Synthesis of C-5-thioglycopyranosides and their sulfonium derivatives from 1-C-(2′-oxoalkyl)-5-S-acetylglycofuranosides Original Research Article

1-C-(2′-oxoalkyl)-5-S-acetylglycofuranosides of l-arabinose, d-ribose, and d-xylose were converted to 1-C-(2′-oxoalkyl)-5-thioglycopyranosides by base treatment. The transformation was achieved through β-elimination to an acyclic α,β-conjugated aldehyde (ketone or ester), followed by an intramolecular hetero-Michael addition by the 5-thiol group. The cycloaddition was highly stereoselective in favor of an equatorial 1-C-substitution. The resultant C-5-thioglycopyranosides were further converted to the sulfonium salts by treatment with cyclic sulfate and methyl iodide. Two sulfonium isomers were obtained due to the presence of both S-axial and S-equatorial substitutions. We observed that the chemical shifts of both C-1 and C-5 in the S-axial substituted sulfonium sugars are always shifted up-field (5–10 ppm) in comparison to those in the S-equatorial substitutions (δC 49–53 ppm vs 42–45 ppm at C-1 and 37–42 ppm vs 32–35 ppm at C-5), which provides an easy way for determination of the stereochemistry.