C(O)C.[CH2:4]([O:6][C:7](=[O:21])[CH2:8][S:9]/[C:10](=[C:12]1/[C@H:13]2[C@@H:15]([CH2:16][C:17]/1=O)[C:14]2([CH3:20])[CH3:19])/[CH3:11])[CH3:5]>[OH-].[Na+]>[CH2:4]([O:6][C:7]([C:8]1[S:9][C:10]([CH3:11])=[C:12]2[C:17]=1[CH2:16][C@H:15]1[C:14]([CH3:20])([CH3:19])[C@H:13]12)=[O:21])[CH3:5]A solution of Na (1.70 g, 74.8 mmol) in abs. ethanol (75 mL) is heated to 60° C. before it is treated with a solution of crude {1-[(1S,5R)-6,6-dimethyl-3-oxo-bicyclo[3.1.0]hex-(2Z)-ylidene]-ethylsulfanyl}-acetic acid ethyl ester (18.2 g, 68.0 mmol) in abs. ethanol (200 mL). The mixture is stirred at 75° C. for 20 min, then cooled to rt, diluted with 0.5 M aq. NaOH (500 mL) and extracted with DCM (450+200 mL). The combined extracts are dried over Na2SO4, filtered and the solvent is removed in vacuo. This yields crude (1aS,5aR)-1,1,2-trimethyl-1,1a,5,5a-tetrahydro-3-thia-cyclopropa[a]pentalene-4-carboxylic acid ethyl ester (10.5 g) as a yellow oil of 87% purity (LC-MS, UV 280 nm). LC-MS: tR=1.11 min, [M+1]+=251.14; 1H NMR (CDCl3): δ 4.26 (q, J=7.0 Hz, 2H), 2.95 (dp, Jd=18.8 Hz, Jp=3.5 Hz, 1H), 2.79 (d, J=19.3, 1H), 2.37 (s, 3H), 1.89-1.84 (m, 2H), 1.34 (t, J=7.0 Hz, 3H), 1.12 (s, 3H), 0.72 (s, 3H).
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