To a solution of 4-oxo-4H-pyran-2,6-dicarboxylic acid (99 mg. 0.535 mmol) in DMF (10 mL) is added HOBt (98 mg. 0.643 mmol) and EDCI (123 mg, 0.643 mmol) and the mixture is stirred at room temperature for 10 minutes. To this is then added (2R,4S)-4-amino-5-biphenyl-4-yl-2-methyl-pentanoic acid benzyl ester hydrochloride (200 mg, 0.535 mmol) and triethylamine (0.224 mL, 1.61 mmol) and the mixture is stirred at room temperature for 48 hours. Water is added and the mixture is extracted with ethyl acetate. The organic phase is washed with water and brine and is dried over magnesium sulfate. The solvent is removed under reduced pressure and the residue is purified by preparative HPLC using a gradient of 10-100% MeCN/water (0.1% TFA) to elute the product, 6-((1S,3R)-3-benzyloxycarbonyl-1-biphenyl-4-ylmethyl-butylcarbamoyl)-4-oxo-4H-pyran-2-carboxylic acid. MS 540.2 (M+1). Next, to a solution of 6-((1S,3R)-3-benzyloxycarbonyl-1-biphenyl-4-ylmethyl-butylcarbamoyl)-4-oxo-4H-pyran-2-carboxylic acid (100 mg, 0.185 mmol) in methylene chloride (5 mL) is added BCl3 (65.1 mg, 0.556 mmol) and the mixture is stirred at room temperature for 10 minutes. The mixture is acidified to pH 2-3 with aqueous 1M HCl and is extracted with ethyl acetate. The organic phase is washed with water and brine and is dried over magnesium sulfate. The solvent is removed under reduced pressure and the residue is purified by preparative HPLC using a gradient of 10-100% MeCN/water (0.1% TFA) to elute the product. The proper fractions are lyophilized to furnish 6-((1S,3R)-1-biphenyl-4-ylmethyl-3-carboxy-butylcarbamoyl)-4-oxo-4H-pyran-2-carboxylic acid. MS 450.1 (M+1); 1H-NMR (400Hz, DMSO-d6); δ ppm 1.07 (d, J=7.07Hz, 3H), 1.59 (m, 1H), 1.88 (m, 1H), 2.45 (m, 1H), 2.84 (d, J=6.69Hz, 2H), 4.19 (m, 1H), 6.84 (s, 1H), 6.93 (s, 1H), 7.32 (dd, J=8.08Hz, 6.57Hz, 3H), 7.45 (t, J=7.83Hz, 2H), 7.58 (d, J=8.21Hz, 2H), 7.64 (d, J=7.33Hz, 2H), 8.61 (d, J=8.72Hz, 1H).
