Stereoselective Formation of Glycosyl Sulfoxides and Their Subsequent Equilibration:  Ring Inversion of an α-Xylopyranosyl Sulfoxide Dependent on the Configuration at Sulfur

Abstract

A series of four S-allyl d-thiopyranosides, α- and β-manno and xylo, were oxidized with MCPBA at low temperature to give seven of the eight possible sulfoxides, whose configuration at sulfur was determined either directly by X-ray crystallography or by correlation with closely related structures. For the axial thioglycosides oxidation leads very predominantly to the (R)S-diastereomer in the xylo series and exclusively so in the manno series; the configuration at C2 is of little importance in determining the stereoselectivity of oxidation of axial thioglycopyranosides. In the equatorial series the configuration at C2 has a significant effect on the outcome of the reaction as, although both series favored the (S)S-sulfoxide, selectivity was significantly higher in the case of the β-mannoside than of the β-xyloside. The two α-xylo sulfoxides have different conformations of the pyranoside ring with the (R)S-isomer adopting the 1C4 chair and the (S)S-diastereomer the 4C1. Each pair of diastereomeric sulfoxides was thermally equilibrated in C6D6 and in CD3OD. In the mannose series the kinetic isomers are also thermodynamically preferred. In the xylose series, on the other hand, the nature of the thermodynamic isomer in both the α- and β-anomers is a function of solvent with a switch observed on going from C6D6 to CD3OD. The results are rationalized in terms of the exo-anomeric effect, steric shielding provided by H3 and H5 in the axial series, the interaction of the C2−O2 and sulfoxide dipoles, and increased steric interactions on hydrogen bonding of the sulfoxides to CD3OD