carbohydrate (6-deoxy-1,2:3,4-di-O-isopropylidene-alpha-D-galactose) (prepared, 1 equiv.),
1 % aqueous sulfuric acid (5 mL/mmol),
sodium hydrogen carbonate (powder)

The carbohydrate (6-deoxy-1,2:3,4-di-O-isopropylidene-alpha-D-galactose) (14.9 g, 61.07 mmol, 1 equiv.) was suspended in 1 % aqueous sulfuric acid (228 mL) and the mixture heated at reflux for 3 hours (110 ºC). The reaction mixture (now a clear and colourless solution) was allowed to cool to room temperature and neutralised (pH 7) by slow, portionwise addition of sodium hydrogen carbonate. This followed solvent removal in vacuo (0.5 mmHg, water bath temperature below 30 ºC), and finally complete removal of all water on a freeze drier to give the crude product (16.4 g). Further purification was not necessary, and the sugar was reprotected as the acetate (>99 % over 2 steps).

This transformation was carried out on 1 to 15 g of starting material without difficulties. It should however be noted that, in our hands, recrystallisation was not successful. Solvent removal at low temperature ensures that the sugar will not decompose and complete solvent removal can only be achieved using a freeze drier. Other nonaqueous conditions for removal of the protecting groups have proven less successful, resulting in incomplete conversion or difficulties isolating the product. An alternative method that should give good results in this case and has not been investigated yet is p-toluenesulfonic acid (monohydrate, cat.) in methanol (room temperature) (see other references). The free sugar was not purified (i.e. the sodium sulfate was not removed), but rather reprotected with acetic anhydride/pyridine (alternatively other protecting groups may be introduced at this point).

No data available. The crude product was reprotected as the acetate straight away.

R. L. Whistler, M. L. Wolfrom, (T. Schmidt); Methods in Carbohydrate Chemistry I; Academic Press; New York; 1962; 190