期刊
TETRAHEDRON
卷 64, 期 24, 页码 5773-5778出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.tet.2008.04.007
关键词
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Ab initio MO calculations were carried out, at the MP2/6-311++G(d,p)//MP2/6-311G(d,p) level, to investigate the Gibbs energy of conformational isomers of (R)-alpha-phellandrene and related 5-alkyl-1,3-cyclohexadienes. It has been found that the conformer bearing the 5-alkyl group in axial orientation is more stable than the equatorial congener. The result is consistent with experimental evidence that the axial-isopropyl conformer prevails in the conformational equilibrium of a-phellandrene. The reason for the stability of the folded conformer has been sought in the context of the CH/pi hydrogen bond. A number of short non-bond distances have been disclosed in the axial conformers, between CHs in the 5-alkyl group and sp(2)-carbons of the cyclohexadiene ring. We suggest that the stability of the folded conformation often observed in conjugated diene compounds of natural origin, such as alpha-phellandrene and levopimaric acid, is attributed to all attractive molecular force, the CH/pi hydrogen bond. (C) 2008 Elsevier Ltd. All rights reserved.
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