Juanes M, Saragi RT, Enríquez L, Jaraíz M, Lesarri A. Molecular Rotation Spectrum of Tetracyclic Quinolizidines: Observation of
trans-Matrine and the Elusive
cis-Matrine.
J Org Chem 2021;
86:1861-1867. [PMID:
33405924 DOI:
10.1021/acs.joc.0c02689]
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Abstract
We characterized the bis-quinolizidine tetracyclic alkaloid (5S, 6S, 7R, 11R)-matrine in a supersonic jet expansion, using chirped-pulsed broadband microwave spectroscopy. Previous crystal diffraction analyses suggested 16 diastereoisomers associated with matrine's four carbon stereocenters but were inconclusive whether the lactamic nitrogen atom would additionally produce separated trans-/cis- diastereoisomers or if both species may interconvert through low potential barriers. Our experiment simultaneously detected trans- and cis-matrine through their rotational spectrum, confirming the possibility of conformational rearrangement in matrine alkaloids. The two matrine conformers mainly differ in the envelope or half-chair lactamic ring, as evidenced by the experimental rotational and nuclear quadrupole coupling parameters. Molecular orbital calculations with ab initio (MP2) and density functional methods (B3LYP-D3(BJ) and MN15) were tested against the experiment, additionally offering an estimation of the cis-/trans- barrier of 24.9-26.9 kJ mol-1. The experiment illustrates the structural potential of chirped-pulsed broadband microwave spectroscopy for high-resolution rotational studies of biomolecules in the range of 20-40 atoms.
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