Stereoselectivity in the synthesis of unnatural disaccharides.

: The synthesis of branched-chain sugars with two hexose moieties linked by a -CH2- bridge in the positions (3-->3) is described. The synthesis starts from protected 3-deoxy-3-(formylmethyl)-alpha-D-allofuranose in which the original formyl group is converted in four reaction steps into a new hexopyranose of D- or L-configuration. The relative configuration of the prepared compounds was determined on the basis of coupling constants in the 1H NMR spectra whereas the absolute configuration of the newly created pyranose was unequivocally assigned by single crystal X-ray crystallography of the 3-deoxy-3-[(2R,4R,5S,6R)-2-ethoxy-5-acetoxy-6- acetoxymethyl-tetrahydropyran-4-ylmethyl]-1,2:5,6-di-O-isopropylid ene- alpha-D-allofuranose (10).