Cesium carbonate → tert-butyl methyl 2-{(1S)-[3-(benzyloxy)phenyl][(tert-butoxycarbonyl)amino]methyl}propanedioate

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Tert-butyl {[3-(benzyloxy)phenyl](phenylsulfonyl)methyl}carbamate (3.0 g, 5.93 mmol) was suspended in a toluene (21.6 mL) water (16 mL) mixture. At 0° C. tert-butyl methyl malonate (1.2 mL, 7.1 mmol), cesium carbonate (1.94 g, 5.9 mmol) and 1-[3,5-bis(trifluoromethy) phenyl]-3-[(1R,2R)-(−)-2-(dimethylamino)cyclohexyl]thiourea (246 mg, 0.59 mmol) was added and the mixture was stirred for 72 hours at 0° C. After storage at −20° C. for 72 hours the mixture was diluted with water and ethyl acetate, after filtration the phases were separated and the aqueous phase extracted with ethyl acetate. The combined extracts were dried over sodium sulfate and concentrated. The residue was purified by chromatography on silica gel (ethyl acetate in hexane 0 to 35%) to yield 2.17 g of enantiomerically enriched tert-butyl methyl 2-{(1S)-[3-(benzyloxy)phenyl][(tert-butoxycarbonyl)amino]methyl}propanedioate.

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