A mixture of triethylamine (0.101 ml) and Intermediate 11 (2.0 g) in dichloromethane (20 ml) was added dropwise to a solution of 4-nitrophenyl chloroformate (1.62 g) in dichloromethane (10 ml) with stirring at 0° C. under nitrogen. The solution was stirred for 3 h at 0° C., allowed to warm to 20° C., and stand for 18 h. The volatile components were evaporated in vacuo, and the residue was purified by Biotage flash chromatography on silica gel (90 g cartridge), eluting initially with 10% methanol in dichloromethane and subsequently with 20% methanol in dichloromethane, to give the title compound as a light brown solid (2.5 g).
