Thionyl Chloride → Ethyl (4S,5S)-5-[(S)-3-methyl-1-(3-methylbutylcarbamoyl)butylcarbamoyl]-1,3,2-dioxathiolane-4-carboxylate-2-oxide

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Ethyl (2S,3S)-2,3-dihydroxy-3-[(S)-3-methyl-1-(3-methylbutylcarbamoyl)butylcarbamoyl]propionate (305 mg) obtained in Example 2 was dissolved in carbon tetrachloride (6 ml). Thionyl chloride (69 μl) was added to the resultant solution at room temperature, and the solution was heated under reflux for 40 minutes. The reaction solution was condensed under reduced pressure to obtain a crude product. This crude product was purified by silica gel column chromatography (a developing solvent; hexane:ethyl acetate=2:1) to obtain 174 mg of a target compound as white crystals. This product was a diastereomer mixture due to 2-position asymmetry. Therefore, the mixture was subjected to silica gel column chromatography again to obtain a high-polarity isomer (compound A) and a low-polarity isomer (compound B). NMR data and SIMS data of the compounds A and B were as follows.

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