Compound W (3.33 g, 20 mmol), dissolved in abs. DCM (15 ml), was added dropwise at 0° C. within 10 min to a solution of compound B (2.81 g, 14.3 mmol in abs. DCM (22 ml)). A solution of triethylamine (11.2 ml, 80 mmol) in abs. DCM (10 ml) was added to this mixture at 0° C. within 10 min. As the reaction was highly exothermic, cooling was carried out with a mixture of ice and sodium chloride. The reaction mixture was stirred for 16 h at RT. For working up the mixture, DCM (15 ml) and water (45 ml) were added and the mixture was stirred for 15 min. After separation of the phases, the aqueous phase was extracted with DCM (2×45 ml). The phases were separated. The organic phase was washed successively with 10% citric acid solution (2×30 ml) and sat. NaCl solution (30 ml). The organic phase was dried and concentrated (5.3 g, brown oil). The residue was separated by chromatography [silica gel 60 (180 g); cyclohexane/EtOAc 7:1 (1,200 ml); 5:1 (900 ml)]. The desired product X was obtained in a yield of 681 mg (11%).
