π-Bonding contribution to restricted internal rotations in saccharides

Area of Science:

• Computational Chemistry
• Carbohydrate Chemistry
• Molecular Orbital Theory

Background:

• The exo-anomeric effect is a key phenomenon influencing carbohydrate conformation and reactivity.
• Previous studies have explored the exo-anomeric effect using various theoretical approaches.
• Understanding this effect is crucial for predicting molecular behavior in glycosides.

Purpose of the Study:

• To investigate the molecular orbital interactions responsible for the exo-anomeric effect in O-, N-, and S-glycosides.
• To utilize semiempirical self-consistent field molecular orbital (SCF-MO) calculations to quantify these interactions.
• To correlate calculated π-bond orders with observed conformational preferences in complex carbohydrates.

Main Methods:

• Extensive semiempirical SCF-MO calculations were performed on various glycoside models.
• Calculations included simpler model compounds (X-CH2-Y) and complex carbohydrates.
• Analysis focused on π-bond orders between specific atomic orbitals as a measure of interaction.

Main Results:

• The exo-anomeric effect in methyl O-, N-, and S-glycosides is confirmed to involve π-character interaction along the C1-Y1 bond.
• Calculated π-bond orders accurately reflect known anomeric effect analyses and conformational preferences.
• Maximal π-bond orders were observed for the synclinal conformation around the C1-Y1 bond.

Conclusions:

• The study confirms that π-orbital interactions are the primary drivers of the exo-anomeric effect.
• Semiempirical methods, while not perfect for energetics, are valuable for predicting conformational aspects of carbohydrates.
• The findings provide a molecular orbital basis for understanding conformational preferences in glycosides and related structures.