Isopropanol (1.5 ml, 18.7 mmol) was added to a solution of [7-(benzyloxy)-5-hydroxy-4-oxoquinazolin-3(4H)-yl]methyl pivalate (see WO01/094341, 6.5 g, 17.0 mmol) and triphenylphosphine (6.2 g, 23.8 mmol) in dichloromethane (300 ml). The reaction mixture was cooled to 0° C. and then a solution of di-tert-butylazodicarboxylate (4.7 g, 20.4 mmol) in dichloromethane (25 ml) was added dropwise over 30 minutes. The mixture was allowed to warm to room temperature and then stirred for 2 hours. The mixture was evaporated under reduced pressure to afford crude [7-(benzyloxy)-5-isopropoxy-4-oxoquinazolin-3(4H)-yl]methyl pivalate which was used in the next stage without further purification.
