Piperazine-Amberlyst®15-catalysed synthesis of 2-amino-4H-chromenes, chromeno[2,3-b]pyridines and chromeno[2,3-d]pyrimidines

Abstract

Chromenes, biologically-active scaffolds, and their variants have been often synthesized by the combination of salicylaldehyde, malononitrile, and nucleophilic species (indoles, naphthols, nitro compounds, thiols). Whereas this combination furnishes specifically 2-amino-4H-chromenes, other related compounds such as chromeno[2,3- b]pyridines and chromeno[2,3-d]pyrimidines may be also attained using similar readily available reactants: salicylaldehyde, two equivalents of malononitrile and a thiol for the former and malononitrile, two salicylaldehyde equivalents and an amine for the latter. To the best of our knowledge, there are no reported studies which have attempted to synthesize these products using the same catalyst. Hence, the aim of the below study was to find a cheap and recyclable catalyst that would be able to drive the synthesis of all three products. Positively, piperazine supported on the polymeric sulfonic acid resin Amberlyst® 15 was found to be an inexpensive and easily-prepared novel catalyst that could be used to synthesize all three derivatives (33 examples, 18–82%) in fairly good yields whilst also being recyclable and reusable (for up to four or five runs).