[C:1]([N:8]1[CH2:12][CH2:11][C@@H:10](O)[CH2:9]1)([O:3][C:4]([CH3:7])([CH3:6])[CH3:5])=[O:2].C(N(S(F)(F)[F:20])CC)C.C([O-])(O)=O.[Na+]>ClC(Cl)C.O>[C:1]([N:8]1[CH2:12][CH2:11][C@H:10]([F:20])[CH2:9]1)([O:3][C:4]([CH3:7])([CH3:6])[CH3:5])=[O:2]To a well stirred and cooled (−30° C.) solution of Step 1 intermediate (1.5 g, 8.01 mmol) in dichloroethane (50 ml) was added diethylaminosulphur trifluoride (1.94 mg, 12.01 mmol) under nitrogen and the reaction mixture was maintained at this temperature for 1 h. The reaction mixture was gradually allowed to warm to room temperature and stirring was continued for another 14 h. The reaction mixture was poured onto a mixture of ice and solid NaHCO3 and stirred till no effervescence was seen. The mixture was diluted with water and extracted with dichloromethane (3×100 ml). The combined organic extracts were washed with water, brine and dried (Na2SO4). The solvent was evaporated under reduced pressure and the residue obtained was purified by silica gel column chromatography (25% ethyl acetate in petroleum ether) to give 840 mg of the desired compound as yellow oil; IR (neat) 3500, 2978, 1698, 1407 cm−1; 1H NMR (CDCl3, 300 MHz) δ 1.44 (s, 9H), 1.89-2.28 (m, 2H), 3.44-3.77 (m, 4H), 5.11-5.31 (m, 1H).
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