After 5 min the reaction mixture was transferred into an external vial containing a solution of sulfuric acid (34 mL, 0.12 M). Nitrogen was passed through the mixture for 20 s to encourage mixing. The crude product was extracted on to two MCX SepPak cartridges, using syringe 2 for loading in a stepwise manner. A slower loading was achieved by breaking down the syringe plunger movements into multiple programmed stop/start commands (12 × 5 second intervals).