[Cl:1][C:2]1[CH:3]=[C:4]([C:9](=[O:11])[CH3:10])[CH:5]=[C:6]([Cl:8])[CH:7]=1.C[Si]([N-][Si](C)(C)C)(C)C.[Li+].[C:22](OC(C)(C)C)(=[O:30])[C:23]([O:25][C:26]([CH3:29])([CH3:28])[CH3:27])=[O:24]>C(OCC)C>[Cl:1][C:2]1[CH:3]=[C:4]([C:9](=[O:11])[CH2:10][C:22](=[O:30])[C:23]([O:25][C:26]([CH3:29])([CH3:28])[CH3:27])=[O:24])[CH:5]=[C:6]([Cl:8])[CH:7]=1To a solution of the product from step B (4.1 g, 21.7 mmol) in 100 mL of diethyl ether at −78° C. was added lithium bis(trimethylsilyl)amide (23.9 mL, 23.9 mmol). After 50 min at −78° C. di-tert-butyl oxalate (4.18 g, 20.66 mmol) was added as a solid. The cold bath was removed and after 2 h an additional portion of di-tert-butyl oxalate (530 mg, 2.6 mmol) was added and stirring continued at ambient temperature for 16 h. The reaction was quenched by adding 150 mL of 1N hydrochloric acid and stirring for 1 h. The mixture was extracted with ethyl acetate and the organic layer washed with brine, dried over magnesium sulfate, and concentrated in vacuo to provide the title compound as an oil which was used without further purification. 1H NMR (500 MHz, CDCl3) δ: 7.84 (d, J=1.9 Hz, 2H); 7.59 (d, J=1.9 Hz, 1H); 6.93 (s, 1H); 1.56 (s 9H).
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