[F-:1].[F-:2].[F-].C(N(SN(CC)CC)CC)C.[C:15]([O:18][CH2:19][C:20]1[O:24][C:23]([CH:25]=O)=[CH:22][CH:21]=1)(=[O:17])[CH3:16].C([O-])(O)=O.[Na+]>C(Cl)Cl>[C:15]([O:18][CH2:19][C:20]1[O:24][C:23]([CH:25]([F:2])[F:1])=[CH:22][CH:21]=1)(=[O:17])[CH3:16]A solution containing 7.26 cm3 of DAST (diethylamino sulphide trifluoride) and 30 cm3 of methylene chloride is added at 5° C. to a solution containing 10 g of the product prepared in Stage A and 100 cm3 of methylene chloride. The reaction mixture is agitated at 20° C. for half an hour and is taken to reflux for 1 hour. It is maintained under agitation for 18 hours at 20° C. and taken to reflux for 3 hours, then treated with sodium acid carbonate and extracted with methylene chloride. After drying and concentrating, 12 g of a product is obtained which is chromatographed on silica eluting with a hexane--ethyl acetate mixture (8-2). In this way 5.5 g of desired product is obtained.
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