[C:1]1(=O)[CH2:4][CH2:3][CH2:2]1.C(O)(=O)C.[CH:10]12[CH:15]([C:16]([O:18][CH2:19][CH3:20])=[O:17])[CH:14]1[CH2:13][NH:12][CH2:11]2.C(O[BH-](OC(=O)C)OC(=O)C)(=O)C.[Na+].C(=O)(O)[O-].[Na+]>ClCCl>[CH:1]1([N:12]2[CH2:11][CH:10]3[CH:14]([CH:15]3[C:16]([O:18][CH2:19][CH3:20])=[O:17])[CH2:13]2)[CH2:4][CH2:3][CH2:2]1A mixture of cyclobutanone (157 mg, 2.19 mmol) in acetic acid (0.11 mL, 1.56 mmol) was added to a stirred solution of ethyl 3-aza bicyclo[3.1.0]hexane-6-carboxylate (243 mg, 1.56 mmol, obtained in the above step) in dichloromethane, cooled at 0° C. Solid sodium triacetoxy borohydride (727 mg, 3.43 mmol) was added portion wise over a period of 15 minutes. The reaction mixture was gradually warmed to room temperature and stirred for 16 hours. The reaction mixture was cooled to 0° C. and basified with saturated sodium bicarbonate solution (pH: 7.5). The two layers were separated, the organic layer was washed with brine, dried over anhydrous sodium sulfate and the volatiles were removed under reduced pressure to obtain ethyl 3-cyclobutyl-3-aza bicyclo[3.1.0]hexane-6-carboxylate (219 mg).
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