Stereoselective synthesis and structure elucidation of spiro-ketodisaccharides

Cycloglycosylation of 3,4,5,7-tetra-O-benzyl-alpha -D-hept-2-ulopyranoses ( 2a-c) was carried out stereoselectively under the catalysis of Lewis acid t o afford two spiro-cyclodisaccharides 3a-c and 4a-c in good yields. The rea ction 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 5a-c were prepared by catalytic hydrogenation and followed by acetylat ion. The structures of compounds 4a-c, 6a-c and 8a-c were confirmed to be a lpha,beta -anomeric configuration with chair-chair-chair forin for the tri- cycles based on the X-ray crystallographic analysis of 6a-c. The alpha,alph a -anomeric configurations of compounds 3a-c, 5a-c and 7a-c were determined based on the measurements of the three bond coupling constants (3)J(C,H) b etween the C-1 and the H-3 of 7a-c. (C) 2001 Elsevier Science Ltd. All righ ts reserved.