One-Pot Diastereoselective Synthesis of Tetrahydroquinolines from Star Anise Oil in a Choline Chloride/Zinc Chloride Eutectic Mixture

In this contribution, a sustainable one-pot diastereoselective synthesis of tetrahydroquinolines through the aza Diels-Alder reaction is reported starting from star anise oil, substituted anilines, and aromatic aldehydes in a choline chloride/zinc chloride eutectic mixture as new reaction media. Star anise extracts and essential oils obtained by Soxhlet, ultrasound-assisted extraction, steam distillation, and hydrodistillation were analyzed by gas chromatography-mass spectrometry finding the highest content of trans-anethol from hydrodistillation; in addition, the eutectic composition and temperature for the choline chloride/zinc chloride mixture were revisited. All the compounds were obtained in 28-98% yield with diastereoselectivity toward the cis-(2e,4e) product. Furthermore, the deep eutectic solvent was reused in three cycles without observing a detrimental catalytic activity, and green metric analysis showed a favorable environmental impact of this synthetic method.