Direct stereoselective synthesis of β-thiomannoside

A highly diastereoselective synthesis of beta-thiomannopyranosides is described in which S-phenyl 2,3-di-0-benzyl-4,6-O-benzylidene-1-deoxy-1-thia-alpha-D-mannopyranoside S-oxide is treated with triflic anhydride and 2,6-di-tert-butyl-4-methylpyridine in CH2Cl2 at -78 degrees C leading to the formation of an intermediate alpha-mannosyl triflate. Addition of primary, secondary, or tertiary thiols then leads to the beta-thiomannosides, by an S(N)2-like displacement, in good yield and with excellent stereoselectivity. Deprotection is achieved either by Birch reduction or by Zemplen deacetylation, of the acetyl protected aglycons, followed by hydrogenolysis over Pearlman's catalyst. The assignment of configuration of the beta-thiomannopyranosides is discussed in terms of the chemical shift of the mannose H5 resonance and the (1)J(CH) of the mannose anomeric carbon.