Synthesis and photoinitiated thiol-ene reactions of exo-mannals - a new route to C-ß-d-mannosyl derivatives.

Syntheses of acyl protected exo-mannal derivatives were developed starting from O-peracylated mannopyranoses via the corresponding anhydro-aldose tosylhydrazones under modified Bamford-Stevens conditions. The synthesis of analogous O-peralkylated (benzylated and isopropylenated) derivatives was carried out from pyranoid and furanoid mannonolactones using methylene transfer reagents. Photoinitiated thiol-ene additions of these exo-mannals resulted in the corresponding C-(mannopyranosyl/mannofuranosyl)methyl sulfides in medium to good yields with exclusive regio- and ß(d) stereoselectivities.

Photoinitiated hydrothiolation of pyranoid exo-glycals: the d-galacto and d-xylo cases.

Radical-mediated addition reactions of thiols to O-peracetylated exo-galactal and exo-xylal with 2,2-dimethoxy-2-phenylacetophenone as the photoinitiator resulted in high yielding formation of the corresponding ß-d-glycopyranosylmethyl-sulfide derivatives (2,6-anhydro-1-deoxy-1-S-substituted-1-thio-alditols) with exclusive regio- and very high stereoselectivity, including disaccharide mimicks with Gly-CH2-S-Gly scaffolds.