25 mL of 3% hydrochloric acid and extracted with two 25-mL portions of ethyl acetate. The combined organic phases were washed with a saturated ammonium chloride solution (2 x 10 mL), dried over anhydrous MgSO,, and concentrated under reduced pressure. The resulting yellow oil was then filtered in methanol on a KC 18 reverse-phase cartridge (SEP PAK) to yield 96% of the diol7. which was used in the next step without further purification.
10: To a solution of the eryfhro diol 7 (266.9 mg, 0.43 mmol) in anhydrous benzene (1 mL) was added dropwise a suspension of N.N'-carbonyldiimidazole (76 mg. 0.47 mmol; 1.09 equiv.) in anhydrous benzene (10 mL). The mixture was stirred for 1 hat 45 'C, then a second portion of N,N'-carbonyldiimidazole (76 mg, 0.43 mmole; 1.09 equiv.) was added at 64 'C. Stirring was continued for 1.5 h at this temperature and the mixture was then transferred to a separatory funnel containing water (25 mL) and extracted with ethyl acetate (3 x 25 mL). The combined organic layers were washed with water (3 x 25 mL), dried over anhydrous MgSO,, and concentrated under reduced pressure. The resulting cake was purified by flash chromatography on silica gel (eluant: cyclohexane:ethyl acetate 80:20), yielding 209 mg (75%) of pure yellow crystalline carbonate 10. 11/12. The CdrbOnatlOn of the mixture of rhreo diols 8 and 9 was simpler, since it occurred within 1.5 h at 20 "C following one addition of N,N'-carbonyldiimidazole (1 70 mg. 5.0 equiv.). Extraction and chromatographic separation of the two yellow crystalline rhreo carbonates 11 and 12 were performed as described above for 10 (I1 + 12: ll5.9mg. 85% overall yield, 11:12 = 90: 10).