Abstract
Internal rotation of cannabidiol ( 1), its mono-( 2) and dimethyl ( 3) derivatives has been studied by 1H and 13C dynamic NMR spectroscopy. The free energy of activation for the rotation of C 3-C 2, bond that we have observed for cannabidiol of 14.7 kcal/mol at 22° in CDCl 3 is in contrast with values previously reported in the literature. Molecular mechanics simulation of the rotation of C 3-C 2, bond of a model structure ( 4) well reproduced the presently determined barrier height. Analysis of the calculated rotational transition state structure revealed severe nonbonded C 2H/O and C 8/O interactions as the source of barrier. Calculations of other model structures indicate general occurrence of restricted rotation about the pivot bond of phenylcyclohexanes appropriately substituted at β-carbon of cyclohexane ring and at ortho position of phenyl ring. The fully substituted 2,6-dimethyl-1-( o,o'-dimethylphenyl) cyclohexane ( 10) is predicted to show atropisomerism.
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