Carboxylic acid 12 (2.50 g, 5.08 mmol), HATU (2.13 g, 5.59 mmol) and (S)-allyl-glycine methyl ester hydrochloride (0.925 g, 5.59 mmol) were dissolved in DMF (30 mL). DIPEA was added (3.54 mL, 20.3 mmol) and the reaction mixture was stirred at rt for 18 h before being partitioned between EtOAc and 1M hydrochloric acid. The organic phase was washed sequentially with 1M hydrochloric acid and brine before being dried (MgSO4), filtered and concentrated in vacuo. Purification was achieved using flash chromatography, eluting with a gradient of (50/70) petroleum ether to 2/1 EtOAc/(50/70) petroleum ether to yield a white solid, 2.70 g, 89%. Rf=0.38 (1/1 EtOAc/(50/70) petroleum ether).
