Ethyl chloroformate (3.29 g) in acetone (10 ml) was added dropwise to a solution of the product of stage (iii) (10.0 g) and triethylamine (3.0 g, 30 mmol) in acetone (100 ml) and water (10 ml) at 0°. The mixture was stirred at 0° for 40 min and sodium azide (2.0 g) in water (25 ml) was added dropwise. The resulting suspension was stirred at room temperature for 45 min, diluted with water (200 ml) and extracted with T (2×200 ml). The dried (Na2SO4) extract was heated at 75°-80° for 2.5 h and evaporated. The residue was treated with benzyl alcohol (20 ml), heated at 75°-80° for 60 h and benzyl alcohol was removed by distillation (~1 mmHg). The resulting oil was purified on a column of silica (Merck 9385) eluted with ER-CX (1:2) to give the title compound, as a yellow oil (4.74 g). T.l.c. ER-CX (1:2) Rf 0.13.
