[CH2:1]([O:3][C:4](=[O:21])[CH2:5][CH:6]1[CH2:13][CH:12]2[CH:8]([CH2:9][C:10]3(OCC(C)(C)C[O:14]3)[CH2:11]2)[CH2:7]1)[CH3:2].O.C1(C)C=CC(S(O)(=O)=O)=CC=1.CCOC(C)=O>CC(C)=O.O>[CH2:1]([O:3][C:4](=[O:21])[CH2:5][CH:6]1[CH2:13][CH:12]2[CH:8]([CH2:9][C:10](=[O:14])[CH2:11]2)[CH2:7]1)[CH3:2]
A solution of the product of Step 3 (400 mg, 1.350 mmol) in acetone (8.0 mL) and water (8.00 mL) was stirred at room temperature 16 hrs with p-toluenesulfonic acid monohydrate (257 mg, 1.35 mmol). The volatiles were removed under reduced pressure and the residue was partitioned between EtOAc and brine. The separated organic layer was dried over Na2SO4, filtered and concentrated under reduced pressure. Flash chromatography (10 to 60% EtOAc in hexanes) afforded ethyl[5-oxooctahydropentalen-2-yl]acetate (38 mg, 0.181 mmol, 13.4% yield) as a colorless oil. 1H NMR (500 MHz, CDCl3) δ 4.14-4.09 (m, 2H), 2.73-2.71 (m, 2H), 2.53-2.47 (m, 2H), 2.37-2.32 (m, 3H), 2.29-2.25 (m, 2H), 2.08-2.02 (m, 2H), 1.27-1.24 (m, 3H).
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