(E)-2-Benzylidenecyclanones: Part XIX. Reaction of (E)-2-(4′-X-Benzylidene)-1-tetralones with Cellular Thiols: Comparison of Thiol Reactivities of Open-Chain Chalcones and Their Six- and Seven-Membered Cyclic Analogs

Non-enzyme-catalyzed thiol addition onto the α,β-unsaturated carbonyl system is associated with several biological effects. Kinetics and diastereoselectivity of non-enzyme catalyzed nucleophilic addition of reduced glutathione (GSH) and N-acetylcysteine (NAC) to the six-membered cyclic chalcone analogs 2a and 2b were investigated at different pH values (pH 3.2, 7.4 and 8.0). The selected compounds displayed in vitro cancer cell cytotoxicity (IC50) of different orders of magnitude. The chalcones intrinsically reacted with both thiols under all incubation conditions. The initial rates and compositions of the final mixtures depended both on the substitution and the pH. The stereochemical outcome of the reactions was evaluated using high-pressure liquid chromatography with UV detection (HPLC-UV). The structures of the formed thiol-conjugates and the retro-Michael products (Z)-2a and (Z)-2b were confirmed by high-pressure liquid chromatography-mass spectrometry (HPLC-MS). Frontier molecular orbitals and the Fukui function calculations were carried out to investigate their effects on the six-membered cyclic analogs. Data were compared with those obtained with the open-chain (1) and the seven-membered (3) analogs. The observed reactivities do not directly relate to the difference in in vitro cancer cell cytotoxicity of the compounds.