Reaction Steps
5 g (3R,5S)-5-biphenyl-4-ylmethyl-3-methyl-2-oxo-pyrrolidine-1-carboxylic acid tert-butyl ester (2-a, R1=BOC) was dissolved in 50 ml of a mixture of THF and a 2 M lithium hydroxide solution (ratio 1:1). After 1 hour at room temperature, phosphoric acid was added to neutralise the excess of lithium hydroxide. The slurry was concentrated under vacuum to remove most of the solvent and extracted with iso-propyl acetate. The organic phase was then washed with water, partially concentrated under vacuum and brought to crystallisation upon addition of heptane. The crystalline solid obtained was collected and dried under vacuum to yield about 3.6 g of [(2R,4S)-5-biphenyl-4-yl-4-tert-butoxycarbonylamino-2-methylpentanoic acid] (3-a, R1=BOC, R2=R3=H). Mpt 146-147° C.; δH (500 MHz; DMSO) 1.07 (3H, d, J 7.0, 1-CH3), 1.34 (9H, s, (CH3)3), 1.38 (1H, m, 3-HA), 1.77 (1H, m, 3-HB), 2.43 (1H, m, 2-H), 2.70 (2H, d, J 7.0, 5-H), 3.69 (1H, m, 4-H), 6.74 (1H, d, J 9.0, NH), 7.27 (2H, d, J 8.0, Ar-ortho-H(Ph)), 7.36 (1H, t, J 7.0, Ar-(Ph)-para-H), 7.46 (2H, t, J 7.5, Ar-(Ph)-meta-H), 7.57 (2H, d, J 8.0, Ar-meta-H(Ph), 7.64 (2H, d, J 7.5, Ar-(Ph)-ortho-H), 12.01 (1H, s, CO2H); δC (500 MHz, DMSO) 18.1 (1-CH3), 28.3 [(CH3)3], 35.9 (2-C), 37.9 (3-C), 40.7 (5-C), 50.0 (4-C), 77.4 [(C(CH3)3], 126.3, 126.5, 127.2, 128.9, 129.8 (Ar—CH), 137.7 (Ar-ipso-C(Ph)), 138.3 (Ar-para-C(Ph)), 140.1 (Ar-(Ph)-ipso-C), 155.2 (NCO), 177.2 (CO2H); m/z (+ESI) 406 ([MNa]+, 6%), 384 ([MH]+, 31), 328 (100), 284 (19). Found: [MH]+, 384.21691. C23H30NO4 requires MH 384.21693.
