Influence of anthraquinone scaffold on E/Z isomer distribution of two thiosemicarbazone derivatives. 2D NMR and DFT studies

• 2D NMR: two anthraquinone-thiosemicarbazones exist as the E-isomers.
• The most stable conformers of the E- and Z-isomers of both compounds were revealed.
• DFT: E-isomers are thermodynamically more stable than Z-isomers.
• Perfect agreement between the experimental and calculated 13C NMR spectra.
• The E-isomers dominate due to the ten-membered NH⋯O hydrogen bonds.

A distribution of possible isomeric and tautomeric forms of two tautomerizable anthraquinone-thiosemicarbazones with pronounced cytotoxic potential was investigated using 2D NMR and DFT studies. Conformational analysis of the E and Z isomers of both thiosemicarbazones was performed to find out the most stable conformation for each molecule. It was found that superior stability of E-isomers results from ten-membered intramolecular hydrogen bond between thiosemicarbazone N2H and anthraquinone carbonyl group. This hydrogen bond is stronger than that between thiosemicarbazone N2H and ester oxygen, owing to the large partial negative charge on the anthraquinone oxygen.

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