Tuning of Chemo- and Regioselectivities in Multicomponent Condensations of 5-Aminopyrazoles, Dimedone, and Aldehydes

Abstract

Regio- and chemoselective multicomponent protocols for the synthesis of 1,4,6,7,8,9-hexahydro-1H-pyrazolo[3,4-b]quinolin-5-ones, 5,6,7,9-tetrahydropyrazolo[5,1-b]quinazolin-8-ones, and 5a-hydroxy-4,5,5a,6,7,8-hexahydropyrazolo[4,3-c]quinolizin-9-ones starting from 5-amino-3-phenylpyrazole, cyclic 1,3-dicarbonyl compounds and aromatic aldehydes are described. Whereas the three-component coupling in ethanol under reflux conditions provides mixtures of pyrazoloquinolinones and pyrazoloquinazolinones, the condensation can be successfully tuned toward the formation pyrazoloquinolinones (Hantzsch-type dihydropyridines) by performing the reaction at 150 °C in the presence of triethylamine base applying sealed vessel microwave or conventional heating. On the other hand, using sonication at room temperature under neutral conditions favors the formation of the isomeric pyrazoloquinazolinones (Biginelli-type dihydropyrimidines). These products are also obtained when the three-component condensation is executed in the presence of trimethylsilylchloride as reaction mediator at high temperatures. A third reaction pathway leading to pyrazoloquinolizinones in a ring-opening/recyclization sequence can be accessed by switching from triethylamine to a more nucleophilic base such as sodium ethoxide or potassium tert-butoxide. The reaction mechanism and intermediates leading to these three distinct tricyclic condensation products are discussed.