# Pentacyclic aromatic heterocycles from Pd-catalyzed annulation of 1,5-diaryl-1,2,3-triazoles

## TL;DR

This paper reports a new method to synthesize complex aromatic compounds using palladium catalysis, which showed antimicrobial activity.

## Contribution

A novel Pd-catalyzed annulation method to create pentacyclic aromatic heterocycles with antimicrobial properties.

## Key findings

- Annulation of triazole N1-connected subunits succeeded under microwave irradiation with yields of 31–90%.
- Annulated compounds showed antimicrobial activity with MIC values of 2–16 μM against Gram-positive bacteria and yeast.
- Annulated systems exhibited structural rigidity with increased emission intensity and decreased Stokes shifts.

## Abstract

A series of pentacyclic aromatic heterocycles representing functionalized phenanthridines, naphthyridines, and phenanthrolines were prepared via Pd-catalyzed annulation of 1,5-diaryl-1,2,3-triazoles. Analogs with triazole N1-connected ortho-bromobenzene subunits successfully underwent annulation under microwave irradiation in yields of 31–90%. In contrast, annulations of triazole C1-connected ortho-bromobenzene subunit analogs failed under microwave irradiation but were successful using conventional thermal heating in yields of 31–65%. The expanded nature of the aromatic ring system following annulation was reflected by the downfield shifting of aromatic 1H NMR signals and the red-shifting of UV–visible absorbance signals relative to their non-annulated counterparts. Structural rigidity of annulated systems compared to non-annulated counterparts was reflected by emission signals with increased intensity and decreased Stokes shifts. Five benzotriazolophenanthroline regioisomers sharing structural similarity regarding N center placement showed antimicrobial activity, as measured by minimum inhibitory concentration assays. MIC values of 2–16 μM towards Gram-positive bacteria Staphylococcus epidermidis and Bacillus subtilis and 8–16 μM towards Saccharomyces cerevisiae yeast were observed for these annulated molecules, while their analogous non-annulated control compounds were not bioactive.