SYNTHESIS OF METHYL 2,3,4,6-TETRA-0-ACETYL-𝛽-d-GLUCOPYRANOSIDE

• In a 500-ml two-necked flask fitted with a mechanical stirrer and calcium chloride guard-tube place 110 ml of dry methanol, 110 ml of pure chloroform, 22 g of anhydrous calcium sulphate, 7.2 g of yellow mercury(n) oxide and 0.55 g of mercury(n) bromide. 

• Stir the suspension for 30 minutes and add 16.5 g (0.04 mol) of 2,3,4,6-tetra-O-acetyl-𝛼 -D-glucopyranosyl bromide in one portion. 

• The temperature of the mixture will rise to about 25-30 °C, the pH of the solution will fall from 7 to 2 and the yellow coloration of the mercury(n) oxide will disappear (1).

• Stir the suspension for a further 90 minutes, filter through a pad of Celite filter-aid and evaporate the filtrate on a rotary evaporator under reduced pressure. 

• Dissolve the viscous oil which remains in 10 ml of chloroform, remove the inorganic salts which are precipitated by filtration and wash the residue well with further portions of chloroform. 

• Evaporate the chloroform and triturate the resulting viscous oil with methanol until it solidifies. 

• Recrystallise from methanol to give pure methyl 2,3,4,6-tetra-O-acetyl-𝛼 -D-glucopyranoside, m.p. 104-105 °C, [𝛼]ᴅ²º    - 18.2° (c 1 in CHC1 3 ). The yield is 13.7g (95%).

Notes to keep in mind:

1.  The yellow coloration in the solution disappears within a few minutes of addition of the glucosyl halide and t.l.c. analysis (solvent system benzene-methanol, 98:2) reveals virtual completion of the reaction.

Cognate preparation: Methyl 2,3,4,6-tetra-O-acetyl-𝛽-D-galactopyranoside

•  Use 13.5 g (0.033 mol) of 2,3,4,6-tetra-O-acetyl-𝛼-D-galactopyranosyl bromide, 19 g of anhydrous calcium sulphate, 5.6 g of yellow mercury(n) oxide, 0.5 g of mercury(n) bromide, 90 ml of dry chloroform and 90 ml of dry methanol under the reaction conditions and subsequent isolation procedure described above; 7.5 g (63%) of methyl 2,3,4,6-tetra-0-acetyl-𝛽-D- galactopyranoside, m.p. 96-97 °C, [𝛼]ᴅ²º-28.0° (c2.5 in CHC1 3 ), is obtained after several recrystallisations from ethanol.