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%).