To a solution of the compound of step 5 (6.290 g, 17.85 mmol) in 180 ml of anhydrous DMF under nitrogen were added ethyl 3,3-diethoxy-propanoate (10.42 ml, 53.55 mmol) and p-toluenesulfonic acid hydrate (0.509 g, 2.68 mmol), and the solution was heated at 90° C. for 1.5 h. After cooling to 60° C., DBU (13.32 ml, 89.24 mmol) was added dropwise and the mixture was stirred for 45 min at 60° C. The solution was then cooled to room temperature, water (600 ml) was added and the solution was extracted four times with EA (200 ml). The combined organic phases were washed with 0.5 N hydrochloric acid (300 ml) and brine (300 ml), dried over magnesium sulfate, filtered and evaporated to dryness under reduced pressure. The residue was triturated several times with cyclohexane and dried in vacuo at 50° C. 5.43 g of the title compound were obtained as a brown solid melting at 176-177° C.
