Methanesulfonic Acid → (2R)-2-phenylcarbonyloxypropyl (2S)-2-amino-3-(3,4-dihydroxyphenyl)propanoate mesylate

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To an aqueous solution of the hydrochloride salt 3 (65.0 g, 164 mmol, 200 mL) was added aqueous NaHCO3 solution (20.7 g, 246 mmol, 200 mL) and then extracted with EtOAc (2×400 mL). The pooled organic extracts were washed with brine and dried over Na2SO4. After filtration, methanesulfonic acid (12.8 mL, 197 mmol) was slowly added to the filtrate while stirring at room temperature. The resulting white crystals were filtered through a fritted funnel, washed with EtOAc (3×1000 mL) and dried under high vacuum at 50° C. to afford 73.6 g (98.4% yield) of the mesylate salt 1. 1H NMR (400 MHz, CD3OD): δ 1.40 (d, J=6.4 Hz, 3H), 2.70 (s, 3H), 2.98 (dd, J=7.8, 14.6 Hz, 1H), 3.10 (dd, J=5.6, 14.4 Hz, 1H), 4.24 (dd, J=5.8, 7.8 Hz, 1H), 4.38 (dd, J=6.8, 12.0 Hz, 1H), 4.52 (dd, J=3.4, 11.8 Hz, 1H), 5.40 (dp, J=3.2, 6.4 Hz, 1H), 6.52 (dd, J=2.2, 8.2 Hz, 1H), 6.67 (d, J=2.2 Hz, 1H), 6.69 (d, J=8.0 Hz, 1H), 7.47 (t, J=7.6′ Hz, 2H), 7.60 (br t, J=7.4 Hz, 1H), 8.01 (d, J=7.6 Hz, 2H). MS (ESI) m/z 360.07 (M+H)+ and 358.01 (M−H)−.

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