Step 2:Preparation of intermediate 4
To a solution of 3 (5.0g, 41.3mmol, 1.0eq) in DCM (100ml, c=0.5) was added pyridine (4.9g, 61.95mmol, 1.5eq). To the solution was added 2 (9.2g, 49.59mmol, 1.2eq) in DCM (40mL, c=1.2). The reaction mixture was stirred at room temperature overnight. Then to the solution was added water (50mL). The organic phase was washed with brine, dried over Na2SO4, filtered and concentrated under reduce pressure. The residue was washed with n-hexane to give intermediate 4 (7.8g, yield=70%, HPLC: 98.6%).