Treatment of the known methyl 3,4-di-O-acetyl-2,6-anhydro-alpha-D-altropyranoside (I) with acetic anhydride in the presence of sulfuric acid resulted in an inseparable mixture of the two diastereomers of methyl 2,6-anhydro-D-altrose tetraacetate (IIa-b) and 1,3,4-tri-O-acetyl-2,6-anhydro-beta-D-altropyranoside (III). Reaction of this mixture with 4-sulfanylbenzonitrile (IV) in the presence of trimethylsilyl triflate produced a mixture of compounds (V), (VIa-b) and (VII) that could be partially separated by column chromatography affording a 2:1 mixture of dithioketal (V) and one of the diastereoisomers of the monothioketal (VI). Deacetylation of this mixture by means of NaOMe produced the corresponding mixture of the title compound and monothioketal (VIII), which were finally separated by column chromatography.

A different method, amenable to large-scale synthesis, was further developed. Methyl 3,4-di-O-acetyl-2,6-anhydro-alpha-D-altropyranoside (I) was converted into the pentaacetate (IX) with acetic anhydride in the presence of trimethylsilyl triflate. Coupling of acylal (IX) with 4-cyanobenzenethiol (IV) produced the corresponding dithioketal, which was subsequently deacetylated with NaOMe to furnish the title compound.