A highly efficient and recyclable silica-supported tungstic acid (STA) catalyst for the synthesis of pyrano[3,2- c ]chromen-5-ones under solvent free conditions

Clean chlorination of silica surfaces by a single-site substitution approach

“…Following protonation of the supported tungstate with aqueous HCl, the ensuing supported tungstic acid could be employed as a catalyst for several organic condensations. [27][28][29][30][31] These advances highlight the potential importance of chlorinated silica as a catalyst and a support, however, the chlorination of the silica surface has not been studied in detail. It is not known whether the chlorination reaction involves the silanol moieties and/or the siloxane bridges.…”

A catalyst- and solvent-free protocol for the sustainable synthesis of fused 4H-pyran derivatives

“…The structure of the compound 4a was conrmed by its melting point, IR, 1 H and 13 C NMR spectroscopies, and the data matched well with the reported data. [20][21][22][23][24] Encouraged by these results, we proceeded to optimize the reaction conditions. For this process, the model reaction was executed in the presence of different acid and base catalysts (10 mol%) in EtOH (4 mL) as well as without catalyst in EtOH with reux (Table 1, entries 2-6).…”

“…All the synthesized compounds were characterized by spectroscopic techniques including IR, 1 H and 13 C NMR spectroscopies and elemental analysis; the resulting data for several of the reported compounds are well-matched with the literature. [20][21][22][23][24] Further, the structure and stereochemistry of the compounds 4a (CCDC 1901104) and 6b (CCDC 1901105) were unambiguously conrmed by single-crystal X-ray diffraction analysis, and the ORTEP diagrams are shown in Fig. 2 and 3, respectively.…”

“…20) and piperidine, 21 whereas coumarin-fused 4H-pyran scaffolds were prepared using catalysts such as STA, 22 6,6 0 -thiobis(methylene)b-cyclodextrin dimer 23 and piperidine. 24 Regardless of the adequacy of these reported methods, each exhibits one or more shortcomings, such as the required usage of catalysts [20][21][22][23][24] or solvents 20,24 and prolonged reaction time. 20,24 Therefore, there still remains a scope of improvement of the synthetic strategy to access such important fused 4H-pyrans.…”

NitroketeneN,S-acetals: synergistic building blocks for the synthesis of heterocycles

“…The reaction was carried out under solvent-free conditions using 4-hydroxycoumarin 43, aryl aldehydes 38 and NMSM 9, and silicasupported tungstic acid (STA) was employed in a catalytic amount; they showed recyclability of the catalyst up to the 4 th cycle and achieved an 89% yield (Scheme 13). 43 Similarly, the b-CD-S dimer and piperidine have also been used as catalysts for the synthesis of 44. 41,42 Moreover, Mao et al developed a sustainable method for the synthesis of the new type of coumarin-fused pyrano[4,3-b]pyran-5-one derivatives 46 using 9 as a building block.…”