Pyrolytic elimination of hydrogen halide and halogen from 2,3-dihalogenoketones
The thermal elimination of hydrogen chloride from 2′-hydroxychalcone dichlorides occurred in two stages, initially forming 3-chloroflavanones and then flavones; α-chloro-2′-hydroxychalcones, the likely intermediates in the formation of 3-chloroflavanones, also yielded the same products. The pyrolysis of 2′-hydroxychalcone dibromides resulted in debromination as well as dehydrobromination with the consequent appearance of brominated flavones among the products. The formation of 3-bromoflavanones was not observed but the isomeric (E)-α-bromo-2′-hydroxychalcones thermally isomerized to their (Z)-isomers and then underwent cyclization and dehydrobromination. The bromides were considerably more reactive than the dichlorides but, as a synthetic procedure, the pyrolysis of the dichlorides is cleaner and more productive.