Crude (R)-5-Biphenyl-4-ylmethyl-3-[1-dimethylaminometh-(E/Z)-ylidene]-2-oxo-pyrrolidine-1-carboxylic acid tert-butyl ester (7-a, R1=Boc, R6=Me, R7=Me) (67.5 g) is dissolved in THF (340 ml) and cooled to 0° C. Hydrochloric acid (37%; 13.1 ml) is added followed by water (143.4 ml). Mixture is stirred at room temperature for 2 h. Phases are separated. To the organic phase is added formaldehyde solution (37% in water; 138 ml). Potassium carbonate (31.8 g) is added portionwise over 4 h. 1% Tetra-n-butylammoniumhydroxide solution (14.2 ml) is added followed by sodium hydroxide solution (30% in water) until pH 10.5. The mixture is stirred for 2 h. The phases are separated. The organic phase is washed with water (100 ml) and sodium bisulfite (20 g) added. Toluene (100 ml) and sodium chloride solution (20%; 50 ml). The phases are separated. The organic phase is extracted with a sodium chloride solution (20%; 50 ml), dried (Na2SO4) and concentrated in vacuo to give (R)-5-biphenyl-4-ylmethyl-3-methylene-2-oxo-pyrrolidine-1-carboxylic acid tert-butyl ester (4-a, R1=Boc) (63 g). The material can be recrystallised as follows: crude (R)-5-biphenyl-4-ylmethyl-3-methylene-2-oxo-pyrrolidine-1-carboxylic acid tert-butyl ester (4-a, R1=Boc) (58.4 g) is dissolved in methanol (525 ml) at 50° C. Water (175 ml) is added and the mixture is cooled to 0° C. The solid is collected by filtration and the cake washed with a mixture of methanol (53 ml) and water (18 ml). The solid is then dried in vacuo, giving (R)-5-biphenyl-4-ylmethyl-3-methylene-2-oxo-pyrrolidine-1-carboxylic acid tert-butyl ester (4-a, R1=Boc). Spectroscopic data as Example 23.
