Furans and Thiophenes 103Peracids convert thiophenes into S,S-dioxides that are no longer aromatic (no electrons on the sulfur to make up an
aromatic sextet) and which react as dienes in Diels–Alder reactions. Generally sulfur dioxide is extruded from the ini-
tial adducts, leading to aromatisation, as in the example shown (see also page 104).
Controlled oxidation of furans
The conversion of furans into 2,5-dihydro-2,5-dimethoxyfurans (synthetic equivalents of unsaturated 1,4-dicarbonyl
compounds, see page 100) via halogenation in methanol is effectively an oxidation. There are other ways in which
desirable unsaturated 1,4-diketones can be accessed from furans. Clearly, oxidative cleavage of the ring must produce
Z-isomers initially and these can be the isolated products using m-chloroperbenzoic acid or hydrogen peroxide cata-
lysed by methyltrioxorhenium. Acid catalyses isomerisation to the more stable E-isomers.
The oxidation of furyl-2-carbinols can produce 6-hydroxy-2H-pyran-3(6H)-ones, which can be used for the forma-
tion of pyrylium-3-olates (page 73).
Pericyclic reactions
Furan, the ‘least aromatic’ of the trio furan/thiophene/pyrrole, reacts readily with electron-defi cient dienophiles; the
exo structure of its product using maleic anhydride is the result of a readily reversible cycloaddition that allows the
kineticendo product to be converted into the more stable exo isomer. At higher reaction temperatures or under very
high pressures, alkynes and even electron-rich alkenes will cycloadd to furan. Pyrroles will only act as 4 components
in Diels–Alder reactions when an electron-withdrawing substituent is present on the nitrogen to make them ‘less
aromatic’ – more like a diene (page 82).