Dimethyl Sulfoxide → Dimethyl 5-oxocyclohexane-1,3-dicarboxylate

To a mixture of 5-hydroxy-cyclohexane-1,3-dicarboxylic acid dimethyl ester from step 1 (3.00 g, 13.9 mmol) and triethylamine (5.80 mL, 41.6 mmol) in dimethyl sulfoxide (6.00 mL, 84.5 mmol) and DCM (0.60 mL, 9.36 mmol) was added portion-wise sulfur trioxide-pyridine complex (5.08 g, 31.9 mmol) at 10° C. The reaction mixture was stirred at room temperature for 2 hrs and then quenched with water (50 mL). The aqueous layer was extracted with EtOAc (3×50 mL). The combined organic layers were washed with brine (2×40 mL) and concentrated under reduced pressure. The residue was purified by CombiFlash® system (gradient: 0 to 50% ethyl acetate in DCM over 30 min) to give 2.2 g (74%) the title compound, 5-oxo-cyclohexane-1,3-dicarboxylic acid dimethyl ester. 1H NMR (400 MHz, CDCl3): δ 3.72 (s, 6H), 2.61-2.75 (m, 2H), 2.31-2.47 (m, 3H), 1.83 (ddt, J=2.9, 13.5, 34.4 Hz, 2H), 1.53 (q, J=13.2 Hz, 1H).