[C:1]([O:5][C:6]([C@H:8]1[CH2:11][C@@H:10]([C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])[N:9]1CC1C=CC=CC=1)=[O:7])([CH3:4])([CH3:3])[CH3:2].O.[H][H]>CO.[OH-].[OH-].[Pd+2]>[C:1]([O:5][C:6]([C@H:8]1[CH2:11][C@@H:10]([C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])[NH:9]1)=[O:7])([CH3:4])([CH3:3])[CH3:2]307 mg (884 μmol) of cis-N-benzylazetidine-2,4-dicarboxylic acid di-tert-butyl ester in 25 ml of methanol is hydrogenated in a Parr shaker over 100 mg of 20% Pd(OH)2 /C (containing 31% water) under 4 atm of hydrogen for 7.5 h. The catalyst is filtered off through a double paper filter and thoroughly washed with methanol. Evaporation and filtration over SiO2 (EtOAc/hexanes 1:1) affords 149 mg (66%) of cis-azetidine-2,4-dicarboxylic acid di-tert.butyl ester as a yellowish oil which solidifies spontaneously. mp. 47°-48.5° C. 1H NMR (CDCl3): δ4.04 (dd, 2H, J =5.5, 9), 3.2 (br., 1H), 2.97 (dt, 1H, J =12 (d), 9 (t)), 2.38 (dt, 1H, J =12 (d), 5.5 (t)), 1.48 (s, 18H). IR (neat): 3328, 2979, 2934, 1731, 1480, 1458, 1394, 1369, 1280, 1257, 1156, 1103, 1048, 1031, 892, 846, 734 cm-1.
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