Potassium tert-butoxide (1 eq.) was added to polyethylene glycol (8 eq.) in anhydrous THF (0.2 mL/mmol) at 0 °C. The resulting mixture was stirred at 60 °C for 0.5 h, then it was cooled to r.t. A solution of tert-butyl-bromoacetate (1.0 eq.) in anhydrous THF (0.1 mL/mmol) was added to the reaction mixture at r.t. The resulting mixture was stirred at r.t. for 24 h. The reaction was quenched with brine and the aqueous phase was extracted with ethyl acetate. The combined organic phase was evaporated to dryness. The crude material was purified by column chromatography (from 0 to 8% of methanol in DCM) to afford the desired compound.