8.29 ml (57.9 mmol) of ethyl 4-bromobutanoate and 28.21 g (86.58 mmol) of cesium carbonate were added to 21.5 g (57.9 mmol) of tert-butyl N-benzyloxycarbonyltyrosine in 280 ml of acetone and the mixture was heated to reflux with stirring. After 2 h, a further 2 ml of ethyl 4-bromobutanoate and 2 g of cesium carbonate were added, after a further 2 h a further 2 ml of ethyl 4-bromobutanoate and 3 g of cesium carbonate were added and after standing at room temperature overnight 9 ml of ethyl 4-bromobutanoate were added again and the mixture was heated to reflux for a further 6 h. After cooling, it was filtered, the residue was washed with acetone and the filtrate was concentrated. The residue was taken up in diethyl ether and the organic phase was washed successively with 3% strength citric acid solution, 3×H2O and saturated NaCl solution. The ether phase was dried over MgSO4, the drying agent was filtered off and the filtrate was concentrated in vacuo. The residue was chromatographed on silica gel using CH2Cl2 and CH2Cl2/MeOH (99/1). 31.3 g of a pale yellow oil were obtained, which was employed without further purification for the synthesis of (1.2).
