(R)-4-Benzyl-3-(2-(4-chlorophenyl)acetyl)oxazolidin-2-one (3.00 g, 9.10 mmol) was dissolved in DCM (91 mL) and cooled to −78° C. A 1M toluene solution of TiCl4 (11.4 mL, 11.4 mmol) was added to the solution followed by DIEA (1.66 mL, 9.55 mmol) to afford a dark purple reaction. This was stirred for 15 minutes, and then tert-butyl 2,4-dimethoxybenzyl(methoxymethyl)carbamate (3.40 g, 10.9 mmol) was added dropwise as a solution in DCM (10 mL). The reaction was allowed to stir for 15 minutes at −78° C., and then allowed to warm to −18° C. in a brine-ice bath for one hour. This reaction was allowed to warm slowly to 0° C. over a 2.5 hour period and then quenched with the addition of saturated NH4Cl solution (100 mL). The layers were separated, and the organic portion was extracted once with DCM. The combined organic portions were dried over MgSO4, filtered, and concentrated in vacuo to afford a yellow oil. The residue was purified by chromatography (silica gel eluted with 4:1 hexanes:ethyl acetate) to afford the pure material as a colorless oil, tert-butyl (S)-3-((R)-4-benzyl-2-oxooxazolidin-3-yl)-2-(4-chlorophenyl)-3-oxopropyl(2,4-dimethoxybenzyl)carbamate (4.07 g, 73.5% yield). tert-Butyl (S)-3-((R)-4-benzyl-2-oxooxazolidin-3-yl)-2-(4-chlorophenyl)-3-oxopropyl(2,4-dimethoxybenzyl)carbamate (680 mg, 1.12 mmol) was dissolved in DCM (10.6 mL) and water (560 uL; 19:1 DCM:water) at ambient temperature. The solution was treated with DDQ (380 mg, 1.67 mmol), and the reaction was allowed to stir for one day to afford reaction completion by TLC and LCMS analysis. The reaction was diluted with DCM and washed twice with half saturated NaHCO3 solution. The organic portion was dried over MgSO4, filtered, and concentrated in vacuo to afford a yellow-orange oil. The residue was purified by chromatography (silica gel eluted with 9:1 hexanes:ethyl acetate) to afford a mixture of the aldehyde by-product and tert-butyl (S)-3-((R)-4-benzyl-2-oxooxazolidin-3-yl)-2-(4-chlorophenyl)-3-oxopropylcarbamate (not separable) as a pale-yellow oil (729 mg combined mass). LC/MS (APCI+) m/z 359.1 [M-BOC+H]+.
