4-Methylmorpholine → tert-butyl (2S,4EZ)-2-({[(2S)-2-hydroxy-2-phenylethyl]amino}carbonyl)-4-(methoxyimino)-1-pyrrolidine-carboxylate

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To a solution of the central building block, e.g. (2S,4EZ)-1-(tert-butoxycarbonyl)-4-(methoxyimino)-2-pyrrolidinecarboxylic acid (Intermediate 7) (24.2 mmol, 6.24 g) in dry THF (125 ml) at −25° C. was added NMM (2.5 eq, 60.4 mmol, 6.64 ml) followed by isobutylchloroformate (1.05 eq, 25.4 mmol, 3.3 ml). The resulting mixture was stirred at −25° C. for 30 min and an amine or an amine salt, e.g. (S)-2-amino-1-phenylethanol (1.51 eq, 36.5 mmol, 5 g) was then added. The mixture was allowed to gradually warm to rt. After 16 h, the solvents were removed. The residue was dissolved in AcOEt, washed twice with NH4Cl saturated solution, then twice with 10% NaHCO3 solution. The organic layer was dried over Na2SO4, filtrated and concentrated to afford the desired product, e.g. tert-butyl (2S,4EZ)-2-({[(2S)-2-hydroxy-2-phenylethyl]amino}carbonyl)-4-(methoxyimino)-1-pyrrolidine-carboxylate (8.76 g, 96%) as a pale yellow oil in 88.5% purity by HPLC.

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