Furans catechols

As depicted in the following scheme, in the presence of sodium iodate and pyridine, several 5,6-dihydroxylated benzofuran derivatives were synthesized via an oxidation-Michael addition of P-dicarbonyl compounds to catechols in a one-pot procedure <06TL2615 06JHC1673>. A novel additive Pummerer reaction of 2-benzo[fc]furan sulfilimines with carbon nucleophiles derived from P-dicarbonyl compounds was also employed to the synthesis of 2,3-disubstituted benzo[b]furans <06TL595>. 

The furanoid ring in benzo[6 ]furan is susceptible to attack by oxidants. Permanganate and chromic acid give derivatives of 2-hydroxybenzoic acid with compounds unsubstituted at the 3-position, but compounds with a 3-methyl or a 3-aryl substituent give derivatives of 2-hydroxyacetophenone or 2-hydroxybenzophenone. Ozonolysis of benzo[6]furan affords 2-hydroxybenzoic acid, 2-hydroxybenzaldehyde and some catechol produced via its diformate. Before the advent of NMR spectroscopy these methods were used in structural elucidation of benzofuranoid natural products, as in the case of O-methyleuparin (Scheme 26). 

As shown below, polyphenol-based benzo[A furans were biosynthetically made from catechols and 1,3-dicarbonyl compounds in the presence of laccase <07T10958 07TL5073>. A similar type of benzo[A]furan was also obtained via electrochemical oxidation of catechols and methyl acetoacetate <07T3894>. 

Furans with suitable substituents in the 2-position can be transformed in acid conditions into phenols. The reaction proceeds through cyclic acetals of 1,4-dicarbonyl compounds, which then undergo an intramolecular condensation. Equations 94 and 95 show some examples where different catechols have been prepared by refluxing several types of tetrahydrofuran dimethyl acetals with dilute hydrochloric acid. 

Previously, Ascic et al. [43] reported the same reaction with the Hoveyda-Grubbs II catalyst and trifluoroacetic acid as a bicatalytic system, but as a racemic version [43]. The intermediate acyliminium ion readily attacks also other tethered nucleophiles, such as trimethoxybenzene, benzothiophene, thiophene, 0,0-dimethyl catechol, and furan, intramolecularly. When tryptophane or the corresponding alanine derivatives are employed, reasonable levels of diastere-oselectivity can be observed for the terminal Pictet-Spengler acyhminium cyclization. Even a tethered alcohol functionahty diastereoselectively traps the chiral acyhminium intermediate, furnishing bicychc OAf-acetals. 

Detoxification procedures are often required for acidic and steam-exploded lignocellulosic hydrolysates, as variable concentrations of toxic furans (furfural and 5-HMF), lignin degradation compounds (aromatics and phenolics including vanillin, catechol, guaiacol, ferulic acid, syringaldehyde, and 4-hydroxy-... 

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