3,3′-Dideoxytrehaloses via dimerisation of 2-hydroxyglycal esters

The susceptibility of 2-hydroxyglycal esters i.e., 2,3,4,6-tetra-O-acetyl-1,5-anhydro-d-arabino-hex-1-enitol and its d-lyxo analog towards dimerization to 2,3-unsaturated α,α-trehaloses is described, smoothly occurring in moist dichloromethane or acetone solution in the presence of iodine or boron trifluoride. The reaction involves Ferrier ionization and addition of water at C-1 of the delocalized glycosyl oxocarbenium ion, followed by addition of the hex-2-enopyranose thus formed, with its 1-OH, onto the educt still present. Hydrogenation of the dimers thus formed, i.e., 2,4,6-tri-O-acetyl-3-deoxy-α-d-erythro-hex-2-enopyranosyl 2,4,6-tri-O-acetyl-3-deoxy-α-d-erythro-hex-2-enopyranoside and its d-threo/d-threo analog proceeds with high stereoselectivity to give the peracetylated 3,3′-dideoxy-α,α-trehaloses with d-ribo/d-ribo and d-lyxo/d-lyxo configuration.