8.64 g of benzyl (S)-2-[(2-cyanobiphenyl-4-ylmethyl)-(4-oxo-pentanoyl)-amino]-3-methyl-butanoate in 20 ml of xylene and 12.71 g of Tributyltinazid (Aldrich) were refluxed for 28 hours. The mixture was treated with 0.5N aqueous sodium hydroxide, the aqueous phase was washed with ether and the ether phase was extracted once with water. The combined aqueous phases were acidified with concentrated aqueous hydrochloric acid, extracted with dichloromethane, washed with water, suspended in activated carbon, filtered, dried with sodium sulfate, concentrated and dried under vacuum. The product (S)-3-methyl-2-{(4-oxo-pentanoyl)-[2-(1H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-amino}-butanoic acid benzyl ester was obtained as a brown foam.
7.9 g of benzyl (S)-3-methyl-2-{(4-oxo-pentanoyl)-[2-(1H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-amino}-butanoate in 160 ml of tetrahydrofuran was hydrogenated in the presence of 1.5 g of lapped palladium-carbon (10%) at atmospheric pressure and room temperature until saturation was reached. The mixture was filtered and concentrated in vacuo to afford (S)-3-methyl-2-{(4-oxo-pentanoyl)-[2-(1H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-amino}-butanoic acid as almost white foam
