Stereoselective synthesis and structure elucidation of spiro-ketodisaccharides

Cycloglycosylation of 3,4,5,7-tetra-O-benzyl-α-d-hept-2-ulopyranoses (2a–c) was carried out stereoselectively under the catalysis of Lewis acid to afford two spiro-cyclodisaccharides 3a–c and 4a–c in good yields. The reaction provided the kinetic products 3a–c or the thermodynamic products 4a–c as the predominant products under different conditions, respectively. The unprotected disaccharides 5a–c and 6a–c and the acetylated derivatives 7a–c and 8a–c were prepared by catalytic hydrogenation and followed by acetylation. The structures of compounds 4a–c, 6a–c and 8a–c were confirmed to be α,β-anomeric configuration with chair–chair–chair form for the tri-cycles based on the X-ray crystallographic analysis of 6a–c. The α,α-anomeric configurations of compounds 3a–c, 5a–c and 7a–c were determined based on the measurements of the three bond coupling constants 3JC,H between the C-1 and the H-3 of 7a–c.