A 2 mL dichloromethane solution of Compound 515 (320 mg, 0.68 mmol) was added to a dichloromethane solution of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (209 mg, 1.09 mmol) and 1-hydroxybenzotriazole hydrate (167 mg, 1.09 mmol). The solution was stirred for 15 minutes at ambient temperature. N-hydroxyacetamidine (76 mg, 1.02 mmol) was added in one portion as a solid, and the mixture was stirred overnight. After diluting with additional DCM, the reaction mixture was washed with saturated aqueous NaHCO3 solution and brine, dried over anhydrous Na2SO4, and concentrated to a pale yellow foaming solid (Intermediate 49A). LCMS (+ESI) m/z 527.2 [M+H]+.
