l-Proline-catalysed sequential four-component “on water” protocol for the synthesis of structurally complex heterocyclic ortho-quinones

The L-proline-catalyzed synthesis of 7-(aryl)-8-methyl-10-phenyl-5H-benzo[h]pyrazolo-[3,4-b]quinoline-5,6(10H)-diones via the four-component sequential reaction of phenylhydrazine, 3-aminocrotononitrile, substituted benzaldehydes and 2-hydroxynaphthalene-1,4-dione is described. This “on water” protocol proceeds in high atom economy and leads to the generation of two rings, together with two C–C, one C–N and two CN bonds in a single operation. The environmental advantages of the method include short reaction time, excellent yield, easy work-up, and the absence of extraction and chromatographic purification steps.