Benzyltrimethylammonium hydroxide → (R)-4-Acetyl-1-((S)-1-(4-methoxyphenyl)ethyl)pyrrolidin-2-one

103 g of a mixture of diastereoisomers (R/S)-4-acetyl-1-[(S)-1-(4-methoxyphenyl)-ethyl]-pyrrolidine-2-one is dissolved in 155 mL 1-butanol at 25° C. 18 mL benzyltrimethylammonium hydroxide (40% solution in methanol) is added. The solution is stirred for 30 minutes at 25° C. The solution is cooled to 0° C. Precipitation starts. The suspension is stirred for 15 minutes at 0° C. 100 mL n-heptane is added slowly and the suspension is stirred for 30 minutes at 0° C. The addition of 100 mL portions of n-heptane is repeated 4 times with subsequent stirring of the suspension at 0° C. for 30 minutes. The precipitate is isolated, washed with n-heptane and dried at 50° C.