Quinolone-Hydroxyquinoline Tautomerism in Quinolone 3-Esters. Preserving the 4-Oxoquinoline Structure To Retain Antimalarial Activity

Horta P., Kus N., Henriques M. S. C., Paixao J. A., Coelho L., Nogueira F., ...More

JOURNAL OF ORGANIC CHEMISTRY, vol.80, no.24, pp.12244-12257, 2015 (SCI-Expanded) identifier identifier identifier

  • Publication Type: Article / Article
  • Volume: 80 Issue: 24
  • Publication Date: 2015
  • Doi Number: 10.1021/acs.joc.5b02169
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.12244-12257
  • Anadolu University Affiliated: Yes


Recent publications report in vitro activity of quinolone 3-esters against the bc(1) protein complex of Plasmodium falciparum and the parasite. Docking studies performed in silica at the yeast Q(o) Site established a key role for the 4-oxo and N-H groups in drug-target interactions. Thus, the possibility of 4-oxoquinoline/4-hydroxyquinoline tautomerism may impact in pharmacologic profiles and should be investigated. We describe the synthesis, structure, photochemistry, and activity against multidrug-resistant P. falciparum strain Dd2 of ethyl 4-oxo-7-methylquinoline-3-carboxylate (7Me-OQE) and ethyl 4-hydroxy-5-methylquinoline-3-carboxylate (5Me-HQE), obtained from diethyl 2-[((3-methylphenyl)amino)methylene]malonate. Theoretically (B3LYP/6-311+ +G(d,p)), 5Me-HQE and 7Me-OQE show clear preference for the hydroxyquinoline form. The difference between the lowest energy hydroxyquinoline and quinolone forms is 27 and 38 kJ mol(-1), for 5Me-HQE and 7Me-OQE, respectively. Calculations of atomaticity indexes show that in 5Me-HQE,both rings are aromatic, while in the corresponding oxo tautomers the nitrogen-containing ring is essentially non-aromatic. The structure of monomeric 5Me-HQE was studied using matrix-isolation coupled to FTIR spectroscopy. No traces of 4-oxoquinoline tautomers were found in the experimental IR spectra, revealing that:the species present in the crystal, 5Me-HQE center dot HCl, was lost HCl upon sublimation but did not tautomerize. Continuous broadband irradiation (lambda > 220 nm; 130 min) of the matrix led to only partial photodecomposition of 5Me-HQE (ca. 1/3).