Condensation of D-mannitol (I) with acetone by means of zinc chloride gives 1,2,5,6-di-O isopropylidene-D-mannitol (II), which on treatment with dimethyl sulfate in DMSO in the presence of NaOH gives the 3,4-di-O-methyl derivative (III). Removal of the isopropylidene groups of (III) by HCl in boiling ethanol gives 3,4-di-O-methyl-o-mannitol (IV), a solution of which in pyridine is treated with methanesulfonyl chloride and subsequently with acetic anhydride to give 1,6-di-O-methylsulfonyl-2,5-di-O-acetyl-3,4-di-O-methyl-D-mannitol (V), deacetylation of which is carried out in methanol by means of hydrochloric acid.