To a cold solution of compound 38 (1.80 mmol), TBTU (2.17 mmol), and N-methylhex-5-en-1-amine tosylate salt 31a (2.17 mmol) in DMF (100 mL) was added diisopropyl ethyl amine (5.4 mmol) dropwise under nitrogen at 0° C. The reaction mixture was stirred overnight at room temperature, then quenched with water and extracted with diethyl ether. The organic layer was washed with brine, then dried on magnesium sulphate, concentrated under reduced pressure, and purified by chromatography on silica gel to yield compound 39 as a beige solid in 85% yield. MS (ESI, EI+): m/z=578 (MH+).
