Substituted Xanthones as Antimycobacterial Agents,Part 1: Synthesis and Assignment of 1H/13C NMR Chemical Shifts

A series of substituted xanthones was synthesized in order to prove the hypothesis that electron-withdrawing substituents enhance the antimycobacterial activity of these compounds, which is described by means of a QSAR equation with ¹³C NMR chemical shifts as independent parameters. The key step of the synthesis is the formation of substituted 2-phenoxybenzoic acids by Ullmann reaction followed by intramolecular Friedel-Crafts acylation, leading to methyl-, carboxy-, nitro-, cyano-, and aminoxanthones as a test set for QSAR investigations. Spectroscopic data (¹H and ¹³C chemical shifts, IR, UV) of these xanthones are presented and analyzed. Specific shift increments for xanthones depending on the substituent position and on the position of the respective proton/carbon atom as well as additivity rules were developed.