[O:1]=[C:2]1[CH2:6][CH2:5][C@@H:4]([CH2:7][C:8]([OH:10])=[O:9])[C@H:3]1[CH2:11][CH2:12][CH2:13][CH2:14][CH3:15].[CH3:16]OS(OC)(=O)=O.CCOCC>CC(C)=O.O>[O:1]=[C:2]1[CH2:6][CH2:5][C@@H:4]([CH2:7][C:8]([O:10][CH3:16])=[O:9])[C@H:3]1[CH2:11][CH2:12][CH2:13][CH2:14][CH3:15]To 104 mg (0.49 mmole) of (±)cis-3-oxo-2-pentyl-1-cyclopentaneacetic acid (93:7 cis/trans ratio) in 1 ml of (CH3)2CO there were added, first 56 μl of (CH3)2SO4 (74 ml, 0.59 mmole, 1.2 eq.) and then 81 mg (0.59 mmole, 1.2 eq.) of anhydrous solid K2 CO3. The resulting suspension was heated at r.t. during 4 h and taken in water and with ether. The ether phase was separated and the aqueous phase once more extracted with ether. The combined organic phases were washed with water, dried over MgSO4, filtered and concentrated. The residue was distilled in a bulb-to-bulb apparatus (oven t.: 180°/7 Pa) to provide 93 mg (0.44 mmole, yield 84%) of methyl (±)-cis-3-oxo-2-pentyl-1-cyclopentaneacetate (92/8 cis/trans ratio).
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