C(N[CH2:4][CH3:5])C.C(OC(=O)[CH:10]([CH2:14][CH:15]1[CH2:19][CH2:18][N:17]([C:20]([O:22][C:23]([CH3:26])([CH3:25])[CH3:24])=[O:21])[CH2:16]1)[C:11](O)=O)C.[CH2:28]=[O:29].C(Cl)Cl.[OH2:33]>>[C:23]([O:22][C:20]([N:17]1[CH2:18][CH2:19][CH:15]([CH2:14][C:10]([O:33][CH2:4][CH3:5])=[C:11]=[C:28]=[O:29])[CH2:16]1)=[O:21])([CH3:24])([CH3:25])[CH3:26]Diethyl amine (0.34 mL; 3.3 mmol) was added to a mixture of 2-(1-tert-butoxycarbonyl-pyrrolidin-3-ylmethyl)-malonic acid monoethyl ester (0.69 g, 2.19 mmol) in 36% aqueous solution of formaldehyde (0.27 mL, 3.5 mmol), CH2Cl2 (1.6 mL) and water (1.6 mL) at 0° C. The reaction mixture was stirred at room temperature overnight, poured onto ice-water (500 mL) and extracted with CH2Cl2. The combined organic phases were washed with 5% NaHCO3, dried and concentrated under reduced pressure to yield 3-(2-ethoxy-carbonyl-allyl)-pyrrolidine-1-carboxylic acid tert-butyl ester (0.55 g, 87%) as a colourless oil.
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