Ascorbic acid: an efficient organocatalyst for environmentally benign synthesis of indole-substituted 4H‑chromenes

p-Toluenesulfonic acid-promoted organic transformations for the generation of molecular complexity

Since the beginning of this century, p-toluenesulfonic acid (p-TSA) catalysed organic transformations have been an active area of research for developing efficient synthetic methodologies. Often, catalysis using p-TSA is associated with many advantages, such as operational simplicity, high selectivity, excellent yields, and ease of product isolation, which make organic synthesis convenient and versatile. Notably, p-TSA is a non-toxic, commercially available, inexpensive solid organic compound that is soluble in water, alcohols, and other polar organic solvents. p-TSA is a strong acid compared to many protic or mineral acids and its high acidity helps activate different organic functional groups. p-TSA-promoted conversions are fast, have a high atom and pot economy, and feature a multiple bond-forming index. Therefore, the utilization of p-TSA enables the synthesis of many important structural scaffolds without any hazardous metals, making it desirable in numerous applications of sustainable and green chemistry. Recently, this emerging area of research has become one of the pillars of synthetic organic chemistry to synthesise biologically relevant, complex carbocycles and heterocycles. This study provides a comprehensive summary of methods, applications, and mechanistic insights into p-TSA-catalysed organic transformations, covering the literature reports that have appeared since 2012.

Magnetic UiO-66 functionalized with 4,4'-diamino-2,2'-stilbenedisulfonic as a highly recoverable acid catalyst for the synthesis of 4H-chromenes in green solvent

According to 4H-chromenes importance, we synthesized a novel magnetic UiO-66 functionalized with 4,4′-diamino-2,2′-stilbenedisulfonic as an efficient and reusable solid acid catalyst for synthesizing 4H-chromene skeletons via a one-pot three components reaction in a green solvent. The structure of the synthesized catalyst was confirmed by various techniques including FT-IR, XRD, BET, TGA, TEM, EDX, and SEM, and also the product yields were obtained in 83–96% of yields for all the reactions and under mild conditions. The reported procedure presents an environmentally friendly approach for synthesizing a significant number of 4H-chromene derivatives. Correspondingly, MOF-based catalyst makes it easy to separate from reaction media and reuse in the next runs.

Water extract of red mud: an efficient and renewable medium for environmentally benign synthesis of 2-amino-4H-chromenes

In this study, an efficient and convenient domino Michael addition/intramolecular cyclization protocol is presented for the synthesis of biologically relevant 2-amino-4H-chromenes in short reaction times using water extract of red mud (WERM) at room temperature. Red mud is generated abundantly as wastes in aluminum industries and this is the first report to utilize WERM as an effective and renewable medium in organic synthesis. As the precursor material is a waste, the present method is environmentally benign and economical. The final 2-amino-4H-chromenes were obtained in high yields by simple precipitation and subsequent washing by aqueous ethanol which eliminates the chromatographic separation. The present method is tolerated by electronically diverse functional groups and also applicable for large-scale synthesis. Moreover, WERM was recovered from the reaction medium and reused for several cycles without significant loss of reactivity.