(R)-5-Biphenyl-4-ylmethyl-3-methylene-2-oxo-pyrrolidine-1-carboxylic acid tert-butyl ester (4-a, R1=Boc) (27.7 g) is dissolved in THF (270 ml) at room temperature. Tetrabutylammonium bromide (0.24 g) is added followed with water (10 ml). The mixture is then cooled to 10° C. A solution of lithium hydroxide (7.3 g) in water (92 ml) is added over 2 h. Phosphoric acid (37 g, 85%) is added until pH 3. Phases are then separated. The organic phase is diluted with toluene (100 ml) and washed with brine. Phases are separated. The organic phase is then concentrated in vacuo. The residue is dissolved in acetonitrile (350 ml) at 80° C. and azeotropically distilled. Further acetonitrile is added (150 ml) and the mixture cooled to 0° C. The solid is collected by filtration and dried, to afford (R)-5-biphenyl-4-yl-4-tert-butoxycarbonylamino-2-methylenepentanoic acid (2-a, R1=Boc, R2=H, R3=CO2H) (25.7 g). δH (400 MHz, DMSO) 1.30 (9H), 2.29 (1H), 2.50 (1H), 2.75 (2H), 3.91 (8H), 5.62 (1H), 6.09 (1H), 6.66 (1H), 7.28 (2H), 7.33 (1H), 7.44 (2H), 7.56 (2H), 7.63 (2H).
