Lithium chloride (5.12 g, 121 mmol) was heat dried with a heat gun under reduced pressure. Under a nitrogen atmosphere, a mixture of No. 5499685 (E)-10-(2-heptyl-[1,3]dioxolan-2-yl)-1-((S)-4-isopropyl-5,5-diphenyl-2-thioxo-oxazolidin-3-yl)-dec-3-en-1-one (25.0 g, 40.3 mmol) and THF (80 mL) was added at room temperature. Further THF (145 mL) was then added to the reaction vessel. The mixture was stirred until it became homogeneous and then cooled to −78° C. Lithium hexamethyldisilazide (1 M solution in THF, 7.74 mL, 7.74 mmol) was added, and the mixture was stirred for 2 h. To the reaction mixture was added dropwise a mixture of tert-butyl 4-methoxy-2-oxo-butyrate (10.6 g, 56.5 mmol) and THF (25 mL) and the mixture was stirred for further 1 hour. To the reaction mixture was added acetic acid (4.61 mL, 80.6 mmol) and the cooling bus was removed. Water was added and the mixture was extracted with ethyl acetate. The organic layer was then washed with water, an aqueous solution of 1% NaHCO3, water, and a saturated brine in this order, and dried over anhydrous sodium sulfate. The solvent was then distilled off under reduced pressure. The resulting residue was purified with SP1 (SiO2 cartridge, 20% ethyl acetate/n-hexane, Rf=0.3) to obtain No. 6801291, tert-butyl (E)-(2S,3S)-11-(2-heptyl-[1,3]dioxolan-2-yl)-2-hydroxy-3-((S)-4-isopropyl-5,5-diphenyl-2-thioxo-oxazolidine-3-carbonyl)-2-(2-methoxy-ethyl)-undec-4-enoate (25.9 g, 80% yield).
