To a cooled (0° C.) solution of (S)-2-benzyl-4-(tert-butyl-diphenyl-silanyloxy)-2-hydroxy-N-((1S,2R)-2-hydroxy-indan-1-yl)-butyramide (21e) (0.325 g, 0.56 mmol) and pyridinium p-toluenesulphonic acid (15 mg, 0.05 mmol) in dry dichloromethane (20 mL), 2-methoxypropene (0.4 g, 5.6 mmol) was added and stirred for 6 h at the same temperature. Saturated NaHCO3 solution was added, organic layer and washed with sat. NaHCO3, brine, dried over anhydrous MgSO4 and evaporated under reduced pressure. The crude title product [(0.33 g), MS (ESI+): 620 (M+)] was used as such for the next reaction.
