SYNTHESIS OF METHYL 𝛼-D-GALACTOPYRANOSIDE

• In a 2-litre flask fitted with a reflux condenser place 100 g (0.56 mol) of dry 𝛼-D-galactose and 700 ml of an anhydrous methanolic solution of hydrogen chloride (about 0.6 m) (1). 

• Heat the mixture under reflux for 14 hours, cool, add 1 50 ml of distilled water and treat the light brown solution with solid lead carbonate until all the acid has been neutralised (2). 

• Filter off the inorganic salts and remove the solvent on a rotary evaporator under reduced pressure. 

• Triturate the resulting brown syrup with absolute ethanol with cooling in ice to cause the product to crystallise and recrystallise it from the minimum quantity of absolute ethanol to obtain 62 g of crude material, m.p. 85-90 °C.

• Isolate the methyl a-D-galactopyranoside as the hydrate by dissolving the crude product in 30 ml of water and allowing the solution to stand for one day at room temperature and two days at 4 °C. 

• Repeat the recrystallisations several times using proportionate amounts of water until pure hydrated product, m.p. 109-110 °C, [𝛼]ᴅ²⁰ + 173.4° (c 1 in H 2 0), is obtained; the yield is 38g(35%)(3).

Notes to keep in mind: 

1. Dry hydrogen chloride gas is passed into dry methanol (contained in a flask protected by a calcium chloride guard-tube) until analysis of aliquot portions by titration with standard aqueous sodium hydroxide solution reveals the required concentration has been reached. It is usually more convenient to prepare initially a smaller volume of a more concentrated solution and dilute it to the

appropriate concentration with dry methanol. The aliquot portions (say 5 ml) should be diluted with distilled water (20 ml) before titration.

2. Universal indicator paper moistened with distilled water gives a satisfactory indication of neutralisation.

3. Pure methyl 𝛽-D-galactopyranoside (m.p. 1 77-180 °C) may be isolated from the combined aqueous filtrates of these several crystallisations by removal of water and recrystallisation of the residue from absolute ethanol.