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N-Methyl Inversion and Accurate Equilibrium Structures in Alkaloids: Pseudopelletierine
Montserrat Vallejo-López1, Patricia Écija1, Natalja Vogt2
1Departamento de Química Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV/EHU), Apartado 644, 48080, Bilbao, Spain.
Rotational spectroscopy revealed two distinct conformers of the alkaloid pseudopelletierine in the gas phase. The study determined their structural properties and conformational equilibrium, favoring the axial form.
Area of Science:
- Molecular spectroscopy
- Physical organic chemistry
- Alkaloid chemistry
Background:
- Pseudopelletierine is an alkaloid with a complex bridged ring structure.
- Understanding its conformational preferences is crucial for its chemical and biological properties.
- Conformational analysis of alkaloids provides insights into structure-activity relationships.
Purpose of the Study:
- To investigate the conformational equilibrium of pseudopelletierine in the gas phase.
- To determine the precise structural properties of its different conformers.
- To compare its conformational behavior with related tropane alkaloids.
Main Methods:
- Rotational spectroscopy, specifically Fourier-transform microwave spectroscopy in a jet expansion.
- Recording and analysis of nine independent isotopologues.
- Quantum-chemical calculations up to the coupled-cluster level of theory.
- Semiexperimental mixed-estimation method for structure determination.
Main Results:
- Two distinct conformers of pseudopelletierine were identified, arising from N-methyl group inversion.
- Both conformers adopt a chair-chair configuration of the bridged six-membered rings.
- The conformational equilibrium favors the axial conformer over the equatorial conformer (approx. 2:1 ratio).
- Accurate structural parameters for both conformers were determined.
Conclusions:
- The study provides a detailed understanding of pseudopelletierine's conformational landscape in the gas phase.
- The N-methyl group inversion is a key factor influencing its conformational equilibrium.
- The findings contribute to the broader knowledge of alkaloid conformational dynamics and structural characterization.
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