Cesium bicarbonate → (2R)-2-Phenylcarbonyloxypropyl(2S)-2-(tert-Butoxycarbonyl)amino-3-(3,4-dihydroxyphenyl)propanoate

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A suspension of (1R)-2-bromo-1-methylethyl benzoate 1b (4.98 g, 20.6 mmol), (2S)-3-(3,4-dihydroxyphenyl)-2-[(tert-butoxycarbonyl)amino]propanoic acid, tetrabutylammonium salt 1a (7.3 g, 25 mmol), and cesium bicarbonate (4.85 g, 25 mmol) in N,N-dimethylacetamide (100 mL) was stirred at 55° C. for 16 h. The solvent was evaporated under vacuum. Ethyl acetate was added to the residue and the resulting solution was washed with water, then 5% NaHCO3 and brine, and dried over Na2SO4. After removing the solvent under reduced pressure, chromatography (silica gel, 30% ethyl acetate in hexane) of the residue gave 6.3 g (68% yield) of the title compound 1c as a white solid. 1H NMR (400 MHz, CD3OD): δ 1.25 (s, 9H), 1.40 (d, J=6.4 Hz, 3H), 2.99 (dd, J=7.6, 14.4 Hz, 1H), 3.10 (dd, J=5.6, 14.4 Hz, 1H), 4.24 (dd, J=5.6, 7.4 Hz, 1H), 4.38 (dd, J=6.8, 11.6 Hz, 1H), 4.52 (dd, J=3.2, 11.6 Hz, 1H), 5.40 (m, 1H), 6.53 (dd, J=2.2, 8.4 Hz, 1H), 6.66 (d, J=2.2 Hz, 1H), 6.69 (d, J=8.4 Hz, 1H), 7.47 (t, J=7.6 Hz, 2H), 7.60 (t, J=7.6 Hz, 1H), 8.02 (d, J=7.6 Hz, 2H). MS (ESI) m/z 360.15 (M+H)+ and 358.09 (M−H)−.

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