SYNTHESIS OF L-GULOSE, L-GALACTOSE, AND THEIR ACETYLATED ALDEHYDO FORMS FROM 6-S-PHENYL-6-THIO-D-HEXOSES

Methyl 6-S-phenyl-6-thio-a-d-glucopyranoside, prepared in high yield from methyl a-d-glucopyranoside by the action of diphenyl disulfide and tributylphosphine in pyridine, was converted into 6-S-phenyl-6-thio-d-glucitol pentaacetate (7) by sequential hydrolysis, borohydride reduction, and acetylation. Oxidation of 7 with 3-chloroperoxybenzoic acid gave the corresponding S-epimeric sulfoxides, which underwent Pummerer rearrangement to 1-epimeric l-gulose S-phenyl monothiohemiacetal hexaacetates. Boron trifluoride-catalyzed reaction of the latter with thiophenol gave the analogous diphenyl dithioacetal, whereas base-catalyzed methanolysis led to free l-gulose. Treatment of 7 with N-chlorosuccinimide afforded 1-epimeric 1-chloro-1-S-phenyl-1-thio-l-gulitol pentaacetates, which were hydrolyzed to provide aldehydo-l-gulose pentaacetate. The same reaction sequences were performed with 6-S-phenyl-6-thio-d-galactose, synthesized in two steps from 1,2:3,4-di-O-isopropylidene-a-d-galactopyranose, furnishing ultimately l-galactose, its diphenyl dithioacetal pentaacetate, and aldehydo-l-galactose pentaacetate. Similar reaction sequences for the chain-terminal interchange of oxidation state in other ω-S-phenyl-ω-thioaldoses may prove useful for the preparation of less-common sugar derivatives. © 1990.