Chemical and spectroscopic properties of the 3-hydroxythiophene [thiophen-3(2H)-one] system

Abstract

3-Hydroxythiophene 1 spontaneously dimerises to 4,5-dihydro-5-(3-hydroxythien-2-yl)thiophen-3(2H)-one 14. 3-Hydroxythiophenes 1E and 4–10E exist in solvent-dependent equilibrium with their thiophen-3(2H)-one 1K and 4–10K tautomers; the amount of hydroxy tautomer is greater than in the case of the corresponding 3-hydroxypyrroles. 3-Hydroxythiophenes are much less reactive to electrophiles than corresponding 3-hydroxypyrroles, but the 5-methylsulfanyl derivative 10 reacts at the 2-position with methoxymethylene Meldrum's acid and undergoes Vilsmeier formylation. The enolates derived from 3-hydroxythiophenes by treatment with base can be O-alkylated and O-acylated with high regioselectivity. 2,2-Disubstituted thiophen-3(2H)-ones undergo equilibrium conjugate addition with nucleophiles, but the resulting adducts could not be isolated.