Direct stereoselective synthesis of beta-thiomannoside

A highly diastereoselective synthesis of beta-thiomannopyranosides is descr ibed in which S-phenyl 2,3-di-0-benzyl-4,6-O-benzylidene-1-deoxy-1-thia-alp ha-D-mannopyranoside S-oxide is treated with triflic anhydride and 2,6-di-t ert-butyl-4-methylpyridine in CH2Cl2 at -78 degrees C leading to the format ion of an intermediate alpha-mannosyl triflate. Addition of primary, second ary, or tertiary thiols then leads to the beta-thiomannosides, by an S(N)2- like displacement, in good yield and with excellent stereoselectivity. Depr otection 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-thiomannopyranosid es is discussed in terms of the chemical shift of the mannose H5 resonance and the (1)J(CH) of the mannose anomeric carbon.