Facile synthesis of pyronin-9-thione via a trisulfur radical anion mechanism

Pyronin-9-thione (PY-S) is a key synthetic intermediate to develop various fluorescent probes for biological labeling and imaging. We developed a facile synthesis of PY-S from pyronin with a high yield up to 90%. The investigation of the reaction mechanism disclosed that the trisulfur radical anion (S3˙−) played a key role in the formation of the CS bond. The new established methodology was proved to have a short reaction time, operation easiness, and low toxicity and is suitable for the gram-scale synthesis of PY-S.