[CH3:1][O:2][C:3](=[O:16])[C:4]([NH:8][C:9]([O:11][C:12]([CH3:15])([CH3:14])[CH3:13])=[O:10])([CH3:7])[CH2:5][OH:6].CO[C:19](OC)([CH3:21])[CH3:20].B(F)(F)F.CCOCC>CC(C)=O>[CH3:1][O:2][C:3]([C:4]1([CH3:7])[CH2:5][O:6][C:19]([CH3:21])([CH3:20])[N:8]1[C:9]([O:11][C:12]([CH3:15])([CH3:14])[CH3:13])=[O:10])=[O:16]To a stirring solution of 3 (9.342 g, 40 mmol) in acetone (15 mL) in a flame dried round bottom flask under an inert atmosphere was added 2,2-dimethoxypropane (66 mL). To this solution was added BF3.OEt2 (0.30 mL, cat.) and stirring was continued at room temperature for 2 hours. The reaction mixture was then concentrated in vacuo to an orange oil which was purified by flash chromatography to provide 4 (9.392 g, 85%) as a white solid. Rf=0.55 (1:3 EtOAc/hexanes). Compound was observed as an uneven mixture of rotomers.
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