Dimethyl Sulfoxide → Ethyl 2-(tert-butoxycarbonylamino)-2-formyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)hexanoate

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While under an argon atmosphere, a solution of oxalyl chloride (1.46 g, 0.88 mL, 11.6 mmol) in dichloromethane (15 mL) was cooled to −78° C. and treated with dimethyl sulfoxide (1.8 g, 1.64 mL, 23.2 mmol) and stirred for 10 minutes. To this mixture ethyl 2-(tert-butoxycarbonylamino)-2-(hydroxymethyl)-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)hexanoate (2.4 g, 5.78 mmol) in anhydrous dichloromethane (15 mL) was added and the mixture was stirred for 30 minutes. The reaction mixture was quenched by adding triethylamine (3.5 g, 4.8 mL, 34.7 mmol) at −78° C. and slowly warming the reaction mixture to room temperature. The mixture was diluted with dichloromethane (25 mL) and washed successively with water (2×25 mL) and saturated aqueous sodium chloride (25 mL). Organic layer was dried over magnesium sulfate, filtered and evaporated to dryness. The resulting residue was purified using a combiflash system (40 g column, eluted with 20-40% ethyl acetate in heptanes) to give ethyl 2-(tert-butoxycarbonylamino)-2-formyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)hexanoate (1.35 g, 57%). 1HNMR (CDCl3) δ 9.58 (s, 1H), 5.64 (bs, 1H), 4.25 (m, 2H), 2.13 (m, 2H), 1.45 (m, 9H), 1.28 (m, 4H), 1.24 (s, 12H), 0.88 (t, J=6.4 Hz, 3H), 0.77 (t, J=7.9 Hz, 2H). MS found for C20H36BNO7 m/z[436(M+Na)].

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