Lawesson's reagent → (R)-2-(6-bromo-3-thioxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)-propionic acid ethyl ester

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A solution of (R)-2-(6-bromo-3-oxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)-propionic acid ethyl ester (82.4 g, 252 mmol) and Lawesson's reagent (61.1 g, 151 mmol) in toluene (900 mL) was heated at reflux for 3 h and then cooled to ambient temperature. The solvent was removed in vacuo and the residue was purified by chromatography on silica gel (eluting with 5-10% EtOAc in petroleum ether) to give (R)-2-(6-bromo-3-thioxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)-propionic acid ethyl ester (75.2 g, 87%) as a yellow solid. 1H NMR (400 MHz, CDCl3): δ 7.19 (d, J=8.8 Hz, 1H), 7.04 (s, 1H), 6.93 (d, J=8.8 Hz, 1H), 6.57 (m, 2H), 4.96-4.87 (m, 2H), 4.29-4.16 (m, 2H), 1.71 (d, J=6.8 Hz, 3H), 1.18 (t, J=6.8 Hz, 3H). SFC (Table 1, Method 29) Rt=3.529 min.; [α]D20=19.25 (C=0.002 g/mL in methanol).

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