Formaldehyde → (S)-benzyl 4-(3-(1,3-dioxoisoindolin-2-yl)propyl)-5-oxooxazolidine-3-carboxylate

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A mixture of (S)-5-amino-2-(benzyloxycarbonylamino)pentanoic acid (10.0 g, 37.56 mmol), ethyl 1,3-dioxoisoindoline-2-carboxylate (9.04 g, 41.28 mmol) in NMP (40.0 mL) was heated to 150° C. for 30 min in 4 microwave reaction tubes. The mixture was diluted with EtOAc (500 mL). The organic layer washed with water (100 mL×2), brine, dried over Na2SO4, filtered, and concentrated under reduced pressure to give an oil, which was dissolved in toluene. To this toluene solution was added paraformaldehyde, PTSA and molecular sieves (3 Å) and the mixture was heated to 150° C. for 20 min. The mixture was diluted with ether (800 mL), washed with NaHCO3, brine, dried over Na2SO4, filtered, and concentrated under reduced pressure. The crude was purified on silica gel column using a mixture of EtOAc and hexanes to give (S)-benzyl 4-(3-(1,3-dioxoisoindolin-2-yl)propyl)-5-oxooxazolidine-3-carboxylate (11.6 g, 76% for 2 steps).

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