Equilibration of 2-chloro-1,2-dihydrosantonin conformers; A theoretical approach using X-ray diffraction and MO calculations.

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2α-Chloro-1, 2-dihydro-l-α-santonin (1) with the cyclohexenone A-ring of the ordinary half-chair form and the 2β-chloro isomer (2) with that of the unique half-boat from were found to equilibrate through acid-catalyzed epimerization and thermodynamic conformational change. In this, the former was favored over the latter by a free energy difference of 0.7-1.0 kcal/mol. The crystal structure and the half-chair conformation of the molecule of 1 were confirmed by X-ray diffraction methods. Semi-empirical (MNDO and CNDO) and ab initio molecular orbital calculations of these chloroenone conformers using input parameters obtained from their X-ray analytical data reproduced the observed conformational energy.

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