Furans hydrolytic ring opening

Furan and the simple alkyl-furans are relatively stable to aqueous mineral acids, though furan is instantly decomposed by concentrated sulfuric acid or by Lewis acids, such as aluminium chloride. Furan reacts only slowly with hydrogen chloride, either as the concentrated aqueous acid or in a non-hydroxylic organic solvent. Hot, dilute aqueous mineral acids cause hydrolytic ring opening. 

No pA aH value is available for O-protonation of furan, but it is probably much less basic at oxygen than an aliphatic ether. Acid-catalysed deuteration occurs at an a-positionf 3/4-deuterio-furans are not obtained because, although P-protonation probably occurs, the cation produced is more susceptible to water, leading to hydrolytic ring opening. An estimate of p/faH -10.0 was made for the 2-protonation of 2,5-di-t-butylfuran, which implies a value of about -13 for furan itself. An a-protonated cation, stable in solution, is produced on treatment of 2,5-di-t-butylfuran with concentrated sulfuric acid. ... 

Treatment of 2-phenyl-l,3-benzoxazepines with hydrochloric acid in acetic acid gives benzo-furans by hydrolytic opening of the oxazepine ring, followed by loss of benzamide and re-cyclization.23... 

The furan ring of such substrates may be opened by reduction or hydrolytic processes to afford pyrazines, as illustrated in the following examples ... 

The transformation 123 124 is formally an internal redox process and can be interpreted by a mechanism in which proton-induced formation of a carboxonium intermediate 125 and ROH addition at the 5-position of 125 are the crucial steps, followed by tautomerization and (H+-catalyzed) hydrolytic opening of the furan ring. 

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