Reaction of glyoxal at the ortho position of phenols. Synthesis of 5a,10b-dihydrobenzofuro[2,3-b]benzofurans and 2-(3-benzofuranyl)phenols

Abstract

The acid-catalyzed reaction of glyoxal at the ortho position of phenols has been shown to yield, as initial products, substituted 5a,10b-dihydrobenzofuro[2,3-b]benzofurans, such structures being indicated by the nuclear magnetic resonance spectra, and confirmed in several cases by synthesis via an unambiguous route. At higher temperatures in acidic media, and in the absence of glyoxal, these initial products were converted into the corresponding 2-(3-benzofuranyl)phenols. When glyoxal was present in acidic media, the 2-(3-benzofuranyl)phenols reacted further to give unidentified products of appreciably higher molecular weight.

Polycondensation of bis(cyanoacetate) and a,10bdihydrobenzofuro[2,3‐b]benzofuran‐2,9‐dicarbaldehyde via knoevenagel reaction: Synthesis of donor–acceptor polymers containing shoulder‐to‐shoulder main chains

“…The reaction of phenols and glyoxal is a known reaction leading to benzofuro[2,3‐ b ]benzofuran‐type molecules, which was discussed fully by Coxworth23 and this reaction was used to prepare a thermally stable kind of polymers by Maravigna 24. Further complimentary reactions on the product of p ‐cresol and glyoxal were reported to yield benzofuro[2,3‐ b ]benzofuran‐2,9‐dicarboxylic acid 25.…”

“…In this work, a dialdehyde molecule with the benzofuro[2,3‐ b ]benzofuran structure was synthesized to introduce a Knoevenagel polycondensation reaction. Dialdehyde compound II was prepared in two steps from p ‐cresol and glyoxal: The first step involves the reaction of p ‐cresol and aqueous glyoxal which produced 5 a ,10 b ‐dihydro‐2,9‐dimethylbenzofuro[2,3‐ b ]benzofuran ( I ) 23. The second step was the oxidation of methyl groups to aldehyde groups on ( I ) using copper sulfate and peroxydisulfate (Scheme ) 25.…”

Polymers with benzofuro‐benzofuran structures

“…There was, therefore, an inconsistency between the structures of the products which were recognized and reported by these two authors. Decisive structural assignment on the reaction product of glyoxal and phenols was finally carried out by Coxworth 3. He removed this ambiguity over the products, by synthesizing molecules similar to those of the proposed products by using reactions other than the reaction of glyoxal and phenols.…”

“… 100 MHz 1 H NMR of a benzofuro‐benzofuran type molecule synthesized by Coxworth3 to give structural recognition of the reaction product. The upper inset is an expansion of region 6.5–7.5 ppm for better appearance of the 5a hydrogen's doublet.…”

“…The main problem with these two was an inconsistency in the structure assigned to the product by the authors, ie two different structures for the reaction product. Several years later Coxworth3 tried to resolve this inconsistency and this which will be discussed fully in the following parts of the present article. After these initial efforts in relation to the reaction of glyoxals and phenols, several authors used the reaction for various purposes.…”