[CH2:1]([O:3][C:4]([C@H:6]1[CH2:11][CH2:10][C@H:9]([OH:12])[CH2:8][CH2:7]1)=[O:5])[CH3:2].Cl([O-])(=O)(=O)=O.[Mg+2].Cl([O-])(=O)(=O)=O>ClCCl>[CH2:1]([O:3][C:4]([C@H:6]1[CH2:11][CH2:10][C@H:9]([O:12][C:6]([CH3:11])([CH3:7])[CH3:4])[CH2:8][CH2:7]1)=[O:5])[CH3:2].[CH2:1]([O:3][C:4]([C@H:6]1[CH2:11][CH2:10][C@@H:9]([O:12][C:6]([CH3:11])([CH3:7])[CH3:4])[CH2:8][CH2:7]1)=[O:5])[CH3:2]To a solution of cis/trans-4-hydroxycyclohexane carboxylic acid ethyl ester (2:1) (1.0 g, 5.8 mmol) and di-tort-butyl dicarbonate (2.9 g, 13.4 mmol) in dichloromethane (6 ml) was added anhydrous magnesium perchlorate (0.13 g, 0.58 mmol) which was previously activated in high vacuo (ca. 1 mbar) at 150° C. for 1 h. Stirring for 16 h at 40° C. was followed by partitioning between dichloromethane (50 ml) and water (30 ml). The layers were separated. The aqueous layer was extracted with one 50-ml portion of dichloromethane. The combined organic layers were dried over anhydrous sodium sulfate and concentrated in vacuo. Purification by flash-chromatography with n-heptane/tert-butyl methyl ether as eluent gave pure trans-4-tert-butoxy-cyclohexanecarboxylic acid ethyl ester and cis-4-tert-butoxy-cyclohexanecarboxylic acid ethyl ester.
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