conclusion
In conclusion we have described a simple, fast and efficient synthetic route to ferrocenyl oxindoles, potential model compounds for chemical and pharmacological studies.

Table 1 .
Condensation of oxindole and FcC(O)H in ethanol, at room temperature, in the presence of various bases

Table 2 .
Condensation reactions of oxindole derivatives with FcC(O)H in the presence of KOH as catalyst a yields are for isolated products after flash column chromatography on silica-gel; b Fc = ferrocenyl.