Propanoic acid (140.6 ml, 1878.6 mmol, 6 eq) was added to a suspension of sodium borohydride (23.2 g, 626.2 mmol, 2 eq) in ethyl acetate (600 ml) and held until the reaction was deemed to be complete. The resulting solution was added to a solution of benzyl (2S)-5-[(benzyloxy)imino]piperidine-2-carboxylic acid benzyl ester in ethyl acetate (600 ml total volume) and sulphuric acid (83.4 ml, 1565 mmol, 5 eq) at −20° C. and held until reaction was deemed to be complete. The reaction was quenched by the addition of water (1000 ml), then neutralized with aqueous ammonia solution. The organic layer was washed with water and concentrated in vacuo to 400 ml. The solution was warmed to 45° C. and held at this temperature. Methanol (200 ml) at 40° C. was added, followed by a freshly prepared solution of oxalic acid dihydrate (39.5 g, 313.1 mmol) in methanol (100 ml). The mixture was cooled and the product was isolated by filtration. The solid was washed with an ethyl acetate/methanol mixture, then with ethyl acetate. The solid was dried to give benzyl (2S,5R)-5-[(benzyloxy)amino]piperidine-2-carboxylate ethanedioate (1:1) as a single isomer (79.4 g, 185 mmol, 59%).
