Structural studies of crystalline oligosaccharides VI: Crystal structure and conformation of benzyl‐4,6‐O‐benzylidene‐3‐O‐benzoyl‐β‐D‐galactoside

Abstract

Monsaccharides are the building blocks of polysaccharides and hence are the simplest sugar molecules to study the conformation and molecular structures of sugars. Benzyl-4,6-O-benzylidene-3-O-benzoyl-β-Dgalactoside is a key intermediate in the synthesis of polysaccharides. Crystal structural investigation of the title compound was undertaken to establish their chemical structure as well as to study their solid state conformation. Crystals of the title compound, obtained from water/methanol, are orthorhombic, space group P2 1 2 1 2 1 , with cell dimensions a=11.290 (4), b=9.941 (1) , 2886 reflections were collected on a CAD-4 diffractometer. The structure was solved by direct methods and refined to a final reliability index of 4.7%. The galactoside sugar has the chair conformation with C2' and C5' deviating from the mean plane of the other atoms of the sugar. The 4,6-Obenzylidene ring also has a chair conformation with the benzoyl group proximal to O6'. The crystal structure is stabilized by O-H…O hydrogen bonds involving O2' as donor and three C-H…O hydrogen bond interactions.

Unexpected Nucleophile Masking in Acyl Transfer to Sterically Crowded and Conformationally Restricted Galactosides

“…Second, the crystallographic structure of 2a reveals that the 4,6- O -benzylidene-protected galactoside assumes a 4 C 1 chair conformation (Figure , bottom). The geometric constraints of the fused ring system and the bulky substituents hinder chair flip conformational dynamics and likely stabilize an all-adjacent equatorial locked system. , The masking of O -2 in 2a and 2b therefore could stem from steric crowding by adjacent substituents. However, masking through π–π stacking of the tolyl group and the aromatic ether could not be ruled out.…”