To a solution of the compound 60 (498 mg, 1.45 mmol) in 7.5 mL of Et2O were added sequentially 0.24 mL of triethylamine (1.2 eq), 0.23 mL of isobutyl chloroformate (1.2 eq) at −30° C. After being stirred at −30° C. for 15 min, diazomethane (prepared by mixing MNNG and KOH in diethyl ether) (excess) was added to the above mixture at 0° C. and stirred there for 5 hours. Glacial HOAc (1.5 mL) was added to destroy the excess diazomethane and the solution was extracted with EtOAc. The organic phase was washed with sat NaHCO3, water, brine and dried (MgSO4). LCMS 368.1 (M+1).
