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 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]+.
