Benzene → (2R,3R)-2,3-diphenyl-1,4-dioxaspiro[4.5]dec-6-ene

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To a 2 L flask was added (1R,2R)-1,2-diphenyl-1,2-ethanediol (200 g, 924 mmol), 2-cyclohexen-1-one (101 g, 1017 mmol), benzene (1232 mL) and PPTS (11.61 g, 46.2 mmol). The flask was fitted with a condenser and a dean-stark trap filled with benzene. The reaction was heated to 115° C. for 18 h (meanwhile trap contained 16.8 mL water indicating reaction completion). The reaction was cooled to RT and diluted with ether. The mixture was washed with saturated aq NaHCO3. The organic layer was dried over MgSO4, filtered and concentrated to give an orange liquid. This was passed over a silica gel plug eluting with 100% hexane (500 mL), then 5% EtOAc/Hexane (2 L), then 10% EtOAc/Hexane (500 mL), then 25% EtOAc/Hexane (500 mL). The product eluted in the first ˜2.5 L of solvent. Removal of solvent afforded a light yellow oil. To this residue was added ˜275 mL of hexane and the solution was let stand and crystals started to rapidly form. After about 1 h at RT, the solution was cooled in the freezer overnight. The next day, the hexane was decanted off, the solids washed with cold hexane. The solids were dried under reduced pressure at 75° C., and 223.75 g of (2R,3R)-2,3-diphenyl-1,4-dioxaspiro[4.5]dec-6-ene was obtained. The hexane solution was concentrated and purified by silica gel (ISCO, 330 g column, 100 mL/min, 0-10% EtOAc/Hexane over 40 min). Concentration of the pure fractions afforded (2R,3R)-2,3-diphenyl-1,4-dioxaspiro[4.5]dec-6-ene as an oil that solidified on sitting (30.47 g).

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