Synthesis of a potential tyrosine kinase inhibitor by knoevenagel condensation of oxindole with 4-[¹⁸F]Fluorbenzaldehyde

Objectives: Receptor tyrosine kinases (RTKs) play an important role in tumour angiogenesis through their involvement in proliferation, migration and differentiation between tumour and endothelial cells. Recently, small molecule tyrosine kinase inhibitors like Imatinib mesylate (Gleevec®), Gefitinib (Iressa®) and SU11248 (Sutent®) have been radiolabelled to study tumour angiogenesis in vivo^1,2,3. The aim of the present work is the development of an sufficient radiosynthesis of 3-(4’-[¹⁸F]fluoro-benzylidenyl]-indolin-2-one 2 as derivative of potent RTK inhibitor Semaxinib® based on Knoevenagel condensation of 4-[¹⁸F]fluorobenzaldehyde with oxindole.
Methods: Synthesis of 4-[¹⁸F]fluorobenzaldehyde was performed by reacting [¹⁸F]fluoride with 4-trimethylammoniumbenzaldehyde triflate in an automated synthesis module (Nuclear Interface). Briefly, 15 mg of precursor dissolved in acetonitrile (1 ml) was heated with dried [¹⁸F]KF at 90 °C for 10 min. After addition of water (11 ml), 4-[¹⁸F]fluorobenzaldehyde was purified via solid-phase extraction on a HLB-plus cartridge (250 mg, Waters). Purified 4-[¹⁸F]fluorobenzaldehyde was eluted from the cartridge with ethanol (3 ml) and transferred into a separate reaction vial containing oxindole 1 (10 mg) and a base. Knoevenagel condensation occurred at 90 °C for 20 min (Fig. 1). The radiochemical yield of product 2 was determined by radio-HPLC.

Results: Knoevenagel condensation of oxindole 1 with 4-[¹⁸F]fluorobenzaldehyde was optimised by screening the influence of different bases on the radiochemical yield (RCY) of the carbonyl olefination reaction. The results are summarised in Table 1. Knoevenagel condensation strongly depends on the used base. Best results could be obtained by using amine bases piperidine (18% to 29%) or diethylamine (48%). Application of stronger amine bases (DABCO, diisopropyl-ethyl amine) or weaker bases (ammonium acetate, 2,6-di-tert.-butyl pyridine) resulted in lower or no product formation. Phosphazane as a very strong base yielded 26% of desired product 2 along with formation of large amounts of non-identified side products.

Table 1. Knoevenagel condensation between 4-[¹⁸F]fluorobenzaldehyde and oxindole

Conclusions: Knoevenagel condensation of 4-[18F]Fluorobenzaldehyde with oxindole is a suitable labelling technique for the synthesis of radiotracer 2. Within a series of different bases, diethylamine provides highest radiochemical yields of up to 48%. The Knoevenagel condensation reaction optimised under the object of radiolabeling involving readily available 4-[18F]Fluorobenzaldehyde should be applicable for the convenient radiosynthesis of other compounds containing a benzylidene motif.

References: [1] Kil KE et al. Nucl Med Biol, 2007, 34, 153. [2] Wang JQ et al. Bioorg Med Chem Lett, 2006, 16, 4102. [3] Wang JQ et al. Bioorg Med Chem Lett, 2005, 15, 4380.