Towards potential nanoparticle contrast agents: Synthesis of new functionalized PEG bisphosphonates

The use of nanotechnologies for biomedical applications took a real development during these last years. To allow an effective targeting for biomedical imaging applications, the adsorption of plasmatic proteins on the surface of nanoparticles must be prevented to reduce the hepatic capture and increase the plasmatic time life. In biologic media, metal oxide nanoparticles are not stable and must be coated by biocompatible organic ligands. The use of phosphonate ligands to modify the nanoparticle surface drew a lot of attention in the last years for the design of highly functional hybrid materials. Here, we report a methodology to synthesize bisphosphonates having functionalized PEG side chains with different lengths. The key step is a procedure developed in our laboratory to introduce the bisphosphonate from acyl chloride and tris(trimethylsilyl)phosphite in one step.

Then, benzyl bromide for 2 (6.1 mL, 51 mmol) or ethyl bromoacetate for 18 (6.1 mL, 51 mmol) was added dropwise and the resulting solution was stirred at room temperature. The reaction is monitored by TLC and 1 H NMR. After completion of the reaction, the reaction mixture was filtered and evaporated under reduced pressure. The residue was dissolved in dichloromethane (7 mL), washed three times with water (20 mL) and the combined organic layer was dried with MgSO 4 and concentrated in vacuo.    8, 9a,b, 21). 13-Phenyl-3,6,9,12-tetraoxatridecan-1-oyl chloride (8 [11]. General procedure for the preparation of azide compounds (12a,b).
Sodium azide (9.34 g, 143.7 mmol) was added to a solution of the monotosylated PEG derivative 11a-b (28.8 mmol) in anhydrous DMF (100 mL). The reaction mixture was then heated at 60 °C for 5 hours. The solvent was removed under reduced pressure. Water (50 mL) and dichloromethane (50 mL) were added and the aqueous layer was extracted with dichloromethane (3 × 100 mL). The combined organic layers were washed successively with water and brine, dried with MgSO 4 and concentrated in vacuo.  [12].
A solution of azide 12a or 12b (23.8 mmol) in anhydrous DMF ( General procedure for the preparation of acid compounds (14a,b).
Aqueous 2 M NaOH solution (23 mL) was added to a solution of 13a or 13b (11.5 mmol) in methanol (23 mL). After 10 minutes, the methanol was removed under vacuum to obtain the crude product. The reaction mixture was acidified to pH = 3.5 and extracted with 3 × 30 mL of chloroform. The combined organic layers were concentrated in vacuo yielding the pure product.
General procedure for the preparation of acid chloride compounds (15a,b).
A 50 mL flame-dried three-necked flask equipped with an argon inlet and a dropping funnel was charged with the carboxylic acid derivative 14a,b (10 s14 mmol) and 40 mL of freshly distilled dichloromethane. The flask was placed in an ice bath (0 °C) and a solution of oxalyl chloride (6.3 g, 50 mmol) in 10 mL of dichloromethane was added dropwise. The reaction mixture was then stirred at room temperature for 48 hours. The reaction evolution was monitored by infrared spectroscopy. Then, volatile fractions were co-evaporated under reduced pressure (0.01 Torr) twice with anhydrous dichloromethane and anhydrous diethyl ether.