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Chair interconversion and reactivity of mannuronic acid esters
Jerk Rönnols1, Marthe T C Walvoort, Gijsbert A van der Marel
1Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden.
Organic & Biomolecular Chemistry
|November 1, 2013
Summary
Mannopyranosyluronic acids exhibit unique (1)C4 chair conformations, influencing their high reactivity as glycosyl donors. Conformational flexibility and C2 substituents impact glycosylation donor reactivity, with 2-O-benzyl ethers being most reactive.
Area of Science:
- Carbohydrate chemistry
- Organic chemistry
- NMR spectroscopy
Background:
- Mannopyranosyluronic acids often adopt an unusual (1)C4 chair conformation.
- These compounds exhibit high reactivity as glycosyl donors in glycosylation reactions.
Purpose of the Study:
- To investigate the unusual conformational behavior of mannuronic acid derivatives.
- To correlate conformational dynamics with glycosyl donor reactivity.
Main Methods:
- Dynamic Nuclear Magnetic Resonance (NMR) experiments, including lineshape analysis of (1)H and (19)F NMR spectra at variable temperatures (-80 °C to 0 °C).
- Low-temperature (19)F exchange spectroscopy.
- Competition reactions monitored by (1)H NMR spectroscopy at low temperature.
Main Results:
- Conformational exchange rates between (4)C1 and (1)C4 chairs depend on C2 substituent electronic properties/size and aglycon.
- Glycosyl triflates show higher exchange rates, especially with smaller C2 substituents.
- Anomeric triflates did not exchange with free triflate species.
- Relative reactivity in glycosylation: 2-O-benzyl ether > others > 2-fluoro compound.
- Ring-flip is crucial for donor reactivity, but flip rate has minor influence on relative reactivity.
Conclusions:
- The conformational preferences and dynamics of mannuronic acid derivatives significantly influence their reactivity as glycosyl donors.
- Substituent effects at C2 and the nature of the leaving group modulate both conformation and reactivity.
- Understanding these relationships aids in designing more efficient glycosylation strategies.
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