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

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A solution of NaHCO3 (9.87 g, 117.5 mmol) in water (80 mL) was slowly added to a solution of the hydrochloride salt 3 (31.0 g, 78.3 mmol) in water (300 mL). The resulting aqueous suspension was extracted with EtOAc (2×400 mL). The combined EtOAc extract was washed with water, then brine, and dried through MgSO4. Methanesulfonic acid (6.04 mL, 93.12 mmol) was slowly added to the EtOAc solution while stirred. White precipitate formed as soon as the addition of methanesulfonic acid was complete. The suspension was stirred for another 30 min and then filtered. The filter cake was washed three times with EtOAc and vacuum dried overnight to afford 35.4 g (quantitative) of the mesylate salt 1 as a white solid. 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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