A glycerol mediated domino reaction: an efficient, green synthesis of polyheterocycles incorporating a new thiochromeno[2,3-b]quinoline unit

One-pot synthesis of some polyheterocycles, all of which incorporate a new thiochromeno[2,3-b]quinoline unit, has been demonstrated via a domino/Knoevenagel-hetero-Diels-Alder (DKHDA) reaction, which combines a typical aldehyde substrate, 2-methyl-2-(4-methylpent-3-en-1-yl)-2H-thiopyrano[2,3-b]quinoline- 3-carbaldehyde derived from 2-mercapto-quinoline-3-carbaldehyde and citral, with 5-pyrazolone, in glycerol. As evidenced by TLC (thin layer chromatography), all the new quinoline-based aldehyde substrates can be assembled with pyrazolone in a new environmentally benign glycerol as the reaction medium without a catalyst at 120 degrees C. The cis-fusion between the pyran and central carbocyclic (of thiochromeno) rings in all the new polyheterocycles was confirmed from both 2D NMR experiments - COSY and NOESY - and single crystal X-ray diffraction data.