ELSEVIER THEO CHEM Journal of Molecular Structure (Theochem)390 (1997) 209-215 Theoretical study of cyclization of 2’-hydroxychalcone L.J. Yamin, S.E. Blanco, J.M. Luco, F.H. Ferretti Area de Quimica-Fisica, Facultad de Quimica, Bioquimica y Farmocia, UN de San Luis, Chacabuco y Pedernera, 5700 San Luis, Argentina Received 26 September 1995; accepted 18 March 1996 Abstract The isomerization mechanism of 2’(0H)chalcone (1) in flavanone (2) was studied. The calculations were performed with the semiempirical method AMl, using totally optimized molecular geometries. A 6-step mechanism including several equilibrium states was proposed. It was concluded that: (a) At the conformational equilibrium of 1 there could be 43.9% of s-cis conformer; (b) The acid dissociation of 1 truns-s-truns is considerable; (c) The EE, AHr and net charges show that the rotation of ring A of 1 and the formation of ring C of 2 occurs without greater impairments; (d) Although the keto structure is the most stable one, the enolate of 2 is present in the reaction medium; (e) The conversion of enol of 2 in the keto form would be the limiting step of the analyzed isomerization rate. 0 1997 Elsevier Science B.V. Keywords: AM1 calculation; 2’-Hydroxylchalcone; Isomerization mechanism 1. Introduction Both the chemical and physical behavior and the different properties exhibited by 2’(0H)chalcone (1) are matters of great importance [l]. This /3-phenyl- &non saturated ketone has attracted the interest of theoretical and experimental chemists because of its stereochemical features [2] and high chemical reactivity [3,4]. Also, due to its bacteriostatic activity, it is considered of interest in biochemistry [S]. Cycli- zation of 1 to give flavanone (2) occurs photochemi- tally [6] in acid medium [7] and easily in alkaline medium [8]. The basic catalysis of the isomerization of substituted 2’(0H)chalcones-flavanones has been explained by various authors through different mechanisms [g-11]. Following the line of earlier studies [12,13], a theoretical study of the mechanism of reaction 1 - 2 has been performed in alkalinized aqueous ethanol. 2. Calculations The structures of 2’(0H)chalcone and flavanone are shown in Fig. 1. The reactivity indexes and energy parameters of 1 and 2 were calculated through the semiempirical AM1 (optional Saddle) method [14,1.5], included in the MOPAC version 6.0 [16], using totally optimized molecular geometries. The energy values of the involved structures in the con- formational equilibrium zyxwvutsrqponmlkjihgfedcbaZYXWVU tram-s-cis e tram-s-tram and also of rotation of ring A from 2’(0H)chalconate tram-s-trans (Scheme 1, steps 1 and 3 respectively), were obtained for different molecular structures with dihedral angles (8, 4) increasing by 15” in the 0” to 180” interval. The dihedral angle 6 is formed by the planes containing benzoyl and cinnamoyl groups and its intersection corresponds to the single bond C carbonyhc -Ca. The dihedral angle 4 is formed by the plane that involves ring A of 3 and the plane containing 0166-1280/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PI1 SO166-1280(96)04776-S