Hydrochloric Acid → (S)-Vigabatrin

A solution of (5S)-N-((2R)-2-(2,4-di-t-butylphenyloxy)-propanoyl)-5-ethenylpyrrolidin-2-one (19a, 1.0 g, 2.5 mmol) and 87% potassium hydroxide (317 mg, 5.7 mmol) in water (2 mL) is refluxed for 1.5 hours. The solution is allowed to cool to room temperature. Water is added (15 mL) and the solution is acidified to pH 7 with concentrated hydrochloric acid and extracted with diethyl ether (3×10 mL) to remove the chiral auxiliary. The aqueous solution is then acidified to pH 5.5 with concentrated hydrochloric acid and extracted with diethyl ether (3×15 mL). The second series of organic extracts are combined and dried (MgSO4). Concentration by rotary evaporation affords (4S)-4-amino-5-hexenoic acid (B).