A solution of 6.62 g (19.8 mmol, purity 90%) of the compound from Example 53A in dichloromethane (211 ml) was admixed with 9.65 ml (7.00 g, 69.2 mmol) of triethylamine and then admixed at 0° C. with 3.99 g (19.8 mmol) of 4-nitrophenyl chloroformate. The mixture was warmed to RT and stirred for 1 h. The reaction solution was washed with saturated aqueous sodium hydrogencarbonate solution and water, and the organic phase was dried over magnesium sulphate, filtered and concentrated under reduced pressure. Yield: 10.3 g (91% of theory, purity 81%)
