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This article presents the results of an experimental X-ray diffraction study on the natural product 3′-methylaminoavarone ( 3 ) coupled to a theoretical analysis of the conformational stability of avarol-type molecules performed by molecular mechanics. In the crystal of 3 , the two molecules (A and B) forming the asymmetric unit differ substantially for what concerns the reciprocal orientation of sesquiterpene and quinone residues. It is noteworthy that the A molecule is the only conformer, among the avarol-type structures, whose two residues do not arrange roughly perpendicular to one another. Molecular mechanics calculations have been performed on the model-molecule of avarone ( 2 ) by MM2 and AMBER methods and have proved the highest energy stability of the conformers with torsion angle θ 1 (C12–C9–C11–C1′) close to 180° and θ 2 (C9–C11–C1′– C2′) close to −90° or 90°. The theoretical results are in good agreement with the conformations observed in the crystal state. Furthermore, the comparison between the energetic barriers separating minima points to a preferential path through θ 1 =180°, that corresponds just to perpendicularity between the residues. This preferential path favors the conformational changes required to optimize in each molecule the intramolecular interactions in dependence on the different bulkiness and position of substituents. Really, the perpendicularity between sesquiterpene and quinone systems allows a rotational freedom for the θ 2 torsion angle, whereas the same rotation is much more hindered in the other θ 1 rotamers ( θ 1 far from 180°). |
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