Unexpected nonplanar quinoidal substructure in an abietane terpene from Salvia candelabrum—an ab initio computational search for the simplest substitution pattern causing the nonplanarity

Pálinkó, István and Forgó, Péter (2005) Unexpected nonplanar quinoidal substructure in an abietane terpene from Salvia candelabrum—an ab initio computational search for the simplest substitution pattern causing the nonplanarity. JOURNAL OF MOLECULAR STRUCTURE-THEOCHEM, 717 (1-3). pp. 153-155. ISSN 0166-1280

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Abstract

A recently isolated natural diterpene (candelabroquinone) unexpectedly displayed nonplanarity in its quinoidal substructure on using a variety quantum chemical methods for unrestricted geometry optimisation (PM3, HF 3-21G, HF 6-31G* and HF 6-31G**). By gradual structural simplification, it was possible to identify the minimum substitution pattern causing the nonplanarity. Appreciable alteration from planarity was found when the original three-ring compound was simplified to a two-ring structure containing a cyclohexene ring fused to the 1,4-benzoquinoidal substructure having an oxo substituent in the cyclohexene ring next to the anellation. A decreased but definite nonplanarity was still observed when the oxo group was replaced by a methyl group. On removing the methyl substituent the quinoidal ring became planar and it remained so even when the two-ring molecule was further simplified to a fully substituted one-ring system.

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