To a solution of diol (1 eq.) in dichloromethane (DCM) (4 mL per mmol), tert-butyl bromoacetate (8 eq.), TBABr (1.1 eq.) and 37% w/w aqueous NaOH (4 mL per mmol) were added. The biphasic reaction was vigorously stirred at room temperature (r.t.) overnight. The organic phase was separated from the aqueous layer and then the aqueous phase was extracted with DCM (x3). Organic layers were collected, dried over MgSO 4 and evaporated under reduced pressure. The crude was purified by flash chromatography (using a gradient from 10 to 100% of ethyl acetate in heptane).