p-Toluenesulfonic acid monohydrate (0.35 g, 1.8 mmol) was added to a solution of (3aR,4R,5R,6aS)-4-((R,E)-4-(3-(trifluoromethyl)phenoxy)-3-hydroxybut-1-enyl)-hexahydro-5-hydroxycyclopenta[b]furan-2-one (15.0 g, 40.3 mmol) and 3,4-dihydro-2H-pyran (8.47 g, 100.7 mmol) in THF (200 mL) at room temperature and the mixture was stirred for 2.5 hr (TLC monitoring). Saturated aqueous solution of sodium bicarbonate (200 mL) was poured into the reaction mixture and the mixture was stirred for 5 minutes. The organic layer was separated and the aqueous layer was extracted with ethyl acetate (200 mL). The combined organic layers were dried over magnesium sulfate, filtered off solid and concentrated under reduced pressure to give 24.0 g of crude product. The crude product was purified by column chromatography and then concentrated under reduced pressure to provide 19.0 g of the titled compound (87.5% yield).
