A Simple and Clean Method for O-Isopropylidenation of Carbohydrates

Experimental Section

General Methods. Sulfonated polystyrene cation exchange resin CAT600 was donated by Xian Electric Power Resin Factory, China. It showed a parameter as the density of SO3H groups in 4.9-5.1 mmol/g, specific surface area in 35.0-38.5 m2/g, average pore size in 50-100 Å. Unless otherwise stated, other chemicals (AR) were purchased from commercial sources and used without purification. 1H NMR spectra were recorded using CDCl3 as a solvent on a Bruker Avance 300 MHz spectrometer (75 MHz for 13C NMR). GC analysis was performed on a Shimadzu GC-2010 apparatus equipped with an Rxi-1MS capillary column (30 m × 0.25 mm × 0.25 μm) connected to a hydrogen flame ionization detector. The GC condition was used as follows: a sampling/ detector temperature of 250 ℃, a column temperature of 190 ℃, nitrogen as carrier gas, nitrogen pressure 100 kPa, total flow rate: 50 mL/min, hydrogen flow rate: 40 mL/min, air flow rate: 400 mL/min. The temperature programme: 190 ℃ for 3 min, then rose to 250 ℃ with the rate 10 ℃/min, finally held at 250 ℃ for 1 min.

General Procedure for the O-Isopropylidenation. To a suspension of the substrate (2 mmol) in dry acetone (10 mL), CAT600 (100 mg) was added. Then the mixture was stirred at 40 ℃ till the TLC (n-hexane-EtOAc 2:1) showed the completion of the reaction. The catalyst was separated by filtration, washed with acetone, dried, and reused for a consecutive run under the same reaction conditions. The filtrate was condensed to dry in vacuum, and the residue was dissolved in CH2Cl2 (10 mL) and washed with 3 × 5 mL brine. The organic layer was dried over anhydrous Na2SO4, filtered and evaporated to afford the crude product. Then the desired pure product was obtained by recrystallization from n-hexane. While, the silica gel column chromatography was used if the product existed in the liquid form.