Study On The Metal Modified Zeolite-catalyzed Synthesis Of 2,3-unsubsituded Benzofurans

The benzofurans are presenting in a wide variety of natural products and synthetic compounds have attracted considerable attention owing to their biophysical and pharmacological activities.As medicine and synthesis intermediates,the benzofurans and their derivatives have been extensively investigated in the selection of medical intermediates and other fields.As the basic skeleton,2,3-unsubstituted benzofuran nucleus is one of important class in benzofuran chemistry.Normally,they are widely used as the important intermediates for the synthesis of complicated benzofuran derivatives by selective 2-or 3-functionalization.In this thesis,a general method for the synthesis of 2,3-unsubstituted benzofurans has been developed via intramolecular electrophilic cyclization of aryloxyacetaldehyde acetals.Sn(Ⅱ)-β which is a metal modified zeolite with exhibited excellent catalytic ability in this transformation.Thus,using Sn(Ⅱ)-β as catalyst,a wide range of 2,3-unsubstituted benzofurans were obtained from corresponding aryloxyacetaldehyde acetals in good to excellent yields.This work mainly based on the following two aspects.Firstly,Using the method reported in the literature,twenty O-(2,2-diethoxyethyl)phenols were synthesized from corresponding phenol compounds and 2-bromo acetaldehyde diethyl acetal via nucleophilic substitution reaction.Using KOH as base,both electron-deficient and electron-rich phenol compounds could be smoothly nucleophilic substitution reaction with 2-bromo acetaldehyde diethyl acetal to form corresponding O-(2,2-diethoxyethyl)phenols in 68-97%yields.Secondly,the intramolecular cyclization of O-(2,2-diethoxyethyl)phenols was studied under the catalysis of various acids.Various zeolites including MCM-41,MCM-41-Al,MCM-41-S03H,HY,HZSM-5(aluminosilicate zeolite,Si/Al=50 and 25),Hβ and M-β(M represents a metal ion)were screened as catalyst in this cyclization reaction.At the same time,sulfonic acid resin(Amberlyst-15)was tested for comparison.The results showed that Sn(II)-β exhibited the remarkable catalytic ability in this transformation.Under the catalysis of Sn(II)-P,all the synthesized O-(2,2-diethoxyethyl)phenols could be rapidly cyclized to form corresponding 2,3-unsubstituted benzofurans and good to excellent yields were resulted.Especially,when the benzene ring is substituted with electron-containing group,there was a higher yiled.Moreover,reusability study showed that the catalyst Sn(II)-β could be reused at least for 6 runs without losing significant catalytic activity after a simple washing.In summary,a novel protocol for the synthesis of 2,3-unsubstituted benzofurans have been successfully developed from commercially available raw materials.The newly developed procedure was general,facile and efficient.