A new kind of fasciolicide: molecular and electronic structures of some o-hydroxybenzenesulfonanilides

The molecular and electronic structures of some fasciolicidal o-hydroxybenzenesulfonanilides (HBSA) have been studied using X-ray diffraction and semiempirical MO calculation. In these compounds, the phenolic hydroxyl forms a strong intramolecular hydrogen bond with an adjacent sulfonyl oxygen atom and the strength of the d-p dative S<--N bond, which may control the electron delocalization throughout the entire molecule, is affected by substituents on the phenyl rings on both sides. Owing to the poor delocalization, the contribution of the keto-form of the resonance structure is larger for some phenolate anions of HBSA in solution, and this may be a key factor determining the potency of fasciolicidal activity of HBSA.