Synthesis of bioactive heterocycles using reusable heterogeneous catalyst HClO₄–SiO₂ under solvent-free conditions

Abstract

We are reporting a simple, efficient and green protocol for the synthesis of chromenes and dihydropyrimidines (products of Knoevenagel and Biginelli reaction, respectively) by the use of silica-supported perchloric acid (HClO 4-SiO 2) as an effective heterogeneous catalyst. Short reaction times, high product yields, simple procedure and reusability of the catalyst are the superior characteristics of this protocol.

Recent developments in the solvent-free synthesis of heterocycles

“…In 2018, L. V. Chanu and his co-workers reported an HClO 4 –SiO 2 catalyzed solvent-free synthesis of chromenes by Knoevenagel condensation and dihydro-pyrimidinone via Biginelli-type reaction with β-oxodithioesters as starting compound (Scheme 19). 32 They initiated the Knoevenegel condensation with salicaldehyde 69a (1 mmol) and methyl 3-oxo-3-phenylpropanedithioate 68a (1 mmol) using the HClO 4 –SiO 2 catalyst. The best results were obtained when the reaction temperature was elevated to 80 °C for two hours while using a 5 mol% catalyst in solvent-free conditions.…”

An Expedient Catalytic Process to Obtain Solketal from Biobased Glycerol

“…Anyway, going on with further bibliographic research, we were surprised to note the reported activity of silica perchloric acid (HClO 4 •SiO 2 ) [33][34][35][36], or silica triflic (trifluoromethanesulfonic) acid [37][38][39], even in the acetalization processes. Potassium bisulfate on silica (KHSO 4 •SiO 2 ) showed good activity too [40], although it was not tested in acetalization processes, until now.…”

“…Thus, according to a use-inspired basic research, we judged the employment of these "special" acids to be not justified. Perchloric acid, in particular, features known safety problems on concentration [36], so loading on silica was limited to 0.75 mmol/g [35]. A series of bisulfates never employed in acetalization processes were also considered, namely: sodium (entry 3, Table 2), potassium (entry 4), ammonium (entry 5), and N-Me imidazolium (entry 6).…”

Dysprosium–balsalazide complex trapped between the functionalized halloysite and g‐C3N4: A novel heterogeneous catalyst for the synthesis of annulated chromenes in water

“…[57][58][59][60][61][62] Among the various heterocyclic molecules, heterocycles having oxygen are considered as the main class of drugs. [63][64][65][66] Moreover, chromenes, as chemical intermediates, are broadly utilized in numerous industrial formulations including biological and pharmaceutical ones. [63,[67][68][69][70] It is worth mentioning that various approaches have been reported for the synthesis of chromenes.…”