CC1(C)C2C1CCC2=O.C(OCC)(=O)C(OCC)=O.CC(C)([O-])C.[K+].[CH2:26]([O:28][C:29](=[O:41])[C:30]([CH:32]1[CH2:37][CH:36]2[CH:34]([C:35]2([CH3:39])[CH3:38])[C:33]1=O)=O)[CH3:27].Cl.[NH2:43][NH2:44]>C(O)C.O>[CH2:26]([O:28][C:29]([C:30]1[NH:44][N:43]=[C:33]2[C:32]=1[CH2:37][CH:36]1[C:35]([CH3:39])([CH3:38])[CH:34]12)=[O:41])[CH3:27](±)-6,6-Dimethyl-bicyclo[3.1.0]hexan-2-one (1.49 g, 12.0 mmol), diethyl oxalate (2.46 g, 16.8 mmol) and potassium t-butoxide (18.0 mL, 1M in THF, 18.0 mmol) were stirred in ethanol (40 mL) at room temperature for 2 hours. The desired (6,6-dimethyl-2-oxo-bicyclo[3.1.0]hex-3-yl)-oxo-acetic acid ethyl ester was observed by LCMS (m/z (ES+): 247 [M+Na]+, 225 [M+H]+)but not isolated. Hydrazine monohydrochloride (0.168 g, 24.4 mmol) in water (2.0 mL) was added and the solution heated to 80° C. for 18 hours. Solvent was removed under reduced pressure and the resulting oil poured into 0.1M aqueous hydrochloric acid (30 mL) and extracted into DCM (200 mL). Solvent was removed under reduced pressure and the residue purified by column chromatography (0-50% EtOAc/n-hexane, silica) to give (±)-1,1-dimethyl-1a,3,5,5a-tetrahydro-1H-2,3-diaza-cyclopropa[a]pentalene-4-carboxylic acid ethyl ester as a pale yellow oil which solidified upon standing. m/z (ES+): 243 [M+Na]+, 221 [M+H]+, 175 [M−OEt]+; 1H NMR (CD3OD): δ 4.4-4.3 (m, 2H, OCH2), 2.90 (dd, 1H, J1=17.5, J2=6.9), 2.65 (d, 1H, J=17.5), 2.1-2.0 (m, 1H), 1.95 (t, 1H, J=12.9), 1.37 (td, J1=7.1, J2=2.0), 1.13 (s, 3H, exo-CH3), 0.74 (d, 3H, J=2.0, endo-CH3).
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