Our aim was to develop a green and industrially acceptable process for the synthesis of pyrrole derivatives. In view of the fact that the lactic XL388 can indeed be used as a green solvent for organic synthesis, and ultrasonic-assisted organic synthesis is also a green synthetic approach  that is being used more and more, we turned our attention to evaluate Knorr condensation reaction in lactic acid medium under ultrasonic radiation. Surprisingly, Knorr condensation reaction was carried out very efficiently under ultrasonic irradiation in lactic acid medium. As shown in Scheme 2 and Table 1, the synthesis of pyrrole derivatives was readily achieved in high yields in lactic acid medium under low power ultrasonic irradiation.
Scheme 2. The synthetic route of compounds 1–18 in lactic acid medium under ultrasonic irradiation, (a) NaNO2/lactic acid/0–5 °C; (b) 1,3-dicarbonyl compound/Zn/lactic acid; (c) lactic acid.Figure optionsDownload full-size imageDownload as PowerPoint slide