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To a solution of cis and trans-5-t-butyldimethylsilyloxymethyl-2-(diethyloxyphosphinoyl)-1,3-oxathiolane (example 25) (4.00 g, 10.8 mmol) in MeOH (30 mL) was added a few drops of acetyl chloride (200 μL) at 0° C. The mixture was stirred at 0° C. for 1 hr. and at room temperature for 18 hr. A few drops of ammonium chloride was added and the solvent was evaporated under reduced pressure. The crude material was purified by several flash chromatography with a mixture of dichloromethane and methanol (96:4) to give 382 mg of the cis isomer , 1.19 g of the trans isomer and 332 mg of the mixture in a total yield of 80% after the first purification. ##STR52## 1H NMR (300 MHz, CDCl3) δ: 1.37 (t, 6H, J=7.0 Hz, CH3), 3.03 (dd, 1H, J=5.4 Hz, 10.8 Hz, H-4a), 3.31 (dd, 1H, J=9.0 Hz, 10.9 Hz, H-4b), 3.74 (dd, 1H, J=4.0 Hz, 12.3 Hz, CHAHBOH), 3.98 (dd, 1H, J=2.9 Hz, 12.4 Hz, CHAHBOH), 4.24 (m, 4H, CH2), 4.40 (m, 1H, H-5), 5.30 (d, 1H, 2JHP =9.6 Hz, H-2). ##STR53## 1H NMR (300 MHz, CDCl3) δ: 1.37 (m, 6H, CH3), 2.05 (bs, 1H, OH), 3.00 (dd, 1H, J=7.0 Hz, 10.2 Hz, H-4a), 3.25 (dd, 1H, J=6.3 Hz, 10.4 Hz, H-4b), 3.69 (dd, 1H, J=4.9 Hz, 11.9 Hz, CHAHBOH), 3.85 (dd, 1H, J=3.9 Hz, 11.9 Hz, CHAHBOH), 4.23 (m, 4H, CH2), 4.65 (m, 1H, H-5), 5.36 (d, 1H, 2JHP =8.6 Hz, H-2).

1. waitand at room temperature for 18 hr
2. customthe solvent was evaporated under reduced pressure
3. customThe crude material was purified by several flash chromatography with a mixture of dichloromethane and methanol (96:4)