Collect. Czech. Chem. Commun. 1999, 64, 1761-1769

Sterically Crowded Heterocycles. XI. A Semiempirical Prediction of Enantiomerization Barriers for Substituted (Z)-3-(Imidazo[1,2-a]pyridin-3-yl)-1-phenylprop-2-en-1-ones

Stanislav Böhm*, Radek Pohl and Josef Kuthan

Department of Organic Chemistry, Prague Institute of Chemical Technology, 166 28 Prague 6, Czech Republic


Conformational behaviour and racemization paths of the parent skeleton are discussed using a threedimensional Φ,Ψ-energy map calculated by the semiempirical PM3 method. To restrain the number of possible racemization paths, a less sophisticated approach based on the PM3 heat of formation - torsion angle Φ relationships has been used for 5-substituted title molecules; the role of non-planar enantiomeric transition states is postulated. Plausibility of the simple theoretical procedure has been tested using two phenyl derivatives of the title compounds for which experimental barriers to racemization are accessible.

Keywords: Imidazo[1,2-a]pyridines; Axial chirality; Transition states; PM3 method; Rotation barriers; Hindered rotation; Atropisomerism; Semiempirical calculations.